Effects Of Soil Water Stress Research Articles

High temperatures and low soil moisture synergistically reduce switchgrass yields from marginal field sites and inhibit fermentation

Abstract‘Marginal lands’ are low productivity sites abandoned from agriculture for reasons such as low or high soil water content, challenging topography, or nutrient deficiency. To avoid competition with crop production, cellulosic bioenergy crops have been proposed for cultivation on marginal lands, however on these sites they may be more strongly affected by environmental stresses such as low soil water content. In this study we used rainout shelters to induce low soil moisture on marginal lands and determine the effect of soil water stress on switchgrass growth and the subsequent production of bioethanol. Five marginal land sites that span a latitudinal gradient in Michigan and Wisconsin were planted to switchgrass in 2013 and during the 2018–2021 growing seasons were exposed to reduced precipitation under rainout shelters in comparison to ambient precipitation. The effect of reduced precipitation was related to the environmental conditions at each site and biofuel production metrics (switchgrass biomass yields and composition and ethanol production). During the first year (2018), the rainout shelters were designed with 60% rain exclusion, which did not affect biomass yields compared to ambient conditions at any of the field sites, but decreased switchgrass fermentability at the Wisconsin Central–Hancock site. In subsequent years, the shelters were redesigned to fully exclude rainfall, which led to reduced biomass yields and inhibited fermentation for three sites. When switchgrass was grown in soils with large reductions in moisture and increases in temperature, the potential for biofuel production was significantly reduced, exposing some of the challenges associated with producing biofuels from lignocellulosic biomass grown under drought conditions.

Accumulated soil moisture deficit better indicates the effect of soil water stress on light use efficiency of grasslands during drought years

Light use efficiency (LUE) models have been widely used in the estimation of gross primary productivity (GPP). However, many studies indicated that current LUE models generally overestimate GPP during drought years. Recent studies also showed that soil moisture (SM) regulated LUE during drought years, indicating the necessity of adding SM-related stress scalars in LUE models. Meanwhile, considering the persistence characteristic of drought, the effect of SM stress on LUE is expected to be not only instantaneous but also accumulated. However, it is unclear whether the accumulated SM stress can better capture LUE. Thus, based on observations from grassland flux sites in the United States and Australia, we compared the effectiveness of concurrent soil moisture deficit (SMD), lagged soil moisture deficit (LSMD), and accumulated soil moisture deficit (ASMD) in capturing actual LUE during drought years. ASMD had more stable and stronger linear correlations with LUE than SMD and LSMD at almost all the sites. Furthermore, ASMD outperformed water stress scalars commonly used in LUE models such as the Land Surface Water Index (LSWI) and vapor pressure deficit (VPD) for tracking actual LUE during drought years. The introduction of ASMD improved the accuracy of the LUE model for simulating grassland GPP during drought years (with a R2 and RMSE of 0.88 and 1.06 g C m−2 d−1). Our study highlights the accumulated effect of SM in regulating LUE, and proves that ASMD has the potential in improving the performance of the existing LUE models during drought years.

Effect of Water Stress during Seed Production and Storage Time on Germination and Seedling Growth of Cowpea Grown in the Dry Zone of Sri Lanka

The water stress during the flowering stage of cowpea reduces the seed yield of the same season, but its effect on seed germination and seedling growth in the following season remains poorly understood. This study was carried out to determine the effect of soil water stress maintained during the onset of flowering stage and storage period of seeds on seed germination, vigor index, and seed physical characteristics of five cowpea varieties (<em>Waruni</em>, <em>Dhawala</em>, <em>MI 35</em>, <em>ANKCP 01 </em>and <em>Bombay</em>) in the following season in the dry zone of Sri Lanka. Seed production under well-watered and water-stressed conditions was done at the Field Crops Research and Development Institute (FCRDI), Mahailuppallama, in a Randomized Complete Block Design. The germination study was conducted in a polytunnel at the Faculty of Agriculture, Rajarata University of Sri Lanka, Anuradhapura, following the Completely Randomized Design. Seeds of five cowpea varieties produced after exposing the plants to two soil moisture levels (water stress: -50 kPa and well-watered: -30 kPa) were used in a series of germination trials conducted after 1, 1½, 2, and 2½ months of storage. Three hundred seeds from each genotype, randomly sub-divided into three replicates of 100 seeds each, were sown in sterilized sand media. Germination percentage was recorded on the 4^th and 6^th days after sowing. The seedling characters were assessed on the 6^th day after sowing, using five randomly-selected seedlings from each replicate. The results revealed that the storage time significantly affected germination (<em>p</em>= 0.0133). However, the soil water status maintained during seed production did not affect the germination percentage of any variety in the following season. The majority of the varieties showed an increase in germination percentage with storage time. Although the vigor index did not change with either the well-watered or water-stressed treatments in all five varieties, the vigor index of all varieties declined with storage period irrespective of the water treatment (<em>p</em>&lt; 0.0001). The findings suggest that moderate water deficits during cowpea seed production do not compromise seed germination and seedling vigor in the following season while emphasizing the importance of utilizing the seeds for subsequent field cultivation in the following season before their vigor is decreased.

