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Sap velocity measurements are useful in fields ranging from plant water relations to hydrology at a variety of scales. Techniques based on pulses of heat are among the most common methods to measure sap velocity, but most lack ability to measure velocities across a wide range, including very high, very low and negative velocities (reverse flow). We propose a new method, the double-ratio method (DRM), which is robust across an unprecedented range of sap velocities and provides real-time estimates of the thermal diffusivity of wood. The DRM employs one temperature sensor upstream (proximal) and two sensors downstream (distal) to the source of heat. This facilitates several theoretical, heat-based approaches to quantifying sap velocity. We tested the DRM using whole-tree lysimetry in Eucalyptus cypellocarpa L.A.S. Johnson and found strong agreement across a wide range of velocities.

High CO2 concentrations stimulate net photosynthesis by increasing CO2 substrate availability for Rubisco, simultaneously suppressing photorespiration. Previously, we reported that silencing the chloroplast vesiculation (cv) gene in rice increased source fitness, through the maintenance of chloroplast stability and the expression of photorespiration-associated genes. Because high atmospheric CO2 conditions diminished photorespiration, we tested whether CV silencing might be a viable strategy to improve the effects of high CO2 on grain yield and N assimilation in rice. Under elevated CO2 , OsCV expression was induced, and OsCV was targeted to peroxisomes where it facilitated the removal of OsPEX11-1 from the peroxisome and delivered it to the vacuole for degradation. This process correlated well with the reduction in the number of peroxisomes, the decreased catalase activity and the increased H2 O2 content in wild-type plants under elevated CO2 . At elevated CO2 , CV-silenced rice plants maintained peroxisome proliferation and photorespiration and displayed higher N assimilation than wild-type plants. This was supported by higher activity of enzymes involved in NO3- and NH4+ assimilation and higher total and seed protein contents. Co-immunoprecipitation of OsCV-interacting proteins suggested that, similar to its role in chloroplast protein turnover, OsCV acted as a scaffold, binding peroxisomal proteins.

Water-use efficiency (WUE) is a common concept in the agronomy literature. Although definitions of WUE vary, the most common relates the ratio of grain yield (GY) to evapotranspiration (ET) e.g. WUEo=oGY/ET or WUEo=oET/GY. But using this metric has issues, as it is a ratio and is often analyzed relative to one of its components. Here it is shown that plotting WUE versus GY or ET causes spurious correlations, leading to artificial elevation of the proportion of variance explained. This may lead to higher confidence in WUE data than is warranted by the raw data. When WUE is regressed on GY, fitted slopes can be biased leading to predictions of high GY gain for little change in water use. A survey found ˜23% of WUE focused literature presented spurious correlations. To avoid these issues, future analyses could focus on interpretation of raw GY versus ET data and infer WUE changes from this plot. Treatments and site variation are shown to have effects on the interpretation of the GY to ET relationship. Statistical methods are reviewed to evaluate the mean/median response of GY to ET, and thus allow interpretation of patterns of WUE. Isolines of constant WUE are introduced as a general tool and illustrated relative to contrasting datasets. Other statistical methods are illustrated for evaluating the benchmark WUE, or boundary lines. Although no one statistical method will work for all datasets, the following conclusion is general across agronomy: spurious correlations can be avoided by not relating variables that share measured data.

Background Wild crop relatives have been potentially subjected to stresses on an evolutionary time scale prior to domestication. Among these stresses, drought is one of the main factors limiting crop productivity and its impact is likely to increase under current scenarios of global climate change. We sought to determine to what extent wild common bean (Phaseolus vulgaris) exhibited adaptation to drought stress, whether this potential adaptation is dependent on the climatic conditions of the location of origin of individual populations, and to what extent domesticated common bean reflects potential drought adaptation. Methods An extensive and diverse set of wild beans from across Mesoamerica, along with a set of reference Mesoamerican domesticated cultivars, were evaluated for root and shoot traits related to drought adaptation. A water deficit experiment was conducted by growing each genotype in a long transparent tube in greenhouse conditions so that root growth, in addition to shoot growth, could be monitored. Results Phenotypic and landscape genomic analyses, based on single-nucleotide polymorphisms, suggested that beans originating from central and north-west Mexico and Oaxaca, in the driest parts of their distribution, produced more biomass and were deeper-rooted. Nevertheless, deeper rooting was correlated with less root biomass production relative to total biomass. Compared with wild types, domesticated types showed a stronger reduction and delay in growth and development in response to drought stress. Specific genomic regions were associated with root depth, biomass productivity and drought response, some of which showed signals of selection and were previously related to productivity and drought tolerance. Conclusions The drought tolerance of wild beans consists in its stronger ability, compared with domesticated types, to continue growth in spite of water-limited conditions. This study is the first to relate bean response to drought to environment of origin for a diverse selection of wild beans. It provides information that needs to be corroborated in crosses between wild and domesticated beans to make it applicable to breeding programmes.

