Ambient Vapor Pressure Deficit Research Articles

Microclimate for Cryptomeria japonica seedlings in treeshelters – Mitigation of severe condition by seedling transpiration

ABSTRACT We studied the microclimate inside treeshelters to examine how they affected growth of Japanese cedar (Cryptomeria japonica) seedlings in western Japan. Air temperature (T), relative humidity (RH), and photosynthetic active radiation (PAR) were measured inside and outside of commonly used design of tubular treeshelters made of translucent polypropylene at a nursery throughout a year. Vapor pressure deficit (VPD) was calculated from T and RH data. Results indicated that the inside T and VPD above the seedling foliage were extremely higher than ambient air. However, T within the seedling foliage showed 2.7°C higher than ambient T even though 10.3°C higher above the foliage in August. The VPD within seedling foliage was lower than ambient VPD in summer, and increased with decreasing RH when the precipitation was low. Microclimate in an empty treeshelter showed higher T and VPD than those in treeshelter with seedling. These results indicated that high T and VPD were mitigated by transpiration from needle leaves of seedling, and Japanese cedar seedlings could grow in treeshelters at sufficient water supply.

Role of guard-cell ABA in determining steady-state stomatal aperture and prompt vapor-pressure-deficit response

Abscisic acid (ABA) is known to be involved in stomatal closure. However, its role in stomatal response to rapid increases in the vapor pressure deficit (VPD) is unclear. To study this issue, we generated guard cell-specific ABA-insensitive Arabidopsis plants (guard-cell specific abi1-1; GCabi). Under non-stressed conditions, the stomatal conductance (gs) and apertures of GCabi plants were greater than those of control plants. This supports guard-cell ABA role as limiting steady-state stomatal aperture under non-stressful conditions. When there was a rapid increase in VPD (0.15 to 1 kPa), the gs and stomatal apertures of GCabi decreased in a manner similar that observed in the WT control, but different from that observed in WT plants treated with fusicoccin. Low VPD increased the size of the stomatal apertures of the WT, but not of GCabi. We conclude that guard-cell ABA does not play a significant role in the initial, rapid stomatal closure that occurs in response to an increase in VPD, but is important for stomatal adaptation to ambient VPD. We propose a biphasic angiosperm VPD-sensing model that includes an initial ABA-independent phase and a subsequent ABA-dependent steady-state phase in which stomatal behavior is optimized for ambient VPD conditions.

Alpine wetland ecosystem carbon sink and its controls at the Qinghai Lake

Wetland ecosystems play an important role in regulating the Earth’s climate. This paper presents the results of a study on the alpine wetland ecosystem CO2 fluxes at the Qinghai Lake at different vegetative development stages and CO2 controlling factors measured by eddy covariance system. The results showed that the alpine wetland ecosystem at the Qinghai Lake area was a carbon sink on the whole year basis. The average net ecosystem CO2 exchange (NEE) amount was −904.42 g CO2/m2; the average amount of the ecosystem reparation (Re) was 1450 g CO2/m2; the average gross primary productivity (GPP) was 2354.42 g CO2/m2 from July 2011 to June 2013 year. The NEE, Re and GPP amounts were the maximum during the growing stage of the whole year. There were significant negative linear correlations between monthly NEE amount and monthly average total solar radiation (SR) (R 2 = 0.27, p < 0.05), incident photosynthetically active radiation (PAR) (R 2 = 0.28, p < 0.05), ambient vapor pressure deficit (VPD) (R 2 = 0.38, p < 0.01), monthly total precipitation (P) (R 2 = 0.65, p < 0.0001), monthly mean leaf area index (LAI) (R 2 = 0.85, p < 0.0001) and enhanced vegetation index (EVI) (R 2 = 0.84, p < 0.0001). Extreme significant polynomial nonlinear negative correlations were found between NEE amounts and monthly average air temperature (T a ) (R 2 = 0.84, p < 0.0001), soil temperature at 0–5 cm depth (T s ) (R 2 = 0.90, p < 0.0001) and air relative humidity (RH) (R 2 = 0.76, p < 0.0001). The monthly Re amount of alpine wetland ecosystem at the Qinghai Lake at different stages showed significant nonlinear positive correlations with all the identified affecting factors except for monthly mean EVI, which showed a linear relationship (R 2 = 0.72, p < 0.0001). The monthly GPP amount at different vegetative development stages showed a significant exponential correlation with monthly average T a (R 2 = 0.92, p < 0.0001), a power function with VPD (R 2 = 0.24, p < 0.05), quadratic polynomial ones with monthly average T s (R 2 = 0.93, p < 0.0001), RH (R 2 = 0.74, p < 0.0001) and monthly P (R 2 = 0.55, p < 0.05). Extremely significant linear correlations of the monthly GPP amount at different developing stages with monthly average LAI (R 2 = 0.85, p < 0.0001) and EVI (R 2 = 0.92, p < 0.0001) were also found.