Scaling Up from Leaf to Whole-Plant Level for Water Use Efficiency Estimates Based on Stomatal and Mesophyll Behaviour in Platycladus orientalis

Prediction of whole-plant short-term water use efficiency (WUEs,P) is essential to indicate plant performance and facilitate comparison across different temporal and spatial scales. In this study, an isotope model was scaled up from the leaf to the whole-plant level, in order to simulate the variation in WUEs,P in response to different CO2 concentrations (Ca; 400, 600, and 800 μmol·mol−1) and soil water content (SWC; 35–100% of field capacity). For WUEs,P modelling, leaf gas exchange information, plant respiration, and “unproductive” water loss were taken into account. Specifically, in shaping the expression of the WUEs,P, we emphasized the role of both stomatal (gsw) and mesophyll conductance (gm). Simulations were compared with the measured results to check the model’s applicability. The verification showed that estimates of gsw from the coupled photosynthesis (Pn,L)-gsw model accounting for the effect of soil water stress slightly outperformed the model neglecting the soil water status effect. The established coupled Pn,L-gm model also proved more effective in estimating gm than the previously proposed model. Introducing the two diffusion control functions into the whole-plant model, the developed model for WUEs,P effectively captured its response pattern to different Ca and SWC conditions. Overall, this study confirmed that the accurate estimation of WUEs,P requires an improved predictive accuracy of gsw and gm. These results have important implications for predicting how plants respond to climate change.

Global decadal variability of plant carbon isotope discrimination and its link to gross primary production.

Carbon isotope discrimination (Δ13C) in C3 woody plants is a key variable for the study of photosynthesis. Yet how Δ13C varies at decadal scales, and across regions, and how it is related to gross primary production (GPP), are still incompletely understood. Here we address these questions by implementing a new Δ13C modelling capability in the land‐surface model JULES incorporating both photorespiratory and mesophyll‐conductance fractionations. We test the ability of four leaf‐internal CO2 concentration models embedded in JULES to reproduce leaf and tree‐ring (TR) carbon isotopic data. We show that all the tested models tend to overestimate average Δ13C values, and to underestimate interannual variability in Δ13C. This is likely because they ignore the effects of soil water stress on stomatal behavior. Variations in post‐photosynthetic isotopic fractionations across species, sites and years, may also partly explain the discrepancies between predicted and TR‐derived Δ13C values. Nonetheless, the “least‐cost” (Prentice) model shows the lowest biases with the isotopic measurements, and lead to improved predictions of canopy‐level carbon and water fluxes. Overall, modelled Δ13C trends vary strongly between regions during the recent (1979–2016) historical period but stay nearly constant when averaged over the globe. Photorespiratory and mesophyll effects modulate the simulated global Δ13C trend by 0.0015 ± 0.005‰ and –0.0006 ± 0.001‰ ppm−1, respectively. These predictions contrast with previous findings based on atmospheric carbon isotope measurements. Predicted Δ13C and GPP tend to be negatively correlated in wet‐humid and cold regions, and in tropical African forests, but positively related elsewhere. The negative correlation between Δ13C and GPP is partly due to the strong dominant influences of temperature on GPP and vapor pressure deficit on Δ13C in those forests. Our results demonstrate that the combined analysis of Δ13C and GPP can help understand the drivers of photosynthesis changes in different climatic regions.

