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Large sample datasets of in situ evapotranspiration (ET) measurements with well documented data provenance and quality assurance are critical for water management and many fields of earth science research. We present a post-processed ET oriented dataset at daily and monthly timesteps, from 161 stations, including 148 eddy covariance flux towers, that were chosen based on their data quality from nearly 350 stations across the contiguous United States. In addition to ET, the data includes energy and heat fluxes, meteorological measurements, and reference ET downloaded from gridMET for each flux station. Data processing techniques were conducted in a reproducible manner using open-source software. Most data initially came from the public AmeriFlux network, however, several different networks (e.g., the USDA-Agricultural Research Service) and university partners provided data that was not yet public. Initial half-hourly energy balance data were gap-filled and aggregated to daily frequency, and turbulent fluxes were corrected for energy balance closure error using the FLUXNET2015/ONEFlux energy balance ratio approach. Metadata, diagnostics of energy balance, and interactive graphs of time series data are included for each station. Although the dataset was developed primarily to benchmark satellite-based remote sensing ET models of the OpenET initiative, there are many other potential uses, such as validation for a range of regional hydrologic and atmospheric models.

Irrigation and other agricultural management practices play a key role in land surface fluxes and their interactions with atmospheric processes. California's Central Valley agricultural productivity is strongly linked to water availability associated with conveyance infrastructure and groundwater, but greater scrutiny over agricultural water use requires better practices particularly during extended and severe drought conditions. The future of irrigated agriculture in California is expected to be characterized neither by perpetual scarcity nor by widespread abundance. Thus, further advancing irrigation technologies and improving management practices will be key for California's agriculture sustainability. In this study, we present micrometeorological observations from the Grape Remote Sensing Atmospheric Profile and Evapotranspiration eXperiment (GRAPEX) project. Daily, seasonal, and inter-seasonal surface flux patterns and relationships across five vineyards over three distinct California wine production regions were investigated. Vineyard actual evapotranspiration showed significant differences at the sub-daily and daily scale when comparisons across wine production regions and varieties were performed. Water use in vineyards in the Central Valley was about 70% greater in comparison to the vineyards at the North Coast area due to canopy size, atmospheric demand, and irrigation inputs. Inter-annual variability of surface fluxes was also significant, even though, overall weather conditions (i.e., air temperature, vapor pressure deficit, wind speed, and solar radiation) were not significantly different. Thus, not only irrigation but also other management practices played a key role in seasonal water use, and given these differences, we conclude that further advancing ground-based techniques to quantify crop water use at an operational scale will be key to facing California's agriculture present and future water challenges.Supplementary informationThe online version contains supplementary material available at 10.1007/s00271-022-00784-0.

Background and aimsLiving root tissues significantly constrain plant water uptake under drought, but we lack functional traits to feasibly screen diverse plants for variation in the drought responses of these tissues. Water stress causes roots to lose volume and turgor, which are crucial to root structure, hydraulics and growth. Thus, we hypothesized that root pressure-volume (p-v) curve traits, which quantify the effects of water potential on bulk root turgor and volume, would capture differences in rootstock drought tolerance.MethodsWe used a greenhouse experiment to evaluate relationships between root p-v curve traits and gas exchange, whole-plant hydraulic conductance and biomass under drought for eight grapevine rootstocks that varied widely in drought performance in field trials (101-14, 110R, 420A, 5C, 140-Ru, 1103P, Ramsey and Riparia Gloire), grafted to the same scion variety (Vitis vinifera 'Chardonnay').Key resultsThe traits varied significantly across rootstocks, and droughted vines significantly reduced root turgor loss point (πtlp), osmotic potential at full hydration (πo) and capacitance (C), indicating that roots became less susceptible to turgor loss and volumetric shrinkage. Rootstocks that retained a greater root volume (i.e. a lower C) also maintained more gas exchange under drought. The rootstocks that previous field trials have classified as drought tolerant exhibited significantly lower πtlp, πo and C values in well-watered conditions, but significantly higher πo and πtlp values under water stress, than the varieties classified as drought sensitive.ConclusionsThese findings suggest that acclimation in root p-v curve traits improves gas exchange in persistently dry conditions, potentially through impacts on root hydraulics or root to shoot chemical signalling. However, retaining turgor and volume in previously unstressed roots, as these roots deplete wet soil to moderately negative water potentials, could be more important to drought performance in the deep, highly heterogenous rooting zones which grapevines develop under field conditions.

The supply of water to a plant canopy is dependent on the xylem pathway connecting roots to leaves. In some plants, sectored xylem pathways can restrict resource distribution, resulting in variable quality of organs in the shoots, yet little is known about the effects of sectoring in crop cultivars. In this study, we combined sap flow measurements and infusion of xylem-specific dyes to document functional conductive area and flow pathways from roots to shoots of 20-year-old Thompson Seedless and 8-year-old Chardonnay grapevines. Sap flow measurements and dye infusion demonstrated that water flowed predominantly in discrete xylem (visually identifiable from the trunk surface) sectors along the trunk axis, each supplying limited portions of the canopy. Functional conductive area in the trunk was proportional to that in the shoots even though sector size varied considerably between vines. Leaf area removal experiments further demonstrated sectoring in grapevines; sap flow decreased by >90 % in trunk sectors connected to excised shoots while it remained constant in trunk sectors supplying intact portions of the canopy. Despite the functional sectoring in grapevines, a high degree of interconnectivity of trunk xylem in the tangential direction was confirmed with synchrotron-based micro-computed tomography (microCT) and dye crossover infusion studies. Fruit attached to dyed canes was also similarly sectored; no clusters exhibited dye on non-dyed canes, while 97 % of clusters attached to dyed canes exhibited dye infusion. The dye travelled down the cluster rachis and appeared to accumulate at the pedicel/berry junction, but only on dyed canes. These findings suggest that xylem in grapevine trunks is integrated anatomically, but functions in a sectored manner due to high axial hydraulic conductivity. The functional sectoring of grapevine xylem documented here has important implications for management practices in vineyards and for fruit cluster uniformity within single grapevine.

Cortical lacunae caused by drought, especially observed in hybrids originating from Vitis rupestris, disrupt the connection between roots and soil. Yet, the physiological processes behind lacuna formation during drought and its consistency across Vitis species remain unclear. Here, we used a root pressure probe to investigate fine root hydraulic and mechanical properties, in the arid-adapted R-65 and drought-susceptible 101-14Mgt cultivars. We then performed P-V curves, root sap osmolality, and electrolyte leakage (EL) and used fluorescent light microscopy techniques. Only 101-14Mgt showed lacunae formation during drought due to its stiffer cortical tissue, unlike R-65. Lacunae resulted in a notable decline in root hydraulic conductivity during severe drought, with increased EL and root sap osmolality, indicating potential cellular damage. R-65 displayed different and xerophyte-like characteristics featuring a higher turgor loss point and decreased root capacitance, essential for maintaining root structural integrity in arid conditions. Our findings highlight lacuna formation is impacted by root tissue elasticity possibly linked to specific Vitis species favoring deeper rooting. In arid-adapted grapevines, hydraulic regulators such as reduced turgor loss point, and root capacitance could contribute to enhanced drought tolerance., (© 2024 John Wiley & Sons Ltd.)

