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Plant function arises from a complex network of structural and physiological traits. Explicit representation of these traits, as well as their connections with other biophysical processes, is required to advance our understanding of plant-soil-climate interactions. We used the Terrestrial Regional Ecosystem Exchange Simulator (TREES) to evaluate physiological trait networks in maize. Net primary productivity (NPP) and grain yield were simulated across five contrasting climate scenarios. Simulations achieving high NPP and grain yield in high precipitation environments featured trait networks conferring high water use strategies: deep roots, high stomatal conductance at low water potential (“risky” stomatal regulation), high xylem hydraulic conductivity, and high maximal leaf area index. In contrast, high NPP and grain yield was achieved in dry environments with low late-season precipitation via water conserving trait networks: deep roots, high embolism resistance, and low stomatal conductance at low leaf water potential (“conservative” stomatal regulation). We suggest that our approach, which allows for the simultaneous evaluation of physiological traits and their interactions (i.e., networks), has potential to improve crop growth predictions in different environments. In contrast, evaluating single traits in isolation of other coordinated traits does not appear to be an effective strategy for predicting plant performance.Summary statementOur process-based model uncovered two beneficial but contrasting trait networks for maize which can be understood by their integrated effect on water use/conservation. Modification of multiple, physiologically aligned, traits were required to bring about meaningful improvements in NPP and yield.

Determining water stress levels of vegetated surfaces is crucial for irrigation scheduling. This paper aims to evaluate a new method for obtaining crop water stress index (CWSI) based on the estimation of sensible heat flux using an aerodynamic temperature gradient approach. Data were collected on a deficit irrigated maize field at a research farm located in Greeley, Colorado, USA, in 2017 and 2018. The irrigation treatment used subsurface drip. Weather data were measured on-site at 3.3 m above ground level. RED and NIR surface reflectance data were obtained on-site through multispectral radiometer measurements. Nadir surface temperature data were measured using infra-red thermometers at 1 m above canopy. CWSI estimated values were used to assess daily soil water stress index (SWSI), calculated from measurements of volumetric soil water content (VWC) and management allowed depletion (MAD) of 40%. Results show that SWSI is best represented through a non-linear rational CWSI function. Modeled CWSI estimates were compared to measured surface heat fluxes, resulting in a mean bias error of − 0.02 and a root mean square error of 0.09, while errors were 0.02 and 0.06 when compared with observed CWSI based on canopy transpiration measured with plant sap flow devices. Results seem to validate the proposed sensible heat flux-based CWSI model. The CWSI approach presented could be used to manage irrigation and conserve water resources for maize in semi-arid regions.

Precise water and fertilizer application can increase crop water productivity and reduce agricultural contributions to greenhouse gas (GHG) emissions. Regulated deficit irrigation (DI) and drip fertigation control the amount, location, and timing of water and nutrient application. Yet, few studies have measured GHG emissions under these practices, especially for maize (Zea mays L.). The objective was to quantify N

We evaluated sixteen traits related to water acquisition and transport, stomatal conductance, and photosynthesis within a diverse panel of maize inbred lines, founders of the U.S. maize nested association mapping (NAM) population, with the aim to determine which traits confer improved growth under water deficit and well-watered conditions. Lasso regression revealed that three key traits explained meaningful and independent proportions of variation in total end-of-season biomass under deficit irrigation (multiple r2 = 0.86): 1) the maximal net CO2 assimilation rate (P = 0.007), 2) the achievable stomatal conductance during the hottest part of the day (P = 0.005), and 3) the width-to-depth ratio of the root system at the seedling stage (P = 0.060), i.e., initial deep root system development facilitated growth. Under well-watered conditions, maximal stomatal conductance in the morning (P = 0.014) and the width-to-depth ratio of the root system (P = 0.043) were identified as key traits contributing to improved performance (multiple r2 = 0.68). Structural equation models revealed that growth under water deficit was linked more strongly to stomatal conductance occurring during the middle of the day (std coef = 0.75; P = 0.006), rather than the maximal stomatal conductance (std coef = −0.25; P = 0.368). In turn, the maintenance of stomatal conductance through the middle of the day depended on the capacity of the xylem tissue to supply water (per unit cross-sectional area) (std coef = 0.48; P = 0.046). Aligned with the transport of water to the stomata and growth, root system depth ( r = 0.77; P = 0.003) and width-to-depth ratio ( r = −0.55; P = 0.064) at seedling stages were also correlated with the capacity of the xylem to transport water, thus suggesting close coordination between root, xylem, and stomatal traits to achieve greater growth under water deficit and well-watered conditions. We propose that maize performance under the drought conditions considered here, could likely be improved via lower stomatal sensitivity to hydraulic and atmospheric cues, greater xylem conductivity, and a deeper, but not necessarily more extensive, root system.

Water shortages are responsible for the greatest crop losses around the world and are expected to worsen. In arid areas where agriculture is dependent on irrigation, various forms of deficit irrigation management have been suggested to achieve high yields with less water used by the crop (i.e. evapotranspiration, ET). This study of maize evaluated twelve treatments with varying levels of deficit irrigation during late vegetative and maturation (grain filling) growth stages in semi-arid Northern Colorado. In particular, application of greater deficit during the late vegetative state with full or nearly full ET during the rest of the season consistently resulted in yield similar to full ET treatments while saving approximately 15–17% of ET. Maize given 40% of full ET during the late vegetative period had slightly reduced leaf area index (LAI) with significant leaf curling, thus reduced light interception during vegetative growth. However, when plants were fully watered during anthesis, all treatments had full canopy cover with no differences in light interception. The efficiency of photosystem II (quantum yield) declined with water stress but recovered with re-watering. The ability of photosystem II and light interception to recover after stress when well-watered suggests that reductions in biomass and yield resulted from stomatal closure, reduced photochemistry, or loss of xylem conductance that was temporary. With little indication of permanent decline in carbon assimilation after reducing ET in vegetative stages, maize appears able to achieve high grain yield if soil water is readily available during the reproductive and maturation stages. However, plants given full or nearly full irrigation during the entire vegetative period followed by stress later on during the maturation period, had dramatically greater yield loss than ET savings. Thus, while strategic deficit irrigation might maintain yield with less water, it may be especially important for buffering crops against yield losses due to end of season water shortfalls in water limited environments.