SummaryMaintaining high rates of photosynthesis in leaves requires efficient movement of CO2 from the atmosphere to the chloroplasts inside the leaf where it is converted into sugar. Throughout the evolution of vascular plants, CO2 diffusion across the leaf surface was maximized by reducing the sizes of the guard cells that form stomatal pores in the leaf epidermis1,2. Once inside the leaf, CO2 must diffuse through the intercellular airspace and into the mesophyll cells where photosynthesis occurs3,4. However, the diffusive interface defined by the mesophyll cells and the airspace and its coordinated evolution with other leaf traits are not well described5. Here we show that among vascular plants variation in the total amount of mesophyll surface area per unit mesophyll volume is driven primarily by cell size, the lower limit of which is defined by genome size. The higher surface area enabled by smaller cells allows for more efficient CO2 diffusion into photosynthetic mesophyll cells. Our results demonstrate that genome downsizing among the flowering plants6 was critical to restructuring the entire pathway of CO2 diffusion, facilitating high rates of CO2 supply to the leaf mesophyll cells despite declining atmospheric CO2 levels during the Cretaceous.

Maintaining high rates of photosynthesis in leaves requires efficient movement of CO2 from the atmosphere to the mesophyll cells inside the leaf where CO2 is converted into sugar. CO2 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 CO2 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 CO2 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 CO2 diffusion into and through the leaf, maintaining high rates of CO2 supply to the leaf mesophyll despite declining atmospheric CO2 levels during the Cretaceous.

More frequent, intense, and more extended droughts and heatwaves are challenging agricultural productivity and management. This study introduces a new modeling approach to represent the impact of heat and water stress on energy and mass fluxes from orchards. A photosynthesis model that explicitly accounts for dynamic responses of Rubisco and RuBP limitations to stress was linked to the Advanced Canopy-Atmosphere-Soil Algorithm (ACASA). ACASA is a multilayer soil-vegetation-atmosphere numerical model based on higher-order closure of turbulence equations to calculate plant-atmosphere exchanges of carbon dioxide, water, and heat. Field measurements of leaf area index, stomatal conductance, and photosynthesis were performed to estimate model parameters. Model results were compared to data from an eddy covariance flux tower deployed at an almond orchard. Overall, this new approach, ACASA-DynPM, is in closer agreement with observations of H, λE, and Fc compared to ACASA. On days of high temperatures and water stress, ACASA-DynPM outperforms ACASA. Thus, we argue that accounting for plant physiological stress might be especially relevant when estimating energy and mass fluxes over intensively irrigated agricultural regions. Such biases can have implications for the modeled degree of crop stress and water demands, and eventually, it can impact climate regional estimates when coupled with an atmospheric model.

The leaf intercellular airspace (IAS) is generally considered to have high conductance to CO2 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 (gIAS). Here, we reevaluated the standard equation for gIAS 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 m3 m−3), tortuosity (1.09–3.33 m2 m−2), lateral path lengthening (1.12–3.19 m m−1), and IAS connectivity (0.81–0.97 m2 m−2) across all bromeliad leaves. The revised gIAS model predicted significantly lower gIAS 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 gIAS 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 gIAS 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.

Ceptometry (the measurement of average photosynthetically active radiation under a plant canopy using many individual light sensors connected in parallel on a long bar) is commonly used to infer canopy light interception as well as leaf area index. Typically, the user places a ceptometer below the canopy and records a measurement, which is compared with a concurrent above-canopy measurement to produce a value of transmittance and then corrected for the effects of solar zenith and solar beam fraction. In row crops, canopy architecture is often systematically clumped in ways that may bias inferences based on ceptometer measurements. The role of spatial sampling in bias due to clumping has been extensively studied in the literature, but time-dependent bias has not. We assessed this bias using 68 handmade continuously recording ceptometers in 239 wheat ( L.) genotypes in Australia and made confirmatory measurements in California. We found that canopy properties inferred from instantaneous ceptometry varied widely, both randomly and systematically in time, when compared with properties inferred from continuous measurements. However, these biases tended to be somewhat similar across genotypes on a given day, arising from the interaction of solar zenith and non-random features of the canopy distribution caused by row planting. We recommend that continuous ceptometry be used in conjunction with instantaneous ceptometry to correct for these biases, and we provide schematics for low-cost handmade ceptometers.

The degree of coupling to the environment of almond (Prunus dulcis) orchards during periods of transient water stress was investigated in a two-year study in California. Plant water status was monitored weekly, before and/or after irrigation, measuring midday stem water potential (Ψstem) that ranged from −0.5 to −2 MPa, while actual evapotranspiration (ETa) was measured with an eddy covariance tower. Irrigation was applied weekly following common commercial practice, resulting in weekly cycles of Ψstem. Despite Ψstem reaching levels shown to induce substantial stomatal closure, the ratio actual to reference evapotranspiration (ETa/ETo = Ka) did not show a decrease during weekly periods of low Ψstem in the two years of the study. Midday average canopy surface resistance (rcmid), computed by reversing the Penman-Monteith equation from eddy covariance ET data, yielded a statistically significant increase with a decrease in Ψstem, but just in the first year of the study. However, rcmid did not show a significant relationship with stomatal resistance measured at the leaf level with porometry and scaled-up to the canopy level. In the first year, rcmid showed a sharp increase after harvest, when Ka also decreased, perhaps produced by the composite effect of defoliation associated with harvest and stomatal closure associated with water stress. During the growing season, rcmid ranged from 0 to 100 s m−1 and midday average aerodynamic resistance (ramid) ranged between 0 and 50 s m−1. Despite rcmid being generally larger than ra, the midday average decoupling factor (Ω) averaged 0.7 during the irrigation season, indicating decoupled conditions. However, there was a large day to day fluctuation of midday Ω ranging from 0.16 to 0.98 mostly associated with rcmid and wind speed. This study indicated that tall and rough canopies can be relatively decoupled depending on the effect of wind speed and canopy resistance on the decoupling factor. From a water management point of view, this result suggests that inducing transient mild to moderate water stress may not produce substantial water savings in areas having low to moderate winds.

