Changes In Vapor Pressure Deficit Research Articles

Cacao trees under different shade tree shelter: effects on water use

We asked how shade tree admixture affects cacao water use in agroforests. In Central Sulawesi, Indonesia, cacao and shade tree sap flux was monitored in a monoculture, in a stand with admixed Gliricidia trees and in a mixture with a multi-species tree assemblage, with both mixtures having similar canopy openness. A Jarvis type sap flux model suggested a distinct difference in sap flux response to changes in vapor pressure deficit and radiation among cacao trees in the individual cultivation systems. We argue that differences originate from stomatal control of transpiration in the monoculture and altered radiation conditions and a different degree of uncoupling of the VPD from the bulk atmosphere inside shaded stands. Probably due to high sap flux variability among trees, these differences however did not result in significantly altered average daily cacao water use rates which were 16 L day−1 in the multi-species assemblage and 22 L day−1 in the other plots. In shaded stands, water use of single cacao trees increased with decreasing canopy gap fraction in the overstory since shading enhanced vegetative growth of cacao fostering transpiration per unit ground area. Estimated transpiration rates of the cacao tree layer were further controlled by stem density and amounted to 1.2 mm day−1 in the monoculture, 2.2 mm day−1 for cacao in the cacao/Gliricidia stand, and 1.1 mm day−1 in the cacao/multi-species stand. The additional transpiration by the shade trees is estimated at 0.5 mm day−1 for the Gliricidia and 1 mm day−1 for the mixed-species cultivation system.

Carbon and water vapor fluxes over four forests in two contrasting climatic zones

The inter- and seasonal patterns of water vapor and canopy carbon fluxes were compared for four forest ecosystems in two contrasting climatic zones in Europe. The eddy covariance and ancillary data were taken from the Carboeurope and FLUXNET databases and a linear modeling statistical analysis was made. The four sites were a high-density poplar (Populus spp.) short rotation coppice plantation (in Lochristi, Belgium) and a mature Scots pine (Pinus sylvestris) forest (in Brasschaat, Belgium) in the Temperate climate versus a fast-growing Eucalypt (Eucalyptus) plantation (in Espirra, Portugal) and a Holm oak (Quercus ilex) forest (in Puechabon, France) in the Mediterranean climate.•The Eucalypt stand showed an efficient stomatal control in response to changes in vapor pressure deficit (VPD), suggesting an ideal adaptation of this species to the severe Mediterranean climate.•The fast-growing poplar stand did not show a similar stomatal control under conditions of moderate water stress. But during an intensive dry period a decrease in the development of the leaf area index (LAI) was observed.•The Holm oak stand showed a low GPP, which is typical for a low productive species with a long rotation cycle. The GPP showed low diurnal variability, even under high solar radiation. This behavior suggested a strong stomatal control caused by the severe water stress, a mechanism that allowed this stand to cope with diurnal and seasonal water deficits.•The mature Scots pine forest in the Temperate climate showed no variation in the GPP – radiation relationship. In this forest no water stress was observed, probably because the trees always had access to the water table. Irrespective of the climate the evapotranspiration of the Scots pine forest presented a tight coupling with the atmosphere, i.e. a low decoupling factor, Ω, comparable with the Holm oak and the Eucalypt forests.The high Ω values of the young poplar plantation were not typical for forest canopies. These values confirmed the strong influence of solar radiation and available energy on evapotranspiration and on the dynamics of this fast-developing canopy. At all four sites the forests showed their capacity to react to the environmental drivers, characteristic from their respective climatic types. However, drastic climatic changes – such as heat waves or long drought spells – may compromise the productivity of fast-growing plantations such as the Eucalypt and poplar stands. The response of the poplars to these events is mainly achieved through LAI control in contrast to the stomatal control in the Eucalypts.

Global change-type drought-induced tree mortality: vapor pressure deficit is more important than temperature per se in causing decline in tree health.