Plant hydraulic transport controls transpiration sensitivity to soil water stress

Abstract. Plant transpiration downregulation in the presence of soil water stress is a critical mechanism for predicting global water, carbon, and energy cycles. Currently, many terrestrial biosphere models (TBMs) represent this mechanism with an empirical correction function (β) of soil moisture – a convenient approach that can produce large prediction uncertainties. To reduce this uncertainty, TBMs have increasingly incorporated physically based plant hydraulic models (PHMs). However, PHMs introduce additional parameter uncertainty and computational demands. Therefore, understanding why and when PHM and β predictions diverge would usefully inform model selection within TBMs. Here, we use a minimalist PHM to demonstrate that coupling the effects of soil water stress and atmospheric moisture demand leads to a spectrum of transpiration responses controlled by soil–plant hydraulic transport (conductance). Within this transport-limitation spectrum, β emerges as an end-member scenario of PHMs with infinite conductance, completely decoupling the effects of soil water stress and atmospheric moisture demand on transpiration. As a result, PHM and β transpiration predictions diverge most for soil–plant systems with low hydraulic conductance (transport-limited) that experience high variation in atmospheric moisture demand and have moderate soil moisture supply for plants. We test these minimalist model results by using a land surface model at an AmeriFlux site. At this transport-limited site, a PHM downregulation scheme outperforms the β scheme due to its sensitivity to variations in atmospheric moisture demand. Based on this observation, we develop a new “dynamic β” that varies with atmospheric moisture demand – an approach that overcomes existing biases within β schemes and has potential to simplify existing PHM parameterization and implementation.

Soil moisture\u2013atmosphere feedback dominates land carbon uptake variability

Year-to-year changes in carbon uptake by terrestrial ecosystems have an essential role in determining atmospheric carbon dioxide concentrations1. It remains uncertain to what extent temperature and water availability can explain these variations at the global scale2–5. Here we use factorial climate model simulations6 and show that variability in soil moisture drives 90 per cent of the inter-annual variability in global land carbon uptake, mainly through its impact on photosynthesis. We find that most of this ecosystem response occurs indirectly as soil moisture–atmosphere feedback amplifies temperature and humidity anomalies and enhances the direct effects of soil water stress. The strength of this feedback mechanism explains why coupled climate models indicate that soil moisture has a dominant role4, which is not readily apparent from land surface model simulations and observational analyses2,5. These findings highlight the need to account for feedback between soil and atmospheric dryness when estimating the response of the carbon cycle to climatic change globally5,7, as well as when conducting field-scale investigations of the response of the ecosystem to droughts8,9. Our results show that most of the global variability in modelled land carbon uptake is driven by temperature and vapour pressure deficit effects that are controlled by soil moisture.

Estimating stomatal conductance and evapotranspiration of winter wheat using a soil-plant water relations-based stress index

Stomatal conductance, closely related to water flow in the soil-plant-atmosphere continuum, is an important parameter in the Penman-Monteith (P-M) model for estimating evapotranspiration (ET). In this study, a novel soil water stress index ω, considering intrinsic soil-plant water relations, was introduced into the Jarvis empirical estimation model of stomatal conductance to improve the representation of the effect of soil water stress on stomatal conductance. The index ω accounted not only for current water availability by combing the effects of relative distribution of soil water to roots and nonlinear stomatal response, but also for the hysteresis effect of water stress by means of the inclusion of a recovery coefficient. Combined plant and soil-based measurements from a greenhouse experiment provided the basis for investigating the relationship between leaf stomatal conductance gs and root zone soil water stress represented by ω. The response of gs to root-weighted soil matric potential was found to be nonlinear. The relationship between gs and the extent of previous water stress (i.e. the water stress recovery coefficient curve) was generalized by a power function and was verified and confirmed using results obtained from the literature. The reliability of ω was tested by coupling it into the Jarvis model to estimate leaf (gs) and canopy (gc) stomatal conductance, and thereupon into the P-M model to estimate cumulative ET (CET) in the greenhouse experiment and two field experiments. The estimated gs, gc and CET agreed well with the measurements, with root mean squared error not more than 0.0006 m s−1, 0.0020 m s−1 and 8.2 mm, respectively, and determination coefficient (Nash-Sutcliffe efficiency coefficient) consistently greater than 65% (0.14). Therefore, ω should be feasible and reliable to delineate the response of stomatal physiological reaction to water stress, and hence helpful for accurate estimation of ET using Jarvis-based P-M models.