Climate change adaptation requires building agricultural system resilience to warmer, drier climates. Increasing temporal plant diversity through crop rotation diversification increases yields of some crops under drought, but its potential to enhance crop drought resistance and the underlying mechanisms remain unclear. We conducted a drought manipulation experiment using rainout shelters embedded within a 36-year crop rotation diversity and no-till experiment in a temperate climate and measured a suite of soil and crop developmental and eco-physiological traits in the field and laboratory. We show that diversifying maize-soybean rotations with small grain cereals and cover crops mitigated maize water stress at the leaf and canopy scales and reduced yield losses to drought by 17.1 ± 6.1%, while no-till did not affect maize drought resistance. Path analysis showed a strong correlation between soil organic matter and lower maize water stress despite no significant differences in soil organic matter between rotations or tillage treatments. This positive relationship between soil organic matter and maize water status was not mediated by higher soil water retention or infiltration as often hypothesized, nor differential depth of root water uptake as measured with stable isotopes, suggesting that other mechanisms are at play. Crop rotation diversification is an underappreciated drought management tool to adapt crop production to climate change through managing for soil organic matter.

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 CO2 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.

Given the changing climate and increasing impact of agriculture on global resources, it is important to identify phenotypes which are global and sustainable optima. Here, an in silico framework is constructed by coupling evolutionary optimization with thermodynamically sound crop physiology, and its ability to rationally design phenotypes with maximum productivity is demonstrated, within well‐defined limits on water availability. Results reveal that in mesic environments, such as the North American Midwest, and semi‐arid environments, such as Colorado, phenotypes optimized for maximum productivity and survival under drought are similar to those with maximum productivity under irrigated conditions. In hot and dry environments like California, phenotypes adapted to drought produce 40% lower yields when irrigated compared to those optimized for irrigation. In all three representative environments, the trade‐off between productivity under drought versus that under irrigation was shallow, justifying a successful strategy of breeding crops combining best productivity under irrigation and close to best productivity under drought.

Short AbstractDetailed instructions on how to build, calibrate and collect research quality data from PARbar ceptometers are presented.Long AbstractCeptometry is a technique used to measure the transmittance of photosynthetically active radiation through a plant canopy using multiple light sensors connected in parallel on a long bar. Ceptometry is often used to infer properties of canopy structure and light interception, notably leaf area index (LAI) and effective plant area index (PAIeff). Due to the high cost of commercially available ceptometers, the number of measurements that can be taken is often limited in space and time. This limits the usefulness of ceptometry for studying genetic variability in light interception, and precludes thorough analysis of, and correction for, biases that can skew measurements depending on the time of day. We developed continuously logging ceptometers (called PARbars) that can be produced for USD $75 each and yield high quality data comparable to commercially available alternatives. Here we provide detailed instruction on how to build and calibrate PARbars, how to deploy them in the field and how to estimate PAI from collected transmittance data. We provide representative results from wheat canopies and discuss further considerations that should be made when using PARbars.

Plant physiological responses to stress and subsequent damage have not been successfully integrated into leaf, plant or land surface models. Water deficit leads to a series of plant responses: stomatal closure decreases evaporative cooling leading to higher leaf temperature, which it can affect photosynthetic dissipation of absorbed energy, especially under high irradiance. Excess absorbed energy by PSII results in photoinhibition, which is not typically included when modeling photosynthesis using the Farquhar-von Caemmerer-Berry (FvCB) model. We introduce a novel approach to represent plant-atmosphere interactions: a non-steady state model of photoinhibition linked to the RuBP-regeneration component of the FvCB model that accounts for the interactions between high temperature, irradiance, and water deficits. Gas exchange and chlorophyll fluorescence dynamics showed that, under severe water stress, the maximum quantum efficiency of PSII (Fv/Fm) quickly decreases, and it takes days to recover. We found a clear relationship between maximum electron transport rate and Fv/Fm and that short-term acclimation to irradiance affected photosynthetic FvCB model parameters. Fv/Fm functioned well as a coupling variable, able to scale RuBP-regeneration limited photosynthesis parameters proportional to photoinhibition. These results indicate that plant stress, photo-inhibitory damage, and recovery can be explicitly represented when modeling photosynthesis.

High CO

© 2017 Elsevier B.V. Water potential is a useful predictive tool in irrigation scheduling as it, or a component, is associated with physiological responses to water deficit. Increasing atmospheric demand for water increases transpiration and decreases water potential for the same stomatal conductance. However, based on supply by the soil-plant-atmosphere-continuum, decreasing soil water potential should decrease stomatal conductance and thus transpiration but also decrease water potential. Such contradictory behavior of supply and demand responses, may limit the value of water potential as an indicator of plant water status. This work studied the relationship between plant water potential and transpiration affected by supply (soil moisture) and atmospheric evaporative demand, and has implications for interpretation of water potentials and irrigation management. Results were that plant water potential has a narrow range of sensitivity to variation in supply and demand in hydrated soils, but greatly varying sensitivity in dry soils, limiting interpretation under dry conditions. Loss of soil conductance in dry, coarse soil types affects the trajectory of plant water potential response to supply and demand. Sapflow measurements on almonds indicated that variation in reference evapotranspiration and/or soil moisture deficit led to similar variation in stem water potentials to that predicted by the model. The model indicates hypotheses that with further testing may have important repercussions on the measurement of plant water use and irrigation scheduling.