Drought-induced tree mortality is occurring across all forested continents and is expected to increase worldwide during the coming century. Regional-scale forest die-off influences terrestrial albedo, carbon and water budgets, and land-surface energy partitioning. Although increased temperatures during drought are widely identified as a critical contributor to exacerbated tree mortality associated with “global-change-type drought”, corresponding changes in vapor pressure deficit (D) have rarely been considered explicitly and have not been disaggregated from that of temperature per se. Here, we apply a detailed mechanistic soil–plant–atmosphere model to examine the impacts of drought, increased air temperature (+2°C or +5°C), and increased vapor pressure deficit (D; +1 kPa or +2.5 kPa), singly and in combination, on net primary productivity (NPP) and transpiration and forest responses, especially soil moisture content, leaf water potential, and stomatal conductance. We show that increased D exerts a larger detrimental effect on transpiration and NPP, than increased temperature alone, with or without the imposition of a 3-month drought. Combined with drought, the effect of increased D on NPP was substantially larger than that of drought plus increased temperature. Thus, the number of days when NPP was zero across the 2-year simulation was 13 or 14 days in the control and increased temperature scenarios, but increased to approximately 200 days when D was increased. Drought alone increased the number of days of zero NPP to 88, but drought plus increased temperature did not increase the number of days. In contrast, drought and increased D resulted in the number of days when NPP = 0 increasing to 235 (+1 kPa) or 304 days (+2.5 kPa). We conclude that correct identification of the causes of global change-type mortality events requires explicit consideration of the influence of D as well as its interaction with drought and temperature.This study disaggregates the influence of temperature and vapour pressure deficit on net primary productivity of an Australian woodland and their interactions with drought as potential causal agents in recent widespread forest mortality.

Stomatal responses to changes in vapor pressure deficit reflect tissue‐specific differences in hydraulic conductance

The vapor pressure deficit (D) of the atmosphere can negatively affect plant growth as plants reduce stomatal conductance to water vapor (g(wv)) in response to increasing D, limiting the ability of plants to assimilate carbon. The sensitivity of g(wv) to changes in D varies among species and has been correlated with the hydraulic conductance of leaves (K(leaf) ), but the hydraulic conductance of other tissues has also been implicated in plant responses to changing D. Among the 19 grass species, we found that K(leaf) was correlated with the hydraulic conductance of large longitudinal veins (K(lv), r(2) = 0.81), but was not related to K(root) (r(2) = 0.01). Stomatal sensitivity to D was correlated with K(leaf) relative to total leaf area (r(2) = 0.50), and did not differ between C3 and C4 species. Transpiration (E) increased in response to D, but 8 of the 19 plants showed a decline in E at high D, indicative of an 'apparent feedforward' response. For these individuals, E began to decline at lower values of D in plants with low K(root) (r(2) = 0.72). These results show the significance of both leaf and root hydraulic conductance as drivers of plant responses to evaporative demand.

Stomatal encryption by epicuticular waxes as a plastic trait modifying gas exchange in a Mediterranean evergreen species (Quercus coccifera L.)

The adaptive benefit of stomatal crypts remains a matter of controversy. This work studies the effect on gas exchange of cuticular rims that overarch the stomatal pore in the Mediterranean species Quercus coccifera L. growing under Mediterranean (lower relative humidities and high summer temperatures) or oceanic conditions (higher daily relative humidities and mild temperatures). After microscopic assessment of the leaf surfaces and stomatal architecture, the impact of the cuticular 'cup' on gas exchange was evaluated by employing three-dimensional finite element models. Here, we provide evidence for a high plasticity of the Q. coccifera cuticular cup, with much larger vents under oceanic conditions compared to small vents under Mediterranean conditions. This structure adds a substantial fixed resistance thereby strongly decreasing gas exchange under Mediterranean conditions. The cuticular cup, which also increases leaf internal humidity, might buffer the rapid changes in vapour pressure deficit (VPD) often observed under Mediterranean conditions. Since water loss of guard and adjacent epidermal cells regulates stomatal aperture, we suggest that this structure allows an efficient regulation of stomatal conductance and optimum use of resources under high VPD. This study provides evidence that plasticity of stomatal architecture can be an important structural component of hydraulic adaptation to different climate conditions.

