Vapor Pressure Deficit Conditions Research Articles

Influence of soil water status and atmospheric vapor pressure deficit on leaf gas exchange in field-grown winter wheat

Abstract Soil water status and atmospheric vapor pressure deficit (VPD) are important environmental parameters that influence plant gas exchange. The objective of this study was to investigate the effect of soil water potential (Ψs) on the responses of gas exchange to VPD in four field-grown winter wheat (Triticum aestivum L.) cultivars. The gas exchange parameters: net CO2 assimilation rate (An), stomatal conductance (gs), transpiration rate (E), and intercellular CO2 concentration (ci) were measured under different VPD and Ψs conditions during the period from heading to the middle of grain filling. The soil water potential had a significant effect on the responses of gas exchange parameters to VPD. At high Ψs (−0.09 MPa), An did not respond to increased VPD. However, An was sensitive to increased VPD at low Ψs, and the most pronounced decrease in An as VPD increased was observed under severe water stress (Ψs=−0.7 MPa). Stomatal conductance decreased with increased VPD even at high Ψs (−0.09 MPa). Transpiration rate increased as VPD increased at high Ψs, but decreased as VPD increased under low Ψs. The response of gs to VPD cannot be explained as feedback mechanism, i.e. the decrease in gs is not due to increase in E at high VPD. The response of gs to VPD in this study was consistent with a feedforward mechanism. Among the four cultivars, Karl 92 had the lowest An, gs, and E, while the other three cultivars (Arapahoe, Cheyenne, and Scout 66) had higher An, gs, and E than Karl 92 under low Ψs and high VPD conditions.

Regulation of canopy conductance and transpiration and their modelling in irrigated grapevines

Whole-vine transpiration was estimated for well-watered nine-year-old Sultana grapevines (Vitis vinifera L. cv. Sultana) from xylem sap flow measured with Granier's heat-dissipation probes. Canopy conductance of the grapevine was calculated by inverting the Penman-Monteith equation. Transpiration from grapevine canopies was strongly controlled by the canopy conductance. Canopy conductance decreased exponentially with increasing vapour pressure deficit (VPD) except in the morning when solar radiation was less than 200 W m-2 and the canopy conductance was predominantly limited by the solar radiation. A non-linear model of canopy conductance as a function of the solar radiation and VPD explained > 90% of the variation observed in canopy conductance. Under contrasting VPD conditions (daytime maximum of 3 kPa vs 8 kPa), grapevines were able to regulate their canopy conductance from 0.006 to 0.001 m s-1 to maintain a near constant transpiration. Whole-canopy transpiration calculated from modelled canopy conductance using the Penman-Monteith equation was highly correlated with the measured transpiration (sap flow) values over the range of 0-0.20 mm h-1 (R2 > 0.85). Cross-validation shows that these mechanistic models based on solar radiation and VPD provide good predictions of canopy conductance and transpiration under the conditions of the study.

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.

Effects of Soil Drought and Atmospheric Humidity on Yield, Gas Exchange, and Stable Carbon Isotope Composition of Barley

The combined effects of water status, vapour pressure deficit (VPD), and elevated temperature from heading to maturity were studied in barley. Plants growing at high VPD, either under well-watered or water deficit conditions, had higher grain yield and grain filling rate than plants growing at low VPD. By contrast, water stress decreased grain yield and individual grain dry matter at any VPD. Water regime and to a lesser extent VPD affected δ13C of plant parts sampled at mid-grain filling and maturity. The differences between treatments were maximal in mature grains, where high VPD increased δ13C for both water regimes. However, the total amount of water used by the plant during grain filling did not change as response to a higher VPD whereas transpiration efficiency (TE) decreased. The net photosynthetic rate (PN) of the flag leaves decreased significantly under water stress at both VPD regimes. However, PN of the ears was higher at high VPD than at low VPD, and did not decrease as response to water stress. The higher correlation of grain yield with PN of the ear compared with that of the flag leaf support the role of ear as the main photosynthetic organ during grain filling under water deficit and high VPD. The deleterious effects of combined moderately high temperature and drought on yield were attenuated at high VPD.

