A coupled model of stomatal conductance, photosynthesis and transpiration

A model that couples stomatal conductance, photosynthesis, leaf energy balance and transport of water through the soil–plant–atmosphere continuum is presented. Stomatal conductance in the model depends on light, temperature and intercellular CO₂ concentration via photosynthesis and on leaf water potential, which in turn is a function of soil water potential, the rate of water flow through the soil and plant, and on xylem hydraulic resistance. Water transport from soil to roots is simulated through solution of Richards’ equation. The model captures the observed hysteresis in diurnal variations in stomatal conductance, assimilation rate and transpiration for plant canopies. Hysteresis arises because atmospheric demand for water from the leaves typically peaks in mid‐afternoon and because of uneven distribution of soil matric potentials with distance from the roots. Potentials at the root surfaces are lower than in the bulk soil, and once soil water supply starts to limit transpiration, root potentials are substantially less negative in the morning than in the afternoon. This leads to higher stomatal conductances, CO₂ assimilation and transpiration in the morning compared to later in the day. Stomatal conductance is sensitive to soil and plant hydraulic properties and to root length density only after approximately 10 d of soil drying, when supply of water by the soil to the roots becomes limiting. High atmospheric demand causes transpiration rates, LE, to decline at a slightly higher soil water content, θ_s, than at low atmospheric demand, but all curves of LE versus θ_s fall on the same line when soil water supply limits transpiration. Stomatal conductance cannot be modelled in isolation, but must be fully coupled with models of photosynthesis/respiration and the transport of water from soil, through roots, stems and leaves to the atmosphere.     

Integration of hydraulic and chemical signalling in the control of stomatal conductance and water status of droughted plants

We describe here an integration of hydraulic and chemical signals which control stomatal conductance of plants in drying soil, and suggest that such a system is more likely than control based on chemical signals or water relations alone. The determination of xylem [ABA] and the stomatal response to xylem [ABA] are likely to involve the water flux through the plant. (1) If, as seems likely, the production of a chemical message depends on the root water status (Ψ_r), it will not depend solely on the soil water potential (Ψ_s) but also on the flux of water through the soil-plant-atmosphere continuum, to which are linked the difference between Ψ_r and Ψ_s. (2) The water flux will also dilute the concentration of the message in the xylem sap. (3) The stomatal sensitivity to the message is increased as leaf water potential falls. Stomatal conductance, which controls the water flux, therefore would be controlled by a water-flux-dependent message, with a water-flux-dependent sensitivity. In such a system, we have to consider a common regulation for stomatal conductance, leaf and root water potentials, water flux and concentration of ABA in the xylem. In order to test this possibility, we have combined equations which describe the generation and effects of chemical signals and classical equations of water flux. When the simulation was run for a variety of conditions, the solution suggested that such common regulation can operate. Simulations suggest that, as well as providing control of stomatal conductance, integration of chemical and hydraulic signalling may also provide a control of leaf water potential and of xylem [ABA], features which are apparent from our experimental data. We conclude that the root message would provide the plant with a means to sense the conditions of water extraction (soil water status and resisance to water flux) on a daily timescale, while the short-term plant response to this message would depend on the evaporative demand.     

Soil textures rather than root hairs dominate water uptake and soil–plant hydraulics under drought