Variability in the carbon isotopic composition of foliage carbon pools (soluble carbohydrates, waxes) and respiration fluxes in southeastern U.S. pine forests

We measured the δ13C of assimilated carbon (foliage organic matter (δCOM), soluble carbohydrates (δCSC), and waxes (δCW)) and respiratory carbon (foliage (δCFR), soil (δCSR) and ecosystem 13CO2 (δCER)) for two years at adjacent ecosystems in the southeastern U.S.: a regenerated 32 m tall mature Pinus palustrisforest, and a mid‐rotation 13 m tallPinus elliottii stand. Carbon pools and foliage respiration in P. palustris were isotopically enriched by 2‰ relative to P. elliottii. Despite this enrichment, mean δCER values of the two sites were nearly identical. No temporal trends were apparent in δCSC, δCFR, δCSR and δCER. In contrast, δCOM and δCW at both sites declined by approximately 2‰ over the study. This appears to reflect the adjustment in the δ13C of carbon storage reserves used for biosynthesis as the trees recovered from a severe drought prior to our study. Unexpectedly, the rate of δ13C decrease in the secondary C32–36 n‐alkanoic acid wax molecular cluster was twice that observed forδCOM and the predominant C22–26 compound cluster, and provides new evidence for parallel but separate wax chain elongation systems utilizing different carbon precursor pools in these species. δCFR and δCER were consistently enriched relative to assimilated carbon but, in contrast to previous studies, showed limited variations in response to changes in vapor pressure deficit (D). This limited variability in respiratory fluxes and δCSC may be due to the shallow water table as well as the deep taproots of pines, which limit fluctuations in photosynthetic discrimination arising from changes in D.

Novel applications of carbon isotopes in atmospheric CO<sub>2</sub>: what can atmospheric measurements teach us about processes in the biosphere?

Abstract. Conventionally, measurements of carbon isotopes in atmospheric CO2 (δ13CO2) have been used to partition fluxes between terrestrial and ocean carbon pools. However, novel analytical approaches combined with an increase in the spatial extent and frequency of δ13CO2 measurements allow us to conduct a global analysis of δ13CO2 variability to infer the isotopic composition of source CO2 to the atmosphere (δs). This global analysis yields coherent seasonal patterns of isotopic enrichment. Our results indicate that seasonal values of δs are more highly correlated with vapor pressure deficit (r = 0.404) than relative humidity (r = 0.149). We then evaluate two widely used stomatal conductance models and determine that the Leuning Model, which is primarily driven by vapor pressure deficit is more effective globally at predicting δs (RMSE = 1.6‰) than the Ball-Woodrow-Berry model, which is driven by relative humidity (RMSE = 2.7‰). Thus stomatal conductance on a global scale may be more sensitive to changes in vapor pressure deficit than relative humidity. This approach highlights a new application of using δ13CO2 measurements to validate global models.

Patchy stomatal behavior during midday depression of leaf CO2 exchange in tropical trees

We investigated effects of heterogeneous stomatal behavior on diurnal patterns of leaf gas exchange in 10 tree species. Observations were made in middle and upper canopy layers of potted tropical rainforest trees in a nursery at the Forest Research Institute Malaysia. Measurements were taken from 29 January to 3 February 2010. We measured in situ diurnal changes in net photosynthetic rate and stomatal conductance in three leaves of each species under natural light. In both top-canopy and sub-canopy species, midday depression of net assimilation rate occurred in late morning. Numerical analysis showed that patchy bimodal stomatal behavior occurred only during midday depression, suggesting that the distribution pattern of stomatal apertures (either uniform or non-uniform stomatal behavior) varies flexibly within single days. Direct observation of stomatal aperture using Suzuki's Universal Micro-Printing (SUMP) method demonstrated midday patchy stomatal closure that fits a bimodal pattern in Shorea leprosula Miq., Shorea macrantha Brandis. and Dipterocarpus tempehes V.Sl. Inhibition of net assimilation rate and stomatal conductance appears to be a response to changes in vapor pressure deficit (VPD). Variable stomatal closure with increasing VPD is a mechanism used by a range of species to prevent excess water loss from leaves through evapotranspiration (viz., inhibition of midday leaf gas exchange). Bimodal stomatal closure may occur among adjacent stomata within a single patch, rather than among patches on a single leaf. Our results suggest theoccurrence of patches at several scales within single leaves. Further analysis should consider variable spatial scales in heterogeneous stomatal behavior between and within patches and within single leaves.