© 2017 Elsevier B.V. A limited transpiration rate under high vapor pressure deficit (VPD) could be used to conserve soil water for later use under drought conditions. Many crops show this behavior either as limited transpiration or decreases in stomatal conductance. However, little work has been done in Phaseolus. Four experiments evaluated stomatal closure across a range of VPD for well-watered plants, each experiment using varying combinations of genotypes of common (15), lima (6) and tepary beans (7 genotypes). A two-year experiment found genotypic variation in average stomatal conductance, but genotypes only had 14% stomatal closure between a VPD of 1–4 kPa. In comparison, soybean, which is known to close stomata, had a 40% decrease for similar conditions in Davis, CA, USA. In a second field experiment and outdoor pot experiments, genotypes from the three species displayed, on average, a 34, 50–45% increase in stomatal conductance with increasing VPD. Six genotypes were statistically indistinguishable from a 40% decrease, but all had low probability (p < 0.21) of having 40% closure, and some showed little closure in other experiments. The VPD range measured in this study was large relative to the range for hot, arid California, thus the results are generalizable: most Phaseolus beans are not expected to have appreciable stomatal closure under well-watered conditions. Thus, there is limited evidence that Phaseolus has somegenetic diversity in stomatal responses to VPD, relative to that shown in other species. However, there was constitutive genetic variation in species and genotypic stomatal conductance under low VPD conditions.

© 2016 John Wiley & Sons Ltd Mesophyll conductance to CO2(gm) may respond to light either through regulated dynamic mechanisms or due to anatomical and structural factors. At low light, some layers of cells in the leaf cross-section approach photocompensation and contribute minimally to bulk leaf photosynthesis and little to whole leaf gm(gm,leaf). Thus, the bulk gm,leafwill appear to respond to light despite being based upon cells having an anatomically fixed mesophyll conductance. Such behaviour was observed in species with contrasting leaf structure using the variable J or stable isotope method of measuring gm,leaf. A species with bifacial structure, Arbutus × ‘Marina’, and an isobilateral species, Triticum durum L., had contrasting responses of gm,leafupon varying adaxial or abaxial illumination. Anatomical observations, when coupled with the proposed model of gm,leafto photosynthetic photon flux density (PPFD) response, successfully represented the observed gas exchange data. The theoretical and observed evidence that gm,leafapparently responds to light has large implications for how gm,leafvalues are interpreted, particularly limitation analyses, and indicates the importance of measuring gmunder full light saturation. Responses of gm,leafto the environment should be treated as an emergent property of a distributed 3D structure, and not solely a leaf area-based phenomenon.

The mesophyll surface area exposed to intercellular air space per leaf area (Sm ) is closely associated with CO2 diffusion and photosynthetic rates. Sm 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 Sm 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 Sm estimates were, on average, 15-30% lower. Two of the four 2D Sm methods typically fell within 10% of 3D values. For most species, only a few 2D slices were needed to accurately estimate Sm 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 Sm values.

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 (

© 2016 Elsevier Ltd The mechanism, or hypothesis, of how a plant might be adapted to drought should strongly influence experimental design. For instance, an experiment testing for water conservation should be distinct from a damage-tolerance evaluation. We define here four new, general mechanisms for plant adaptation to drought such that experiments can be more easily designed based upon the definitions. A series of experimental methods are suggested together with appropriate physiological measurements related to the drought adaptation mechanisms. The suggestion is made that the experimental manipulation should match the rate, length, and severity of soil water deficit (SWD) necessary to test the hypothesized type of drought adaptation mechanism.

To increase soybean [Glycine max (L.) Merr.] productivity, it will be necessary to improve yields in water-deficit regions. Genotype PI 471938, which exhibits a slow-wilting phenotype under water-deficit conditions, has proven to be a good genetic resource in developing droughtresistant progeny even though the physiological basis for this advantage is not known. The objective of this study was to investigate the involvement of four water-saving, physiological mechanisms as candidates contributing to drought tolerance of PI 471938. (i) In response to soil drying, the soil water content at which leaf gas exchange began to decrease in PI 471938 was not different from the other tested genotypes. (ii) Measurement of leaf photosynthetic capacity failed to show that PI 471938 had a high capacity allowing high CO 2 assimilations even with partial stomata closure. (iii) Plant Introduction 471938 failed to exhibit a limited transpiration rate with increasing vapor pressure deficit (VPD), which would have allowed this genotype to conserve soil water during midday periods of high VPD. (iv) Finally, PI 471938 did not show an ability to maintain high leaf water potential (Ψ Leaf) when VPD was increasing. In fact, there was a dramatic decrease in Ψ Leaf with increasing VPD, which usually implies a decrease in leaf turgor pressure. Overall this study resulted in the rejection of four major hypotheses to explain the slow-wilting phenotype exhibited by PI 471938, and the basis for its drought resistance remains unknown. © Crop Science Society of America.