Ecophysiology of the Soft Tree Fern, Dicksonia antarctica Labill

Abstract Environmental constraints on gas exchange, stomatal conductance and water relations were investigated in the Soft Tree Fern, Dicksonia antarctica, at sites across its natural distribution and in the glasshouse. Dicksonia antarctica exhibited strong stomatal response down to a vapour pressure deficit (VPD) of 0.25 kPa, an unusual characteristic when compared with other ground fern species. Net photosynthetic rate may be a response of the microenvironment prevalent during frond development, reflecting acclimatory capacity. Both these ecophysiological characteristics are consistent with the ecological niche of D. antarctica, a long‐lived, fire‐resistant species that, during its lifetime, may be exposed to: (i) a humid environment beneath a rainforest canopy; and (ii) an exposed environment following wildfire. Maximum net photosynthesis and quantum yield of photosynthesis correlated strongly with VPD and the maximum net photosynthetic rate of 10.8 µmol m−2 s−1 was the highest yet recorded for a fern. These observations are consistent with the relatively low growth typically observed in D. antarctica on sunny, exposed sites and vice versa on cool, humid sites exposed to sunflecks. Favourable water relations maintained under conditions of moderate VPD (2.03 kPa) were probably due to stomatal control. However, inadequate rainfall or high VPD (4.98 kPa) caused water stress, recovery of which was limited by slow water transport through fronds. These observations are consistent with the limitation of D. antarctica distribution to sites sheltered from hot winds and with reliable water supply. The funnel‐shaped rosette of fronds of D. antarctica may harvest rainfall and make it accessible to aerial roots situated at the base of fronds. This process may maintain favourable water relations independently of a subterranean root system. This proposed strategy of water acquisition is unique for a fern species and may eliminate a need for soil moisture competition with surrounding plant species. It is suggested that the ecophysiological characteristics observed in D. antarctica in this study may contribute to the ecological niche it occupies, which is characterized by a variable environment.

Measurement of evapotranspiration in a winter wheat field

AbstractDaily evapotranspiration from a winter wheat field on the North China Plain measured by large‐scale weighing lysimeter was linearly related to that measured by the Bowen ratio energy balance (BREB) technique. Soil evaporation averaged about 23·6% of evapotranspiration from the post‐winter dormancy revival stage to the grain ripening stage in 1999. On clear days during winter dormancy, about half of the net radiation flux Rn was used to warm soil. During the revival stage, conductive heat flux G also used most of the incoming Rn, but the ratio of latent heat flux λE to Rn increased. During the stem‐extension stage, λE was about 50% of Rn; thereafter, λE/Rn increased continually, but G remained less than 10% of Rn. During the ripening stage, λE was almost 90% of Rn. Evaporative fraction (EF) can be expressed as a function of plant status and atmospheric boundary layer conditions. The relationship between EF and available energy under moderate air temperature and vapour pressure deficit conditions was examined for five combinations of aerodynamic and canopy conductance. Although the theoretical relationship indicates that EF should be highly correlated to soil water content, the correlation has been difficult to identify under field conditions. However, we observed that there exists a threshold value of Rn − G, above which EF is less than 1·0, and that the threshold value is lower under soil‐water deficit conditions than under abundant soil‐water conditions. Copyright © 2002 John Wiley & Sons, Ltd.

Genetic variation in summer gas exchange patterns of interior spruce (Picea glauca (Moench) Voss times Picea engelmannii Parry ex Engelm.)

Interior spruce (Picea glauca (Moench) Voss x Picea engelmannii Parry ex Engelm.) somatic seedlings from a range of clones were measured for gas exchange processes in relation to summer atmospheric parameters. Carbon isotope composition ( delta 13C) of needles was compared with gas exchange parameters and intrinsic water use efficiency (WUEi). Needle conductance (gwv) decreased as vapour pressure deficit (VPD) increased. Clone T703 had the lowest gwv level across all VPD conditions, with clones G351, N366, and W460 having significantly greater gwv than clone T703 at VPD levels <2.0 kPa. Response of net photosynthesis (Pn) to photosynthetically active radiation (PAR) for all clones showed Pn to increase to PAR levels of around 1000 µmol ·m-2 ·s-1 and then, Pn rates were stable at higher PAR levels. Clones N366 and W460 had significantly greater Pn than clone T703 at all PAR levels. All clones showed Pn to decline as VPD increased, with clone W460 having the highest predicted Pn level across all VPD conditions and significantly greater Pn than clone T703. Higher WUEi was related to an increase in delta 13C, with clone T703 having the highest WUEi and delta 13C, followed by clone W460. Changes in delta 13C, for all clones, indicated a stronger relationship with gwv than with Pn.