Heat waves (HWs) pose a significant threat to California agriculture, with potential adverse effects on crop photosynthetic capacity, quality and yield, all of which contribute to significant economic loss. Lack of heat-resilient cultivars puts perennial crop production under severe threat due to increasing HW frequency, duration and intensity. Currently, available walnut cultivars are highly sensitive to abiotic stress, and germplasm collections provide potential solutions via genotypes native to varied climates. We screened nine English walnut accessions (Juglans regia) for physiological heat stress resilience and recovery in the USDA-ARS National Clonal Germplasm over 2-years, and identified accessions with superior resilience to heat stress. Heat stress impacted photosynthetic capacity in most accessions, as evidenced by reductions in net (A n ) and maximum (A max ) assimilation rates, quantum efficiency of PSII, and changes in stomatal conductance (g s ). However, two accessions exhibited either higher or complete recovery post-irrigation. This aligns with the established practice of using irrigation to mitigate heat stress, as it improved recovery for several accessions, with A3 and A5 demonstrating the most resilience. One of these two superior accessions is native to one of the hottest and driest habitats of all studied accessions. These same accessions exhibited the highest A n under non-stressed conditions and at higher temperatures of 35° to 45°C. Higher performance for A3 and A5 under HWs was associated with greater carboxylation rates, electron transport rates, and A max . All accessions suffered significant declines in photosynthetic performance at 45°C, which were the ambient leaf temperatures approached during record-setting heat waves in California during September 2022., (© 2024 John Wiley & Sons Ltd.)

Leaf architecture impacts the ease of gases diffusion, biochemical process, and photosynthetic performance. For balsam poplar, a widespread North American species, the influence of water availability on leaf anatomy and subsequent photosynthetic performance remains unknown. To address this shortcoming, we characterized the anatomical changes across the leaf profile in three-dimensional space for saplings subjected to soil drying and rewatering using X-ray microcomputed tomography. Our hypothesis was that higher abundance of bundle sheet extensions (BSE) minimizes drought-induced changes in intercellular airspace volume relative to mesophyll volume (i.e. mesophyll porosity, θ IAS ) and aids recovery by supporting leaf structural integrity. Leaves of 'Carnduff-9' with less abundant BSEs exhibited greater θ IAS , higher spongy mesophyll surface area, reduced palisade mesophyll surface area, and less veins compared with 'Gillam-5'. Under drought conditions, Carnduff-9 showed significant changes in θ IAS across leaf profile while that was little for 'Gillam-5'. Under rewatered conditions, drought-induced changes in θ IAS were fully reversible in 'Gillam-5' but not in 'Carnduff-9'. Our data suggest that a 'robust' leaf structure with higher abundance of BSEs, reduced θ IAS , and relatively large mesophyll surface area provides for improved photosynthetic capacity under drought and supports recovery in leaf architecture after rewatering in balsam poplar., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

California contains a broad geography over which climate conditions can be suitable for cultivating multiple varieties of winegrapes. However, climate change is projected to make winegrape cultivation more challenging across many of California's winegrowing regions. In order to understand the potential effects of climate change on winegrapes, this study models variety-specific phenology for six winegrape varieties and quantifies the change in phenology and viticulturally-important agroclimate metrics over 12 of California's American Viticultural Areas (AVAs) by the mid-21st century. Results show more rapid development for winegrapes with earlier budburst, flowering, veraison, and maturation across all varieties and AVAs. Cabernet Sauvignon shows the greatest change in phenology timing, while Chardonnay shows the least change. Likewise, the West Sonoma Coast AVA shows the greatest average change in phenology timing across varieties and development stages and Lodi AVA shows the least. Projected changes in agroclimatic metrics include an additional month of potentially damaging heat days (above 35 °C) in some AVAs, and decreases in frost days. These results have implications for numerous factors related to viticultural production, including water resources management and crop yield and quality, and underscore the need for California winegrape growers to improve their resilience to climate change by adopting strategies such as increasing soil health and water use efficiency and selecting cultivars suited for future climate conditions. By conducting climate effects analyses at the variety-specific and AVA scale, important information is provided to the winegrowing industry at a resolution that can support decision-making towards resilience., (© 2024. The Author(s).)

Background and Aims: Many agricultural areas are expected to face hotter, drier conditions from climate change. Understanding the mechanisms that crops use to mitigate these stresses can guide breeding for more tolerant plant material. We tested relationships between traits, physiological function in hot conditions and historical climate associations to evaluate these mechanisms for winegrapes. We expected a more negative leaf osmotic potential at full hydration (πo), which reduces leaf turgor loss during drought, and either a metabolically cheaper or more osmoprotectant leaf chemical composition, to allow cultivars associated with hot, dry regions to maintain greater gas exchange in hot growing conditions., Methods: We measured πo, gas exchange and leaf chemistry for seven commercially important winegrape cultivars that vary widely in historical climate associations. Vines were grown in common-garden field conditions in a hot wine-growing region (Davis, CA, USA) and measured over the hottest period of the growing season (July-September)., Key Results: The value of πo varied significantly between cultivars, and all cultivars significantly reduced πo (osmotically adjusted) over the study period, although osmotic adjustment did not vary across cultivars. The value of πo was correlated with gas exchange and climate associations, but in the direction opposite to expected. Photosynthesis and πo were higher in the cultivars associated with hotter, less humid regions. Leaf chemical composition varied between cultivars but was not related to climate associations., Conclusions: These findings suggest that maintenance of leaf turgor is not a primary limitation on grapevine adaptation to hot or atmospherically dry growing conditions. Thus, selecting for a more negative πo or greater osmotic adjustment is not a promising strategy to develop more climate-resilient grape varieties, contrary to findings for other crops. Future work is needed to identify the mechanisms increasing photosynthesis in the cultivars associated with hot, dry regions., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Annals of Botany Company.)

Background and Aims: Drought events have devasting impacts on grape berry production. The aim of this study was to investigate berry growth in the context of leaf stomatal closure under progressive drought stress., Methods: Potted grapevine plants (varieties 'Syrah' and 'Cabernet Sauvignon') were evaluated at pre-verasion (30-45 d after anthesis, DAA) and post-veraison (90-107 DAA). Berry diameter, berry absolute growth rate (AGR), leaf stomatal conductance (Gs) at midday, plant water potential at predawn and midday (ΨPD and ΨMD, respectively), and soil relative water content were measured repeatedly. The ΨPD-threshold of 90 % loss in stomatal conductance (Gs10, i.e. complete stomatal closure) was determined. Data were related to plant dehydration phases I, II and III with corresponding boundaries Θ1 and Θ2, using the water potential curve method., Key Results: At pre-veraison, berry AGR declined together with leaf Gs in response to soil drying in both varieties. Berry AGR transitioned from positive to negative (shrinkage) values when leaf Gs approached zero. The Gs10-threshold was -0.81 MPa in 'Syrah' and -0.74 MPa in 'Cabernet Sauvignon' and was linked to boundary Θ1. At post-veraison, berry AGR was negligible and negative AGR values were not intensified by increasing drought stress in either variety., Conclusion: Leaf complete stomatal closure under progressive drought stress coincides with cessation of berry growth followed by shrinkage at pre-veraison (growth stage 1)., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Many insects feed on xylem or phloem sap of vascular plants. Although physical damage to the plant is minimal, the process of insect feeding can transmit lethal viruses and bacterial pathogens. Disparities between insect-mediated pathogen transmission efficiency have been identified among xylem sap-feeding insects; however, the mechanistic drivers of these trends are unclear. Identifying and understanding the structural factors and associated integrated functional components that may ultimately determine these disparities are critical for managing plant diseases. Here, we applied synchrotron-based X-ray microcomputed tomography to digitally reconstruct the morphology of three xylem sap-feeding insect vectors of plant pathogens: Graphocephala atropunctata (blue-green sharpshooter; Hemiptera, Cicadellidae) and Homalodisca vitripennis (glassy-winged sharpshooter; Hemiptera, Cicadellidae), and the spittlebug Philaenus spumarius (meadow spittlebug; Hemiptera, Aphrophoridae). The application of this technique revealed previously undescribed anatomical features of these organisms, such as key components of the salivary complex. The visualization of the 3D structure of the precibarial valve led to new insights into the mechanism of how this structure functions. Morphological disparities with functional implications between taxa were highlighted as well, including the morphology and volume of the cibarial dilator musculature responsible for extracting xylem sap, which has implications for force application capabilities. These morphological insights will be used to target analyses illuminating functional differences in feeding behavior., (© 2023 The Authors. Journal of Morphology published by Wiley Periodicals LLC.)