Effects of drought and changes in vapour pressure deficit on water relations of Populus deltoides growing in ambient and elevated CO2

The means by which growth CO(2) concentration ([CO(2)]) affects anatomy and water relations responses to drought and vapour pressure deficit (VPD) were studied for yearly coppiced, 4-year-old Populus deltoides clones that were grown in either 400 mumol mol(-1) (ambient) or 800 mumol mol(-1) (elevated) CO(2) for 3 years. It was hypothesized that, during drought, trees growing in elevated [CO(2)] would have a lower volume flux density of water (J(V)), stomatal conductance (g(s)) and transpiration per leaf area (E), as well as a lower stomatal density and a greater stomatal response to drought and changes in VPD than would trees in ambient [CO(2)]. Trees in elevated [CO(2)] actually had higher J(V) values throughout the study, but did not differ from trees in ambient [CO(2)] with respect to g(s) or E under saturating light or E scaled from J(V) (E(scaled)), all of which indicates that the higher J(V) in elevated [CO(2)] resulted from those trees having greater leaf area and not from differences in g(s). Furthermore, although plants in elevated [CO(2)] had greater absolute leaf loss during the drought, the percentage of leaf area lost was similar to that of trees in ambient [CO(2)]. g(s) and E under saturating light were affected by changes in VPD after the first 9 days of the experiment, which coincided with a large decrease in water potential at a soil depth of 0.1 m. Trees in elevated [CO(2)] had a greater stomatal density and a lower wood density than trees in ambient [CO(2)], both traits that may make the trees more susceptible to xylem cavitation in severe drought. Drought and VPD effects for the P. deltoides clone were not ameliorated by long-term growth in elevated [CO(2)] compared with ambient [CO(2)], and plants in elevated [CO(2)] possessed anatomical traits that may result in greater stress associated with long-term drought.

A Comparison of Australian Open Water Body Evaporation Trends for Current and Future Climates Estimated from Class A Evaporation Pans and General Circulation Models

Abstract Trends of decreasing pan evaporation around the world have renewed interest in evaporation and its behavior in a warming world. Observed pan evaporation around Australia has been modeled to attribute changes in its constituent variables. It is found that wind speed decreases have generally led to decreases in pan evaporation. Trends were also calculated from reanalysis and general circulation model (GCM) outputs. The reanalysis reflected the general pattern and magnitude of the observed station trends across Australia. However, unlike the station trends, the reanalysis trends are mainly driven by vapor pressure deficit changes than wind speed changes. Some of the GCMs modeled the trends well, but most showed an average positive trend for Australia. Half the GCMs analyzed show increasing wind speed trends, and most show larger changes in vapor pressure deficit than would be expected based on the station data. Future changes to open water body evaporation have also been assessed using projections for two emission scenarios. Averaged across Australia, the models show a 5% increase in open water body evaporation by 2070 compared to 1990 levels. There is considerable variability in the model projections, particularly for the aerodynamic component of evaporation. Assumptions of increases in evaporation in a warming world need to be considered in light of the variability in the parameters that affect evaporation.