096 QUANTIFYING THE EFFECT OF SUPPLEMENTAL LIGHTING ON PLANT TEMPERATURES IN GREENHOUSES

The effects of supplemental lighting on vinca (Catharanthus roseus L.) plant temperature were quantified in greenhouses maintained at air temperatures of 15. 25, and 35C. High-pressure sodium (HPS) lamps delivering 100 μmol·m-2·s-1 PPF provided 73 W · m-2 of total radiation (400 to 50,000 nm) to lighted plants. Plant shoot-tip temperature was measured by using 40-gauge thermocouples. Relative to air temperature, plant shoot-tip temperature depended on the irradiance and vapor-pressure deficit (VPD). Irrespective of VPD, the additional irradiance absorbed by plants under the HPS lamps increased plant temperature 1 to 2°C. Under relatively low VPD conditions (1 kPa), plant temperature was greater than air temperature, while under high VPD conditions (4 to 5 kPa), temperature of both lighted and unlighted plants remained below air temperature throughout the day. Temperature of lighted plants however, remained 1 to 2°C above that of unlighted plants. Analysis of a degree-day model of vinca development showed hastened development associated with supplemental lighting could be explained by increased plant temperature rather than any specific photosynthetic effect.

Inflence of dormancy induction treatments on the photosynthetic response of field planted western hemlock seedlings

Abstract Nursery cultural treatments can improve field performance by pre-acclimatizing seedlings to field site conditions. Four dormancy induction treatments (DIT) (i.e. long-day wet, long-day dry, short-day wet, and short-day dry) were examined for their influence on net photosynthesis ( P n ) of western hemlock ( Tsuga heterophylla (Raf.) Sarg.) seedlings. Seedlings were planted on a control site with high soil water potential, adequate nitrogen levels and no plant competition. Net photosynthesis was measured in response to a full range of photosynthetically active radiation (PAR) and vapour pressure deficit (VPD) conditions on 1-year-old and current-year needles. The response of P n to both PAR and VPD was defined by response surface models used to forecast field performance over the range of anticipated environmental conditions. All models had greater P n values as PAR increased, but increased VPD resulted in a suppression of P n . For 1-year-old needles, short-day compared with long-day seedlings, had greater or equivalent P n response over the range of measured PAR and VPD conditions. For current-year needles, short-day treated seedlings had greater P n response at high VPD (e.g. 3.8 kPa). Current-year, compared with 1-year-old needles had from 45 to 110% greater P n over a wide range of PAR (0.5–2.2 mmol m −2 s −1 ) and VPD (0.5–2.4 kPa) conditions. The results were compared with the testing of these DIT on a reforestation site to determine the validity of measuring physiological responses on a control site as a method for improving nursery cultural treatment selections.

A Physiological Comparison of Leaves and Phyllodes in Acacia melanoxylon

Leaves and phyllodes of A. melanoxylon were compared in several aspects of their physiology. Changes in gas exchange and water use efficiency (WUE) under controlled conditions of vapour pressure deficit (vpd) and foliar water potential were examined. Water use efficiency in phyllodes remained constant under a wide range of evaporative demand due to high stomatal sensitivity to vpd. Leaf stomata were less sensitive to changes in vpd causing decreased WUE with increased vpd. Under water stress phyllodes survived longer and produced higher WUE than leaves. Maximum photosynthetic rates per unit foliar area were higher in phyllodes than leaves. Thus, phyllodes have a number of advantages over leaves under conditions of long and short term water stress and high irradiance. Leaves had a higher photosynthetic rate per unit of photosynthetic investment than phyllodes, suggesting that their function is to maximise growth during the seedling phase.

Evaluating canopy temperature-based indices for irrigation scheduling

Since the development of commercial versions of infrared sensors, they have been increasingly used to determine canopy temperature and schedule irrigations. However, some shortcomings of the technique have been identified, among them the sensitivity of canopy temperature measurements to weather fluctuations. Based on field and computer simulated data, an analysis of the suitability of crop water stress indices (CWSI's) developed from canopy temperature under variable weather conditions was done. Important day to day fluctuations of CWSI values determined using an empirical baseline (empirical CWSI) appeared common for nonstressed crops, particularly under low vapor pressure deficit conditions. These fluctuations generate uncertainty in the use of this empirical index to determine needs for irrigation. The use of an improved index (theoretical CWSI) requiring measurements of net radiation, soil heat flux and wind speed, and estimates of aerodynamic and canopy resistances reduced but did not eliminate these fluctuations. Results using a simulation model showed that the empirical CWSI provided late indication of irrigation needs, after some water stress has developed, which may limit its application for crops sensitive to water stress. These simulations also indicated that the theoretical CWSI was able to track the development of water stress and provide reasonable indication of irrigation needs. However, this result may not be fully realized in field applications where the determination of CWSI may be affected by various sources of variability which are not accounted for by the model.

