土壤水势对水曲柳幼苗水分生态的影响

采用根区渗灌控水技术,将土壤水势长期控制在0～-20kPa(W1)、-20～-40kPa(W2)、-40～-60kPa(W3)、-60～-80kPa(W4)、-80～-160kPa(W5)范围内,系统地研究了不同土壤水势条件下水曲柳幼苗的蒸腾过程、吸水过程、根叶水势13动态过程及SPAC体系的水流阻力,结果表明,在亚饱和土壤水分状态下(W1),细根水势最高,水分由土壤进入细根的阻力最小,根系吸水速率最高,从而支持了13间强烈的蒸腾作用．在田间持水量土壤水分状态下(W2),细根吸水阻力成倍增加,吸水速率和蒸腾速率显著下降,但尚未改变蒸腾作用13动态过程的单峰模式．当土壤水分在田间持水量状态以下(W3-W5)时,随着土壤水势递降,细根吸水阻力急剧增加至几倍乃至几十倍,根系吸水速率过低,吸水与蒸腾矛盾加剧,叶水势降至很低,气孔关闭,蒸腾作用受到严重抑制,呈现明显的午休低谷．在实验范围内(0～-160kPa),土壤水分对水曲柳幼苗是非等效的,当土壤水分在田间持水量状态以下(<-40kPa)时,水曲柳全光苗发生显著的水分胁迫。     

土壤水势对水曲柳幼苗水分生态的影响

采用根区渗灌控水技术,将土壤水势长期控制在0～-20kPa(W₁)、-20～-40kPa(W₂)、-40～-60kPa(W₃)、-60～-80kPa(W₄)、-80～-160kPa(W₅)范围内,系统地研究了不同土壤水势条件下水曲柳幼苗的蒸腾过程、吸水过程、根叶水势日动态过程及SPAC体系的水流阻力.结果表明,在亚饱和土壤水分状态下(W₁),细根水势最高,水分由土壤进入细根的阻力最小,根系吸水速率最高,从而支持了日间强烈的蒸腾作用.在田间持水量土壤水分状态下(W₂),细根吸水阻力成倍增加,吸水速率和蒸腾速率显著下降,但尚未改变蒸腾作用日动态过程的单峰模式.当土壤水分在田间持水量状态以下(W₃～W₅)时,随着土壤水势递降,细根吸水阻力急剧增加至几倍乃至几十倍,根系吸水速率过低,吸水与蒸腾矛盾加剧,叶水势降至很低,气孔关闭,蒸腾作用受到严重抑制,呈现明显的午休低谷.在实验范围内(0～-160kPa),土壤水分对水曲柳幼苗是非等效的,当土壤水分在田间持水量状态以下(<-40kPa)时,水曲柳全光苗发生显著的水分胁迫.     

水分胁迫下盆栽冬小麦根系生长与苗系生长及某些生理特性的关系

水分胁迫下,盆栽冬小麦根干重和根长密度呈直线正相关。鉴于根长密度反映了土壤中根系最活跃的部分［５］,是研究植物根系吸收水分和养分的最优参数之一［６］,本文用之研究了它与地上部生物量、净同化速率、叶水势和叶片相对含水量、气孔阻力和蒸腾速率的关系。结果表明,根长密度与净同化速率和地上部干重呈直线负相关,与叶水势和叶片相对含水量呈直线正相关;与气孔阻力呈直线负相关,与蒸腾速率呈直线正相关。为实验室进行冬小麦生长控制与生理特性控制提供了一定的基础。     