Toward using δ13C of ecosystem respiration to monitor canopy physiology in complex terrain

In 2005 and 2006, air samples were collected at the base of a Douglas-fir watershed to monitor seasonal changes in the delta13CO2 of ecosystem respiration (delta13C(ER)). The goals of this study were to determine whether variations in delta13C(ER) correlated with environmental variables and could be used to predict expected variations in canopy-average stomatal conductance (Gs). Changes in delta13C(ER) correlated weakly with changes in vapor pressure deficit (VPD) measured 0 and 3-7 days earlier and significantly with soil matric potential (psi(m)) (P value <0.02) measured on the same day. Midday G (s) was estimated using sapflow measurements (heat-dissipation method) at four plots located at different elevations within the watershed. Values of midday Gs from 0 and 3-7 days earlier were correlated with delta13C(ER), with the 5-day lag being significant (P value <0.05). To examine direct relationships between delta13C(ER) and recent Gs, we used models relating isotope discrimination to stomatal conductance and photosynthetic capacity at the leaf level to estimate values of stomatal conductance ("Gs-I") that would be expected if respired CO2 were derived entirely from recent photosynthate. We compared these values with estimates of Gs using direct measurement of transpiration at multiple locations in the watershed. Considering that the approach based on isotopes considers only the effect of photosynthetic discrimination on delta13C(ER), the magnitude and range in the two values were surprisingly similar. We conclude that: (1) delta13C(ER) is sensitive to variations in weather, and (2) delta13C(ER) potentially could be used to directly monitor average, basin-wide variations in Gs in complex terrain if further research improves understanding of how delta13C(ER) is influenced by post-assimilation fractionation processes.

WINE GRAPE RESPONSE TO KAOLIN PARTICLE FILM UNDER DEFICIT AND WELL-WATERED CONDITIONS

Kaolin particle film was developed to reduce heat stress and improve water use efficiency in perennial fruit crops. A study was done to determine how the film impacts water relations of wine grape (I/ills vinijera L.) cultivars '%iognier' and Merlot' grown without rootstock in the high desert region of southwestern Idaho. Vines were either sprayed with kaolin or unsprayed (control) and either well- watered at 100% estimated crop evapotranspiration (ET,), or deficit-irrigated at 35% ET until veraison followed by 70% ET until harvest. As expected, kaolin reduced leaf temperature under both irrigation regimes. However, a reduction in stomatal conductance and less negative leaf water potential values for kaolin-treated vines were observed only under well-watered conditions. Lack of response to kaolin under deficit water conditions was probably due to a water-stress-induced increase in stomatal closure that was independent of leaf temperature. When leaf temperature difference between kaolin and non-kaolin sprayed leaves was at its daily maximum, deficit-irrigated vines had already reached their minimum or most negative values for stomatal conductance and leaf water potential, but values for well-watered vines had just begun to change. Kaolin had no effect on canopy total light interception, pruning weight, yield, berry titratable acidity or soluble solids concentration, though cluster weight of well-watered, kaolin-sprayed Viognier' was larger than well-watered non-kaolin 'Viognier' vines. The leaf water potential of 'Merlot' was more responsive to diurnal changes in vapor pressure deficit than 'Viognier' vet neither showed a response to foliar kaolin application under deficit irrigation. Better understanding of the mechanism by which kaolin film increases water use efficiency will provide insight as to the relative roles of plant hormones, evaporative demand, and plant water status on leaf gas exchange and facilitate determination of its potential for enhancing production efficiency of wine grapes in arid regions.

Grape vine varieties Shiraz and Grenache differ in their stomatal response to VPD: apparent links with ABA physiology and gene expression in leaf tissue