Promotion of Ca Accumulation in Inner Leaves by Air Supply for Prevention of Lettuce Tipburn

The Ca accumulation in the leaves of lettuce was determined by 45Ca tracer measurements to study the effect of air supply to the inner leaves which were susceptible to tipburn, a Ca-related physiological disorder. Lettuce was exposed in a growth cabinet to various conditions of vapor pressure deficit (VPD), flow rate of air supply, and treatment time of air supply. Air was blown onto the inner developing leaves around a growing point. Without air supply in the light period, Ca accumulation in the outer leaves 7-10 cm in length increased with the increase in VPD in the cabinet air, while that in the inner leaves 1-3 cm long was not affected by VPD in the cabinet air and was extremely low. When air was supplied at a flow rate of 160 mL/min, Ca accumulation in the inner leaves was increased 4.6 times over that in the control. In the dark period, Ca accumulation increased in both the inner and the outer leaves when air was supplied. Air supply was shown to prevent tipburn by increasing Ca accumulation in the inner leaves, because it increased VPD in the air around the inner leaves, encouraged transpiration from the leaves, and consequently promoted Ca uptake from the root to the inner leaves.

EFFECT OF MANIPULATION OF WATER AND NITROGEN SUPPLIES ON THE QUANTITATIVE PHENOLOGY OF LARREA TRIDENTATA (CREOSOTE BUSH) IN THE SONORAN DESERT OF CALIFORNIA

Two years of water and nitrogen augmentation experiments on Larrea tridentata (creosote bush) were carried out in a southern Californian warm desert wash plant community. Treatments consisted of control (C), water (W), water and soil nitrogen (W + SN), and soil nitrogen (SN). Quantitative phenological data and microclimatic measurements were collected prior to the onset of and during the growth period and treatments. Predawn and midday water potentials were lower in nonirrigated than irrigated individuals. Leaf conductance was higher in irrigated than in nonirrigated shrubs, with a maximum difference of 1 cm s‐1 observed in July 1984 under relatively low vapor pressure deficit conditions. Leaf production rates were significantly higher in the irrigated (W and W + SN) treatments than in the nonirrigated (C and SN) treatments in 1984. Addition of soil nitrogen caused no increased in vegetative growth rates in 1984. In 1985, a drier year, there was only minimal growth during the spring and summer growth periods in the nonirrigated treatments, while the W and W + SN treatments resulted in significantly higher leafand shoot growth rates. Growth rates in 1985 were significantly higher in the W + SN treatment than in the W treatment. Reproductive growth was higher in the nonirrigated than the irrigated treatments, with the lowest reproductive activity noted in the W treatment.

Problems with porometry: measuring stomatal conductances of potentially transpiring plants

Abstract Porometer measurements of the stomatal conductances ( C s ) of potentially transpiring water hyacinth plants at Phoenix, Arizona in October of 1984, May–June of 1985, and September of 1986 indicate that C s steadily drops as the vapor pressure deficit ( VPD ) of the air in the measuring system's cuvette or leaf chamber rises. Utilizing this relationship to calculate the foliage-air temperature differential ( T F − T A ) response of these leaves to leaf-chamber air VPD , as per the basic equations of standard heat and water vapor transport theory; we obtain a leaf-chamber “non-water-stressed baseline” that is consistent with leaf-chamber measurements of T F − T A vs. air VPD . Free-air T F − T A vs. air VPD data, on the other hand, produce a relationship that is similarly consistent with a plant stomatal conductance which is invariant with respect to the air VPD . Hence, we conclude that the very act of stomatal conductance measurement alters a potentially transpiring plant's evaporative water loss rate in such a way that, for very high air VPD conditions, the directly measured C s value (although correct for the leaf in the cuvette or leaf chamber) may be much reduced from that characteristic of comparable non-chamber-encumbered plants in the free air. We then demonstrate that this instrument-induced reduction in directly measured C s values is a unique function of the leaf-chamber IJ index, evaluated with respect to the plant's free-air non-water-stressed baseline. Similar results obtained by others for cotton suggest that this phenomenon may be quite general, and that the C s vs. air VPD interaction, believed by many to be widely operative throughout the plant kingdom, may not really exist in actual field situations.