植物根系吸水过程中根系水流阻力的变化特征

以植物根系吸水的人工模拟试验所测得的数据为依据,运用水流的电模拟原理,定理分析了不同土壤水分水平处理下植物根系吸水过程中根系水流阻力各主要分量的大小、变化规律及其相对重要性．结果表明,在同一水分水平处理中,植物根内木质部传导阻力（Ｒｃ）随生长时间的推移而减小,随土层深度的加深而增大,土根接触阻力（Ｒｓｒ）、植物根系吸收阻力（Ｒｒ）随生长时间表现出先下降后上升阶段的动态变化特征;在不同水分水平处理中,Ｒｃ、Ｒｓｒ、Ｒｒ均随土壤湿度减小而大幅度增大;在植物根系水流阻力各分量中,Ｒｒ占根系水流阻力的比例为５５％～９６％,Ｒｓｒ约占根系水流阻力的４％～４５％,而Ｒｃ仅占根系水流阻力的７×１０－６,故Ｒｒ是决定植物根系吸水速率的重要因素     

土壤水分亏缺条件下根源信号ABA参与作物气孔调控的数值模拟

建立了根系吸水模型和根源ABA参与作物气孔调控过程相耦合的气孔导度模型,该模型在根源信号ABA的产生项中考虑了根系吸水影响函数和根系密度分布函数.利用该耦合模型模拟大田状况下根源ABA参与玉米气孔行为调控过程,结果表明,由于充分考虑了根区土壤水势和土壤中根长密度分布对根系吸水的影响,较好地反映了土壤不同层次根系吸水强度,更为确切地描述了当土壤水分亏缺时,根系合成ABA的量、各层根系蒸腾流中ABA浓度、木质部ABA浓度以及最终ABA参与对气孔行为的调控作用.     

宽窄行栽植模式下三倍体毛白杨根系分布特征及其与根系吸水的关系

采用剖面法对宽窄行栽植模式下三倍体毛白杨(triploid Populus tomentosa)的根系分布特征进行了研究;采用管式TDR系统对土壤剖面含水率变化动态进行了连续观测,并据此计算林木根系吸水速率,以探讨土壤含水率、根系分布和根系吸水分布之间的相关关系。研究结果表明:毛白杨的总平均根长密度在林带两侧和不同径向距离处非常接近(P>0.05);但在不同土层间变化很大(P<0.01),其中0-20和60-150 cm土层为根系主要分布区域,其根系所占比例共达86%;不同径阶间的根长密度差异显著(P<0.01),且其比例关系会随空间位置的改变而发生变化。不同栽植方位下,林带东侧毛白杨根系分布的浅层化程度高于西侧,且在径向240-280 cm内其0-0.5 mm的极细根显著多于西侧(P<0.05)。因此,宽窄行栽植模式下,深度和径阶是毛白杨根系分布的主要影响因子,而栽植方位会对其形态构型产生影响。毛白杨根系吸水模式受细根分布的影响,但会随土壤剖面水分有效性分布的变化而变化:当表土层水分有效性增加时,根系吸水主要集中在表土层;当表土层水分有效性降低时,深层土壤根系的吸水贡献率会逐渐增加;当土壤剖面水分条件异质性较高时,根系吸水主要集中在根系密度与水分有效性均较高的区域;当土壤剖面水分分布均匀且不存在水分胁迫时,根系吸水分布与细根分布最为一致。     

乌兰布和沙区紫花苜蓿根系吸水模型

在实测获得根重密度和土壤含水量的基础上,运用土壤水分运动方程及Penman-Monteith公式,计算得到干旱沙区不同水分处理下紫花苜蓿(MedicagosativaL.)根系吸水速率和蒸腾强度.结果表明紫花苜蓿根系吸水速率与土壤剖面含水量和根重密度密切相关.苜蓿地水分消耗规律在分枝期以棵间蒸发为主,在开花期和结实期以植株蒸腾为主.建立了干旱沙区紫花苜蓿根系吸水模型,经回归分析得到模型中的各个参数,通过对回归结果的方差分析表明,模型的相关性较好(R2=0.890,p<0.05);另外从模型验证的结果看,土壤剖面含水量模拟值与实测值基本吻合,说明本文提出的根系吸水模型其可靠性较好.     