There is increasing interest in understanding how trait networks can be manipulated to improve the performance of crop species. Working towards this goal, we have identified key traits linking the acquisition of water, the transport of water to the sites of evaporation and photosynthesis, stomatal conductance, and growth across eight maize hybrid lines grown under well-watered and water-limiting conditions in Northern Colorado. Under well-watered conditions, hybrids with higher end-of-season growth and grain yield exhibited higher leaf-specific conductance, lower operating water potentials, higher rates of midday stomatal conductance, higher rates of net CO2 assimilation, and greater leaf osmotic adjustment. This trait network was similar under water-limited conditions with the notable exception that linkages between water transport, midday stomatal conductance, and growth were even stronger than under fully watered conditions. The maintenance of high leaf-specific conductance throughout the day was achieved via higher maximal conductance rates rather than lower susceptibility to conductance loss. Our results suggest that efforts to improve maize performance in well-watered and water-limiting conditions would benefit from considering the physiological trait networks governing water and carbon flux rather than focusing on single traits independently of one another.

There is increasing interest in understanding how trait networks can be manipulated to improve the performance of crop species. Working towards this goal, we have identified key traits linking the acquisition of water, the transport of water to the sites of evaporation and photosynthesis, stomatal conductance, and growth across eight maize hybrid lines grown under well-watered and water-limiting conditions in Northern Colorado. Under well-watered conditions, well-performing hybrids exhibited high leaf-specific conductance, low operating water potentials, high rates of midday stomatal conductance, high rates of net CO2 assimilation, greater leaf osmotic adjustment, and higher end-of-season growth and grain yield. This trait network was similar under water-limited conditions with the notable exception that linkages between water transport, midday stomatal conductance, and growth were even stronger than under fully-watered conditions. The results of this experiment suggest that similar trait networks might confer improved performance under contrasting climate and soil conditions, and that efforts to improve the performance of crop species could possibly benefit by considering the water transport pathway within leaves, as well as within the whole-xylem, in addition to root-level and leaf-level traits.

Root pressure, also manifested as profusive sap flowing from cut stems, is a phenomenon in some species that has perplexed biologists for much of the last century. It is associated with increased crop production under drought, but its function and regulation remain largely unknown. In this study, we investigated the initiation, mechanisms, and possible adaptive function of root pressure in six genotypes of Sorghum bicolor during a drought experiment in the greenhouse. We observed that root pressure was induced in plants exposed to drought followed by re-watering but possibly inhibited by 100% re-watering in some genotypes. We found that root pressure in drought stressed and re-watered plants was associated with greater ratio of fine: coarse root length and shoot biomass production, indicating a possible role of root allocation in creating root pressure and adaptive benefit of root pressure for shoot biomass production. Using RNA-Seq, we identified gene transcripts that were up- and down-regulated in plants with root pressure expression, focusing on genes for aquaporins, membrane transporters, and ATPases that could regulate inter- and intra-cellular transport of water and ions to generate positive xylem pressure in root tissue.

International audience; The effects of plants on the biosphere, atmosphere and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on the literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we reviewed 24 aspects of plant and ecosystem functioning and their relationships with a number of root system traits, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluated the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that belowground traits with the broadest importance in plant and ecosystem functioning are not those most commonly measured. Also, the estimation of trait relative importance for functioning requires us to consider a more comprehensive range of functionally relevant traits from a diverse range of species, across environments and over time series. We also advocate that establishing causal hierarchical links among root traits will provide a hypothesis-based framework to identify the most parsimonious sets of traits with the strongest links on functions, and to link genotypes to plant and ecosystem functioning.

In the context of a recent massive increase in research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting-edge, meaningful and integrated knowledge. Consideration of the below-ground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardised methods and controlled vocabularies. It is meant to be used not only as starting point by students and scientists who desire working on below-ground ecosystems, but also by experts for consolidating and broadening their views on multiple aspects of root ecology. Beyond the classical compilation of measurement protocols, we have synthesised recommendations from the literature to provide key background knowledge useful for: (1) defining below-ground plant entities and giving keys for their meaningful dissection, classification and naming beyond the classical fine-root vs coarse-root approach; (2) considering the specificity of root research to produce sound laboratory and field data; (3) describing typical, but overlooked steps for studying roots (e.g. root handling, cleaning and storage); and (4) gathering metadata necessary for the interpretation of results and their reuse. Most importantly, all root traits have been introduced with some degree of ecological context that will be a foundation for understanding their ecological meaning, their typical use and uncertainties, and some methodological and conceptual perspectives for future research. Considering all of this, we urge readers not to solely extract protocol recommendations for trait measurements from this work, but to take a moment to read and reflect on the extensive information contained in this broader guide to root ecology, including sections I–VII and the many introductions to each section and root trait description. Finally, it is critical to understand that a major aim of this guide is to help break down barriers between the many subdisciplines of root ecology and ecophysiology, broaden researchers’ views on the multiple aspects of root study and create favourable conditions for the inception of comprehensive experiments on the role of roots in plant and ecosystem functioning. ISSN:0028-646X ISSN:1469-8137