Artificially decreased vapour pressure deficit in field conditions modifies foliar metabolite profiles in birch and aspen.

Relative air humidity (RH) is expected to increase in northern Europe due to climate change. Increasing RH reduces the difference of water vapour pressure deficit (VPD) between the leaf and the atmosphere, and affects the gas exchange of plants. Little is known about the effects of decreased VPD on plant metabolism, especially under field conditions. This study was conducted to determine the effects of artificially decreased VPD on silver birch (Betula pendula Roth.) and hybrid aspen (Populus tremula L.×P. tremuloides Michx.) foliar metabolite and nutrient profiles in a unique free air humidity manipulation (FAHM) field experiment during the fourth season of humidity manipulation, in 2011. Long-term exposure to decreased VPD modified nutrient homeostasis in tree leaves, as demonstrated by a lower N concentration and N:P ratio in aspen leaves, and higher Na concentration and lower K:Na ratio in the leaves of both species in decreased VPD than in ambient VPD. Decreased VPD caused a shift in foliar metabolite profiles of both species, affecting primary and secondary metabolites. Metabolic adjustment to decreased VPD included elevated levels of starch and heptulose sugars, sorbitol, hemiterpenoid and phenolic glycosides, and α-tocopherol. High levels of carbon reserves, phenolic compounds, and antioxidants under decreased VPD may modify plant resistance to environmental stresses emerging under changing climate.

Spatio-temporal variation in transpiration responses of maize plants to vapor pressure deficit under an arid climatic condition

The transpiration rate of plant is physically controlled by the magnitude of the vapor pressure deficit (VPD) and stomatal conductance. A limited-transpiration trait has been reported for many crop species in different environments, including Maize (Zea mays L.). This trait results in restricted transpiration rate under high VPD, and can potentially conserve soil water and thus decrease soil water deficit. However, such a restriction on transpiration rate has never been explored in maize under arid climatic conditions in northwestern China. The objective of this study was to examine the transpiration rate of field-grown maize under well-watered conditions in an arid area at both leaf and whole plant levels, and therefore to investigate how transpiration rate responding to the ambient VPD at different spatial and temporal scales. The transpiration rates of maize at leaf and plant scales were measured independently using a gas exchange system and sapflow instrument, respectively. Results showed significant variations in transpiration responses of maize to VPD among different spatio-temporal scales. A two-phase transpiration response was observed at leaf level with a threshold of 3.5 kPa while at the whole plant level, the daytime transpiration rate was positively associated with VPD across all measurement data, as was nighttime transpiration response to VPD at both leaf and whole plant level, which showed no definable threshold vapor pressure deficit, above which transpiration rate was restricted. With regard to temporal scale, transpiration was most responsive to VPD at a daily scale, moderately responsive at a half-hourly scale, and least responsive at an instantaneous scale. A similar breakpoint (about 3.0 kPa) in response of the instantaneous leaf stomatal conductance and hourly canopy bulk conductance to VPD were also observed. At a daily scale, the maximum canopy bulk conductance occurred at a VPD about 1.7 kPa. Generally, the responsiveness of stomatal conductance to VPD at the canopy scale was lower than that at leaf scale. These results indicate a temporal and spatial heterogeneity in how maize transpiration responses to VPD under arid climatic conditions. This could allow a better assessment of the possible benefits of using the maximum transpiration trait to improve maize drought tolerance in arid environment, and allow a better prediction of plant transpiration which underpin empirical models for stomatal conductance at different spatio-temporal scales in the arid climatic conditions.