Plants with limited transpiration rate (TR) under high vapor pressure deficit (VPD) offer the potential to conserve soil water and thus decrease the occurrence of soil water deficit. Genetic variability in TR response to VPD has been observed in the greenhouse for soybean (Glycine max (L.) Merr.) genotypes related to PI416937, but these differences have yet to be measured in the field. The objective of this study was to observe under field conditions leaf gas exchange properties of PI416937 in comparison to nine other genotypes to determine if it expressed limited TR at high VPD. Genotypic differences in stomatal conductance measurements (a proxy for TR) matched those obtained under controlled environment conditions. Genotypes varied from no stomatal response to VPD, to strong negative responses resulting in full stomata closure at ∼4 kPa. There was a greater proportional genetic variability in stomatal conductance in the field (75% at high VPD) than was observed in the greenhouse, but this variation was correlated with greenhouse TR. However, photosynthesis was considerably limited in genotypes that had a stomatal response to VPD. Although field differences in photosynthetic capacity among genotypes were not correlated with greenhouse measurements, there was sufficient genetic variation to allow the possibility of selection of high photosynthetic capacity to overcome about a 34% decrease in stomatal conductance. Thus, a targeted breeding program to combine the water conserving TR-VPD response with increased photosynthetic capacity has the potential to increase soybean yields in field water-deficit environments.

Bundle sheath extensions (BSEs) are key features of leaf structure with currently little-understood functions. To test the hypothesis that BSEs reduce the hydraulic resistance from the bundle sheath to the epidermis (r be) and thereby accelerate hydropassive stomatal movements, we compared stomatal responses with reduced humidity and leaf excision among 20 species with heterobaric or homobaric leaves and herbaceous or woody life forms. We hypothesized that low r be due to the presence of BSEs would increase the rate of stomatal opening (V) during transient wrong-way responses, but more so during wrong-way responses to excision (V e) than humidity (V h), thus increasing the ratio of V e to V h. We predicted the same trends for herbaceous relative to woody species given greater hydraulic resistance in woody species. We found that V e, V h, and their ratio were 2.3 to 4.4 times greater in heterobaric than homobaric leaves and 2.0 to 3.1 times greater in herbaceous than woody species. To assess possible causes for these differences, we simulated these experiments in a dynamic compartment/resistance model, which predicted larger V e and V e/V h in leaves with smaller r be. These results support the hypothesis that BSEs reduce r be. Comparison of our data and simulations suggested that r be is approximately 4 to 16 times larger in homobaric than heterobaric leaves. Our study provides new evidence that variations in the distribution of hydraulic resistance within the leaf and plant are central to understanding dynamic stomatal responses to water status and their ecological correlates and that BSEs play several key roles in the functional ecology of heterobaric leaves.

Plant adaptations minimising costs of burial responses are vital in mobile dune ecosystems. Conventionally, the burial responses of dune plants have been measured as vertical growth. However, a model developed here shows that growth normal to accumulating non-horizontal dune surfaces requires up to 18% less stem production than vertical growth. To determine whether dune plants grow with this optimal geometry a field survey of growth trajectories was made for three coastal plants, and a greenhouse experiment tested whether plants could actively change growth trajectories away from vertical. In the field and greenhouse partial burial resulted in shifts towards vertical growth, indicating that negative gravitropism is the major response to burial, and reduces the amount of stem needed to respond to burial relative to unburied growth trajectories. However, for two species in the field many buried stems grew closer to optimal (shorter) than vertical, mostly on northward, sun-facing dunes. Thus phototropism is the major stimulus for non-vertical growth, and coincides with optimal trajectories on north-facing dunes. Arctotheca populifolia also displayed non-vertical growth after burial on steep south-facing dunes, but responded to burial in the greenhouse with vertical growth. Measurements of the pressure of sand avalanches occurring on dune slipfaces and stem elasticity indicated that deep avalanches were sufficient to orient the flexible shoots of A. populifolia towards optimal trajectories. Thus, dune plants respond to burial by actively modulating growth towards vertical via negative gravitropism, and passively towards non-vertical—more optimal—trajectories via phototropic growth and the influence of sand avalanches.

Intrinsic water use efficiency (WUE(intr)), the ratio of photosynthesis to stomatal conductance to water, is often used as an index for crop water use in breeding projects. However, WUE(intr) conflates variation in these two processes, and thus may be less useful as a selection trait than knowledge of both components. The goal of the present study was to determine whether the contribution of photosynthetic capacity and stomatal conductance to WUE(intr) varied independently between soybean genotypes and whether this pattern was interactive with mild drought. Photosynthetic capacity was defined as the variation in WUE(intr) that would occur if genotypes of interest had the same stomatal conductance as a reference genotype and only differed in photosynthesis; similarly, the contribution of stomatal conductance to WUE(intr) was calculated assuming a constant photosynthetic capacity across genotypes. Genotypic differences in stomatal conductance had the greatest effect on WUE(intr) (26% variation when well watered), and was uncorrelated with the effect of photosynthetic capacity on WUE(intr). Thus, photosynthetic advantages of 8.3% were maintained under drought. The maximal rate of Rubisco carboxylation, generally the limiting photosynthetic process for soybeans, was correlated with photosynthetic capacity. As this trait was not interactive with leaf temperature, and photosynthetic capacity differences were maintained under mild drought, the observed patterns of photosynthetic advantage for particular genotypes are likely to be consistent across a range of environmental conditions. This suggests that it is possible to employ a selection strategy of breeding water-saving soybeans with high photosynthetic capacities to compensate for otherwise reduced photosynthesis in genotypes with lower stomatal conductance.