Climate change is projected to increase the incidence of severe drought in many regions, potentially requiring selection for different traits in crop species to maintain productivity under water stress. In this study, we identified a suite of hydraulic traits associated with high productivity under water stress in four genotypes ofS.melongenaL. We also assessed the potential for recovery of this suite of traits from drought stress after re-watering. We observed that two genotypes, PHL 4841 and PHL 2778, quickly grew into large plants with smaller, thicker leaves and increasingly poor hydraulic status (a water-spender strategy), whereas PHL 2789 and Mara maintained safer water status and larger leaves but sacrificed large gains in biomass (a water-saver strategy). The best performing genotype under water stress, PHL 2778, additionally showed a significant increase in root biomass allocation relative to other genotypes. Biomass traits of all genotypes were negatively impacted by water deficit and remained impaired after a week of recovery; however, physiological traits such as electron transport capacity of photosystem II, and proportional allocation to root biomass and fine root length, and leaf area recovered after one week, indicating a strong capacity for eggplant to rebound from short-term deficits via recovery of physiological activity and allocation to resource acquiring tissues. These traits should be considered in selection and breeding of eggplant hybrids for future agricultural outlooks.

Root systems show a tremendous diversity both between and within species, suggesting a large variability in plant functioning and effects on ecosystem properties and processes. In recent decades, developments in many areas of root research have brought considerable advances in our understanding of root traits and their contribution to plant and ecosystem functioning. However, despite major progress, a comprehensive overview—bridging research fields—is lacking. Furthermore, considerable uncertainties exist in the identification of root entities, and the selection and standardized measurement of traits. Here, we provide a comprehensive overview on root entities, exemplify recent advances in our understanding of both theoretical and demonstrated relationships between root traits and plant or ecosystem functioning, discuss trait-trait relationships and hierarchies among traits, and critically assess current strengths and gaps in our knowledge.

BACKGROUND AND AIMS: Understanding root traits and their trade-off with other plant processes is important for understanding plant functioning in natural ecosystems as well as agricultural systems. The aim of the present study was to determine the relationship between root morphology and the hydraulic characteristics of several orders of fine roots (

Both empirical and theoretical models have been widely used to calculate a crop water stress index (CWSI) − a metric often used to describe crop water status. The purpose of this study was to determine the accuracy, limitation, and uncertainty of an empirical (CWSI-E) and two theoretical models compared with sap flow measurement in maize. One theoretical model used a calculated aerodynamic resistance (CWSI-T1), and the other theoretical model used seasonal average aerodynamic resistance (CWSI-T2). Considering the uncertainty of crop coefficient and sap flow measurement, CWSI-T2 and CWSI-E models gave reasonable overall estimates of water stress. The average root mean square deviation at each growth stage from each model ranged from 0.16 to 0.33. CWSI-T2 and the CWSI-E provided relatively accurate prediction of crop stress, both between growth stages and irrigation events. However, CWSI-T1 did not accurately predict water stress between growth stages or between irrigation events. By including climate factors, crop water stress estimated by CWSI-T2 showed less variation and uncertainty than CWSI-E. The uncertainty of both CWSI-T2 and CWSI-E decreased with increasing vapor pressure deficit (VPD), and CWSI-E show larger crop water stress prediction uncertainty. The intercept of non-water stress baseline was the main source of the uncertainty for CWSI-E and CWSI-T2. Considering both uncertainty and stability, we recommend CWSI-T2 model (i.e., seasonal average aerodynamic resistance) for maize water stress assessment.

In many arid regions of the world, population growth, groundwater depletion, and uncertain supplies have caused water for agricultural production to become increasingly scarce. Deficit irrigation (DI) provides a potential response to water scarcity, but no consensus exists on its economic viability. In this paper, we develop an agro-economic model that connects plant growth-stage-specific evapotranspiration (ET) targets with farm profitability. We use the model to determine the economic conditions under which ET targets of less than 100% are optimal for profit-maximizing maize farmers in Colorado. With 2015 input costs, as maize grain prices increase beyond $0.19 kg−1, DI can become optimal during the late vegetative growth stage but requires a water cost greater than U.S. $0.21 m−3. Under some output price and water cost combinations, DI in the maturation stage also becomes optimal. These results suggest that producers could respond to increasing water scarcity with deficit irrigation, but only in a range of water costs that depends on output price and production costs.

The importance of rhizodeposit C and associated microbial communities in deep soil C stabilization is relatively unknown. Phenotypic variability in plant root biomass could impact C cycling through belowground plant allocation, rooting architecture, and microbial community abundance and composition. We used a pulse-chase 13C labeling experiment with compound-specific stable-isotope probing to investigate the importance of rhizodeposit C to deep soil microbial biomass under two switchgrass ecotypes (Panicum virgatum L., Kanlow and Summer) with contrasting root morphology. We quantified root phenology, soil microbial biomass (phospholipid fatty acids, PLFA), and microbial rhizodeposit uptake (13C-PLFAs) to 150 cm over one year during a severe drought. The lowland ecotype, Kanlow, had two times more root biomass with a coarser root system compared to the upland ecotype, Summer. Over the drought, Kanlow lost 78% of its root biomass, while Summer lost only 60%. Rhizosphere microbial communities associated with both ecotypes were similar. However, rhizodeposit uptake under Kanlow had a higher relative abundance of gram-negative bacteria (44.1%), and Summer rhizodeposit uptake was primarily in saprotrophic fungi (48.5%). Both microbial community composition and rhizodeposit uptake shifted over the drought into gram-positive communities. Rhizosphere soil C was greater one year later under Kanlow due to turnover of unlabeled structural root C. Despite a much greater root biomass under Kanlow, rhizosphere δ13C was not significantly different between the two ecotypes, suggesting greater microbial C input under the finer rooted species, Summer, whose microbial associations were predominately saprotrophic fungi. Ecotype specific microbial communities can direct rhizodeposit C flow and C accrual deep in the soil profile and illustrate the importance of the microbial community in plant strategies to survive environmental stress such as drought.