Inhibitor screen for limited-transpiration trait among maize hybrids

A plant trait to minimize the impact of drought on crop yield is limited-transpiration rate (TR) under high ambient vapor pressure deficit (VPD) so that soil water is conserved to sustain grain fill. Variation among maize (Zea mays L.) hybrids has been identified for the existence of the limited-TR trait at high atmospheric VPD, and the VPD at which TR becomes limited. Further, it has been shown that the TR limitation at high VPD is related to plant hydraulic conductance, which may be due to differences in aquaporin expression. This paper reports studies to relate the TR response of 21 maize hybrids to treatment of leaves and intact plants with cycloheximide (CHX) and four aquaporin inhibitors: silver (AgNO3), gold (HAuCl4), zinc (ZnCl2), and mercury (HgCl2). There was no discrimination among hybrids based on treatment with Hg or CHX. Segregation between hybrids for response to increasing VPD corresponded with differences in leaf response to Ag and Au treatment and intact plant response to Zn. The highest correlation (r=0.90) between VPD breakpoint and TR response to inhibitor was with Ag treatment of leaves. These results indicate that Ag may be an effective initial screen for expression of the limited-TR trait under high VPD.

Limited-transpiration trait evaluated in growth chamber and field for sorghum genotypes

Sorghum [Sorghum bicolor (L.) Moench] is commonly grown in water-limited environments throughout the world. Plant traits could be useful allowing for early-season water conservation so that more water is available for use later in the season when drought is most likely to develop. One trait that might result in early-season water conservation is the expression of a limited-transpiration trait defined as a limitation on further increases in transpiration rate (TR) under high vapor pressure deficit (VPD) conditions. The objective of this study was to compare the expression of the limited-TR trait measured for nine sorghum genotypes under both controlled chamber and field conditions. In the growth chamber, plant TR was measured over a range of imposed VPD to provide a direct measure of plant transpiration under high VPD. In the field, stomatal conductance (gs) was measured over the daily cycle, which resulted in a range of ambient VPD. A decrease in gs under high VPD was evidence of the limited-TR trait. This study identified three sorghum genotypes (DKS 36-06, DKS 44-20, and DKS 54-00) that did not show any limitation on water loss at high VPD in either the greenhouse or field. On the other hand, four genotypes (BTX 2752, SC 599, SC 982, and B 35) exhibited the limited-TR trait in the growth chamber with breakpoints in response to VPD at values of 2.33kPa and above. These four genotypes also expressed a breakpoint in gs in response to increasing VPD in the field. Two genotypes (TX ARG 1, TX 436) that differed between the growth chamber and field showed consistency in response on close examination of the field results. The overall general correspondence within genotypes between the controlled chamber and the field in expression or lack of expression of a breakpoint in response to increasing VPD demonstrated the possibility of selecting genotypes for the TRlim trait under differing environmental conditions.

Seasonal and inter-annual variability of energy exchange above a boreal Scots pine forest

Abstract. Twelve-years of eddy-covariance measurements conducted above a boreal Scots pine forest in Hyytiälä, Southern Finland, were analyzed to assess the seasonal and inter-annual variability of surface conductance (gs) and energy partitioning. The gs had distinct annual course, driven by the seasonal cycle of the Scots pine. Low gs (2–3 mm s−1 in April) cause the sensible heat flux to peak in May–June while evapotranspiration takes over later in July–August when gs is typically 5–7 mm s−1. Hence, during normal years Bowen ratio decreases from 4–6 in April to 0.7–0.9 in August. Sensitivity of gs to ambient vapor pressure deficit (D) was relatively constant but the reference value at D = 1 kPa varied seasonally and between years. Only two drought episodes when volumetric soil moisture content in upper mineral soil decreased below 0.15 m3 m−3 occurred during the period. Below this threshold value, transpiration was strongly reduced, which promoted sensible heat exchange increasing Bowen ratio to 3–4. Annual evapotranspiration varied between 218 and 361 mm and accounted between 50% and 90% of equilibrium evaporation. The forest floor contributed between 16 and 25% of the total evapotranspiration on annual scale. The fraction stayed similar over the observed range of environmental conditions including drought periods. The inter-annual variability of evapotranspiration could not be linked to any mean climate variable while the summertime sensible heat flux and net radiation were well explained by global radiation. The energy balance closure varied annually between 0.66 and 0.95 and had a distinct seasonal cycle with worse closure in spring when a large proportion of available energy is partitioned into sensible heat.