Two cultivars of Vitis vinifera L., namely Grenache and Shiraz, have been described as having near-isohydric and near-anisohydric responses respectively to soil water stress (Schultz, Plant Cell and Environment, 26, 1393–1405, 2003). Given that contrast in sensitivity to soil water, a question arises as to whether atmospheric moisture stress will elicit similar differences. The present study was undertaken to investigate this issue by comparing stomatal responses in these same two cultivars to contrasting vapour pressure deficit (VPD). Test material included field grape vines in the Barossa Valley and pot-grown vines under partial shade in Adelaide. Our experiments showed that the same isohydric/anisohydric distinction as described by Schultz (2003) is apparent in leaf responses to atmospheric moisture stress. In the more isohydric cultivar, Grenache, stomatal conductance is more responsive to changes in VPD. This heightened sensitivity (compared with Shiraz) appears to be associated with higher levels of abscisic acid (ABA) in Grenache xylem sap. Expression studies on the key genes in the ABA biosynthetic pathway indicate that regulation of the V.v.nced1 gene expression in leaf tissue, but not in the root tissues, is associated with the changes in the xylem sap ABA. Moreover, the two cultivars (Grenache and Shiraz) differed with respect to both scale and time course of those responses. We conclude that these two Vitis vinifera cultivars do indeed differ significantly in the way that they respond to potentially stressful atmospheric conditions, and that ABA physiology is a key process in these contrasting responses. An understanding of such mechanisms, including the relative importance of roots and shoots in determining vine response to abiotic stress, is highly relevant to irrigation scheduling, and to management of associated variation in vineyard productivity across diverse environments.

Simulation of carbon isotope discrimination of the terrestrial biosphere

We introduce a multistage model of carbon isotope discrimination during C3 photosynthesis and global maps of C3/C4 plant ratios to an ecophysiological model of the terrestrial biosphere (SiB2) in order to predict the carbon isotope ratios of terrestrial plant carbon globally at a 1° resolution. The model is driven by observed meteorology from the European Centre for Medium‐Range Weather Forecasts (ECMWF), constrained by satellite‐derived Normalized Difference Vegetation Index (NDVI) and run for the years 1983–1993. Modeled mean annual C3 discrimination during this period is 19.2‰; total mean annual discrimination by the terrestrial biosphere (C3 and C4 plants) is 15.9‰. We test simulation results in three ways. First, we compare the modeled response of C3 discrimination to changes in physiological stress, including daily variations in vapor pressure deficit (vpd) and monthly variations in precipitation, to observed changes in discrimination inferred from Keeling plot intercepts. Second, we compare mean δ13C ratios from selected biomes (Broadleaf, Temperate Broadleaf, Temperate Conifer, and Boreal) to the observed values from Keeling plots at these biomes. Third, we compare simulated zonal δ13C ratios in the Northern Hemisphere (20°N to 60°N) to values predicted from high‐frequency variations in measured atmospheric CO2 and δ13C from terrestrially dominated sites within the NOAA‐Globalview flask network. The modeled response to changes in vapor pressure deficit compares favorably to observations. Simulated discrimination in tropical forests of the Amazon basin is less sensitive to changes in monthly precipitation than is suggested by some observations. Mean model δ13C ratios for Broadleaf, Temperate Broadleaf, Temperate Conifer, and Boreal biomes compare well with the few measurements available; however, there is more variability in observations than in the simulation, and modeled δ13C values for tropical forests are heavy relative to observations. Simulated zonal δ13C ratios in the Northern Hemisphere capture patterns of zonal δ13C inferred from atmospheric measurements better than previous investigations. Finally, there is still a need for additional constraints to verify that carbon isotope models behave as expected.

Optimization of irrigation water use in grapevines using the relationship between transpiration and plant water status

Abstract The study was carried out in order to evaluate the thresholds of grapevine internal water status as well as to propose an irrigation management technique that permits the uninterrupted photosynthetic process under drought conditions. The relationships between water relation, gas exchange parameters and sap flow measurements were examined in field grown grapevines ( Vitis vinifera L., cv. Malagouzia) subjected to various levels of water stress. The differences in stem water potential were greater between the treatments compared to leaf water potential indicating that stem water potential represents a more reliable indicator of plant water status. Stomatal conductance and photosynthetic rate were significantly lower in stressed treatments while stomatal sensitivity to changes in vapor pressure deficit seems to be higher in stressed than in well-watered treatment. Maximum diurnal sap flow rates were recorded early in the morning in all treatments and decreased in water stressed treatments over the stress cycle. The SF Ti /SF T1 ratio (SF T1 = mean daily sap flow of the fully irrigated plants; SF Ti = mean sap flow of the stressed plants) also decreased in water stressed treatments. The close relationship between the SF Ti /SF T1 ratio and stem water potential values could be used in developing a sap flow based technique for automatically controlling the irrigation system in field grown grapevines.