The stimulation of dune plant growth in response to burial is a vital attribute allowing survival in areas of mobile sand. Numerous resource-related and physiological mechanisms of growth stimulation have been suggested in the past, but few have been tested comparatively. Manipulation experiments using Scaevola plumieri, an important subtropical coastal dune forming species, demonstrated that physiological shifts were of great importance in determining the nature of the stimulation response to burial. The production of stem length and replacement of leaf area were stimulated by burial, whereas net mass production was similar between buried and unburied treatments. Remobilization of buried leaf resources, seasonal effects, and a shift in biomass allocation to stem production played the greatest role in the compensatory growth response. Other factors, such as increased soil nutrients, changes in photosynthesis, and changes in the costs of producing tissue were of less importance. Thus, the stimulated growth of species adapted to live on mobile dunes is explained by a number of resource-related and physiological mechanisms acting in concert.

The regional abundance of C(4) grasses is strongly controlled by temperature, however, the role of precipitation is less clear. Progress in elucidating the direct effects of photosynthetic pathway on these climate relationships is hindered by the significant genetic divergence between major C(3) and C(4) grass lineages. We addressed this problem by examining seasonal climate responses of photosynthesis in Alloteropsis semialata, a unique grass species with both C(3) and C(4) subspecies. Experimental manipulation of rainfall in a common garden in South Africa tested the hypotheses that: (1) photosynthesis is greater in the C(4) than C(3) subspecies under high summer temperatures, but this pattern is reversed at low winter temperatures; and (2) the photosynthetic advantage of C(4) plants is enhanced during drought events. Measurements of leaf gas exchange over 2 years showed a significant photosynthetic advantage for the C(4) subspecies under irrigated conditions from spring through autumn. However, the C(4) leaves were killed by winter frost, while photosynthesis continued in the C(3) plants. Unexpectedly, the C(4) subspecies also lost its photosynthetic advantage during natural drought events, despite greater water-use efficiency under irrigated conditions. This study highlights previously unrecognized roles for climatic extremes in determining the ecological success of C(3) and C(4) grasses.

The C4 photosynthetic pathway uses water more efficiently than the C3 type, yet biogeographical analyses show a decline in C4 species relative to C3 species with decreasing rainfall. To investigate this paradox, the hypothesis that the C4 advantage over C3 photosynthesis is diminished by drought was tested, and the underlying stomatal and metabolic mechanisms of this response determined. The effects of drought and high evaporative demand on leaf gas exchange and photosynthetic electron sinks in C3 and C4 subspecies of the grass Alloteropsis semialata were examined. Plant responses to climatic variation and soil drought were investigated using a common garden experiment with well-watered and natural rainfall treatments, and underlying mechanisms analysed using controlled drying pot experiments. Photosynthetic rates were significantly higher in the C4 than the C3 subspecies in the garden experiment under well-watered conditions, but this advantage was completely lost during a rainless period when unwatered plants experienced severe drought. Controlled drying experiments showed that this loss was caused by a greater increase in metabolic, rather than stomatal, limitations in C4 than in the C3 leaves. Decreases in CO2 assimilation resulted in lower electron transport rates and decreased photochemical efficiency under drought conditions, rather than increased electron transport to alternative sinks. These findings suggest that the high metabolic sensitivity of photosynthesis to severe drought seen previously in several C4 grass species may be an inherent characteristic of the C4 pathway. The mechanism may explain the paradox of why C4 species decline in arid environments despite high water-use efficiency.

The grass subfamily Danthonioideae is one of the smaller in the family. We utilize DNA sequence data from three chloroplast regions (trnL, rpoC2 and rbcL) and one nuclear region (Internal Transcribed Spacer; ITS) both singly and in combi- nation to elucidate the relationships of the genera in the subfamily. The topology retrieved by the ITS region is not congruent with that of the plastid data, but this conflict is not strongly supported. Nine well- supported clades are retrieved by all data sets. The relationships at the base of the subfamily are clearly established, comprising a series of three clades of Merxmuellera species. The earliest diverging clade probably does not belong in Danthonioideae. The other two clades are centered in the tropical African mountains and Cape mountains respectively. A clade of predominantly North and South American Danthonia species as well as D. archboldii from New Guinea is retrieved, but the African and Asian species of Danthonia are related to African species of Merxmuellera, thus rendering Danthonia polyphy- letic. The relationships of the Danthonia clade remain equivocal, as do those of the two Cortaderia clades, the Pseudopentameris and Rytidosperma clades.

Key message This study identifies a small distal region of the 1RS chromosome from rye that has a positive impact on wheat yield. Abstract The translocation of the short arm of rye (Secale cereale L.) chromosome one (1RS) onto wheat (Triticum aestivum L.) chromosome 1B (1RS.1BL) is used in wheat breeding programs worldwide due to its positive effect on yield, particularly under abiotic stress. Unfortunately, this translocation is associated with poor bread-making quality. To mitigate this problem, the 1RS arm was engineered by the removal and replacement of two interstitial rye segments with wheat chromatin: a distal segment to introduce the Glu-B3/Gli-B1 loci from wheat, and a proximal segment to remove the rye Sec-1 locus. We used this engineered 1RS chromosome (henceforth 1RSWW) to develop and evaluate two sets of 1RS/1RSWW near isogenic lines (NILs). Field trials showed that standard 1RS lines had significantly higher yield and better canopy water status than the 1RSWW NILs in both well-watered and water-stressed environments. We intercrossed the 1RS and 1RSWW lines and generated two additional NILs, one carrying the distal (1RSRW) and the other carrying the proximal (1RSWR) wheat segment. Lines not carrying the distal wheat region (1RS and 1RSWR) showed significant improvements in grain yield and canopy water status compared to NILs carrying the distal wheat segment (1RSWW and 1RSRW), indicating that the 1RS region replaced by the distal wheat segment carries the beneficial allele(s). NILs without the distal wheat segment also showed higher carbon isotope discrimination and increased stomatal conductance, suggesting that these plants had improved access to water. The 1RSWW, 1RSWR and 1RSRW NILs have been deposited in the National Small Grains Collection. Electronic supplementary material The online version of this article (doi:10.1007/s00122-014-2408-6) contains supplementary material, which is available to authorized users.