Although the role of root hairs (RHs) in nutrient uptake is well documented, their role in water uptake and drought tolerance remains controversial. Maize (Zea mays) wild-type and its hair-defective mutant (Mut; roothairless 3) were grown in two contrasting soil textures (sand and loam). We used a root pressure chamber to measure the relation between transpiration rate (E) and leaf xylem water potential (ψ_{leaf_x}) during soil drying. Our hypotheses were: (1) RHs extend root–soil contact and reduce the ψ_{leaf_x} decline at high E in dry soils; (2) the impact of RHs is more pronounced in sand; and (3) Muts partly compensate for lacking RHs by producing longer and/or thicker roots. The ψ_{leaf_x}(E) relation was linear in wet conditions and became nonlinear as the soils dried. This nonlinearity occurred more abruptly and at less negative matric potentials in sand (ca. −10 kPa) than in loam (ca. −100 kPa). At more negative soil matric potentials, soil hydraulic conductance became smaller than root hydraulic conductance in both soils. Both genotypes exhibited 1.7 times longer roots in loam, but 1.6 times thicker roots in sand. No differences were observed in the ψ_{leaf_x}(E) relation and active root length between the two genotypes. In maize, RHs had a minor contribution to soil–plant hydraulics in both soils and their putative role in water uptake was smaller than that reported for barley (Hordeum vulgare). These results suggest that the role of RHs cannot be easily generalized across species and soil textures affect the response of root hydraulics to soil drying.

Root hairs of maize do not show evident contribution to root growth, water uptake, and soil–plant hydraulics, whereas soil textures affect the response of root hydraulics to soil drying.     

Aquaporin regulation in roots controls plant hydraulic conductance,stomatal conductance,and leaf water potential in Pinus radiata under water stress

Stomatal regulation is crucial for forest species performance and survival on drought‐prone sites. We investigated the regulation of root and shoot hydraulics in three Pinus radiata clones exposed to drought stress and its coordination with stomatal conductance (g_s) and leaf water potential (Ψ_leaf). All clones experienced a substantial decrease in root‐specific root hydraulic conductance (K_root‐r) in response to the water stress, but leaf‐specific shoot hydraulic conductance (K_shoot‐l) did not change in any of the clones. The reduction in K_root‐r caused a decrease in leaf‐specific whole‐plant hydraulic conductance (K_plant‐l). Among clones, the larger the decrease in K_plant‐l, the more stomata closed in response to drought. Rewatering resulted in a quick recovery of K_root‐r and g_s. Our results demonstrated that the reduction in K_plant‐l, attributed to a down regulation of aquaporin activity in roots, was linked to the isohydric stomatal behaviour, resulting in a nearly constant Ψ_leaf as water stress started. We concluded that higher K_plant‐l is associated with water stress resistance by sustaining a less negative Ψ_leaf and delaying stomatal closure.     

亚低温与干旱胁迫对番茄植株水分传输和形态解剖结构的影响

为探讨亚低温和干旱对植株水分传输的影响机制,以番茄幼苗为试材,利用人工气候室设置常温(昼25 ℃/夜18 ℃)和亚低温(昼15 ℃/夜8 ℃)环境,采用盆栽进行正常灌水(75%～85%田间持水量)和干旱处理(55%～65%田间持水量),分析了温度和土壤水分对番茄植株水分传输、气孔和木质部导管形态解剖结构的影响。结果表明: 与常温正常灌水处理相比,干旱处理使番茄叶水势、蒸腾速率、气孔导度、水力导度、茎流速率、气孔长度和叶、茎、根导管直径显著减小,而使叶、茎、根导管细胞壁厚度和抗栓塞能力增强;亚低温处理下番茄叶水势、蒸腾速率、气孔导度、水力导度和叶、茎、根导管直径显著降低,但气孔变大,叶、根导管细胞壁厚度和叶、茎、根抗栓塞能力显著升高。亚低温条件下土壤水分状况对番茄叶水势、蒸腾速率、气孔导度、水力导度、气孔形态、叶、根导管结构均无显著影响。总之,干旱处理下番茄通过协同调控叶、茎、根结构使植株水分关系重新达到稳态;亚低温处理下番茄植株水分关系的调控主要通过改变叶和根导管结构实现,且受土壤水分状况的影响较小。     

Amelioration of chilling-induced water stress by abscisic Acid-induced changes in root hydraulic conductance

Pretreatment of soybean (Glycine max L. var Ransom) root systems with abscisic acid (ABA) ameliorates the deleterious effect of low temperatures on root hydraulic conductance. ABA treatment of root systems subsequently chilled to 10°C with shoots at 25°C resulted in higher leaf water potentials and lower stomatal resistances. If the root systems are left at 25°C, ABA causes stomatal closure. Membrane alterations are suggested as a mechanism for the ABA action in plant response to chilling stress.     