Comparison of root water uptake models in simulating CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and H&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O fluxes and growth of wheat

Abstract. Stomatal regulation and whole plant hydraulic signaling affect water fluxes and stress in plants. Land surface models and crop models use a coupled photosynthesis–stomatal conductance modeling approach. Those models estimate the effect of soil water stress on stomatal conductance directly from soil water content or soil hydraulic potential without explicit representation of hydraulic signals between the soil and stomata. In order to explicitly represent stomatal regulation by soil water status as a function of the hydraulic signal and its relation to the whole plant hydraulic conductance, we coupled the crop model LINTULCC2 and the root growth model SLIMROOT with Couvreur's root water uptake model (RWU) and the HILLFLOW soil water balance model. Since plant hydraulic conductance depends on the plant development, this model coupling represents a two-way coupling between growth and plant hydraulics. To evaluate the advantage of considering plant hydraulic conductance and hydraulic signaling, we compared the performance of this newly coupled model with another commonly used approach that relates root water uptake and plant stress directly to the root zone water hydraulic potential (HILLFLOW with Feddes' RWU model). Simulations were compared with gas flux measurements and crop growth data from a wheat crop grown under three water supply regimes (sheltered, rainfed, and irrigated) and two soil types (stony and silty) in western Germany in 2016. The two models showed a relatively similar performance in the simulation of dry matter, leaf area index (LAI), root growth, RWU, gross assimilation rate, and soil water content. The Feddes model predicts more stress and less growth in the silty soil than in the stony soil, which is opposite to the observed growth. The Couvreur model better represents the difference in growth between the two soils and the different treatments. The newly coupled model (HILLFLOW–Couvreur's RWU–SLIMROOT–LINTULCC2) was also able to simulate the dynamics and magnitude of whole plant hydraulic conductance over the growing season. This demonstrates the importance of two-way feedbacks between growth and root water uptake for predicting the crop response to different soil water conditions in different soils. Our results suggest that a better representation of the effects of soil characteristics on root growth is needed for reliable estimations of root hydraulic conductance and gas fluxes, particularly in heterogeneous fields. The newly coupled soil–plant model marks a promising approach but requires further testing for other scenarios regarding crops, soil, and climate.

Simulating the Influences of Soil Water Stress on Leaf Expansion and Senescence of Winter Wheat

Current crop models usually need a lot of input information to simulate crop leaf area index and have relatively large simulation errors under soil water stress, which need to be improved. In this study, based on experiments conducted in glass soil columns and field under rainout shelters in four years (2012-2016) in Yangling, Shaanxi Province in China, we attempted to establish a dynamic model for simulating leaf expansion and senescence of winter wheat under soil water stress. First, a temperature response function was established with four cardinal temperatures (base temperature, lower optimum temperature, higher optimum temperature, and maximum temperature). Then two soil water stress functions were established to quantify the effect of soil water stress on the processes of leaf expansion and leaf senescence of main stem per plant. The first order derivative of a logistic function was then modified with the temperature and soil water stress response function and was used to simulate the daily rate of leaf area expansion and senescence of main stem. The parameters of the new model were estimated using the Solver add-in in MS Excel and then validated based on the data of soil column experiments in 2014-2015 growing season. Then the new model was evaluated with another dataset of column experiments conducted in 2015-2016. Furthermore, the influence of soil water stress on wheat tillering was included in the new model based on open field experiments in 2012-2013 growing season. Then, the new leaf area index (LAI) simulation model was further verified with the data of open field experiments under rainout shelter (2013-2014) and rainfed condition (2004-2005, 2005-2006, and 2008-2009) in different sites. A comparison was conducted between new LAI model and the original LAI module of CERES-Wheat. The results showed that the leaf expansion rate was not affected when the relative soil water availability was greater than 0.7; water stress inhibited the leaf expansion when relative soil water availability was between 0.2 and 0.7; leaves withered and yellowed when relative soil water availability was less than 0.2. The overall average root mean square error (RMSE) and residual accumulation coefficient (CRM) were 9.78 cm2 plant−1, -0.03, and 6.51 cm2 plant−1; 0.05 for model calibration and validation, respectively. The RMSE of LAI decreased by an average of 47.89% compared with the original LAI module of CERES-Wheat. The results of this study can help to improve the current CERES-Wheat model for model applications in the arid and semi-arid regions of China.

Effects of soil water stress and atmospheric CO2 concentration on photosynthetic and post-photosynthetic fractionation

Analysis of plant photosynthesis and post-photosynthetic fractionation can improve our understanding of plant physiology and water management. By measuring δ13C in the atmosphere, and δ13C of soluble compounds in leaves and branch phloem of Platycladus orientalis, we examined discrimination pattern, including atmosphere-leaf discrimination during photosynthesis (ΔCa-leaf) and leaf-twig discrimination during post-photosynthesis (ΔCleaf-phlo), in response to changes of soil water content (SWC) and atmospheric CO2 concentration (Ca). The results showed that ΔCa-leaf reached a maximum of 13.06‰ at 95%-100% field water-holding capacity (FC) and Ca 400 μmol·mol-1, and a minimum of 8.63‰ at 35%-45% FC and Ca 800 μmol·mol-1. Both stomatal conductance and mesophyll cell conductance showed a significant linear positive correlation with ΔCa-leaf, with a correlation coefficient of 0.43 and 0.44, respectively. ΔCleaf-phlo was not affected by SWC and Ca. Our results provide mechanism of carbon isotopes fractionation and a theoretical basis for plant survival strategies in response to future climate change.