Evapotranspiration and crop coefficients of irrigated garlic ( Allium sativum L.) in a semi-arid climate

Despite the importance of garlic ( Allium sativum L.) among vegetable crops, very little information is currently available on its crop water requirements which depend mainly on crop evapotranspiration (ET). The objective of this work was to measure ET of garlic crops and to develop a simple model for its calculation based on the crop coefficient approach. Measurements of ET were performed in two commercial crops of garlic grown in Córdoba (Spain) in 2000 and Ecija (Spain) during 2002. The crop coefficient reached a maximum between 1.2 and 1.3 from canopy closure to maximum leaf area index (LAI), and then declined to values around 0.6 at the end of the growing season. These changes were parallel to changes in surface conductance which showed a maximum around 35 mm/s and decreased to values below 10 mm/s at the end of the growing season, partly due to leaf aging and leaf senescence. The decrease in surface conductance was also related to changes in vapor pressure deficit (VPD), which suggests that the maximum K c of garlic crops should decrease when VPD increases. Seasonal water-use efficiency was 2.8 g dry matter per kg ET which corresponded to around 5 g fresh product per kg ET. Sensitivity of garlic ET to changes in both aerodynamic and surface conductance were rather low which may allow the direct application of the Penman–Monteith equation to calculate crop ET.

Low vapour pressure deficit reduces the beneficial effect of elevated CO2 on growth of N2 -fixing alfalfa plants

Plant responses to elevated CO 2 can be modified by many environmental factors, but very little attention has been paid to the interaction between CO 2 and changes in vapour pressure deficit (VPD). Thirty-day-old alfalfa plants (Medicago saliva L. cv. AragOn), which were inoculated with Sinorhizobium meliloti 102F78 strain, were grown for 1 month in controlled environment chambers at 25/15°C, 14 h photoperiod, and 600 μmol m - 2 s - 1 photosynthetic photon flux (PPF), using a factorial combination of CO 2 concentration (400 μmol mol - 1 or 700 μmol mol - 1 ) and vapour pressure deficit (0.48 kPa or 1.74 kPa, which corresponded to relative humidities of 85% and 45% at 25°C, respectively). Elevated CO 2 strongly stimulated plant growth under high VPD conditions, but this beneficial effect was not observed under low VPD. Under low VPD, elevated CO 2 also did not enhance plant photosynthesis, and plant water stress was greatest for plants grown at elevated CO 2 and low VPD. Moreover, plants grown under elevated CO 2 and low VPD had a lower leaf soluble protein and photosynthetic activity (photosynthetic rate and carboxylation efficiency) than plants grown under elevated CO 2 and high VPD. Elevated CO 2 significantly increased leaf adaxial and abaxial temperatures. Because the effects of elevated CO 2 were dependent on vapour pressure deficit, VPD needs to be controlled in experiments studying the effect of elevated CO 2 as well as considered in the extrapolations of results to a warmer, high-CO 2 world.

Diurnal and spatial variation of xylem dielectric constant in Norway Spruce (Picea abies [L.] Karst.) as related to microclimate, xylem sap flow, and xylem chemistry