Limited rainfall is the main constraint to agriculture, making agricultural research to understand plant behaviour that leads to avoidance of soil water deficit a matter of priority. One focus has screened for crop varieties that decrease stomatal conductance under high vapour pressure deficit (VPD), a proxy for the leaf evaporative gradient. However, the link between stomatal closure and physiological consequences in field environments is not yet clear. A field experiment on soybeans demonstrated that considerable variation in leaf temperature relative to air temperature occurred, leading to evaporative gradients differing substantially from VPD. Thus, transpiration is decreased by stomatal closure at high VPD, but to compensate, transpiration is somewhat increased due to higher leaf temperatures. Soil water deficit led to lower stomatal conductance, particularly under low evaporative conditions, not just under hot conditions. Non-stomatal photosynthetic limitations were observed due to combined occurrence of stomatal closure and high temperature under high VPD. Although leaves reached temperatures higher than the threshold for a decrease in maximum photochemical efficiency, and displayed non-stomatal photosynthetic limitations, no photoinhibition or damage was observed by night-time. The results demonstrate that more understanding of physiological strategies for achieving altered water use is needed to avoid trade-offs and heat stress.

The mechanism of embolism repair in transpiring plants is still not understood, despite significant scientific effort. The refilling process is crucial to maintaining stem transport capacity and ensuring survival for plants experiencing dynamic changes in water stress. Refilling air-filled xylem vessels requires an energy and water source that can only be provided by adjacent living parenchyma cells. Here, we report an analysis of the transcriptome response of xylem parenchyma cells after embolism formation in Populus trichocarpa trees. Genes encoding aquaporins, ion transporters, and carbohydrate metabolic pathways were up-regulated, and there was a significant reduction in the expression of genes responding to oxidative stress. Thus, a novel view of the plant response to embolism emerges that suggests a role for oxygen in embolized vessels as a signal triggering xylem refilling and for the activity of cation transport as having a significant role in the generation of the energy gradient necessary to heal embolized vessels. These findings redefine current hypotheses surrounding the refilling phenomenon and provide insight into the complexity of the biological response to the seemingly simple physical event of xylem embolism formation.

Background and aims The success of C4 plants lies in their ability to attain greater efficiencies of light, water and nitrogen use under high temperature, providing an advantage in arid, hot environments. However, C4 grasses are not necessarily less sensitive to drought than C3 grasses and are proposed to respond with greater metabolic limitations, while the C3 response is predominantly stomatal. The aims of this study were to compare the drought and recovery responses of co-occurring C3 and C4 NADP-ME grasses from the subfamily Panicoideae and to determine stomatal and metabolic contributions to the observed response. Methods Six species of locally co-occurring grasses, C3) species Alloteropsis semialata subsp. eckloniana, Panicum aequinerve and Panicum ecklonii, and C4 (NADP-ME) species Heteropogon contortus, Themeda triandra and Tristachya leucothrix, were established in pots then subjected to a controlled drought followed by re-watering. Water potentials, leaf gas exchange and the response of photosynthetic rate to internal CO2 concentrations were determined on selected occasions during the drought and re-watering treatments and compared between species and photosynthetic types. Key results Leaves of C4 species of grasses maintained their photosynthetic advantage until water deficits became severe, but lost their water-use advantage even under conditions of mild drought. Declining C4 photosynthesis with water deficit was mainly a consequence of metabolic limitations to CO2 assimilation, whereas, in the C3 species, stomatal limitations had a prevailing role in the drought-induced decrease in photosynthesis. The drought-sensitive metabolism of the C4 plants could explain the observed slower recovery of photosynthesis on re-watering, in comparison with C3 plants which recovered a greater proportion of photosynthesis through increased stomatal conductance. Conclusions Within the Panicoid grasses, C4 (NADP-ME) species are metabolically more sensitive to drought than C3 species and recover more slowly from drought.