干旱胁迫对群众杨光合特性与器官干物质分配的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 干旱胁迫对群众杨光合特性与器官干物质分配的影响 DOI: 10.5846/stxb201901270203 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（31400527，31700534）；山西省应用基础研究项目（201701D221190）；山西农业大学引进人才博士科研启动项目（2013YJ18） Effects of water stress on leaf gas exchange and biomass allocation of Populus×popularis ‘35-44’ cuttings Author: Affiliation: Fund Project: National Natural Science Foundation of China (NSFC)、Shanxi applied basic research project、Shanxi agricultural university introduction of doctoral research projects 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:土壤干旱是北方地区杨树人工林生长和生态效益的重要限制因子。以群众杨（Populus×popularis'35-44’）扦插苗为研究材料，采用盆栽称重法控制土壤水分，设置正常供水、中度干旱、重度干旱3种水分梯度。分别于2016年9月与2017年9月测定叶片气体交换参数、各器官干重、根冠比、叶经济性状等指标，研究了持续2年干旱胁迫下群众杨扦插苗光合特性与器官干物质分配的变化规律，分析了土壤干旱程度、树龄在"小老树"形成中的作用过程。2016年与2017年中度、重度干旱胁迫下功能叶净光合速率（Pn）受到非气孔限制，与正常供水相比分别降低了36.08%、56.42%；18.47%、5.13%。第一年干旱胁迫下群众杨气孔导度（Gs）和蒸腾速率（Tr）显著降低；第二年干旱胁迫下Gs与正常供水无显著差异，而Tr显著提高，有利于叶片C、N元素的运输分配进而维持较高的Pn。第一年随干旱胁迫程度加剧，群众杨叶片C/N显著提高，光合产物分配趋向于叶器官。第二年中度干旱胁迫下叶片C/N与正常供水相比无显著差异，各器官干重下降幅度均低于2016年，而根冠比显著增加，表明其光合产物主要用于地下部构建以适应中度干旱环境；持续重度干旱则光合产物向茎中的分配比例提高。干旱胁迫导致群众杨功能叶水分利用效率（WUE，Water use efficiency）与光合氮利用效率（PNUE，Photosynthetic nitrogen use efficiency）显著降低，整株氮利用效率（NUE，Whole plant nitrogen use efficiency）显著提高。严重土壤干旱下叶片耗水增强、PNUE下降以及茎木质部碳投资增加可能是群众杨小老树形成的重要原因。 Abstract:Soil water content is a key limiting factor to development and ecological function of artificial poplar plantations in semi-arid area of northern China. To examine the effects of soil drought on a poplar variety Populus×popularis ‘35-44’, 3 soil water levels, i.e., normal (75%±5% of field capacity), moderate drought (50%±5%) and severe drought (25%±5%), were set up using 1 year old cutting seedlings grown in pots. We measured leaf gas exchange parameters, organ dry mass and leaf economic traits both in September 2016 and September 2017 to analyze interannual changes in leaf gas exchange and biomass allocation in an attempt to elucidate the effects of soil water stress and ageing in formation of the "old dwarf trees" phenomenon. Compared with seedlings grown at the normal soil water level, leaf net photosynthetic rates (Pn) of seedlings grown at moderate and severe soil drought levels were reduced by 36.08% and 56.42% in 2016 respectively and also 18.47% and 5.13% in 2017 respectively, mainly due to non-stoma limitation. In first year of soil water drought treatment, stomatal conductance (Gs) and transpiration rate (Tr) of seedlings grown at both soil drought levels decreased significantly. However, no significant decrease of Gs at both soil drought levels were found in second year, while Tr increased at both drought levels significantly, which could be beneficial to transportation of C and N to leaves and therefore maintenance of Pn. In first year of treatment, with increasing severity of soil drought, leaf C/N ratio increased significantly and more biomass was allocated to foliage. However, no significant differences were found in leaf C/N ratio between normal soil water level and moderate soil drought level in second year of treatment, while the decreases of all component biomass at moderate soil drought level in 2017 were less than those in 2016 with a significantly higher root-shoot ratio, indicating that more dry matter were allocated to underground for root construction in acclimation to moderate soil drought. In comparison, more dry matter were allocated to stem at severe soil drought level. After two years of soil drought treatment, water use efficiency (WUE) and photosynthetic nitrogen use efficiency (PNUE) of functional leaves of seedlings decreased significantly, but nitrogen use efficiency at whole plant level (NUE) increased significantly. Increased leaf water consumption, decreased PNUE and increased stem xylem C input of Populus×popularis ‘35-44’ seedlings at severe soil drought level may be the main drivers of "old dwarf trees" phenomenon. 参考文献 相似文献 引证文献