Spatial and temporal variations in vegetation dielectric properties strongly influence the microwave backscatter characteristics of forested landscapes. This paper examines the relationship between xylem tissue dielectric constant, xylem sap flux density, and xylem sap chemical composition as measured in the stems of two Norway Spruce (Picea abies [L.] Karst.) trees in the Fichtelgebirge region of Northern Bavaria, Germany. Dielectric constant and xylem sap flux were monitored continuously from June through October 1995, at several heights along the tree trunks. At the end of the measurement series, each tree was harvested, and its xylem sap extracted and analyzed to determine the concentrations of amino acids and cations. Results show that the sap flux density was correlated with vapor pressure deficit (VPD) at all heights in the stem. In contrast, the xylem tissue dielectric constant is influenced by VPD but can exhibit a significant temporal lag relative to changes in VPD. This lag varies with position along the tree trunk. The temporal variability of the dielectric constant is compared with both trees at several positions along the tree trunks. Results of xylem sap chemical analysis are presented. We show that spatial and temporal variability in the xylem tissue dielectric constant is influenced not only by water content, but by variations in xylem sap chemistry as well. This has important implications for microwave remote sensing of forested landscapes, as useful information may be acquired regarding stand physiology and water relations and where variations in dielectric properties within individual trees and across geographic areas can be significant error sources for forest inventory mapping.

The effect of vapor pressure deficit on maize transpiration response to a drying soil

A decline in plant transpiration has been widely observed to occur within a fairly stable range of threshold values of fraction transpirable soil water (FTSW), usually 0.3–0.4. However, the stability of this function has not been compared at various levels of atmospheric vapor pressure deficit (VPD). Soil hydraulic conductivity is likely to be involved in determining the threshold where water supply is limiting. Thus, it was hypothesized that at a high VPD resulting in increased transpiration rates, the FTSW threshold for the decline of transpiration rates as a result of drying soil would be increased. This study was undertaken in controlled environment chambers with two maize (Zea mays L.) hybrids (Pioneer Brand Hybrids `3165' and `3737') so as to subject plants to four VPD levels (1.1, 2.0, 2.9 and 3.6 kPa) during a soil drying experiment. In contrast to the original hypothesis, there was little (≤ 0.05 FTSW) change in the threshold FTSW in response to increased VPD for either hybrid. In fact, over the narrow 0.31–0.38 FTSW range observed, the two hybrids showed opposite trends in FTSW threshold as VPD increased. These results supported the view that the FTSW threshold for the decline in transpiration with drying soil is stable, showing little sensitivity to changes in VPD.

Rapid Equilibration of Leaf and Stem Water Potential under Field Conditions in Almonds, Walnuts, and Prunes

Covering a plant leaf with a reflective, water impervious bag ensures that equilibrium is reached between the nontranspiring leaf and the stem, and appears to improve the accuracy of determining plant water status under field conditions. However, the inconvenience of covering the leaf for 1 to 2 hours before measuring stem water potential (SWP) has constrained on-farm adoption of this irrigation management technique. A second constraint has been that the requirement of midafternoon determinations limits the area that can be monitored by one person with a pressure chamber. This paper reports findings from field studies in almonds (Prunus dulcis),prunes (P. domestica), and walnuts (Juglans regia) demonstrating modified procedures to measure midday SWP, making it a more convenient and practical tool for irrigation management. For routine monitoring and irrigation scheduling, an equilibration period of 10 min or longer appears to be suitable to provide accurate SWP measurements. Based on the large sample sizes in this study, we estimate that measurement error related to equilibration time for SWP can be reduced to an acceptable level [0.05 MPa (0.5 bar)] with a sample size of about 10 leaves when using a 10-min equilibration period. Under orchard conditions where tree growth and health appears uniform, a sample of one leaf per tree and 10 trees per irrigation management unit should give an accurate mean indicator of orchard water status. Under more variable orchard conditions a larger sample size may be needed. Midmorning and midday SWP both exhibited similar seasonal patterns and responded alike to irrigation events. On some occasions, midday SWP was accurately predicted from midmorning SWP and the change in air vapor pressure deficit (VPD) from midmorning to midday, but both over- and underestimate errors [to 0.3 MPa (3.0 bar)] appeared to be associated with unusually low or high diurnal changes in VPD, respectively. Hence, direct measurement of SWP under midday conditions (about 1300 to 1500 hr) is still recommended.