Litter decomposition recycles nutrients and causes large fluxes of carbon dioxide into the atmosphere. It is typically assumed that climate, litter quality and decomposer communities determine litter decay rates, yet few comparative studies have examined their relative contributions in tropical forests. We used a short‐term litterbag experiment to quantify the effects of litter quality, placement and mesofaunal exclusion on decomposition in 23 tropical forests in 14 countries. Annual precipitation varied among sites (760–5797 mm). At each site, two standard substrates (Raphia farinifera and Laurus nobilis) were decomposed in fine‐ and coarse‐mesh litterbags both above and below ground for approximately 1 year. Decomposition was rapid, with >95% mass loss within a year at most sites. Litter quality, placement and mesofaunal exclusion all independently affected decomposition, but the magnitude depended upon site. Both the average decomposition rate at each site and the ratio of above‐ to below‐ground decay increased linearly with annual precipitation, explaining 60–65% of among‐site variation. Excluding mesofauna had the largest impact on decomposition, reducing decomposition rates by half on average, but the magnitude of decrease was largely independent of climate. This suggests that the decomposer community might play an important role in explaining patterns of decomposition among sites. Which litter type decomposed fastest varied by site, but was not related to climate. Synthesis. A key goal of ecology is to identify general patterns across ecological communities, as well as relevant site‐specific details to understand local dynamics. Our pan‐tropical study shows that certain aspects of decomposition, including average decomposition rates and the ratio of above‐ to below‐ground decomposition are highly correlated with a simple climatic index: mean annual precipitation. However, we found no relationship between precipitation and effects of mesofaunal exclusion or litter type, suggesting that site‐specific details may also be required to understand how these factors affect decomposition at local scales. Fil: Powers, Jennifer S.. University of Minnesota; Estados Unidos Fil: Montgomery, Rebecca M.. University of Minnesota; Estados Unidos Fil: Adair, Carol E.. University of Minnesota; Estados Unidos Fil: Brealey, Francis Q.. Manchester Metropolitan University; Reino Unido Fil: De Walt, Saarah J.. Clemson University; Estados Unidos Fil: Castanho, Camila T. Universidade de Sao Paulo; Brasil Fil: Chave, Jerome. Centre National de la Recherche Scientifique; Francia. Université Paul Sabatier; Francia Fil: Deiniert, Erica. Organization for Tropical Studies; Costa Rica Fil: Ganzhonr, Jörg U.. Universitat Hamburg; Alemania Fil: Gilbert, Mathew E.. Harvard University; Estados Unidos Fil: Gonzalez Iturbe, José Antonio. Biocenosis A. C.; México Fil: Bunyavejchewin, Sarayudh. National Park. Wildlife and Plant Conservation Department; Tailandia Fil: Grau, Hector Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Laboratorio de Investigaciones Ecológicas de las Yungas; Argentina Fil: Harms, Kyle E.. Smithsonian Tropical Research Institute; Panamá. State University of Louisiana; Estados Unidos Fil: Hiremath, Ankila. Ashoka Trust for Research in Ecology and the Environment; India Fil: Iriarte Vivar, Silvia. Centro de Investigación Científica de Yucatán A.C. Unidad de Recursos Naturales; México Fil: Manzane, Eric. University of Miami; Estados Unidos Fil: De Oliveira, Alexandre A.. Universidade de Sao Paulo. Departamento de Fisiología. Instituto de Biociencias; Brasil Fil: Poorter, Lourens. University of Agriculture Wageningen; Países Bajos. Instituto Boliviano de Investigación Forestal; Bolivia Fil: Ramanamanjato, Jean Baptiste. QIT Madagascar Minerals; Madagascar Fil: Salk, Carl. University of Duke; Estados Unidos Fil: Varela, Amanda. Pontificia Universidad Javeriana; Colombia Fil: Weiblen, George D.. University of Minnesota; Estados Unidos Fil: Lerday Manuel T. University of Virginia; Estados Unidos

The species richness of C(4) grasses is strongly correlated with temperature, with C(4) species dominating subtropical ecosystems and C(3) types predominating in cooler climates. Here, the effects of low temperatures on C(4) and C(3) grasses are compared, controlling for phylogenetic effects by using Alloteropsis semialata, a unique species with C(4) and C(3) subspecies. Controlled environment and common garden experiments tested the hypotheses that: (i) photosynthesis and growth are greater in the C(4) than the C(3) subspecies at high temperatures, but this advantage is reversed below 20 degrees C; and (ii) chilling-induced photoinhibition and light-mediated freezing injury of leaves occur at higher temperature thresholds in the C(4) than the C(3) plants. Measurements of leaf growth and photosynthesis showed the expected advantages of the C(4) pathway over the C(3) type at high temperatures. These declined with temperature, but were not completely lost until 15 degrees C, and there was no evidence of a reversal to give a C(3) advantage. Chronic chilling (5-15 degrees C) or acute freezing events induced a comparable degree of photodamage in illuminated leaves of both subspecies. Similarly, freezing caused high rates of mortality in the unhardened leaves of both subtypes. However, a 2-week chilling treatment prior to these freezing events halved injury in the C(3) but not the C(4) subspecies, suggesting that C(4) leaves lacked the capacity for cold acclimation. These results therefore suggest that C(3) members of this subtropical species may gain an advantage over their C(4) counterparts at low temperatures via protection from freezing injury rather than higher photosynthetic rates.

Past work suggests that burial and low nutrient availability limit the growth and zonal distribution of coastal dune plants. Given the importance of these two factors, there is a surprising lack of field investigations of the interactions between burial and nutrient availability. This study aims to address this issue by measuring the growth responses of four coastal dune plant species to these two factors and their interaction. Species that naturally experience either high or low rates of burial were selected and a factorial burial by nutrient addition experiment was conducted. Growth characteristics were measured in order to determine which characteristics allow a species to respond to burial. Species that naturally experience high rates of burial (Arctotheca populifolia and Scaevola plumieri) displayed increased growth when buried, and this response was nutrient-limited. Stable-dune species had either small (Myrica cordifolia, N-fixer) or negligible responses to burial (Metalasia muricata), and were not nutrient-limited. This interspecific difference in response to burial and/or fertiliser is consistent with the idea that burial maintains the observed zonation of species on coastal dunes. Species that are unable to respond to burial are prevented from occupying the mobile dunes. Species able to cope with high rates of burial had high nitrogen-use efficiencies and low dry mass costs of production, explaining their ability to respond to burial under nutrient limitation. The interaction between burial and nutrient limitation is understudied but vital to understanding the zonation of coastal dune plant species.