Balsam poplar (Populus balsamifera L.) is a widespread tree species in North America with significant ecological and economic value. However, little is known about the susceptibility of saplings to drought-induced embolism and its link to water release from surrounding xylem fibers. Questions remain regarding localized mechanisms that contribute to the survival of saplings in vivo of this species under drought. Using X-ray micro-computed tomography on intact saplings of genotypes Gillam-5 and Carnduff-9, we found that functional vessels are embedded in a matrix of water-filled fibers under well-watered conditions in both genotypes. However, water-depleted fibers started to appear under moderate drought stress while vessels remained water-filled in both genotypes. Drought-induced xylem embolism susceptibility was comparable between genotypes, and a greater frequency of smaller diameter vessels in GIL-5 did not increase embolism resistance in this genotype. Despite having smaller vessels and a total vessel number that was comparable to CAR-9, stomatal conductance was generally higher in GIL-5 compared to CAR-9. In conclusion, our in vivo data on intact saplings indicate that differences in embolism susceptibility are negligible between GIL-5 and CAR-9, and that fiber water release should be considered as a mechanism that contributes to the maintenance of vessel functional status in saplings of balsam poplar experiencing their first drought event., (© 2023 The Authors. Physiologia Plantarum published by John Wiley & Sons Ltd on behalf of Scandinavian Plant Physiology Society.)

Large sample datasets of in situ evapotranspiration (ET) measurements with well documented data provenance and quality assurance are critical for water management and many fields of earth science research. We present a post-processed ET oriented dataset at daily and monthly timesteps, from 161 stations, including 148 eddy covariance flux towers, that were chosen based on their data quality from nearly 350 stations across the contiguous United States. In addition to ET, the data includes energy and heat fluxes, meteorological measurements, and reference ET downloaded from gridMET for each flux station. Data processing techniques were conducted in a reproducible manner using open-source software. Most data initially came from the public AmeriFlux network, however, several different networks (e.g., the USDA-Agricultural Research Service) and university partners provided data that was not yet public. Initial half-hourly energy balance data were gap-filled and aggregated to daily frequency, and turbulent fluxes were corrected for energy balance closure error using the FLUXNET2015/ONEFlux energy balance ratio approach. Metadata, diagnostics of energy balance, and interactive graphs of time series data are included for each station. Although the dataset was developed primarily to benchmark satellite-based remote sensing ET models of the OpenET initiative, there are many other potential uses, such as validation for a range of regional hydrologic and atmospheric models., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

X-ray micro-computed tomography (X-ray μCT) has enabled the characterization of the properties and processes that take place in plants and soils at the micron scale. Despite the widespread use of this advanced technique, major limitations in both hardware and software limit the speed and accuracy of image processing and data analysis. Recent advances in machine learning, specifically the application of convolutional neural networks to image analysis, have enabled rapid and accurate segmentation of image data. Yet, challenges remain in applying convolutional neural networks to the analysis of environmentally and agriculturally relevant images. Specifically, there is a disconnect between the computer scientists and engineers, who build these AI/ML tools, and the potential end users in agricultural research, who may be unsure of how to apply these tools in their work. Additionally, the computing resources required for training and applying deep learning models are unique, more common to computer gaming systems or graphics design work, than to traditional computational systems. To navigate these challenges, we developed a modular workflow for applying convolutional neural networks to X-ray μCT images, using low-cost resources in Google's Colaboratory web application. Here we present the results of the workflow, illustrating how parameters can be optimized to achieve best results using example scans from walnut leaves, almond flower buds, and a soil aggregate. We expect that this framework will accelerate the adoption and use of emerging deep learning techniques within the plant and soil sciences., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Rippner, Raja, Earles, Momayyezi, Buchko, Duong, Forrestel, Parkinson, Shackel, Neyhart and McElrone.)

Similar to other cropping systems, few walnut cultivars are used as scion in commercial production. Germplasm collections can be used to diversify cultivar options and hold potential for improving crop productivity, disease resistance and stress tolerance. In this study, we explored the anatomical and biochemical bases of photosynthetic capacity and response to water stress in 11 Juglans regia accessions in the U.S. department of agriculture, agricultural research service (USDA-ARS) National Clonal Germplasm. Net assimilation rate (A n ) differed significantly among accessions and was greater in lower latitudes coincident with higher stomatal and mesophyll conductances, leaf thickness, mesophyll porosity, gas-phase diffusion, leaf nitrogen and lower leaf mass and stomatal density. High CO 2 -saturated assimilation rates led to increases in A n under diffusional and biochemical limitations. Greater A n was found in lower-latitude accessions native to climates with more frost-free days, greater precipitation seasonality and lower temperature seasonality. As expected, water stress consistently impaired photosynthesis with the highest % reductions in lower-latitude accessions (A3, A5 and A9), which had the highest A n under well-watered conditions. However, A n for A3 and A5 remained among the highest under dehydration. J. regia accessions, which have leaf structural traits and biochemistry that enhance photosynthesis, could be used as commercial scions or breeding parents to enhance productivity., (© 2022 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.)

Leaves balance CO 2 and radiative absorption while maintaining water transport to maximise photosynthesis. Related species with contrasting leaf anatomy can provide insights into inherent and stress-induced links between structure and function for commonly measured leaf traits for important crops. We used two walnut species with contrasting mesophyll anatomy to evaluate these integrated exchange processes under non-stressed and drought conditions using a combination of light microscopy, X-ray microCT, gas exchange, hydraulic conductance, and chlorophyll distribution profiles through leaves. Juglans regia had thicker palisade mesophyll, higher fluorescence in the palisade, and greater low-mesophyll porosity that were associated with greater gas-phase diffusion (g IAS ), stomatal and mesophyll (g m ) conductances and carboxylation capacity. More and highly-packed mesophyll cells and bundle sheath extensions (BSEs) in Juglans microcarpa led to higher fluorescence in the spongy and in proximity to the BSEs. Both species exhibited drought-induced reductions in mesophyll cell volume, yet the associated increases in porosity and g IAS were obscured by declines in biochemical activity that decreased g m . Inherent differences in leaf anatomy between the species were linked to differences in gas exchange, light absorption and photosynthetic capacity, and drought-induced changes in leaf structure impacted performance via imposing species-specific limitations to light absorption, gas exchange and hydraulics., (© 2022 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.)

Improved accuracy of evapotranspiration (ET) estimation, including its partitioning between transpiration (T) and surface evaporation (E), is key to monitor agricultural water use in vineyards, especially to enhance water use efficiency in semi-arid regions such as California, USA. Remote-sensing methods have shown great utility in retrieving ET from surface energy balance models based on thermal infrared data. Notably, the two-source energy balance (TSEB) has been widely and robustly applied in numerous landscapes, including vineyards. However, vineyards add an additional complexity where the landscape is essentially made up of two distinct zones: the grapevine and the interrow, which is often seasonally covered by an herbaceous cover crop. Therefore, it becomes more complex to disentangle the various contributions of the different vegetation elements to total ET, especially through TSEB, which assumes a single vegetation source over a soil layer. As such, a remote-sensing-based three-source energy balance (3SEB) model, which essentially adds a vegetation source to TSEB, was applied in an experimental vineyard located in California's Central Valley to investigate whether it improves the depiction of the grapevine-interrow system. The model was applied in four different blocks in 2019 and 2020, where each block had an eddy-covariance (EC) tower collecting continuous flux, radiometric, and meteorological measurements. 3SEB's latent and sensible heat flux retrievals were accurate with an overall RMSD ~ 50 W/m 2 compared to EC measurements. 3SEB improved upon TSEB simulations, with the largest differences being concentrated in the spring season, when there is greater mixing between grapevine foliage and the cover crop. Additionally, 3SEB's modeled ET partitioning (T/ET) compared well against an EC T/ET retrieval method, being only slightly underestimated. Overall, these promising results indicate 3SEB can be of great utility to vineyard irrigation management, especially to improve T/ET estimations and to quantify the contribution of the cover crop to ET. Improved knowledge of T/ET can enhance grapevine water stress detection to support irrigation and water resource management., Supplementary Information: The online version contains supplementary material available at 10.1007/s00271-022-00787-x., Competing Interests: Conflict of interestOn behalf of all authors, the corresponding author states that there is no conflict of interest., (© The Author(s) 2022.)