Non-hydraulic signals from maize roots in drying soil: inhibition of leaf elongation but not stomatal conductance

Conditions of soil drying and plant growth that lead to non-hydraulic inhibition of leaf elongation and stomatal conductance in maize (Zea mays L.) were investigated using plants grown with their root systems divided between two containers. The soil in one container was allowed to dry while the other container was kept well-watered. Soil drying resulted in a maximum 35% inhibition of leaf elongation rate which occurred during the light hours, with no measurable decline in leaf water potential (_w). Leaf area was 15% less than in control plants after 18 d of soil drying. The inhibition of elongation was observed only when the soil _w declined to below that of the leaves and, thus, the drying soil no longer contributed to transpiration. However, midday root _w in the dry container (-0.29 MPa) remained much higher than that of the surrounding soil (-1.0 MPa) after 15 d of drying, indicating that the roots in drying soil were rehydrated in the dark.To prove that the inhibition of leaf elongation was not caused by undetectable changes in leaf water status as a result of loss of half the watergathering capacity, one-half of the root system of control plants was excised. This treatment had no effect on leaf elongation or stomatal conductance. The inhibition of leaf elongation was also not explained by reductions in nutrient supply.Soil drying had no effect on stomatal conductance despite variations in the rate or extent of soild drying, light, humidity or nutrition. The results indicate that non-hydraulic inhibition of leaf elongation may act to conserve water as the soil dries before the occurrence of shoot water deficits.Symbol _w water potential Contribution from the Missouri Agricultural Experiment Station, Journal Series No. 10881     

Stomatal and hydraulic conductance in growing sugarcane: stomatal adjustment to water transport capacity*

Abstract Stomatal conductance per unit leaf area in well-irrigated field- and greenhouse-grown sugarcane increased with leaf area up to 0.2 m² plant ¹, then declined so that maximum transpiration per plant tended to saturate rather than increase linearly with further increase in leaf area. Conductance to liquid water transport exhibited parallel changes with plant size. This coordiantion of vapour phase and liquid phase conductances resulted in a balance between water loss and water transport capacity, maintaining leaf water status remarkably constant over a wide range of plant size and growing conditions. The changes in stomatal conductance were not related to plant or leaf age. Partial defoliation caused rapid increases in stomatal conductance, to re-establish the original relationship with remaining leaf area. Similarly, pruning of roots caused rapid reductions in stomatal conductance, which maintained or improved leaf water status. These results suggest that sugarcane stomata adjusted to the ratio of total hydraulic conductance to total transpiring leaf area. This could be mediated by root metabolites in the transpiration stream, whose delivery per unit leaf area would be a function of the relative magnitudes of root system size, transpiration rate and leaf area.     

Stomatal conductance in relation to xylem sap abscisic acid concentrations in two tropical trees, Acacia confusa and Litsea glutinosa

Two tropical trees, Acacia confusa and Litsea glutinosa, were grown under controlled conditions with their roots subjected to soil drying and soil compaction treatments. In both species, a decline in stomatal conductance resulting from soil drying took place much earlier than the decline of leaf water potential. Soil compaction treatment also resulted in a substantial decrease in stomatal conductance but had little effect on leaf water potential. A rapid and substantial increase in xylem abscisic acid (ABA) concenation ([ABA]), rather than hulk leaf ABA, was closely related to soil drying and soil compaction. A significant relationship between stomatal conductance (g_s) and xylem [ABA] was observed in both species. Artificially feeding ABA solutions to excised leaves of both species showed that the relationship bet ween g_s and [ABA] was very similar to that obtained from the whole plant, i.e. the relationship between g_s and xylem [ABA]. These results suggest that xylem ABA may act as a stress signal in the control of stomatal conductance.     