Plant capacity for water storage leads to time lags between basal stem sap flow and transpiration in various woody plants. Internal water storage depends on the sizes of woody plants. However, the changes and its influencing factors in time lags of basal stem flow during the development of herbaceous plants including crops remain unclear. A field experiment was conducted in an arid region of Northwest China to examine the time lag characteristics of sap flow in seed-maize and to calibrate the transpiration modeling. Cross-correlation analysis was used to estimate the time lags between stem sap flow and meteorological driving factors including solar radiation (R s ) and vapor pressure deficit of the air (VPD air ). Results indicate that the changes in seed-maize stem sap flow consistently lagged behind the changes in Rs and preceded the changes in VPD air both on hourly and daily scales, suggesting that light-mediated stomatal closures drove sap flow responses. The time lag in the maize’s sap flow differed significantly during different growth stages and the difference was potentially due to developmental changes in capacitance tissue and/or xylem during ontogenesis. The time lags between stem sap flow and Rs in both female plants and male plants corresponded to plant use of stored water and were independent of total plant water use. Time lags of sap flow were always longer in male plants than in female plants. Theoretically, dry soil may decrease the speed by which sap flow adjusts ahead of shifts in VPD air in comparison with wet soil and also increase the speed by which sap flow adjusts to R s . However, sap flow lags that were associated with R s before irrigation and after irrigation in female plants did not shift. Time series analysis method provided better results for simulating seed-maize sap flow with advantages of allowing for fewer variables to be included. This approach would be helpful in improving the accuracy of estimation for canopy transpiration and conductance using meteorological measurements.

Present efforts to improve maize performance in water-limited environments are aggressively pursuing leaf-level traits, such as quantum efficiency, mesophyll osmoregulation, and stress-protein responses. However, it is possible that improvement of these traits will lead directly to hydraulic failure if hydraulics and photosynthesis are closely aligned. Here, we address the whole plant response to drought stress and ask if photosynthetic and hydraulic traits appear bundled together as a “coordinated” response, or if these traits operate largely independently of one another. Xylem conductance of leaves and stems, whole plant conductance, stomatal conductance, rate of electron transport (ETR), maximal catalytic rate of phosphoenolpyruvate carboxylase (V pmax ) (EC 4.1.1.31), and net CO 2 assimilation (A max ) were measured in maize plants subjected to contrasting levels of drought stress in greenhouse and field experiments. Photosynthetic traits (A max , ETR, V pmax ), hydraulic traits (whole plant and stem conductance) and stomatal conductance were all reduced by >80% as leaf water potentials declined below −3.0 MPa. Furthermore, 83% of the variation associated with the photosynthetic and hydraulic traits measured in this study was explained by a single principal component, revealing a remarkable degree of alignment among them. Whole plant transpiration rates recovered to ca 90% of maximal values 4 d after lifting severe drought stress (Ψ leaf ≈ −3.5 MPa). Closely aligned hydraulic, photosynthetic, and stomatal responses to drought stress suggest that improvements to individual traits, in isolation to each other, may lead to the loss of plant functioning (e.g. water transport) rather than leading to marked improvements in growth.

Current maize hybrids have lower critical aboveground biomass nitrogen (N) concentration (TCNP) and grain N concentration (GNC) compared to older hybrids, but few crop models have incorporated this trend. The objective of this study was to evaluate alternative algorithms for calculating TCNP (biomass-based method) and GNC (grain N demand based on N dilution curve) for predicting crop N concentration and N uptake for a current maize hybrid in the CERES-Maize model as implemented in the Root Zone Water Quality Model (RZWQM). Experimental data were obtained from a field study on maize irrigated to meet various percentages (40% to 100%) of evapotranspiration demand at both vegetative and reproductive stages in 2012 and 2013 in Greeley, Colorado. The original RZWQM showed little response of aboveground N concentration (AGBNC) to the irrigation treatments and overpredicted GNC in both years. As a result, crop N uptake was generally overpredicted, with root mean square error (RMSE) values of 28 to 60 kg N ha-1 for the two years. Adjusted coefficients in the original TCNP and GNC algorithms (RZWQM_ADJ) effectively reduced the overpredicted GNC but with less improvement in response to the irrigation treatments in 2013 compared with the original RZWQM simulations. The RZWQM with modified TCNP and GNC algorithms simulated lower GNC and AGBNC than the original version, significantly improved the responses to the irrigation treatments, and captured the variations in measured GNC among seasons. The corresponding crop N uptake simulations improved more in 2012 than in 2013, with lower RMSE values of 16 to 32 kg N ha-1 than the original and RZWQM_ADJ versions. The better-predicted grain N uptake by the alternative algorithms could be helpful to making better crop N management decisions under different deficit irrigation conditions. Keywords: CERES-Maize, Crop N concentration, Crop N demand, Crop N uptake, Deficit irrigation, Maize hybrid, RZWQM.

A new crop water stress indicator, standard deviation of canopy temperature within a thermal image (CTSD), was developed to monitor crop water status. In this study, thermal imagery was taken from maize (Zea mays L.) under various levels of deficit irrigation at different crop growing stages. The Expectation-Maximization algorithm was used to estimate the canopy temperature distribution from thermal imagery under a range of crop coverage and water stress conditions. Soil water deficit (SWD), leaf water potential (ψ), stomatal conductance, and other crop water stress indices were used to evaluate the CTSD. We found that the temperature differences between sunlit and shaded parts of the canopy would increase with larger canopy resistance in the sunlit part of the crop canopy. The CTSD well described impact of irrigation events (timing and depth) on crop water stress. All water stress measurements showed statistically significant relationship with CTSD. Although CTSD is not sensitive to small changes in water stress, the result suggests that the canopy temperature standard deviation could be used as a water stress indicator. This index has strong application potential because it only relies on the canopy temperature itself, and is easy to calculate. Moreover, it may also be applied to high resolution thermal imagery from other remote sensing platforms, such as unmanned aerial vehicles.