Predicting moisture dynamics of fine understory fuels in a moist tropical rainforest system: results of a pilot study undertaken to identify proxy variables useful for rating fire danger

*The use of fire as a land management tool in the moist tropics often has the unintended consequence of degrading adjacent forest, particularly during severe droughts. Reliable models of fire danger are needed to help mitigate these impacts. *Here, we studied the moisture dynamics of fine understory fuels in the east-central Brazilian Amazon during the 2003 dry season. Drying stations established under varying amounts of canopy cover (leaf area index (LAI) = 0 - 5.3) were subjected to a range of water inputs (5-15 mm) and models were developed to forecast litter moisture content (LMC). Predictions were then compared with independent field data. *A multiple linear regression relating litter moisture content to forest structure (LAI), ambient vapor pressure deficit (VPD(M)) and an index of elapsed time since a precipitation event (d(-1)) was identified as the best-fit model (adjusted R(2) = 0.89). Relative to the independent observations, model predictions were relatively unbiased when the LMC was <or= 50%, but consistently underestimated the LMC when the observed values were higher. *The approach to predicting fire danger based on forest structure and meteorological variables is promising; however, additional information to the LAI, for example forest biomass, may be required to accurately capture the influence of forest structure on understory microclimate.

Interactive effects of soil water deficit and air vapour pressure deficit on mesophyll conductance to CO2 in Vitis vinifera and Olea europaea

The present work aims to study the interactive effect of drought stress and high vapour pressure deficit (VPD) on leaf gas exchange, and especially on mesophyll conductance to CO(2) (g(m)), in two woody species of great agronomical importance in the Mediterranean basin: Vitis vinifera L. cv. Tempranillo and Olea europaea L. cv. Manzanilla. Plants were grown in specially designed outdoor chambers with ambient and below ambient VPD, under both well-irrigated and drought conditions. g(m) was estimated by the variable J method from simultaneous measurements of gas exchange and fluorescence. In both species, the response to soil water deficit was larger in g(s) than in g(m), and more important than the response to VPD. Olea europaea was apparently more sensitive to VPD, so that plants growing in more humid chambers showed higher g(s) and g(m). In V. vinifera, in contrast, soil water deficit dominated the response of g(s) and g(m). Consequently, changes in g(m)/g(s) were more related to VPD in O. europaea and to soil water deficit in V. vinifera. Most of the limitations of photosynthesis were diffusional and especially due to stomatal closure. No biochemical limitation was detected. The results showed that structural parameters played an important role in determining g(m) during the acclimation process. Although the relationship between leaf mass per unit area (M(A)) with g(m) was scattered, it imposed a limitation to the maximum g(m) achievable, with higher values of M(A) in O. europaea at lower g(m) values. M(A) decreased under water stress in O. europaea but it increased in V. vinifera. This resulted in a negative relationship between M(A) and the CO(2) draw-down between substomatal cavities and chloroplasts in O. europaea, while being positive in V. vinifera.