Characterization of model errors is important when applying satellite-driven evapotranspiration (ET) models to water resource management problems. This study examines how uncertainty in meteorological forcing data and land surface modeling propagate through to errors in final ET data calculated using the Satellite Irrigation Management Support (SIMS) model, a computationally efficient ET model driven with satellite surface reflectance values. The model is applied to three instrumented winegrape vineyards over the 2017-2020 time period and the spatial and temporal variation in errors are analyzed. We illustrate how meteorological data inputs can introduce biases that vary in space and at seasonal timescales, but that can persist from year to year. We also observe that errors in SIMS estimates of land surface conductance can have a particularly strong dependence on time of year. Overall, meteorological inputs introduced RMSE of 0.33-0.65 mm/day (7-27%) across sites, while SIMS introduced RMSE of 0.55-0.83 mm/day (19-24%). The relative error contribution from meteorological inputs versus SIMS varied across sites; errors from SIMS were larger at one site, errors from meteorological inputs were larger at a second site, and the error contributions were of equal magnitude at the third site. The similar magnitude of error contributions is significant given that many satellite-driven ET models differ in their approaches to estimating land surface conductance, but often rely on similar or identical meteorological forcing data. The finding is particularly notable given that SIMS makes assumptions about the land surface (no soil evaporation or plant water stress) that do not always hold in practice. The results of this study show that improving SIMS by eliminating these assumptions would result in meteorological inputs dominating the error budget of the model on the whole. This finding underscores the need for further work on characterizing spatial uncertainty in the meteorological forcing of ET., Supplementary Information: The online version contains supplementary material available at 10.1007/s00271-022-00808-9., Competing Interests: Conflict of interestOn behalf of all authors, the corresponding author states that there is no conflict of interest., (© The Author(s) 2022.)

Robust information on consumptive water use (evapotranspiration, ET) derived from remote sensing can significantly benefit water decision-making in agriculture, informing irrigation schedules and water management plans over extended regions. To be of optimal utility for operational usage, these remote sensing ET data should be generated at the sub-field spatial resolution and daily-to-weekly timesteps commensurate with the scales of water management activities. However, current methods for field-scale ET retrieval based on thermal infrared (TIR) imaging, a valuable diagnostic of canopy stress and surface moisture status, are limited by the temporal revisit of available medium-resolution (100 m or finer) thermal satellite sensors. This study investigates the efficacy of a data fusion method for combining information from multiple medium-resolution sensors toward generating high spatiotemporal resolution ET products for water management. TIR data from Landsat and ECOSTRESS (both at ~ 100-m native resolution), and VIIRS (375-m native) are sharpened to a common 30-m grid using surface reflectance data from the Harmonized Landsat-Sentinel dataset. Periodic 30-m ET retrievals from these combined thermal data sources are fused with daily retrievals from unsharpened VIIRS to generate daily, 30-m ET image timeseries. The accuracy of this mapping method is tested over several irrigated cropping systems in the Central Valley of California in comparison with flux tower observations, including measurements over irrigated vineyards collected in the GRAPEX campaign. Results demonstrate the operational value added by the augmented TIR sensor suite compared to Landsat alone, in terms of capturing daily ET variability and reduced latency for real-time applications. The method also provides means for incorporating new sources of imaging from future planned thermal missions, further improving our ability to map rapid changes in crop water use at field scales., Competing Interests: Conflict of interestOn behalf of all authors, the corresponding author states that there is no conflict of interest., (© The Author(s) 2022.)

Leaf habit is a major axis of plant diversity that has consequences for carbon balance since the leaf is the primary site of photosynthesis. Nonstructural carbohydrates (NSCs) produced by photosynthesis can be allocated to storage and serve as a resiliency mechanism to future abiotic and biotic stress. However, how leaf habit affects NSC storage in an evolutionary context has not been shown. Using a comparative physiological framework and an analysis of evolutionary model fitting, we examined if variation in NSC storage is explained by leaf habit. We measured sugar and starch concentrations in 51 oak species (Quercus spp.) growing in a common garden and representing multiple evolutions of three different leaf habits (deciduous, brevideciduous and evergreen). The best fitting evolutionary models indicated that deciduous oak species are evolving towards higher NSC concentrations than their brevideciduous and evergreen relatives. Notably, this was observed for starch (the primary storage molecule) in the stem (a long-term C storage organ). Overall, our work provides insight into the evolutionary drivers of NSC storage and suggests that a deciduous strategy may confer an advantage against stress associated with a changing world. Future work should examine additional clades to further corroborate this idea., (© 2021 The Authors. New Phytologist © 2021 New Phytologist Foundation.)

Some grapevine rootstocks perform better than others during and after drought events, yet it is not clear how inherent and stress-induced differences in root morphology and anatomy along the length of fine roots are involved in these responses. Using a variety of growing conditions and plant materials, we observed significant differences in root diameter, specific root length (SRL) and root diameter distribution between two commonly used commercial grapevine rootstocks: Richter 110 (110R; drought resistant) and Millardet et de Grasset 101-14 (101-14Mgt; drought sensitive). The 110R consistently showed greater root diameters with smaller SRL and proportion of root length comprised of fine lateral roots. The 110R also exhibited significantly greater distance from tip to nearest lateral, longer white root length, and larger proportion of root length that is white under drought stress. Mapping of fine root cortical lacunae showed similar patterns between the rootstocks; mechanical failure of cortical cells was common in the maturation zone, limited near the root tip, and increased with drought stress for both genotypes; however, lacuna formed under wetter soil conditions in 110R. Results suggest that drought resistance in grapevine rootstocks is associated with thick, limitedly branched roots with a larger proportion of white-functional roots that tend to form lacuna under more mild water deficit, all of which likely favor continued resource acquisition at depth.

Xylem networks are vulnerable to the formation and spread of gas embolisms that reduce water transport. Embolisms spread through interconduit pits, but the three-dimensional (3D) complexity and scale of xylem networks means that the functional implications of intervessel connections are not well understood. Here, xylem networks of grapevine (Vitis vinifera L.) were reconstructed from 3D high-resolution X-ray micro-computed tomography (microCT) images. Xylem network performance was then modeled to simulate loss of hydraulic conductivity under increasingly negative xylem sap pressure simulating drought stress conditions. We also considered the sensitivity of xylem network performance to changes in key network parameters. We found that the mean pit area per intervessel connection was constant across 10 networks from three, 1.5-m stem segments, but short (0.5 cm) segments fail to capture complete network connectivity. Simulations showed that network organization imparted additional resistance to embolism spread beyond the air-seeding threshold of pit membranes. Xylem network vulnerability to embolism spread was most sensitive to variation in the number and location of vessels that were initially embolized and pit membrane vulnerability. Our results show that xylem network organization can increase stem resistance to embolism spread by 40% (0.66 MPa) and challenge the notion that a single embolism can spread rapidly throughout an entire xylem network., (© American Society of Plant Biologists 2021. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Maintaining high rates of photosynthesis in leaves requires efficient movement of CO 2 from the atmosphere to the mesophyll cells inside the leaf where CO 2 is converted into sugar. CO 2 diffusion inside the leaf depends directly on the structure of the mesophyll cells and their surrounding airspace, which have been difficult to characterize because of their inherently three-dimensional organization. Yet faster CO 2 diffusion inside the leaf was probably critical in elevating rates of photosynthesis that occurred among angiosperm lineages. Here we characterize the three-dimensional surface area of the leaf mesophyll across vascular plants. We show that genome size determines the sizes and packing densities of cells in all leaf tissues and that smaller cells enable more mesophyll surface area to be packed into the leaf volume, facilitating higher CO 2 diffusion. Measurements and modelling revealed that the spongy mesophyll layer better facilitates gaseous phase diffusion while the palisade mesophyll layer better facilitates liquid-phase diffusion. Our results demonstrate that genome downsizing among the angiosperms was critical to restructuring the entire pathway of CO 2 diffusion into and through the leaf, maintaining high rates of CO 2 supply to the leaf mesophyll despite declining atmospheric CO 2 levels during the Cretaceous.