Effects of soil water stress on fruit yield, quality and their relationship with sugar metabolism in ‘Gala’ apple

The aim of this research is to explore apple quality responses to water stress during the fruit growing season in seven-year-old potted ‘Gala’ (Malus domestica Borkh.) trees. Three irrigation levels were applied, including normal irrigation (CK, 70–80%θf), light water stress (LS, 60–70%θf), medium water stress (MS, 50%–60%θf), θf: field capacity. In stage Ⅰ, water stress was applied from stage of young fruit to mature. In stage II, water stress was utilized from stage of fruit expanding to mature. The results showed that MS and LS treatments improve fruit quality by improving the contents of total soluble solid (TSS) and soluble sugar (fructose, glucose and sorbitol) and reducing titratable acidity (TA) levels. But MS and LS treatments applied in stage Ⅰ reduced fruit weight, while LS treatment applied in stage II did not affect fruit weight. Interestingly, the enhancing of activity of Sucrose Synthase (SS) and Acid Invertase (AI) in MS and the increasing of activity of SS in LS promoted the conversion from sucrose to fructose and glucose in stage Ⅰ. In stage II, the enhancing of AI and Sorbitol Oxidase (SOX) in MS and LS promoted the conversion from sucrose to fructose and glucose and the conversion from sorbitol to glucose, respectively. Our observations suggest that different degrees and duration of soil water stress affect different enzymes to promote the increase of fructose and glucose content. In addition, light water control is recommended only during later fruit developmental stages for optimum quality and production.

Application of the Maximum Entropy Production Model of Evapotranspiration over Partially Vegetated Water-Limited Land Surfaces

Abstract The maximum entropy production (MEP) model based on nonequilibrium thermodynamics and the theory of Bayesian probabilities was recently developed to model land surface fluxes, including soil evaporation and vegetation transpiration. This model requires few input data and ensures the closure of the surface energy balance. This study aims to test the capability of such a model to realistically simulate evapotranspiration (ET) over a wide range of climates and vegetation covers. A weighting coefficient is introduced to calculate total ET from soil evaporation and vegetation transpiration over partially vegetated land surfaces, resulting in the MEP-ET model. Using this coefficient, the model outputs are compared with in situ observations of ET at eight FLUXNET sites across the continental United States. Results confirm the close agreement between the MEP-ET predicted daily ET and the corresponding observations at sites characterized by moderately limited water availability. Poor ET results were obtained under high water stress conditions. A regulation parameter was therefore introduced in the MEP-ET model to properly take into account the effects of soil water stress on stomata, yielding the generalized MEP-ET model. This parameter considerably reduced model biases under water stress conditions for various heterogeneous land surface sites. The generalized MEP-ET model outperforms several popular ET models, including Penman–Monteith (PM), modified Priestley–Taylor–Jet Propulsion Laboratory (PT-JPL), and air-relative-humidity-based two-source model (ARTS) at all test sites.

Effect of Soil Water Stress and Nickel Application on Micronutrient Status of Canola Grown on Two Calcareous Soils