Effects of vapour pressure deficit on growth of temperate and tropical evergreen rainforest trees of Australia

Little is known about the effect of vapour pressure deficit (VPD) on the growth of trees. Rainforest trees of eastern Australia provide an opportunity to investigate responses to VPD in species that occur in high precipitation areas but have contrasting dry seasons—summer in the temperate south and winter in the tropical north. Growth responses to VPD were measured in eight species of Australian rainforest trees from different latitudes to investigate possible differences in their response to atmospheric drought. Previous work on these species found that the tropical species have large reductions in gas exchange with increasing VPD whereas the temperate species were mainly unresponsive to increasing VPD. Plants were grown in glasshouses for a year under either low VPD or ambient conditions of a temperate climate. All species had non-significant increases in growth rates (1–9%) of plants grown under low VPD compared with plants grown under ambient VPD. In addition, growing the species under low VPD had no effect on allocation of biomass (leaf area ratio, leaf weight ratio and root/shoot ratio). Therefore, the high sensitivity of gas exchange to increasing VPD found in the tropical rainforest trees did not have a significant, long-term effect on growth under high VPD.

Physiological and environmental control of transpiration by trees in windbreaks

Knowledge of the environmental and physiological variables controlling water use by trees in windbreaks should assist efforts to manage competition for water between trees and crops in agroforestry. Coupling between windbreak trees and the atmosphere was therefore assessed by determining values of the decoupling coefficient ( Ω) for windbreaks during four periods of the year, using experimentally-derived aerodynamic and surface conductances for trees of Azadirachta inidica A. Juss in windbreaks at Sadoré, Niger. Aerodynamic conductances were determined by scaling-up measurements of leaf boundary layer conductances made using leaf replicas. Surface conductances for the trees were derived from measurements of whole-tree transpiration made using the heat-pulse technique. Values of Ω for the windbreaks were ≈0.3 at moderate wind speeds in the more humid seasons of the year, but fell to 0.1 during the driest period of the year. Thus, windbreaks are generally well-coupled to the atmosphere, so that transpiration is predominantly driven by the ambient vapour pressure deficit at leaf surfaces and the trees are able to exert close physiological control over water use. Management strategies to reduce the surface conductances of windbreak canopies should, consequently, effectively reduce demand for water by windbreaks and help to control competition for water between trees and crops.

Seasonal Water Use of a Kiwifruit Vine: Measurements and a Model

We recorded canopy development and stomatal function of a kiwifruit vine for the purpose of calculating the seasonal water use by the crop. Canopy development was described using an empirical “S-shaped” curve fitted to weekly measurements of the vine's leaf area. Stomatal conductance was described using a semi-empirical model based on the incident radiation, and the ambient vapor pressure deficit of the air. These two descriptors, leaf area and stomatal conductance, were combined with meteorological data to calculate vine transpiration via the Penman–Monteith model. Transpiration rates calculated at 30-min intervals were in good agreement with the instantaneous rates of sap flow measured by heat-pulse sensors located in the vine stem. The measured and calculated transpiration remained in concert throughout the experiment, thereby confirming the Penman-Monteith model as a robust and suitable model to describe the seasonal water use by kiwifruit vines. The model validation enables confident predictions of crop water use and thus aids irrigation allocation for kiwifruit crops.

Estimating transpiration from 6‐year‐old Eucalyptus grandis trees: development of a canopy conductance model and comparison with independent sap flux measurements

ABSTRACTDaily patterns of stomatal conductance (gs), xylem pressure potential (P) and canopy microclimatic variables were recorded on 11 sample days as part of a one‐year study of the water use of Eucalyptus grandis Hill ex Maiden in the eastern Transvaal, South Africa. Measured gs was found to be largely controlled by quantum flux density (Q) and ambient vapour pressure deficit (D). Canopy conductance (gc) was determined for hourly intervals using gs measurements and leaf areas in four different canopy levels. A simple model was constructed to allow the prediction of gc and transpiration from Q, D and season of year. The model was used to estimate transpiration rates from 10 trees in a later study of similarly‐aged E. grandis trees, in which sap flow in each tree was measured using the heat pulse velocity (HPV) technique. Five of the trees were monitored on a summer day and five on a winter day. Correspondence between HPV sap flow and modelled transpiration was good for the summertime comparisons, but measured winter‐time sap flow rates were underestimated by the model, especially under conditions of high sap flow. The discrepancy is believed to result from having insufficient data from the conductance study to describe the response of gs to relatively high D in winter. Marked variation in transpiration per unit leaf area indicates that a relatively large number of trees must be sampled for the HPV technique to be used to obtain a mean rate for an entire stand in winter.