Leaf and root control of stomatal closure during drying in soybean

The stomatal conductance of an illuminated 2.5 cm² area of an intact soybean leaflet was the same whether the rest of the shoot was in light or darkness. This was true throughout soil drying cycles. Water potential of tissue immediately outside the illuminated area consistently decreased about 0.3 MPa upon illumination of the shoot. This erroneously suggested that stomatal conductance during soil drying did not respond to diurnal reductions in leaf water potential, but was controlled by root or soil water status. Tests showed that the water potential of tissue in the illuminated area did not change in the steady-state upon illumination of the rest of the shoot. Water potentials of shaded sections of leaves were not different from predawn water potentials, and were higher than leaf xylem pressure potentials as determined with a pressure chamber. These steep local gradients of leaf water potential suggest that there is minimal interchange of water among xylem elements leading from roots to different sections of leaves. The relationship between stomatal conductance and leaf water potential was the same whether leaf water potential was reduced by soil drying, application of polyethylene glycol (PEG) to the root system, lowering root temperature, or leaf excision. In the root cooling experiment, there was no soil drying, and with leaf excision, there was no root drying. The similarity of stomatal responses to leaf water potential in all cases strongly suggests control of conductance by a signal produced by local leaf water potential rather than root or soil water status in these experiments.     

The relations of stomatal closure and reopening to xylem ABA concentration and leaf water potential during soil drying and rewatering

Two tropical tree species, Acacia confusa and Leucaena leucocephala, were used to study the relationships among stomatal conductance, xylem ABA concentration and leaf water potential during a soil drying and rewatering cycle. Stomatal conductance of both A. confusa and L. leucocephala steadily decreased with the decreases in soil water content and pre-dawn leaf water potential. Upon rewatering, soil water content and pre-dawn leaf water potential rapidly returned to the control levels, whereas the reopening of stomata showed an obvious lag time. The length of this lag time was highly dependent not only upon the degree of water stress but also on plant species. The more severe the water stress, the longer the lag time. When A. confusa and L. leucocephala plants were exposed to the same degree of water stress (around –2.0 MPa in pre-dawn leaf water potential), the stomata of A. confusa reopened to the control level 6 days after rewatering. However, it took L. leucocephala about 14 days to reopen fully. A very similar response of leaf photosynthesis to soil water deficit was also observed for both species. Soil drying resulted in a significant increase in leaf and xylem ABA concentrations in both species. The more severe the water stress, the higher the leaf and xylem ABA concentrations. Both leaf ABA and xylem ABA returned to the control level following relief from water deficit and preceded the full recovery of stomata, suggesting that the lag phase of stomatal reopening was not controlled by leaf and/or xylem ABA. In contrast to drying the whole root system, drying half of the root system did not change the leaf water relations, but caused a significant increase in xylem ABA concentration, which could fully explain the decrease of stomatal conductance. After rewatering, the stomatal conductance of plants in which half of the roots were dried recovered more rapidly than those of whole-root dried plants, indicating that the leaf water deficit that occurred during the drying period was related to the post-stress stomatal inhibition. These results indicated that the decrease in stomatal conductance caused by water deficit was closely related to the increase in xylem ABA, but xylem ABA could not fully explain the reopening of stomata after relief of water stress, neither did the leaf ABA. Some unknown physiological and/or morphological processes in the guard cells may be related to the recovery process.     

The influence of arbuscular mycorrhizal colonization on soil–root hydraulic conductance in Agrostis stolonifera L. under two water regimes