乌兰布和沙区紫花苜蓿根系生长及吸水规律的研究

研究了不同水分处理下,乌兰布和沙区紫花苜蓿系生长发育规律及根系吸水速度,结果表明：不同水分处理的根系生长规律最基本一致的,在生长季风均呈增加的趋势;但适度干旱可促进根系的伸长生长。在当地土壤类型条件下,根系主要分布在０－３０cm土层;根重密度在土壤剖面上的分布遵循对数规律,并随深度的增加呈降低趋势。运用一维土壤水分运动方程,计算得到了不同水分处理根系吸水速度在土壤剖面上的分布状况;根系吸水速率与土壤含水量和根密度密切相关。     

土壤水分与温度共同作用对植物根系水分传导的效应

 本文根据不同大气环境温度和土壤温度及不同土壤含水率处理条件下的玉米、向日葵、台湾相思(Acacia confusa)、银合欢(Leucaena glauca)的试验资料，分析了土壤水分和温度以及土壤水分与温度共同作用对植物根系水分传导的效应。台湾相思和银合欢的试验结果表明，在一定的土壤水分范围内，高温(白天/夜晚的温度为40／30℃)环境中的根系水分传导大于低温(30/25℃)环境中的，但当根系水分胁迫十分严重(台湾相思根系水势小于-1.5MPa，银合欢根系水势小于-2.0MPa)时，30/25℃环境的根系水分传导反而大于40／30℃环境的；玉米、向日葵的试验结果表明，在一定土壤温度范围内，根系水分传导随土壤温度增加而增加，其增加的幅度与生育阶段有关；在向日葵生育期土壤温度高于35℃、玉米生育期高于30℃时，其根系水分传导随温度增加而降低。通过植物根区土壤逐渐干旱和干旱复水后的试验，其结果表明复水后根系水分传导上升较快，银合欢复水1.5d、向日葵复水3d后测得的根系水分传导即可达到受旱前的水平，其后的水分传导还略高于一直充分供水处理的，表明根系经受一定程度的干旱锻炼后，对其水分传导具有明显的补偿效应。在干旱和复水过程中根系水分传导与根水势的变化规律相一致。     

3种荒漠灌木的用水策略及相关的叶片生理表现

以新疆古尔班通古特沙漠南缘原始盐生旱生荒漠的3种建群灌木多枝柽柳(Ｔamarix ramosissima)、梭梭(Ｈaloxylon ammodendron)和琵琶柴(Reaumuria soongorica)为对象,跟踪自然降雨过程,利用LI-6400光合作用系统和Model 3005植物水分压力审测定光合作用和叶水势的变化,以研究浅层土壤水分条件改变对荒漠灌木主要叶片生理特征的影响;并在原始生境中将植株根系完整地深挖取出,进行根系形态结构调查,以确定此3种灌木根系功能型与用水策略。当浅层土壤分别处在水分充足及匮缺的条件下时．测定3种灌木的光合作用响应曲线和日过程曲线．以及黎明前和止午叶水势,结果表明：浅层土壤水分状况变化时,3种灌木的光合能力均没有显著改变;多枝柽柳的叶水势亦没有明显波动;而梭梭和琵琶柴的叶水势却表现出显著差异。在两种功能型根中,多枝柽柳为深根型,生存和乍理活动的维持主要依赖于地下水;而梭梭和琵琶柴为非深根型植物,主要水源是降水形成的浅层土壤水,其用水策略是根据水分条件行效调节根系和冠层生长,从而维持正常的光合作用。即荒漠灌木在长期适应的过样中．已形成不同的根系功能型和用水策略;叶水势对浅层土壤水分状况变化的种间差异性响应在一定程度上反映了这一点。同时．此3种荒漠灌小通过不同的个体适应策略都能够实现水分平衡和碳收支的有效调节,这主要体现为浅层土壤水分条件变化时光合响应的种间一致性。     