Pierce's disease (PD) in grapevine (Vitis vinifera) is caused by the bacterial pathogen Xylella fastidiosa. X. fastidiosa is limited to the xylem tissue and following infection induces extensive plant-derived xylem blockages, primarily in the form of tyloses. Tylose-mediated vessel occlusions are a hallmark of PD, particularly in susceptible V. vinifera. We temporally monitored tylose development over the course of the disease to link symptom severity to the level of tylose occlusion and the presence/absence of the bacterial pathogen at fine-scale resolution. The majority of vessels containing tyloses were devoid of bacterial cells, indicating that direct, localized perception of X. fastidiosa was not a primary cause of tylose formation. In addition, we used X-ray computed microtomography and machine-learning to determine that X. fastidiosa induces significant starch depletion in xylem ray parenchyma cells. This suggests that a signalling mechanism emanating from the vessels colonized by bacteria enables a systemic response to X. fastidiosa infection. To understand the transcriptional changes underlying these phenotypes, we integrated global transcriptomics into the phenotypes we tracked over the disease spectrum. Differential gene expression analysis revealed that considerable transcriptomic reprogramming occurred during early PD before symptom appearance. Specifically, we determined that many genes associated with tylose formation (ethylene signalling and cell wall biogenesis) and drought stress were up-regulated during both Phase I and Phase II of PD. On the contrary, several genes related to photosynthesis and carbon fixation were down-regulated during both phases. These responses correlate with significant starch depletion observed in ray cells and tylose synthesis in vessels., (© 2020 The Authors. Molecular Plant Pathology published by British Society for Plant Pathology and John Wiley & Sons Ltd.)

Structural changes during severe drought stress greatly modify the hydraulic properties of fine roots. Yet, the physiological basis behind the restoration of fine root water uptake capacity during water recovery remains unknown. Using neutron radiography (NR), X-ray micro-computed tomography (micro-CT), fluorescence microscopy, and fine root hydraulic conductivity measurements (Lp r ), we examined how drought-induced changes in anatomy and hydraulic properties of contrasting grapevine rootstocks are coupled with fine root growth dynamics during drought and return of soil moisture. Lacunae formation in drought-stressed fine roots was associated with a significant decrease in fine root Lp r for both rootstocks. However, lacunae formation occurred under milder stress in the drought-resistant rootstock, 110R. Suberin was deposited at an earlier developmental stage in fine roots of 101-14Mgt (i.e. drought susceptible), probably limiting cortical lacunae formation during mild stress. During recovery, we found that only 110R fine roots showed rapid re-establishment of elongation and water uptake capacity and we found that soil water status surrounding root tips differed between rootstocks as imaged with NR. These data suggest that drought resistance in grapevine rootstocks is associated with rapid re-establishment of growth and Lp r near the root tip upon re-watering by limiting competing sites along the root cylinder., (© 2020 The Authors New Phytologist © 2020 New Phytologist Trust.)

sUAS (small-Unmanned Aircraft System) and advanced surface energy balance models allow detailed assessment and monitoring (at plant scale) of different (agricultural, urban, and natural) environments. Significant progress has been made in the understanding and modeling of atmosphere-plant-soil interactions and numerical quantification of the internal processes at plant scale. Similarly, progress has been made in ground truth information comparison and validation models. An example of this progress is the application of sUAS information using the Two-Source Surface Energy Balance ( TSEB ) model in commercial vineyards by the Grape Remote sensing Atmospheric Profile and Evapotranspiration eXperiment - GRAPEX Project in California. With advances in frequent sUAS data collection for larger areas, sUAS information processing becomes computationally expensive on local computers. Additionally, fragmentation of different models and tools necessary to process the data and validate the results is a limiting factor. For example, in the referred GRAPEX project, commercial software ( ArcGIS and MS Excel) and Python and Matlab code are needed to complete the analysis. There is a need to assess and integrate research conducted with sUAS and surface energy balance models in a sharing platform to be easily migrated to high performance computing ( HPC ) resources. This research, sponsored by the National Science Foundation FAIR Cyber Training Fellowships, is integrating disparate software and code under a unified language (Python). The Python code for estimating the surface energy fluxes using TSEB2T model as well as the EC footprint analysis code for ground truth information comparison were hosted in myGeoHub site https://mygeohub.org/ to be reproducible and replicable.

Accurate quantification of the partitioning of evapotranspiration (ET) into transpiration and evaporation fluxes is necessary to understanding ecosystem interactions among carbon, water, and energy flux components. ET partitioning can also support the description of atmosphere and land interactions and provide unique insights into vegetation water status. Previous studies have identified leaf area index (LAI) estimation as a key descriptor of biomass conditions needed for the estimation of transpiration and evaporation. LAI estimation in clumped vegetation systems, such as vineyards and orchards, has proven challenging and is strongly related to crop phenological status and canopy management. In this study, a feature extraction model based on previous research was built to generate a total of 202 preliminary variables at a 3.6-by-3.6-meter-grid scale based on submeter-resolution information from a small Unmanned Aerial Vehicle (sUAV) in four commercial vineyards across California. Using these variables, a machine learning model called eXtreme Gradient Boosting (XGBoost) was successfully built for LAI estimation. The XGBoost built-in function requires only six variables relating to vegetation indices and temperature to produce high-accuracy LAI estimation for the vineyard. Using the six-variable XGBoost-based LAI map, two versions of the Two-Source Energy Balance (TSEB) model, TSEB-PT and TSEB-2T were used for energy balance and ET partitioning. Comparing these results with the Eddy-Covariance (EC) tower data, showed that TSEB-PT outperforms TSEB-2T on the estimation of sensible heat flux (within 13% relative error) and surface heat flux (within 34% relative error), while TSEB-2T outperforms TSEB-PT on the estimation of net radiation (within 14% relative error) and latent heat flux (within 2% relative error). For the mature vineyard (north block), TSEB-2T performs better than TSEB-PT in partitioning the canopy latent heat flux with 6.8% relative error and soil latent heat flux with 21.7% relative error; however, for the younger vineyard (south block), TSEB-PT performs better than TSEB-2T in partitioning the canopy latent heat flux with 11.7% relative error and soil latent heat flux with 39.3% relative error.