Understanding the variation of sap flow in female and male parents of maize for hybrid seed production and evapotranspiration ( ET ) partitioning is useful in accurately determining water use of the female and male parents and improving irrigation management of maize for hybrid seed production. Sap flow rate, ET , soil evaporation ( E ), meteorological variable, soil water content ( SWC ) and morphological parameters in female and male parents were measured during April to September in both 2013 and 2014 in the arid region of northwest China. Sap flow rates in female and male parents and E were respectively monitored using heat-balance technique and micro-lysimeter. We found that the variations of sap flow rates in female and male parents were similar during the vegetative growth stage, but the female parents had higher sap flow rate per plant than the male parents after the detasseling, with the maximum difference of daily sap flow rate per plant of 0.28-0.33 L d −1 . Hourly sap flow rates in female and male parents were highly correlated with net radiation, while daily sap flow rates in female and male parents were highly correlated with net radiation, SWC , leaf area index, but not correlated with crop height. Daily E increased exponentially with SWC . Moreover, daily sap flow rates in the male parents were positively correlated with stem diameter. The total ET was 363.31-384.15 mm over the whole growth stage of maize, and the transpiration ( T ) of female parents per ground area accounted for 63–80% of the total ET , while the T of male parents per ground area accounted for 4–14% of the total ET , primarily due to different planting quantities between the female and male parents. And the soil evaporation was 47.77-62.21 mm over the whole growth stage, accounting for 13–16% of the total ET . Our results can provide scientific basis for accurate determination of water use of female and male plants and developing precise irrigation scheduling of maize for hybrid seed production in the arid region.

Regenerating oak seedlings by different methods impacts taproot architecture, root-to-shoot allometry and, potentially, soil water use and plant growth under water limitations. In the present study, Quercus robur (L.) seedlings regenerated by four different methods – sowing (acorn-sown seedlings), sowing followed by shoot clipping (coppiced), sowing followed by root-pruning (root-pruned), and container planting with the root plug intact (containerized) – were subjected to four water-shortage regimes – full watering and 75, 50, and 25% of full watering. These treatments were used to test the hypotheses that root morphology, anatomy and biochemistry varied among regeneration methods, and, thus, differentially affected plant responses to water-shortages. For most traits, the response to water shortage of acorn-sown and containerized seedlings was similar, and opposite to that of coppiced and root-pruned seedlings. Water shortage did not change tree allocation to total root biomass among regeneration methods but altered the partitioning between absorptive and structural roots within the root system. Acorn-sown and containerized seedlings produced more pioneer roots, and coppiced and root–pruned seedlings more highly-branched fine roots with greater specific root length and specific root surface area under greater water shortage. Thus, acorn-sown and containerized oaks appeared to be primed for water foraging, and coppiced and root-pruned oaks for water absorption efficiency in response to water shortage. Water shortage caused a reduction in nonstructural carbohydrate concentrations in fine roots of acorn-sown and containerized seedlings and in the taproot of coppiced and root-pruned seedlings. Results generally indicated that seedling response to water limitation depended on the regeneration method, highlighting the importance of root–to–shoot allometry, taproot development, and carbon reserve mobilization for plant functioning under water shortage. Acorn-sown and containerized oaks seedlings might be more suitable for forest regeneration in sites characterized by severe water shortage periods. The reduction of taproot starch reserves under strong water shortage may render root-pruned oak trees more susceptible to topsoil water deficit than acorn-sown trees. This work contributes to understand drought acclimation and to identify the most adequate forestry regeneration practices to adapt forest ecosystems to ongoing climatic changes.

Switchgrass (Panicum virgatum L.) is a C4, perennial grass that is being developed as a bioenergy crop for the United States. While aboveground biomass production is well documented for switchgrass ecotypes (lowland, upland), little is known about the impact of plant belowground productivity on microbial communities down deep in the soil profiles. Microbial dynamics in deeper soils are likely to exert considerable control on ecosystem services, including C and nutrient cycles, due to their involvement in such processes as soil formation and ecosystem biogeochemistry. Differences in root biomass and rooting characteristics of switchgrass ecotypes could lead to distinct differences in belowground microbial biomass and microbial community composition. We quantified root abundance and root architecture and the associated microbial abundance, composition, and rhizodeposit C uptake for two switchgrass ecotypes using stable-isotope probing of microbial phospholipid fatty acids (PLFAs) after 13CO2 pulse–chase labeling. Kanlow, a lowland ecotype with thicker roots, had greater plant biomass above- and belowground (g m−2), greater root mass density (mg cm−3), and lower specific root length (m g−1) compared to Summer, an upland ecotype with finer root architecture. The relative abundance of bacterial biomarkers dominated microbial PLFA profiles for soils under both Kanlow and Summer (55.4 and 53.5 %, respectively; P = 0.0367), with differences attributable to a greater relative abundance of Gram-negative bacteria in soils under Kanlow (18.1 %) compared to soils under Summer (16.3 %; P = 0.0455). The two ecotypes also had distinctly different microbial communities process rhizodeposit C: greater relative atom % 13C excess in Gram-negative bacteria (44.1 ± 2.3 %) under the thicker roots of Kanlow and greater relative atom % 13C excess in saprotrophic fungi under the thinner roots of Summer (48.5 ± 2.2 %). For bioenergy production systems, variation between switchgrass ecotypes could alter microbial communities and impact C sequestration and storage as well as potentially other belowground processes.