Modelling non-uniform soil water uptake by a single plant root

The assumption of uniform water flow to the root or uniform water potential at the root surface was shown by Hainsworth and Aylmore (1986, 1989) to be erroneous. The present paper demonstrates how the non-uniform uptake of water by a single root can be modeled. Differential equations are numerically solved to describe simultaneous water movement in the plant and in the soil. In the plant, boundary conditions are the water potentials at the root surface (Ψ_s) and in the xylem at the root base (Ψ_b). A set of difference equations describe the flow of water radially through the cortex to the xylem and in the xylem axially upwards to the base. For calculating the water flow in the soil and the values of Ψ_s, i.e. the boundary conditions for flow in the root, the finite element method (FEM) is used, the boundary conditions being the flux of water into the plant root and the zero flow across the wall, bottom and surface of a hypothetical soil cylinder surrounding the root. ei]Section editor: B E Clothier     

Water uptake profile response of corn to soil moisture depletion

The effects of soil moisture distribution on water uptake of drip‐irrigated corn were investigated by simultaneously monitoring the diurnal evolution of sap flow rate in stems, of leaf water potential, and of soil moisture, during intervals between successive irrigations. The results invalidate the steady‐state resistive flow model for the continuum. High hydraulic capacitance of wet soil and low hydraulic conductivity of dry soil surrounding the roots damped significantly diurnal fluctuations of water flow from bulk soil to root surface. By contrast, sap flow responded directly to the large diurnal variation of leaf water potential. In wet soil, the relation between the diurnal courses of uptake rates and leaf water potential was linear. Water potential at the root surface remained nearly constant and uniformly distributed. The slope of the lines allowed calculating the resistance of the hydraulic path in the plant. Resistances increased in inverse relation with root length density. Soil desiccation induced a diurnal variation of water potential at the root surface, the minimum occurring in the late afternoon. The increase of root surface water potential with depth was directly linked to the soil desiccation profile. The development of a water potential gradient at the root surface implies the presence of a significant axial resistance in the root hydraulic path that explains why the desiccation of the soil upper layer induces an absolute increase of water uptake rates from the deeper wet layers.     

Plant and soil resistance to water flow in faba bean (Vicia faba L. major Harz.)

An experiment was conducted to determine soil and plant resistance to water flow in faba bean under field conditions during the growing season. During each sampling period transpiration flux and leaf water potential measured hourly were used with daily measurements of root and soil water potential to calculate total resistance using Ohm's law analogy. Plant growth, root density and soil water content distributions with depth were measured. Leaf area and root length per plant reached their maximum value during flowering and pod setting (0.31 m² and 2200 m, respectively), then decreasing until the end of the growing period. Root distribution decreased with depth ranging, on average, between 34.2% (in the 0–0.25 m soil layer) and 18.1% (in the 0.75–1.0 m soil layer). Mean root diameter was 0.6 mm but most of the roots were less than 0.7 mm in diameter. Changes in plant and soil water potentials reflected plant growth characteristics and climatic patterns. The overall relationship between the difference in water potential between soil and leaf and transpiration was linear, with the slope equal to average plant resistance (0.0165 MPa/(cm³ m^-1 h^-1 10^-3). Different regression parameters were obtained for the various measurement days. The water potential difference was inversely related to transpiration at high leaf stomatal resistance and at high values of VPD. Total resistance decreased with transpiration flux in a linear relationship (r=−0.68). Different slope values were obtained for the different measurement days. Estimated soil resistance was much lower than the observed total resistance to water flow. The change from vegetative growth to pod filling was accompanied by an increase in plant resistance. The experimental results support previous findings that resistance to water flow through plants is not constant but is influenced by plant age, growth stage and environmental conditions. A more complex model than Ohm's law analogy may be necessary for describing the dynamic flow system under field conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Modeling soil water movement with water uptake by roots