Drought decreases water transport capacity of leaves and limits gas exchange, which involves reduced leaf leaf hydraulic conductance (Kleaf) in both the xylem and outside-xylem pathways. Some literature suggests that grapevines are hyper-susceptible to drought-induced xylem embolism. We combined Kleaf and gas exchange measurements, micro-computed tomography of intact leaves, and spatially explicit modeling of the outside-xylem pathways to evaluate the role of vein embolism and Kleaf in the responses of two different grapevine cultivars to drought. Cabernet Sauvignon and Chardonnay exhibited similar vulnerabilities of Kleaf and gs to dehydration, decreasing substantially prior to leaf xylem embolism. Kleaf and gs decreased by 80% for both cultivars by Ψ leaf approximately -0.7 MPa and -1.2 MPa, respectively, while leaf xylem embolism initiated around Ψ leaf = -1.25 MPa in the midribs and little to no embolism was detected in minor veins even under severe dehydration for both cultivars. Modeling results indicated that reduced membrane permeability associated with a Casparian-like band in the leaf vein bundle sheath would explain declines in Kleaf of both cultivars. We conclude that during moderate water stress, changes in the outside-xylem pathways, rather than xylem embolism, are responsible for reduced Kleaf and gs. Understanding this mechanism could help to ensure adequate carbon capture and crop performance under drought., (© The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Mechanistic modeling constitutes a powerful tool to unravel complex biological phenomena. This study describes the construction of a mechanistic, dynamic model for grapevine plant growth and canopy biomass (vigor). To parametrize and validate the model, the progeny from a cross of Ramsey ( Vitis champinii ) × Riparia Gloire ( V. riparia ) was evaluated. Plants with different vigor were grown in a greenhouse during the summer of 2014 and 2015. One set of plants was grafted with Cabernet Sauvignon. Shoot growth rate (b), leaf area (LA), dry biomass, whole plant and root specific hydraulic conductance (k H and L pr ), stomatal conductance (g s ), and water potential (Ψ) were measured. Partitioning indices and specific leaf area (SLA) were calculated. The model includes an empirical fit of a purported seasonal pattern of bioactive GAs based on published seasonal evolutionary levels and reference values. The model provided a good fit of the experimental data, with R = 0.85. Simulation of single trait variations defined the individual effect of each variable on vigor determination. The model predicts, with acceptable accuracy, the vigor of a young plant through the measurement of L pr and SLA. The model also permits further understanding of the functional traits that govern vigor, and, ultimately, could be considered useful for growers, breeders and those studying climate change., Competing Interests: The authors declare no conflict of interest., (© 2020 The Author(s).)

Solar-induced fluorescence (SIF) is a promising tool to estimate photosynthesis across scales; however, there has been limited research done at the leaf level to investigate the relationship between SIF and photosynthesis. To help bridge this gap, a LI-COR LI-6800 gas exchange instrument was modified with a visible-near-infrared (VIS-NIR) spectrometer to measure active and passive fluorescence simultaneously. The system was adapted by drilling a hole into the bottom plate of the leaf chamber and inserting a fibre-optic to measure passive steady-state fluorescence ( F t , λ , analogous to SIF) from the abaxial surface of a leaf. This new modification can concurrently measure gas exchange, passive fluorescence and active fluorescence over the same leaf area and will allow researchers to measure leaf-level F t , λ in the field to validate tower-based and satellite measurements. To test the modified instrument, measurements were performed on leaves of well-watered and water-stressed walnut plants at three light levels and a constant air temperature. Measurements on these same plants were also conducted using a similarly modified Walz GFS-3000 gas exchange instrument to compare results. We found a positive linear correlation between F t , λ measurements from the modified LI-6800 and GFS-3000 instruments. We also report a positive linear relationship between F t , λ and normalized steady-state chlorophyll fluorescence ( F t / F o ) from the pulse-amplitude modulation (PAM) fluorometer of the LI-6800 system. Accordingly, this modification will inform the link between spectrally resolved F t , λ and gas exchange-leading to improved interpretation of how remotely sensed SIF tracks changes in the light reactions of photosynthesis., (Published by Oxford University Press on behalf of the Annals of Botany Company 2020.)

Knowledge about physiological stress thresholds provides crucial information about plant performance and survival under drought. In this study, we report on the triphasic nature of the relationship between plant water potential (Ψ) at predawn and midday and describe a method that predicts Ψ at stomatal closure and turgor loss exclusively from this water potential curve (WP curve). The method is based on a piecewise linear regression model that was developed to predict the boundaries (termed Θ 1 and Θ 2 ) separating the three phases of the curve and corresponding slope values. The method was tested for three economically important woody species. For all species, midday Ψ was much more negative than predawn Ψ during phase I (mild drought), reductions in midday Ψ were minor while predawn Ψ continued to decline during phase II (moderate drought), and midday and predawn Ψ reached similar values during phase III (severe drought). Corresponding measurement of leaf gas exchange indicated that boundary Θ 1 between phases I and II coincided with Ψ at stomatal closure. Data from pressure-volume curves demonstrated that boundary Θ 2 between phases II and III predicted Ψ at leaf turgor loss. The WP curve method described here is an advanced application of the Scholander-type pressure chamber to categorize plant dehydration under drought into three distinct phases and to predict Ψ thresholds of stomatal closure and turgor loss., (© 2020 American Society of Plant Biologists. All Rights Reserved.)

Premise: The young seedling life stage is critical for reforestation after disturbance and for species migration under climate change, yet little is known regarding their basic hydraulic function or vulnerability to drought. Here, we sought to characterize responses to desiccation including hydraulic vulnerability, xylem anatomical traits, and impacts on other stem tissues that contribute to hydraulic functioning., Methods: Larix occidentalis, Pseudotsuga menziesii, and Pinus ponderosa (all ≤6 weeks old) were imaged using x-ray computed microtomography during desiccation to assess seedling biomechanical responses with concurrently measured hydraulic conductivity (k s ) and water potential (Ψ) to assess vulnerability to xylem embolism formation and other tissue damage., Results: In non-stressed samples for all species, pith and cortical cells appeared circular and well hydrated, but they started to empty and deform with decreasing Ψ which resulted in cell tearing and eventual collapse. Despite the severity of this structural damage, the vascular cambium remained well hydrated even under the most severe drought. There were significant differences among species in vulnerability to xylem embolism formation, with 78% xylem embolism in L. occidentalis by Ψ of -2.1 MPa, but only 47.7% and 62.1% in P. ponderosa and P. menziesii at -4.27 and -6.73 MPa, respectively., Conclusions: Larix occidentalis seedlings appeared to be more susceptible to secondary xylem embolism compared to the other two species, but all three maintained hydration of the vascular cambium under severe stress, which could facilitate hydraulic recovery by regrowth of xylem when stress is relieved., (© 2020 Botanical Society of America.)

Extreme heat events will challenge agricultural production and raise the risk of food insecurity. California is the largest agricultural producer in the United States, and climate change and extreme heat may significantly affect the state's food production. This paper provides a summary of the current literature on crop responses to extreme heat, with a focus on perennial agriculture in California. We highlight contemporary trends and future projections in heat extremes, and the range of plant responses to extreme heat exposure, noting the variability in plant tolerance and response across season, crop, and cultivar. We also review practices employed to mitigate heat damage and the capacity for those practices to serve as adaptation options in a warmer and drier future. Finally, we discuss current and future research directions aimed at increasing the adaptive capacity of perennial agriculture to the increased heat exposure anticipated with climate change. Collectively, the literature reviewed makes clear the need to understand crop responses and tolerances to heat within the context of climate change and climate extremes in order to sustain crop production, preserve agricultural communities, and bolster food security at local, national, and global scales., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Leaves lose approximately 400 H 2 O molecules for every 1 CO 2 gained during photosynthesis. Most long-distance water transport in plants, or xylem sap flow, serves to replace this water to prevent desiccation. Theory predicts that the largest vessels contribute disproportionately to overall sap flow because flow in pipe-like systems scales with the fourth power of radius. Here, we confront these theoretical flow predictions for a vessel network reconstructed from X-ray μCT imagery with in vivo flow MRI observations from the same sample of a first-year grapevine stem. Theoretical flow rate predictions based on vessel diameters are not supported. The heterogeneity of the vessel network gives rise to transverse pressure gradients that redirect flow from wide to narrow vessels, reducing the contribution of wide vessels to sap flow by 15% of the total. Our results call for an update of the current working model of the xylem to account for its heterogeneity.

Water transport in vascular plants represents a critical component of terrestrial water cycles and supplies the water needed for the exchange of CO 2 in the atmosphere for photosynthesis. Yet, many fundamental principles of water transport are difficult to assess given the scale and location of plant xylem. Here we review the mechanistic principles that underpin long-distance water transport in vascular plants, with a focus on woody species. We also discuss the recent development of noninvasive tools to study the functional status of xylem networks in planta. Limitations of current methods to detect drought-induced xylem blockages (e.g., embolisms) and quantify corresponding declines in sap flow, and the coordination of hydraulic dysfunction with other physiological processes are assessed. Future avenues of research focused on cross-validation of plant hydraulics methods are discussed, as well as a proposed fundamental shift in the theory and methodology used to characterize and measure plant water use.