The hypothesis that mycorrhizal colonization improves the soil–root conductance in plants was experimentally tested in a growth chamber using pot cultures of Agrostis stolonifera L. colonized by Glomus intraradices. Plants were grown in 50-l pots filled with autoclaved sand/silt soil (1:1), with and without the mycorrhizal fungus. Within the mycorrhizal treatment, half of the pots remained well watered, while the other half was subjected to a progressive water deficit. Soil water potential (estimated as plant water potential measured at the end of the dark period), xylem water potential measured at the tiller base, transpiration rate, and soil water content were monitored throughout the experiment. Soil–root hydraulic conductance was estimated as the ratio between the instantaneous transpiration rate and the soil and xylem water potential difference. To obtain cultures with similar nutritional status, the P in the modified Hoagland’s nutrient solution was withheld from the inoculated pots and applied only once a month. Even though there were no differences on growth or nutrient status for the mycorrhizal treatments, water transport was enhanced by the inoculum presence. Transpiration rate was maintained at lower xylem water potential values in the presence of mycorrhizae. The analysis of the relationship between soil–root hydraulic resistance and soil water content showed that mycorrhizal colonization increased soil–root hydraulic conductance as the soil dried. For these growing conditions, this effect was ascribed to the range of 6–10%.     

Effect of water stress on leaf and root growth,and water uptake ofGmelina arborea ROXB. seedlings

Young seedlings ofGmelina arborea Roxb. were subjected to 2 weeks of drought. Despite the gradual reduction in stomatal conductance, leaf and root growth was not affected until the later part of the stress period. This was attributed to solute adjustment in the roots of the plants. As the severity of water stress increased, root growth was prolific in all the soil segments. As a result, water in the lowest soil segment was used to maintain plant turgor, which in turn sustains the leaf and root growth during the water-stress treatment. The influence of soil water content and soil water potential upon soil water uptake rate was also evaluated on soil profile basis. Rates of extraction began to decline in all soil segments as soon as soil water potential fell below -0.06 MPa, presumably as a result of vapour gaps between the root and soil (root: soil interface resistance). It is suggested that the growth of roots ofGmelina plants away from drying soil will minimize the resistance to water uptake.     

Modelling the impact of heterogeneous rootzone water distribution on the regulation of transpiration by hormone transport and/or hydraulic pressures

Aims

A simulation model to demonstrate that soil water potential can regulate transpiration, by influencing leaf water potential and/or inducing root production of chemical signals that are transported to the leaves.

Methods

Signalling impacts on the relationship between soil water potential and transpiration were simulated by coupling a 3D model for water flow in soil, into and through roots (Javaux et al. 2008) with a model for xylem transport of chemicals (produced as a function of local root water potential). Stomatal conductance was regulated by simulated leaf water potential (H) and/or foliar chemical signal concentrations (C; H?+?C). Split-root experiments were simulated by varying transpiration demands and irrigation placement.

Results

While regulation of stomatal conductance by chemical transport was unstable and oscillatory, simulated transpiration over time and root water uptake from the two soil compartments were similar for both H and H?+?C regulation. Increased stomatal sensitivity more strongly decreased transpiration, and decreased threshold root water potential (below which a chemical signal is produced) delayed transpiration reduction.

Conclusions

Although simulations with H?+?C regulation qualitatively reproduced transpiration of plants exposed to partial rootzone drying (PRD), long-term effects seemed negligible. Moreover, most transpiration responses to PRD could be explained by hydraulic signalling alone.     

The temperature dependence of shoot hydraulic resistance: implications for stomatal behaviour and hydraulic limitation

Recent soil pressurization experiments have shown that stomatal closure in response to high leaf–air humidity gradients can be explained by direct feedback from leaf water potential. The more complex temperature‐by‐humidity interactive effects on stomatal conductance have not yet been explained fully. Measurements of the change in shoot conductance with temperature were made on Phaseolus vulgaris (common bean) to test whether temperature‐induced changes in the liquid‐phase transport capacity could explain these temperature‐ by‐humidity effects. In addition, shoot hydraulic resistances were partitioned within the stem and leaves to determine whether or not leaves exhibit a greater resistance. Changes in hydraulic conductance were calculated based on an Ohm’s law analogy. Whole‐plant gas exchange was used to determine steady‐ state transpiration rates. A combination of in situ psychrometer measurements, Scholander pressure chamber measurements and psychrometric measurements of leaf punches was used to determine water potential differences within the shoot. Hydraulic conductance for each portion of the pathway was estimated as the total flow divided by the water potential difference. Temperature‐induced changes in stomatal conductance were correlated linearly with temperature‐induced changes in hydraulic conductance. The magnitude of the temperature‐induced changes in whole‐plant hydraulic conductance was sufficient to account for the interactive effects of temperature and humidity on stomatal conductance.     