Soil water movement with root water uptake is a key process for plant growth and transport of water and chemicals in the soil-plant system. In this study, a root water extraction model was developed to incorporate the effect of soil water deficit and plant root distributions on plant transpiration of annual crops. For several annual crops, normalized root density distribution functions were established to characterize the relative distributions of root density at different growth stages. The ratio of actual to potential cumulative transpiration was used to determine plant leaf area index under water stress from measurements of plant leaf area index at optimal soil water condition. The root water uptake model was implemented in a numerical model. The numerical model was applied to simulate soil water movement with root water uptake and simulation results were compared with field experimental data. The simulated soil matric potential, soil water content and cumulative evapotranspiration had reasonable agreement with the measured data. Potentially the numerical model implemented with the root water extraction model is a useful tool to study various problems related to flow transport with plant water uptake in variably saturated soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

沙丘多枝柽柳灌丛根层土壤含水量变化特征与根系水力提升证据

分析干旱区深根型荒漠植物的根层土壤水分是揭示荒漠植物与土壤水分关系机理的重要方面。在黑河中游一片风沙侵蚀区域的多枝柽柳(Tamarix ramosissima)人工林地中, 对表层0.3 m到3 m深的土壤不同深度的含水量进行了连续的动态观测。结果显示, 多枝柽柳根系层土壤含水量可以分为明显不同的3层: 浅层(0.2-1.7 m深)相对湿润层、中间(1.7-2.7 m深)相对干层和深层(2.7 m以下)有效含水层。在多枝柽柳生长盛期, 浅层相对湿润层土壤含水量呈现明显的昼夜变化特征, 同时, 在晚上植物根系与浅层土壤之间存在正水势梯度, 这说明存在根系水力提升现象。水力提升是干旱气候下根层浅层土壤含水量保持相对湿润的主要原因, 并因此维系浅层根系的发育, 也为多枝柽柳具备的防风固沙功能提供了可能的解释。据初步估算, 多枝柽柳根系水力提升占每天耗水量的5%-8%, 耗水的主要水分来源仍然是充足的土壤深层有效含水层。     

黄花苜蓿与蒺藜苜蓿对土壤低磷胁迫适应策略的比较研究

土壤有效磷(P)含量低是限制植物生长的主要因素之一。根形态变化和根系大量分泌以柠檬酸为主的有机酸是植物适应土壤P素缺乏的重要机制。以广泛分布于我国北方的重要豆科牧草黄花苜蓿(Medicago falcata)和豆科模式植物蒺藜苜蓿(M. truncatula)为材料, 采用砂培方法, 研究了低P胁迫对其植株生长、根系形态和柠檬酸分泌的影响, 对比了两种苜蓿适应低P胁迫的不同策略。结果表明: 1)低P处理显著抑制了蒺藜苜蓿与黄花苜蓿的地上部生长, 而对地下部生长影响较小, 从而导致根冠比增加。2)低P胁迫显著降低黄花苜蓿的总根长和侧根长, 而对蒺藜苜蓿的上述根系形态指标没有显著影响。3)低P胁迫促进两种苜蓿根系的柠檬酸分泌, 无论是在正常供P还是低P胁迫条件下, 黄花苜蓿根系分泌柠檬酸量显著高于蒺藜苜蓿根系。上述结果表明, 黄花苜蓿和蒺藜苜蓿对低P胁迫的适应策略不同, 低P胁迫下, 黄花苜蓿主要通过根系大量分泌柠檬酸, 活化根际难溶态P来提高对P的吸收, 而蒺藜苜蓿维持较大的根系是其适应低P胁迫的主要策略。     

The role of soil hydraulic conductivity on the spatial and temporal variation of root water uptake in drip-irrigated corn