Water discharge from stem internal storage compartments is thought to minimize the risk of vessel cavitation. Based on this concept, one would expect that water storage compartments involved in the buffering of xylem tensions empty before the onset of vessel cavitation under drought stress, and potentially refill after soil saturation. However, scant in vivo data exist that elucidate this localized spatiotemporal coupling. In this study on intact saplings of American chestnut ( Castanea dentata ), x-ray computed microtomography (microCT) showed that the xylem matrix surrounding vessels releases stored water and becomes air-filled either concurrent to or after vessel cavitation under progressive drought stress. Among annual growth rings, the xylem matrix of the current year remained largely water-filled even under severe drought stress. In comparison, microtomography images collected on excised stems showed that applied pressures of much greater than 0 MPa were required to induce water release from the xylem matrix. Viability staining highlighted that water release from the xylem matrix was associated primarily with emptying of dead fibers. Refilling of the xylem matrix and vessels was detected in intact saplings when the canopy was bagged and stem water potential was close to 0 MPa, and in leafless saplings over the winter period. In conclusion, this study indicates that the bulk of water stored in the xylem matrix is released after the onset of vessel cavitation, and suggests that capillary water contributes to overall stem water storage under drought but is not used primarily for the prevention of drought-induced vessel cavitation in this species., (© 2019 American Society of Plant Biologists. All Rights Reserved.)

Cultivars of grapevine are commonly grafted onto rootstocks to improve resistance against biotic and abiotic stress, however, it is not clear whether known differences in hydraulic traits are conferred from rootstocks to a common scion. We recently found that Vitis riparia and Vitis champinii differed in drought-induced embolism susceptibility and repair, which was related to differences in root pressure generation after rewatering (Knipfer et al. 2015). In the present study, we tested whether these and other physiological responses to drought are conferred to a common V. vinifera scion (Cabernet Sauvignon) grafted on V. riparia and V. champinii rootstocks. We measured xylem embolism formation/repair using in vivo microCT imaging, which was accompanied with analysis of leaf gas exchange, osmotic adjustment and root pressure. Our data indicate that differences in scion physiological behaviour for both rootstock combinations were negligible, suggesting that the sensitivity of Cabernet Sauvignon scion to xylem embolism formation/repair, leaf gas exchange and osmotic adjustment is unaffected by either V. riparia or V. champinii rootstock in response to drought stress.

The influence of the dynamics of leaf hydraulic conductance ( K leaf ) diurnally and during dehydration on stomatal conductance and photosynthesis remains unclear. Using the model species Arabidopsis ( Arabidopsis thaliana ecotype Columbia-0), we applied a multitiered approach including physiological measurements, high-resolution x-ray microcomputed tomography, and modeling at a range of scales to characterize (1) K leaf decline during dehydration; (2) its basis in the hydraulic conductances of leaf xylem and outside-xylem pathways ( K ox ); (3) the dependence of its dynamics on irradiance; (4) its impact on diurnal patterns of stomatal conductance and photosynthetic rate; and (5) its influence on gas exchange and survival under simulated drought regimes. Arabidopsis leaves showed strong vulnerability to dehydration diurnally in both gas exchange and hydraulic conductance, despite lack of xylem embolism or conduit collapse above the turgor loss point, indicating a pronounced sensitivity of K ox to dehydration. K leaf increased under higher irradiance in well-hydrated leaves across the full range of water potential, but no shift in K leaf vulnerability was observed. Modeling indicated that responses to dehydration and irradiance are likely attributable to changes in membrane permeability and that a dynamic K ox would contribute strongly to stomatal closure, improving performance, survival, and efficient water use during drought. These findings for Columbia-0 provide a baseline for assessing variation across genotypes in hydraulic traits and their influence on gas exchange during dehydration., (© 2018 American Society of Plant Biologists. All Rights Reserved.)

The leaf intercellular airspace (IAS) is generally considered to have high conductance to CO 2 diffusion relative to the liquid phase. While previous studies accounted for leaf-level variation in porosity and mesophyll thickness, they omitted 3D IAS traits that potentially influence IAS conductance (g IAS ). Here, we reevaluated the standard equation for g IAS by incorporating tortuosity, lateral path lengthening, and IAS connectivity. We measured and spatially mapped these geometric IAS traits for 19 Bromeliaceae species with Crassulacean acid metabolism (CAM) or C3 photosynthetic pathways using x-ray microcomputed tomography imaging and a novel computational approach. We found substantial variation in porosity (0.04-0.73 m 3 m -3 ), tortuosity (1.09-3.33 m 2 m -2 ), lateral path lengthening (1.12-3.19 m m -1 ), and IAS connectivity (0.81-0.97 m 2 m -2 ) across all bromeliad leaves. The revised g IAS model predicted significantly lower g IAS in CAM (0.01-0.19 mol m -2 s -1 bar -1 ) than in C3 (0.41-2.38 mol m -2 s -1 bar -1 ) plants due to a coordinated decline in these IAS traits. Our reevaluated equation also generally predicted lower g IAS values than the former one. Moreover, we observed high spatial heterogeneity in these IAS geometric traits throughout the mesophyll, especially within CAM leaves. Our data show that IAS traits that better capture the 3D complexity of leaves strongly influence g IAS and that the impact of the IAS on mesophyll conductance should be carefully considered with respect to leaf anatomy. We provide a simple function to estimate tortuosity and lateral path lengthening in the absence of access to imaging tools such as x-ray microcomputed tomography or other novel 3D image-processing techniques., (© 2018 American Society of Plant Biologists. All rights reserved.)

A germplasm collection containing varied Juglans genotypes holds potential to improve drought resistance of plant materials for commercial production. We used X-ray computed microtomography to evaluate stem xylem embolism susceptibility/repair in relation to vessel anatomical features (size, arrangement, connectivity and pit characteristics) in 2-year-old saplings of three Juglans species. In vivo analysis revealed interspecific variations in embolism susceptibility among Juglans microcarpa, J. hindsii (both native to arid habitats) and J. ailantifolia (native to mesic habitats). Stem xylem of J. microcarpa was more resistant to drought-induced embolism as compared with J. hindsii and J. ailantifolia (differences in embolism susceptibility among older and current year xylem were not detected in any species). Variations in most vessel anatomical traits were negligible among the three species; however, we detected substantial interspecific differences in intervessel pit characteristics. As compared with J. hindsii and J. ailantifolia, low embolism susceptibility in J. microcarpa was associated with smaller pit size in larger diameter vessels, a smaller area of the shared vessel wall occupied by pits, lower pit frequency and no changes in pit characteristics as vessel diameters increased. Changes in amount of embolized vessels following 40 days of re-watering were minor in intact saplings of all three species highlighting that an embolism repair mechanism did not contribute to drought recovery. In conclusion, our data indicate that interspecific variations in drought-induced embolism susceptibility are associated with species-specific pit characteristics, and these traits may provide a future target for breeding efforts aimed at selecting walnut germplasm with improved drought resistance.

Water acquisition is thought to be limited to the unsuberized surface located close to root tips. However, there are recurring periods when the unsuberized surfaces are limited in woody root systems, and radial water uptake across the bark of woody roots might play an important physiological role in hydraulic functioning. Using X-ray microcomputed tomography (microCT) and hydraulic conductivity measurements (Lp r ), we examined water uptake capacity of suberized woody roots in vivo and in excised samples. Bark hydration in grapevine woody roots occurred quickly upon exposure to water (c. 4 h). Lp r measurements through the bark of woody roots showed that it is permeable to water and becomes more so upon wetting. After bark hydration, microCT analysis showed that absorbed water was utilized to remove embolism locally, where c. 20% of root xylem vessels refilled completely within 15 h. Embolism removal did not occur in control roots without water. Water uptake through the bark of woody roots probably plays an important role when unsuberized tissue is scarce/absent, and would be particularly relevant following large irrigation events or in late winter when soils are saturated, re-establishing hydraulic functionality before bud break., (No claim to original US Government works New Phytologist © 2018 New Phytologist Trust.)