Recently, application of sewage sludge/effluents to soils as an alternative source of organic matter in arid/semiarid regions and as a practical approach to remove its sanitary adverse effects in urban and industrial regions is gaining increased attention. This may lead to high levels of nickel (Ni) in soil and plants. On the other hand, in these regions water shortage is a major constraint of plant productivity. To determine the combined effects of Ni and water stress on the nutritional status of canola, we studied the effect of five Ni levels (0, 0.05, 0.1, 0.5 and 1 mg Ni kg–1 soil as Ni(NO3)2) and two levels of water status (field capacity, FC, and 0.6 FC) on the contents of iron (Fe), manganese (Mn), zinc (Zn) and copper (Cu) in canola (Brassica napus L.) on two loamy and sandy clay calcareous soils in greenhouse conditions. Shoot/root dry matter yield (SDMY/ RDMY), their Fe, Zn and Mn content decreased under 0.6 FC conditions; whereas, their concentrations increased. Shoot /root Cu concentration /content decreased in water-stressed plants. Application of > 0.05 mg Ni kg–1 to loamy soil increased SDMY, however Ni did not affect SDMY positively or even decreased it in sandy clay soil. Application of 0.01~0.5 mg Ni increased RDMY in loamy soil under FC conditions; whereas, higher levels of Ni decreased it. Nickel decreased RDMY in sandy clay soil, whereas did not affect micronutrient concentrations in the root. The concentration and content of Mn in the shoot followed different patterns in response to applied Ni. Although the highest level of applied Ni increased shoot Zn content. Application of 0.05 and 1 mg Ni decreased the Cu concentration in the shoot on loamy and sandy clay soils, respectively. However, Ni did not affect the Cu content of shoot. Nickel had different impacts on the content of each micronutrient and was not effective in mitigating the adverse effects of drought stress.

The Effect of Water Stress on the Fluorescence Parameters and Growth of&lt;i&gt; Sophora japonica &lt;/i&gt;'Golden Stem'

The effect of soil water stress on the physiology of Sophora japonica Golden stem was studied in order to reduce the irrigation water. The results showed that the growths of the plants were suppressed after treated with flooding for 15 days, which was embodied in the decreasing of qP, ETR, Fv/Fm and the content of pigment, the increasing of the content of MDA and qN. The growth and appearance of plants were suppressed after flooded for 23 days. The drought stress suppressed the growth of plants, which was embodied in the decreasing of qP, ETR, Fv/Fm, the content of pigment, the water content of leaves, the increasing of the content of MDA, qN after treated for 19 days. The qP, ETR, content of pigment were increased after treated with slight drought or middle drought for 19 days. The best irrigation method for Sophora japonica Golden stemin the landscape land was the content of water in the soil between 18% and 30%, which did not affect the growth or landscape effects. The application of S. japonica Golden stem in the landscape and architecture should be avoided in the place which would be flooded for more than 15 days

Coupling a SVAT heat and water flow model, a stomatal-photosynthesis model and a crop growth model to simulate energy, water and carbon fluxes in an irrigated maize ecosystem

Irrigation is practiced on approximately 20% of the agricultural land in the world and accounts for approximately 40% of the total crop production. However, with global warming and an increasing population, the agricultural water consumption increases, leaving generally less water for the natural ecosystems. An increase in water efficiency of agro-ecosystems, especially irrigated agro-ecosystems in arid and semi-arid regions, is an urgent task. The use of computer models to simulate interactions and feedbacks between relevant processes during crop growth is becoming more common and almost a prerequisite for proper management of irrigation water. In this paper, we describe the integration of SHAW, a soil-vegetation-atmosphere transfer (SVAT) model, with a stomatal-photosynthesis model and WOFOST, a crop growth model, to simulate the energy, water and carbon budgets during crop growth. The coupled model was tested and applied for a field study on irrigated maize [38°51′ N, 100°25′ E, altitude 1519m a.s.l.], located in an irrigation oasis of the Heihe river basin in arid Northwest China. The coupled model performs well in simulating the diurnal variation of the leaf water potential, stomatal resistance and transpiration at leaf scale, before and after irrigation. At the canopy scale, the coupled model also reproduces the daily changes in the sensible and latent heat fluxes, carbon dioxide flux, and dynamic soil water content during maize growth and fallow periods. Moreover, there was good agreement between the simulated maize biomass and the field measurements. These results demonstrate that the holistic coupled model not only successfully simulates the actual effect of soil water stress on crop transpiration and photosynthesis, but also can describe the interactions of energy, water, and carbon cycles of the agro-ecosystem and predict crop production under irrigation. This is encouraging for the modelling of crop response to droughts and changed cropping and irrigation regimes aiming at optimized water use. Meanwhile, this study indicates that integrating methods of different physically based models is highly efficient and useful for a better understanding of the interaction between hydrological and ecological processes in the agro-ecosystem.