It is not currently well-understood how much xylem conductance is lost in maize plants during the day, if conductance is recovered during the night, or what soil water conditions are required for recovery to take place. To answer these questions we designed a greenhouse experiment whereby two genetically dissimilar maize genotypes were subjected to a level of water stress commonly experienced in the field (Ψxylem ∼-2 MPa). We then measured the loss of stem-specific conductivity associated with this level of stress, as well as the overnight recovery following three re-watering treatments: Ψsoil ∼ 0 MPa, Ψsoil ∼-0.40 MPa, and Ψsoil ∼-1.70 MPa. Mid-day leaf water potentials of -1.98 MPa resulted in stem-specific conductivity (KS) values that were 31.5% of maximal (i.e., 68% loss). Returning soils to field capacity (Ψsoil ∼ 0 MPa) overnight allowed for the significant recovery of KS (76% of maximal), whereas partial watering (Ψsoil ∼-0.40 MPa) resulted KS values that were 51.7% of maximal values, whereas not watering resulted in no recovery (35.4% of maximal; Ψsoil ∼-1.7 MPa). Recovery of KS was facilitated by the generation of root pressure and low rates of nighttime transpiration.

Root systems are crucial for optimizing plant growth and productivity. There has been a push to better understand root morphological and architectural traits and their plasticity because these traits determine the capacity of plants to effectively acquire available water and soil nutrients in the soil profile. In this study, two sets of germplasm materials were used to investigate the root system of the new oilseed crop Physaria fendleri (syn. Lesquerella fendleri ) and gather preliminary information on available variability in root traits of the taxa and determine their response to temperature previously found optimal for above ground biomass development in the field. One experiment consisted of eighteen Physaria accessions grown in germination pouches for 21 days under two temperature treatments (21/13 °C and at 30/21 °C) then screened for nine root system parameters. Substantial variation in root length was found within the taxon. Apical root length was plastic in response to temperature with plants growing longer apical root zones when grown at a higher temperature and no difference in other root variables associated with temperature. The second experiment consisted of three accessions of P. fendleri and two of its sister genus Paysonia grown for 60 days in the greenhouse. Root trait analyses indicated that total root length, root length density, specific surface area and diameter differed between the two genera. Two accessions of P. fendleri , WCL-LO4 and PI 596456, were represented in both laboratory and greenhouse experiments. PI 596456 exhibited greater root:shoot ratio and root mass ratio than WCL-LO4 in both growth environments. This root trait screening in P. fendleri and Paysonia provided initial information in lesquerella as basis for future genetic and/or physiological studies relating to its root system.

Canopy temperature has been used as an indicator of crop water stress, since a reduction in plant available water results in lower transpiration rates and consequently higher canopy temperatures. In this study, two indices based on remotely-sensed canopy temperature were used in northern Colorado to monitor water stress in sunflower under six levels of regulated deficit irrigation. The two indices included the widely-used Crop Water Stress Index (CWSI) and the simple Degrees Above Non-Stressed (DANS). The goal of the present study was to evaluate the sensitivity of CWSI and DANS to different levels of irrigation and to investigate their relationship with several crop parameters. In estimating CWSI, non-water-stressed baselines developed in this study were similar to those developed previously at another site in northern Colorado. Both CWSI and DANS were sensitive to irrigation timing/depth and soil water deficit in the root zone, increasing as water availability decreased. Four studied crop parameters, namely the fraction of Intercepted Photosynthetically Active Radiation (fIPAR), Leaf Area Index (LAI), Leaf Water Potential, and root growth were all affected by deficit irrigation treatments, with the first three parameters decreasing and the last one increasing with decrease in water application. All of these crop parameters were also strongly correlated with CWSI and DANS. Statistically significant relationships were developed to predict fIPAR, LAI, and relative root growth based on CWSI and DANS, with coefficients of determination that were similar among the two temperature-based stress indices. The results suggest that DANS, based solely on canopy temperature and estimated by a simple subtraction, may be used effectively in monitoring water stress and scheduling irrigations in deficit-irrigated sunflower in arid/semi-arid regions.

Root traits vary enormously among plant species but we have little understanding of how this variation affects their functioning. Of central interest is how root traits are related to plant resource acquisition strategies from soil. We examined root traits of 33 woody species from northeastern US forests that form two of the most common types of mutualisms with fungi, arbuscular mycorrhizas (AM) and ectomycorrhizas (EM). We examined root trait distribution with respect to plant phylogeny, quantifying the phylogenetic signal (K statistic) in fine root morphology and architecture, and used phylogenetically independent contrasts (PICs) to test whether taxa forming different mycorrhizal associations had different root traits. We found a pattern of species forming roots with thinner diameters as species diversified across time. Given moderate phylogenetic signals (K = 0.44-0.68), we used PICs to examine traits variation among taxa forming AM or EM, revealing that hosts of AM were associated with lower branching intensity (r PIC = -0.77) and thicker root diameter (r PIC = -0.41). Because EM evolved relatively more recently and intermittently across plant phylogenies, significant differences in root traits and colonization between plants forming AM and EM imply linkages between the evolution of these biotic interactions and root traits and suggest a history of selection pressures, with trade-offs for supporting different types of associations. Finally, across plant hosts of both EM and AM, species with thinner root diameters and longer specific root length (SRL) had less colonization (r PIC = 0.85, -0.87), suggesting constraints on colonization linked to the evolution of root morphology.

The ability to design plant communities to optimize particular ecosystem functions and thus more effectively provide ecosystem services would improve all types of ecosystem management, including agriculture. We propose a novel quantitative multi-step method for selecting mixtures of plant species to meet ecosystem objectives: collecting trait data; identifying suites of traits related to relevant community processes; relating those processes to the ecosystem functions of interest; optimizing the planted mixture; and evaluating trade-offs. This approach was tested using planted mixtures of 14 grassland species common in managed pastures of the northeastern United States. Ordination of trait data from greenhouse and small plot studies was used to relate species traits to community processes, followed by generalized additive modeling to relate community-weighted mean process scores to field estimates of biomass production and resistance to invasion. Multi-criteria optimization identified seed mixtures that maximized or minimized selected ecosystem functions, facilitating quantification of trade-offs in processual ability within species and between ecosystem functions at the community level. In this study, predicted seasonal and annual biomass values were within expected ranges, but the optimal mixtures differed from forage mixtures traditionally planted in the northeastern United States. Perennial ryegrass was the most commonly-selected grass. Legumes were represented less than expected, possibly due to attributes such as longevity and forage quality that are important in pastures but were not directly included in the optimization. Ecosystem function was not linearly related to species richness. While further research on quantitatively relating species traits and community processes is needed, multi-criteria optimization offers a new path for linking ecosystem function to ecosystem management.