Modeling stomatal resistance: an overview of the 1989 workshop at the Pennsylvania State University

A few years ago, I surveyed the scientific literature in order to identify models that calculate stomatal resistance from atmospheric and soil parameters. I found more than 15 models published over a 15-year period in selected journals. Despite general similarity among the models, the definitions, functional representation and units of variables were often dissimilar. More distressing was the fact that measurements published with one model could not be used to compute coefficients for any other model. This bewildering state of modeling stomatal resistance was the result of differing purposes of the research. In some cases, the intent was to describe stomatal resistance for climate prediction or remote sensing; in other cases, the purpose was to assess crop yield or forest production, or to investigate leaf function. I felt that differing approaches to modeling could be resolved by convening a group of specialists with differing scientific backgrounds, such as meteorology, agronomy and plant physiology. Thus was born a workshop on stomatal resistance, which was held at Penn State University, 10-13 April 1989. The papers in this volume were written in response to a stated goal of the workshop: to summarize the state of the art in modeling stomatal resistance. This topic was broadened during the course of the workshop to include other topics related to plant control of evapotranspiration. Rather than to resolve outstanding issues, the workshop served to highlight opposing points of view. A break in the spectrum of scales and interests was evident between that of the leaf and the canopy. Individuals interested in regional climatology or remote sensing methods tended to express stomatal resistance deterministically. Their formulations empirically related easily measurable environmental parameters (soil water content, ambient vapor pressure deficit, etc. ) to stomatal resistance via arbitrary functions or through the Penman-Monte i th equation. This group of scientists was concerned with

Paraheliotropic Leaf Movements in Mature Alfalfa Canopies1

The relationship between plant water potential and midday sunavoiding leaf movement was studied in mature alfalfa (Medicago sativa L.) canopies. Diurnal solar tracking patterns were determined for three cultivars representing different germplasm sources: ‘African’, a nondormant type introduced from the arid regions of Africa; ‘Lahontan’, a semidormant type of Turkistan origin; and ‘Norseman’, a dormant type introduced from the Ladak province of northern India. Solar tracking was measureduring four regrowth cycles beginning in early spring and concluding in early fall. Early and late season regrowth cycles for African and Lahontan were typified by patterns in which leaflet surfaces were positioned perpendicular to incident radiation (diaheliotropism) throughout the day. Late spring and midsummer diurnal diaheliotropic patterns were interrupted by midday sun‐avoiding paraheliotropic (PHT) movement in both cultivars. Conversely, diurnal tracking in Norseman was interrupted by PHT movement in all cycles. Leaflet PHT movement is generally presumed to be a stress‐avoidance mechanism induced by changes in plant water status. However, regression analyses of PHT activity data for alfalfa showed no clear relationship between diurnal PHT patterns and xylem water potential (XWP) either among cultivars or with the onset of and recovery from PHT activity. Rather, in all three cultivars, cupping was consistently related to ambient vapor pressure deficit (VPD). African and Lahontan did not exhibit midday leaflet PHT movement during the late season regrowth cycle even when the daily ambient VPD and XWP profiles were similar to the midseason patterns.

Evaluation of Cotton Canopy Temperature to Detect Crop Water Stress

ABSTRACT CANOPY temperature of cotton was shown to be useful as an indicator of crop water stress resulting from both soil water deficit (matric) and soil water salinity (osmotic) stresses. The relationship between canopy-air temperature difference and ambient vapor pressure deficit was shown not to depend on cotton row spacing, not to exhibit seasonal trends and to be applicable to a wide cotton production region. Cotton lint yields from two experiments were found to be negatively correlated to the mean seasonal crop water stress index. The crop water stress index was found to be sensitive to crop osmotic stress resulting from high soil water salinity when the soil water matric potential was negligible. Canopy and air temperature difference, in conjunction with ambient vapor pressure measurements, formulated into the crop water stress index appears to be useful in quantifying crop water stress from both soil water deficits and soil salinity. Therefore the crop water stress index appears useful for indicating the need for irrigating cotton.