Critical responses of photosynthetic efficiency in Campsis radicans (L.) Seem to soil water and light intensities

Campsis radicans (L.) Seem is one of the main forestation tree species in semi-arid loess hilly region. Using the CIRAS-2 portable photosynthesis system, the light-response of the photosynthetic efficiency parameters of three-year-old C .radicans leaves, such as net photosynthetic rate (P N ), transpiration rate (Tr), water use efficiency (WUE), and light use efficiency (LUE), were studied under different soil water conditions in order to explore the effects of soil water stress on photosynthesis and the suitable soil water content for water-saving irrigation of this liana. Soil water and light intensity needed by the growth and development of C .radicans were analyzed. The results show that P N , Tr, WUE, and LUE of the leaves had threshold value to soil water and photosynthetically-active radiation (PAR). The nonrectangular hyperbola model was used to simulate light-response curve and the convexity was about 1. With the increase of soil relative water content of field capacity (Wr, ranged from 20.1% to 71.1%), the light compensation point declined while light saturation point, the maximum P N and apparent quantum yield increased. When Wr was about 71.7%, the light compensation point was at the minimum (21.61μmol/m 2 /s) and the light saturation point was at the maximum (1400 μmol/m 2 /s). In order to maintain the normal plant growth and have higher P N , LUE, and WUE synchronously, the range of Wr was from 49.5 to 71.1%. The optimum Wr was 71.1%, and the minimum Wr was 28.2% for the normal growth of C .radicans . The high P N and WUE were recorded when PAR ranged from 800 to 1600 μmol/m 2 /s and the light saturation points ranged from 800 to 1400 μmol/m 2 /s. The peak value of LUE was found when PAR ranged from 100 to 300 μmol/m 2 /s, indicating that the C. radicans had high adaptability to light conditions. Key words : Campsis radicans, soil water content, photosynthetically-active radiation, photosynthetic efficiency, water use efficiency.

Critical water stress levels in Pinus patula seedlings and their relation to measures of seedling morphology

A pot trial was implemented to determine the effect of soil water stress following transplanting on shoot water potential and stomatal conductance of Pinus patula seedlings. The relationship of seedling morphology to measures of water stress was also investigated. The trial consisted of two watering treatments: a control (no water stress) and a severe water stress (no further water after planting). The treatments were applied to recently transplanted seedlings selected to reflect a range of sizes typical in an operational environment. Measures of shoot water potential and stomatal conductance, root and shoot growth and environmental variables were made. The results indicated that P. patula seedlings tolerated high air and soil temperatures (above 35°C) and low soil water availability (−1.5 MPa). The water potential threshold for changes in stomatal conductance was in the region of −0.8 to −0.9 MPa and stomatal closure had occurred at a shoot water potential of between −1.8 and −2.1 MPa. Mortality occurred when shoot water potential declined to below −3.0 MPa. There was variability between seedlings in their potential for survival and growth. Inherently bigger seedlings had a greater capacity for new root growth following planting. Mass of new roots was significantly and positively related to higher rates of transpiration under conditions of low soil moisture.

Critical responses of photosynthetic efficiency of goldspur apple tree to soil water variation in semiarid loess hilly area

Goldspur apple (Malus pumila cv. Goldspur) is one of the main fruit trees planted in semiarid loess hilly areas. The photosynthetic efficiency in leaves of eight-year-old trees were studied under different soil water conditions with a Li-6400 portable photosynthesis system and a Li-Cor1600 portable steady state porometer in order to explore the effects of soil water stress on photosynthesis and the suitable soil water content (SWC) for water-saving irrigation of apple orchards. The results showed that the leaf net photosynthetic rate (P N), transpiration rate (E), water-use efficiency (WUE), stomatal conductance (g s), intercellular CO2 concentration (C i), and stomatal limiting value (L s) displayed different threshold responses to soil water variation. When SWC was within a range of about 60%-86% of field capacity (FC), P N and E were maintained in a relative steady state. At an elevated level but below 60% of FC, both P N and E decreased evidently with decreasing soil moisture. The SWC needed to support WUE in a relatively steady state and at a high level was in the range of about 50%-71% of FC. When SWC was less than 48% of FC, g s and L s declined with decreasing soil moisture, while C i increased rapidly. Based on the analysis of the stomatal limitation of photosynthesis using two criteria (C i and L s) suggested by Farquhar and Sharkey, it was implied that the predominant cause of restricting P N had changed from stomatal limitation to nonstomatal one under severe water stress. In terms of water-saving irrigation for enhancing water-use efficiency, it was concluded that in semiarid loess hilly areas, the suitable range of SWC for water-saving irrigation in goldspur apple orchards is in the range of about 50%-71% of FC, and the most severe degree of soil water stress tolerated for photosynthesis is about 48% of FC.