Plasma membrane aquaporins play a significant role during recovery from water deficit

The role of plasma membrane aquaporins (PIPs) in water relations of Arabidopsis was studied by examining plants with reduced expression of PIP1 and PIP2 aquaporins, produced by crossing two different antisense lines. Compared with controls, the double antisense (dAS) plants had reduced amounts of PIP1 and PIP2 aquaporins, and the osmotic hydraulic conductivity of isolated root and leaf protoplasts was reduced 5- to 30-fold. The dAS plants had a 3-fold decrease in the root hydraulic conductivity expressed on a root dry mass basis, but a compensating 2.5-fold increase in the root to leaf dry mass ratio. The leaf hydraulic conductance expressed on a leaf area basis was similar for the dAS compared with the control plants. As a result, the hydraulic conductance of the whole plant was unchanged. Under sufficient and under water-deficient conditions, stomatal conductance, transpiration rate, plant hydraulic conductance, leaf water potential, osmotic pressure, and turgor pressure were similar for the dAS compared with the control plants. However, after 4 d of rewatering following 8 d of drying, the control plants recovered their hydraulic conductance and their transpiration rates faster than the dAS plants. Moreover, after rewatering, the leaf water potential was significantly higher for the control than for the dAS plants. From these results, we conclude that the PIPs play an important role in the recovery of Arabidopsis from the water-deficient condition.     

Use of the root contact concept,an empirical leaf conductance model and pressure-volume curves in simulating crop water relations

A simulation model “DanStress” was developed for studying the integrated effects of soil, crop and climatic conditions on water relations and water use of field grown cereal crops. The root zone was separated into 0.1 m deep layers of topsoil and subsoil. For each layer the water potential at the root surface was calculated by a single root model, and the uptake of water across the root was calculated by a root contact model. Crop transpiration was calculated by Monteith's combination equation for vapour flow. Crop conductance to water vapour transfer for use in Monteith's combination equation was scaled up from an empirical stomatal conductance model used on sunlit and shaded crop surfaces of different crop layers. In the model, transpirational water loss originates from root water uptake and changes in crop water storage. Crop water capacitance, used for describing the water storage, was derived from the slope of pressure-volume (PV) curves of the leaves. PV curves were also used for deriving crop water potential, osmotic potential, and turgor pressure. The model could simulate detailed diurnal soil-crop water relations during a 23-day-drying cycle with time steps of one hour. During the grain filling period in spring barley (Hordeum distichum L.), grown in a sandy soil in the field, measured and predicted values of leaf water and osmotic potential, RWC, and leaf stomatal conductance were compared. Good agreement was obtained between measured and predicted values at different soil water deficits and climatic conditions. In the field, measured and predicted volumetric soil water contents (θ) of topsoil and subsoil layers were also compared during a drying cycle. Predicted and measured θ-values as a function of soil water deficits were similar suggesting that the root contact model approach was valid. From the investigation we concluded: (I) a model, which takes the degree of contact between root surface and soil water into account, can be used in sandy soil for calculation of root water uptake, so that the root conductance during soil water depletion only varies by the degree of contact; (II) crop conductance, used for calculation of crop transpiration, can be scaled up from an empirical single leaf stomatal conductance model controlled by the level of leaf water potential and micrometeorological conditions; (III) PV curves are usable for describing crop water status including crop water storage.     