Microclimatic and crop responses to center pivot sprinkler and to surface irrigation

Over 1 million hectares are irrigated with center pivot sprinklers in the Great Plains, USA. Microclimatic conditions under center pivot systems will be affected somewhat by periodic sprinkling, but the extent of microclimatic modification to be expected from sprinkling in the High Plains region and the physiological implications have not been reported. We compared the leaf temperature, canopy air temperature, vapor pressure deficit, vapor pressure, soil temperature, and soil heat flux in a corn (Zea mays L.) canopy under center pivot sprinkler and surface irrigation. The crops were grown at Garden City, Kansas, in 1980, a hot, dry year, and in 1981, a relatively cool, wet year. Leaf and air temperatures in 1980 were significantly cooler undder sprinkler irrigation than under surface irrigation. Maximum, minimum, and mean daily leaf temperatures were reduced by 2°, 2°, and 1° C, respectively; and maximum, minimum, and mean canopy air temperatures were reduced by 3°, 1.5°, and 1.5° C, respectively. Leaf and minimum canopy air temperature reductions were significant at the 1 % level. Shorter irrigation intervals may explain the reduced stress on the sprinkled plots. We observed small, nonsignificant temperature reductions under the sprinkler in the 1981 season. No significant effects of irrigation type on vapor pressure deficit or on vapor pressure in the canopy were observed in 1980 or 1981. Analysis of the 1981 data indicated that most of the day-to-day variability in leaf and canopy temperatures is related to ambient air temperature and that canopy vapor pressure deficit and vapor pressure are related to both ambient temperature and ambient vapor pressure deficit. Soil temperatures were significantly reduced and soil heat flux increased under sprinkler irrigation. The diurnal response to sprinkler irrigation cycles was pronounced during early stress periods of the 1980 growing season. Leaf and canopy air temperature and vapor pressure deficit were all significantly lower throughout the day in recently irrigated areas compared to areas that were sprinkled one or two days earlier. Responses to sprinkling during nonstress periods of 1980 and 1981 only persisted while the leaves were wetted; after, conditions returned to levels found in the rest of the field.

Stomatal response to vapour pressure deficit and the effect of plant water stress

Abstract The dynamic response of stomata to changes in atmospheric humidity was investigated in Fragaria × ananassa Duch., Picea engelmannii Parry, and Pseudotsuga menziesii (Mirb.) Franco; and the effect of water stress on this response was determined in Pseudotsuga menziesii.The plants were rotated through three regimes of ambient temperature and vapour pressure deficit: 35°C–3. 5kPa, 35°C–0. 5 kPa, and 20°C–1. 5kPa. Branch and leaflet conductance were measured with a steady‐state porometer, first at ambient vapour pressure deficit and then at one of four treatment conditions achieved by increasing or decreasing vapour pressure within the porometer cuvette. All three species showed similar stomatal response: enhanced conductance at low vapour pressure deficit and depressed conductance at high vapour pressure deficit. Engelmann spruce was more sensitive than Douglas fir and strawberry.Plant water status significantly altered stomatal response to vapour pressure deficit. The relationship of conductance of xylem water potential was linear under ambient conditions but became curvilinear when conductance was measured above and below ambient vapour pressure deficit. Between −0. 5 MPa and −2. 0 MPa xylem water potential, the stomata were sensitive to vapour pressure deficit, but below − 2. 0 MPa, the sensitivity decreased.

The drying pattern of grass swaths in the field

AbstractThe drying pattern of grass swaths was monitored using microclimate techniques during a 48 h summer wilting period of good drying conditions. Swath drying pattem, which is dependent upon the swath energy relations, water relations and resistances to evaporation, was explained in terms of changes in the balance of available energy, ambient vapour pressure deficit and swath internal resistance to water vapour transfer throughout the drying period. A typical drying pattern showed limited swath evaporation between sunrise and 09.00 hours, and rapid evaporation between 09.00 hours and 12.00 hours when available energy and vapour pressure deficit were high and swath resistance to water loss low. Between 12.00 and 18.00 hours, because of a decrease in available energy and increase in swath resistance, swath evaporation was reduced. Between 18.00 hours and sunset swath evaporation was near zero.