1.Ecosystem functioning relies heavily on belowground processes, which are largely regulated by plant fine-roots and their functional traits. However, our knowledge of fine-root trait distribution relies to date on local- and regional-scale studies with limited numbers of species, growth forms and environmental variation. 2.We compiled a worldwide fine-root trait dataset, featuring 1115 species from contrasting climatic areas, phylogeny and growth forms to test a series of hypotheses pertaining to the influence of plant functional types, soil and climate variables, and the degree of manipulation of plant growing conditions on species fine-root trait variation. Most particularly, we tested the competing hypotheses that fine-root traits typical of faster return on investment would be most strongly associated with conditions of limiting versus favourable soil resource availability. We accounted for both data source and species phylogenetic relatedness. 3.We demonstrate that (1) Climate conditions promoting soil fertility relate negatively to fine-root traits favouring fast soil resource acquisition, with a particularly strong positive effect of temperature on fine-root diameter and negative effect on specific root length (SRL), and a negative effect of rainfall on root nitrogen concentration; (2) Soil bulk density strongly influences species fine-root morphology, by favouring thicker, denser fine-roots; (3) Fine-roots from herbaceous species are on average finer and have higher SRL than those of woody species, and N2-fixing capacity positively relates to root nitrogen; (4) Plants growing in pots have higher SRL than those grown in the field. 4.Synthesis. This study reveals both the large variation in fine-root traits encountered globally and the relevance of several key plant functional types and soil and climate variables for explaining a substantial part of this variation. Climate, particularly temperature, and plant functional types were the two strongest predictors of fine-root trait variation. High trait variation occurred at local scales, suggesting that wide-ranging belowground resource economics strategies are viable within most climatic areas and soil conditions. This is the peer reviewed version of an article, which has been published in final form at [http://dx.doi.org/10.1111/1365-2745.12769]. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

PREMISE OF THE STUDY Plant phenology influences resource utilization, carbon fluxes, and interspecific interactions. Although controls on aboveground phenology have been studied to some degree, controls on root phenology are exceptionally poorly understood. METHODS We used minirhizotrons to examine the timing of grape root production over 5 yr in Fredonia, New York, USA, in a humid continental climate; and over 3 yr in Oakville, California, USA, in a Mediterranean climate. We used data from previous experiments to examine the relationship of root phenology with aboveground phenology. We compared interannual variability in root and shoot growth and determined the influence of abiotic factors on the timing of root initiation, peak root standing crop, peak root growth rate, and cessation of root growth. KEY RESULTS Root phenology was not tightly coupled with aboveground phenological periods. Both sites typically had one yearly root flush and high interannual variability in root growth. Root phenology was more variable in California than in New York. In this and other published studies, interannual variation in root phenology was greater than variation in aboveground phenology. The three phenological phases of root growth-root initiation, peak root growth, and root cessation-were related to different suites of abiotic factors. CONCLUSIONS Root phenology is highly variable among years. Analysis of potential controlling factors over several years suggest that belowground phenological phases should be analyzed separately from each other. If aboveground grape phenology responds differently than belowground phenology to changes in air temperature, global warming may further uncouple the timing of aboveground and belowground growth.

On the basis of a synthesis of recent progress in belowground ecology, we advance and discuss a hypothesis that relates root trait evolution to the increased dominance of angiosperms into dry upland habitats and the decline of atmospheric CO2 concentration that began in the Cretaceous. Our hypothesis is built from examining patterns of fine root adaptations during the Cretaceous, when angiosperms dramatically diversified in association with arbuscular and ectomycorrhizal root-fungal symbionts. We then explore the potential effects of root adaptations and mycorrhizas on the geochemical carbon cycle. On the basis of phylogenetic analyses of root traits among extant plant species, we suggest that angiosperm taxa, which diversified since the early Cretaceous, evolved thinner roots with greater root length per unit of biomass invested (i.e., specific root length [SRL]) than earlier diverging taxa. We suggest that these changes in root morphology were facilitated by a decline in atmospheric CO2, which l...

Questions: Can we use plant traits to make predictions about ecosystem functioning of different species mixtures, identify inherent trade-offs of particular species, and design custom communities for desired ecosystem functions? Can we develop a methodology to address plant trait–functioning relationships in species-level units, which are missing from measures of community functional diversity but needed for management? Location: Grazing lands northeastern USA. Method: We measured 53 physiological, morphological and growth traits for 19 plant species from glasshouse and field experiments. We developed a twostep method to link species to ecosystem processes related to management goals of improving desirable forage production in grazing lands in northeastern USA. Results: Species were distributed continuously, rather than clustering into discrete functional types. Grasses, legumes and forbs overlapped considerably in trait values with these common classifications failing to adequately distinguish functional differences. Factor analyses were used to assess variation in species traits, and to rate species for six plant processes through which species contribute to the production of desirable forage. Species performed well in some processes and poorly in others, illustrating complex trade-offs.