Stomatal behavior and water relations of waterlogged tomato plants

The effects of waterlogging the soil on leaf water potential, leaf epidermal conductance, transpiration, root conductance to water flow, and petiole epinasty have been examined in the tomato (Lycopersicon esculentum Mill.). Stomatal conductance and transpiration are reduced by 30% to 40% after approximately 24 hours of soil flooding. This is not due to a transient water deficit, as leaf water potential is unchanged, even though root conductance is decreased by the stress. The stomatal response apparently prevents any reduction in leaf water potential. Experiments with varied time of flooding, root excision, and stem girdling provide indirect evidence for an influence of roots in maintaining stomatal opening potential. This root-effect cannot be entirely accounted for by alterations in source-sink relationships. Although 1-aminocyclopropane-1-carboxylic acid, the immediate precursor of ethylene, is transported from the roots to the shoots of waterlogged tomato plants, it has no direct effect on stomatal conductance. Ethylene-induced petiole epinasty develops coincident with partial stomatal closure in waterlogged plants. Leaf epinasty may have beneficial effects on plant water balance by reducing light interception.     

Estimation of hydraulic conductance within field-grown apricot using sap flow measurements

Using the heat pulse and other techniques, the hydraulic architecture of apricot trees was mapped out. The flows (overall flow, flow across the four main branches) and forces (water potential differences between xylem and leaves) measured allowed us to quantify hydraulic conductance of branches and of the root/soil resistance. The experiment was carried out in a commercial orchard of 11-year-old apricot trees (Prunus armeniaca L., cv. Búlida, on Real Fino apricot rootstock) during 1 week (October 27–November 3, 1998). Three representative trees with a cylindrical trunk divided into four main branches of different sizes, orientation and local microclimate were chosen for the experiment. Sap flow was measured throughout the experimental period. Twelve sets of heat-pulse probes were used, one for each main branch. The diurnal course of the environmental conditions, the fraction of the area irradiated and leaf water relations were also considered in each main branch. The relationships between leaf water potential, xylem water potential and transpiration were established for different branches and also for the total plant. Using the slopes of these regressions, total plant conductance, the hydraulic conductance of the stem and root pathway, the hydraulic conductance of the canopy and the hydraulic conductance of each branch were estimated. Our findings show that the root conductance and the canopy hydraulic conductance are similar in magnitude. Leaf hydraulic conductance per leaf area unit was similar for each of the four branch orientations, indicating that, while the light microclimate has a dominant influence on transpiration, in this case it had little effect on the hydraulic properties of the canopy.     

冬小麦叶片气孔导度模型水分响应函数的参数化

植物气孔导度模型的水分响应函数用来模拟水分胁迫对气孔导度的影响过程, 是模拟缺水环境下植物与大气间水、碳交换过程的关键算法。水分响应函数包括空气湿度响应函数和土壤湿度(或植物水势)响应函数, 该研究基于田间实验观测, 分析了冬小麦(Triticum aestivum)叶片气孔导度对不同空气饱和差和不同土壤体积含水量或叶水势的响应规律。一个土壤水分梯度的田间处理在中国科学院禹城综合试验站实施, 不同水分胁迫下的冬小麦叶片气体交换过程和气孔导度以及其他的温湿度数据被观测, 同时观测了土壤含水量和叶水势。实验数据表明, 冬小麦叶片气孔导度对空气饱和差的响应呈现双曲线规律, 变化趋势显示大约1 kPa空气饱和差是一个有用的阈值, 在小于1 kPa时, 冬小麦气孔导度对空气饱和差变化反应敏感, 而大于1 kPa后则反应缓慢; 分析土壤体积含水量与中午叶片气孔导度的关系发现, 中午叶片气孔导度随土壤含水量增加大致呈现线性增加趋势, 但在平均土壤体积含水量大于大约25%以后, 气孔导度不再明显增加, 而是维持在较高导度值上下波动; 冬小麦中午叶片水势与相应的气孔导度之间, 随着叶水势的增加, 气孔导度呈现增加趋势。根据冬小麦气孔导度对空气湿度、土壤湿度和叶水势的响应规律, 研究分别采用双曲线和幂指数形式拟合了水汽响应函数, 用三段线性方程拟合了土壤湿度响应函数和植物水势响应函数, 得到的参数可以为模型模拟冬小麦的各类水、热、碳交换过程采用。