Starch is the primary energy storage molecule used by most terrestrial plants to fuel respiration and growth during periods of limited to no photosynthesis, and its depletion can drive plant mortality. Destructive techniques at coarse spatial scales exist to quantify starch, but these techniques face methodological challenges that can lead to uncertainty about the lability of tissue-specific starch pools and their role in plant survival. Here, we demonstrate how X-ray microcomputed tomography (microCT) and a machine learning algorithm can be coupled to quantify plant starch content in vivo, repeatedly and nondestructively over time in grapevine stems (Vitis spp.). Starch content estimated for xylem axial and ray parenchyma cells from microCT images was correlated strongly with enzymatically measured bulk-tissue starch concentration on the same stems. After validating our machine learning algorithm, we then characterized the spatial distribution of starch concentration in living stems at micrometer resolution, and identified starch depletion in live plants under experimental conditions designed to halt photosynthesis and starch production, initiating the drawdown of stored starch pools. Using X-ray microCT technology for in vivo starch monitoring should enable novel research directed at resolving the spatial and temporal patterns of starch accumulation and depletion in woody plant species., (No claim to original US Government works New Phytologist © 2018 New Phytologist Trust.)

Embolism removal is critical for restoring hydraulic pathways in some plants, as residual gas bubbles should expand when vessels are reconnected to the transpiration stream. Much of our understanding of embolism removal remains theoretical as a consequence of the lack of in vivo images of the process at high magnification. Here, we used in vivo X-ray micro-computed tomography (microCT) to visualize the final stages of xylem refilling in grapevine (Vitis vinifera) paired with scanning electron microscopy. Before refilling, vessel walls were covered with a surface film, but vessel perforation plate openings and intervessel pits were filled with air. Bubbles were removed from intervessel pits first, followed by bubbles within perforation plates, which hold the last volumes of air which eventually dissolve. Perforation plates were dimorphic, with more steeply angled scalariform plates in narrow diameter vessels, compared with the simple perforation plates in older secondary xylem, which may favor rapid refilling and compartmentalization of embolisms that occur in small vessels, while promoting high hydraulic conductivity in large vessels. Our study provides direct visual evidence of the spatial and temporal dynamics of the final stages of embolism removal., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

Water storage is thought to play an integral role in the maintenance of whole-plant water balance. The contribution of both living and dead cells to water storage can be derived from rehydration and water-release curves on excised plant material, but the underlying tissue-specific emptying/refilling dynamics remain unclear. Here, we used x-ray computed microtomography to characterize the refilling of xylem fibers, pith cells, and vessels under both excised and in vivo conditions in Laurus nobilis In excised stems supplied with water, water uptake exhibited a biphasic response curve, and x-ray computed microtomography images showed that high water storage capacitance was associated with fiber and pith refilling as driven by capillary forces: fibers refilled more rapidly than pith cells, while vessel refilling was minimal. In excised stems that were sealed, fiber and pith refilling was associated with vessel emptying, indicating a link between tissue connectivity and water storage. In contrast, refilling of fibers, pith cells, and vessels was negligible in intact saplings over two time scales, 24 h and 3 weeks. However, those compartments did refill slowly when the shoot was covered to prevent transpiration. Collectively, our data (1) provide direct evidence that storage compartments for capillary water refill in excised stems but rarely under in vivo conditions, (2) highlight that estimates of capacitance from excised samples should be interpreted with caution, as certain storage compartments may not be utilized in the intact plant, and (3) question the paradigm that fibers play a substantial role in daily discharge/recharge of stem capacitance in an intact tree., (© 2017 American Society of Plant Biologists. All Rights Reserved.)

Engineered nanoparticles (NPs) are increasingly used in commercial products including automotive lubricants, clothing, deodorants, sunscreens, and cosmetics and can potentially accumulate in our food supply. Given their size it is difficult to detect and visualize the presence of NPs in environmental samples, including crop plants. New analytical tools are needed to fill the void for detection and visualization of NPs in complex biological and environmental matrices. We aimed to determine whether radiolabeled NPs could be used as a noninvasive, highly sensitive analytical tool to quantitatively track and visualize NP transport and accumulation in vivo in lettuce (Lactuca sativa) and to investigate the effect of NP size on transport and distribution over time using a combination of autoradiography, positron emission tomography (PET)/computed tomography (CT), scanning electron microscopy (SEM), and transition electron microscopy (TEM). Azide functionalized NPs were radiolabeled via a "click" reaction with copper-64 ( 64 Cu)-1,4,7-triazacyclononane triacetic acid (NOTA) azadibenzocyclooctyne (ADIBO) conjugate ([ 64 Cu]-ADIBO-NOTA) via copper-free Huisgen-1,3-dipolar cycloaddition reaction. This yielded radiolabeled [ 64 Cu]-NPs of uniform shape and size with a high radiochemical purity (>99%), specific activity of  2.2 mCi/mg of NP, and high stability (i.e., no detectable dissolution) over 24 h across a pH range of 5-9. Both PET/CT and autoradiography showed that [ 64 Cu]-NPs entered the lettuce seedling roots and were rapidly transported to the cotyledons with the majority of the accumulation inside the roots. Uptake and transport of intact NPs was size-dependent, and in combination with the accumulation within the roots suggests a filtering effect of the plant cell walls at various points along the water transport pathway.

The mesophyll surface area exposed to intercellular air space per leaf area (S m ) is closely associated with CO 2 diffusion and photosynthetic rates. S m is typically estimated from two-dimensional (2D) leaf sections and corrected for the three-dimensional (3D) geometry of mesophyll cells, leading to potential differences between the estimated and actual cell surface area. Here, we examined how 2D methods used for estimating S m compare with 3D values obtained from high-resolution X-ray microcomputed tomography (microCT) for 23 plant species, with broad phylogenetic and anatomical coverage. Relative to 3D, uncorrected 2D S m estimates were, on average, 15-30% lower. Two of the four 2D S m methods typically fell within 10% of 3D values. For most species, only a few 2D slices were needed to accurately estimate S m within 10% of the whole leaf sample median. However, leaves with reticulate vein networks required more sections because of a more heterogeneous vein coverage across slices. These results provide the first comparison of the accuracy of 2D methods in estimating the complex 3D geometry of internal leaf surfaces. Because microCT is not readily available, we provide guidance for using standard light microscopy techniques, as well as recommending standardization of reporting S m values., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

In agricultural and natural systems, diffuse light can enhance plant primary productivity due to deeper penetration into and greater irradiance of the entire canopy. However, for individual sun-grown leaves from three species, photosynthesis is actually less efficient under diffuse compared with direct light. Despite its potential impact on canopy-level productivity, the mechanism for this leaf-level diffuse light photosynthetic depression effect is unknown. Here, we investigate if the spatial distribution of light absorption relative to electron transport capacity in sun- and shade-grown sunflower ( Helianthus annuus ) leaves underlies its previously observed diffuse light photosynthetic depression. Using a new one-dimensional porous medium finite element gas-exchange model parameterized with light absorption profiles, we found that weaker penetration of diffuse versus direct light into the mesophyll of sun-grown sunflower leaves led to a more heterogenous saturation of electron transport capacity and lowered its CO 2 concentration drawdown capacity in the intercellular airspace and chloroplast stroma. This decoupling of light availability from photosynthetic capacity under diffuse light is sufficient to generate an 11% decline in photosynthesis in sun-grown but not shade-grown leaves, primarily because thin shade-grown leaves similarly distribute diffuse and direct light throughout the mesophyll. Finally, we illustrate how diffuse light photosynthetic depression could overcome enhancement in canopies with low light extinction coefficients and/or leaf area, pointing toward a novel direction for future research., (© 2017 American Society of Plant Biologists. All Rights Reserved.)