A multiplexed TDR system and a heat-pulse system for stem sap flow measurements were used to determine the spatial and temporal pattern of root water uptake in field-grown corn. The TDR probes, 0.15 and 0.30 m in length, were buried vertically in the soil profile to a depth of 0.95 m below the soil surface and heat-pulse sensors were installed on the plant base. Nocturnal readings from TDR probes were used successfully to differentiate the two components of moisture change: root uptake and net drainage. The instantaneous rate of water extraction by the plant measured by the heat-pulse system agreed well with the integrated rate of root water uptake measured frequently (at half-hour or hourly intervals) by the TDR probes. This agreement enabled further exploration into the cause of the evolution of the spatial and temporal patterns of root water uptake during a drying cycle. The results indicated that right after irrigation in the well-watered soil profile, it is the spatial distribution of the roots that mainly determines the typical pattern of root extraction, in addition to the fact that the roots near the plant base are more effective than those farther away. The higher density and effectiveness of the roots near the plant base dry the soil rapidly so that soil hydraulic conductivity soon becomes a limiting factor for water uptake. Further analysis revealed that a decrease in root uptake occurs near the plant base under a given atmospheric demand when the relative bulk soil hydraulic conductivity decreases to 0.002K _r. This suggests that low conductivity (high resistance) in the soil near the plant base is the initial cause for downward and lateral shifting of the root uptake pattern. Note that this critical value of hydraulic conductivity is not universal since it depends on the soil type and atmospheric water demand during the period under observation. Therefore, prior to the application of moisture content or suction head as measures of water availability or to control irrigation scheduling, it is suggested that these parameters be calibrated by the soil K() or K() curves, respectively, for the expected atmospheric water demand for the specific crop and growing period.     

Soil solute concentration and water uptake by single lupin and radish plant roots

Application of computer assisted tomography to gamma and X-ray attenuation measurements and Na⁺-LIX microelectrodes were used to determine the spatial distributions of soil water content and Na⁺ concentrations respectively near single roots of eighteen day old lupin and radish plants. These quantities were monitored at root depths of 3, 6 and 9 cm and at zero, 2, 4, 6, and 8 hour intervals from the diurnal commencement of transpiration. The plants were subjected to two levels of transpirational demand and five Na⁺ soil solution concentration levels. Water extraction rates for the lupin and radish roots increased continuously with time but were substantially reduced with increasing Na⁺ concentration in the treatment. Water uptake was uniform along the length of the essentially constant diameter lupin roots but decreased along the tapering radish roots as the diameter and hence the surface area per unit length of the roots decreased. The accumulation of Na⁺ at the root surfaces of both plants increased gradually with time in a near linear fashion and was slightly higher under the higher transpiration demand. These increases were not exponential as would be expected with non-absorption by the roots and this is considered to be due to back diffusion at the relatively high water contents used. At these water contents matric potentials had a much smaller influence on transpiration than osmotic potentials. The relationships between leaf water potentials (Ψ₁) and osmotic potentials at the root surfaces were linear with the decreases in Ψ₁ almost exactly reflecting the decreases in Ψ_π indicating rapid plant adjustment. Leaf water potentials decreased progressively with time and the relationships between leaf water potential and the transpiration rate were also linear supporting the suggestion of constant plant resistances at any given concentration.     

Influence of root density on the critical soil water potential

Estimation of root water uptake in crops is important for making many other agricultural predictions. This estimation often involves two assumptions: (1) that a critical soil water potential exists which is constant for a given combination of soil and crop and which does not depend on root length density, and (2) that the local root water uptake at given soil water potential is proportional to root length density. Recent results of both mathematical modeling and computer tomography show that these assumptions may not be valid when the soil water potential is averaged over a volume of soil containing roots. We tested these assumptions for plants with distinctly different root systems. Root water uptake rates and the critical soil water potential values were determined in several adjacent soil layers for horse bean (Vicia faba) and oat (Avena sativa) grown in lysimeters, and for field-grown cotton (Gossypium L.), maize (Zea mays) and alfalfa (Medicago sativa L.) crops. Root water uptake was calculated from the water balance of each layer in lysimeters. Water uptake rate was proportional to root length density at high soil water potentials, for both horse bean and oat plants, but root water uptake did not depend on root density for horse bean at potentials lower than −25 kPa. We observed a linear dependency of a critical soil water potential on the logarithm of root length density for all plants studied. Soil texture modified the critical water potential values, but not the linearity of the relationship. B E Clothier Section editor