Root allocation and distribution patterns can infl uence forage yield during periods of moisture or nitrogen stress, as deep-rooted species access water and nutrients found deeper in the soil profi le. In a greenhouse study, we examined rooting characteristics to a depth of 50 cm for 21 cool-season forage species (9 grasses, 6 legumes, and 6 forbs) exposed to drought and low N conditions. The goal of this research was to compare root distribution under uniform growing conditions for common grass, legume and non-leguminous forb species found in humidtemperate pastures of the northeast United States. Nitrogen or water stressed grasses generally had greater root biomass and a greater proportion of roots in the 30 to 40 cm soil layer than did stressed legumes or forbs. Low N significantly reduced root weight, but to a lesser extent than the decrease in shoot weight, resulting in an increase in root/shoot ratio. Drought stress reduced shoot weight but had no effect on root weight, also resulting in a greater root/shoot ratio. Drought stress also increased the proportion of deep roots compared with controls, whereas, N stress did not. Comparisons with previously published fi eld studies suggested that information from container-grown plants could provide insights into fi eld results, and also suggested that inherent species differences in root distribution could explain some, but not all, rooting patterns observed in multi-species forage mixtures.

Understanding factors controlling root dynamics and functioning can lead to more efficient and profitable vineyard management. However, our current understanding of root dynamics and their regulation by plant and environmental factors is limited, particularly under field conditions. This paper presents current understanding of grape root dynamics, highlighting studies using minirhizotron cameras, which directly assess root dynamics, and experiments on roots of known age, which link root phenology and function. Data summarised here show timing of grape root production varies widely among different regions, as well as among rootstocks and canopy management systems in the same region. Timing of production can be responsive to differences in soil moisture. Lifespan of grape roots, however, appears less affected by soil moisture because of nocturnal hydraulic redistribution. Root function, such as capacity for P and N uptake, declines rapidly with root age. Differences in timing and spatial distribution of root production can effect above-ground growth and vineyard water-use efficiency. Improving our understanding of when roots grow and are functionally active in agricultural systems can lead to improved water and fertiliser applications, and more precise vineyard management. Because both environmental and biological factors affect root dynamics, simple predictions of timing of root production or standing populations with shoot development are unlikely to be achieved. However, with multi-year data on root dynamics, and environmental and biological factors, regionally specific models of root populations and their functioning may be possible to develop.

Switchgrass (Panicum virgatum L.) is a C4, perennial grass that is being developed as a bioenergy crop for the United States. While aboveground biomass production is well documented for switchgrass ecotypes (lowland, upland), little is known about the impact of plant belowground productivity on microbial communities down deep in the soil profiles. Differences in root biomass and rooting characteristics of switchgrass ecotypes could lead to distinct differences in belowground microbial biomass and microbial community composition. We quantified root biomass and root architecture and the associated microbial abundance, composition and rhizodeposit C uptake for two switchgrass cultivars using stable isotope probing of microbial phospholipid fatty acids (PLFA) after 13CO2 pulse-chase labeling. Kanlow, a lowland cultivar with thicker roots, had greater plant biomass above- and belowground, greater root mass density, and lower specific root length compared to Summer, an upland cultivar with finer root architecture. The relative abundance of bacterial biomarkers dominated microbial PLFA profiles for both Kanlow and Summer soils (55.4% and 53.5%, respectively), with differences attributable to a greater relative abundance of gram-negative bacteria in Kanlow soils (18.1%) compared to Summer soils (16.3%). The two ecotypes also had distinctly different microbial communities process rhizode posit C; greater relative atom % 13C excess in gram-negative bacteria (44.1 ± 2.3%) under the thicker roots of Kanlow and greater relative atom % 13C excess saprotrophic fungi under the thinner roots of Summer (48.5 ± 2.2%). For bioenergy production systems, variation between switchgrass ecotypes could alter microbial communities and impact C sequestration and storage as well as potentially other belowground processes.

Background and Aims Examination of plant growth below ground is relatively scant compared with that above ground, and is needed to understand whole-plant responses to the environment. This study examines whether the seasonal timing of fine root growth and the spatial distribution of this growth through the soil profile varies in response to canopy manipulation and soil temperature. Methods Plasticity in the seasonal timing and vertical distribution of root production in response to canopy and soil water manipulation was analysed in field-grown walnut (Juglans regia ‘Chandler’) using minirhizotron techniques. Key Results Root production in walnuts followed a unimodal curve, with one marked flush of root growth starting in mid-May, with a peak in mid-June. Root production declined later in the season, corresponding to increased soil temperature, as well as to the period of major carbohydrate allocation to reproduction. Canopy and soil moisture manipulation did not influence the timing of root production, but did influence the vertical distribution of roots through the soil profile. Water deficit appeared to promote root production in deeper soil layers for mining soil water. Canopy removal appeared to promote shallow root production. Conclusions The findings of this study add to growing evidence that root growth in many ecosystems follows a unimodal curve with one marked flush of root growth in coordination with the initial leaf flush of the season. Root vertical distribution appeared to have greater plasticity than timing of root production in this system, with temperature and/or carbohydrate competition constraining the timing of root growth. Effects on root distribution can have serious impacts on trees, with shallow rooting having negative impacts in years with limited soil water or positive impacts in years with wet springs, and deep rooting having positive impacts on soil water mining from deeper soil layers but negative impacts in years with wet springs.

Growers plan many of their horticultural activities around certain shoot phenological stages such as bloom and veraison. Timing of root growth in relation to these stages of the shoot is of interest in fertilization scheduling and understanding carbon allocation demands of the root system. With the recent use of minirhizotron root observation tubes, a much greater understanding of patterns of root growth has been made possible. In this paper some published work and present data from ongoing studies on timing of root production are reviewed. Studies discussed include ongoing work in grape vineyards in Fredonia, New York, USA (Vitis lambruscana), and Oakville, California, USA (V. vinifera), and in apple orchards (Malus × domestica) in Summerland, British Columbia, Canada. Root production mainly occurs in the summer, but the specific timing varies widely between bloom and harvest from year to year. Usually there is little root growth in the few weeks prior to harvest and when the vines are dormant. Cultural practices may affect root production, usually from a combination of changes in the soil environment (moisture, temperature) and changes in photosynthetic capacity.