The effects of temperature, vapour pressure deficit and soil moisture stress on growth, flowering and fruit set of custard apple ( Annona cherimola × Annona squamosa) ‘African Pride’

The effects of temperature, vapour pressure deficit (VPD) and soil moisture stress on growth, flowering and fruit set of custard apple (Annona cherimola × Annona squamosa) cultivar ‘African Pride’ were evaluated under controlled-environment conditions. Flowering and fruit set were affected not only by the environmental conditions during flowering, but also by vegetative flushing of the tree. Flowering was closely associated with vegetative flushing. The level of natural fruit set under controlled-environment conditions, irrespective of temperature or humidity, was low (<3.1%) compared to field or container-grown trees flowering under ambient conditions (11 and 28%, respectively). The increased set under ambient conditions appears to be due to the presence of insect pollinators, in particular Nitidulid beetles which were observed in the flowers. High VPD (1.2 kPa) severely reduced both flowering and fruit set. Soil moisture stress (mid-day leaf water potential −2.0 MPa) also reduced flowering and fruit set, particularly at high temperature (28°C). Hand-pollination of flowers, even under high VPD conditions, resulted in a significantly higher set (>20%). Productivity of custard apple in the field may be improved by cultural techniques such as overhead misting, windbreaks and efficient irrigation scheduling.

Transpiration characteristics of banana leaves (cultivar ‘Williams’) in response to progressive depletion of available soil moisture

Abstract Young parent-crop banana plants (cultivar ‘Williams’) were subjected to two soil moisture potential (SMP) regimes, namely, continuous field capacity and progressive depletion of total available soil moisture (TAM). Experiments were conducted in a fibreglass tunnel under mild or enhanced conditions of vapour pressure deficit (VPD), and also in the field during summer. Transpiration rates, stomatal resistance, photosynthetically-active radiation and leaf temperature were measured simultaneously with a steady-state porometer, according to a developed technique which minimised natural sources of error. Under mild conditions of VPD (± 5 hPa) plants responded slowly to soil drying, and at a SMP of −82 kPa, transpiration was reduced by 27% compared to fully-irrigated plants. Under enhanced VPD (15–20 hPa), plants responded rapidly to soil drying, and at a SMP of −75 kPa transpiration was reduced by 73%. In the field, reduction of transpiration in response to SMP at high VPD followed a similar pattern to the covered experiment. At a SMP of −46 kPa the reduction in transpiration rate was 41% in the late afternoon at high VPD, but the following morning transpiration rates of the two treatments were similar due to lower VPD and overnight re-establishment of turgor. At −525 kPa SMP, reduction of transpiration was 70% and plants could not regain turgor overnight. Stomatal conductance seemed more sensitive to water stress than transpiration, but leaf temperature only differentiated advanced levels of stress (± 2°C difference between stressed and non-stressed leaves). These results hold important implications for irrigation scheduling of bananas. Initiation of stress occurred at a SMP of around −20 to −25 kPa which occurred at a 20% depletion of TAM, thus necessitating a summer irrigation frequency of daily or every 2 days.

Relationship between foliage temperature and water stress in potatoes

Field studies were conducted in southern Idaho to evaluate the possibility of using thermal infrared measurements of potato foliage to detect soil water deficits. Concurrent measurements of foliage-air temperature differences (Tf-Ta), leaf water potential (gyleaf) and vapor pressure deficit (VPD) were obtained from differentially-irrigated Russet Burbank and Kennebec potatoes during the 1982 and 1983 growing seasons. Foliageair temperature differences for well-watered potatoes were linearly related to VPD. Differences in Tf-Ta values between stressed and well-watered potatoes were relatively small in the early morning when evaporative demand was low. However, severe soil water deficits caused afternoon Tf-Ta values to rise as much as 8.0 C above non-stressed levels under conditions of high VPD.

Sensitivity of a soil-plant-atmosphere model to changes in air temperature, dew point temperature, and solar radiation

Abstract Air temperature, dew point temperature and solar radiation were independently varied in an hourly soil—plant—atmosphere model in a sensitivity analysis of these parameters. Results suggested that evapotranspiration in eastern Tennessee is limited more by meteorological conditions that determine the vapor-pressure gradient than by the necessary energy to vaporize water within foliage. Transpiration and soil water drainage were very sensitive to changes in air and dew point temperature and to solar radiation under low atmospheric vapor-pressure deficit conditions associated with reduced air temperature. Leaf water potential and stomatal conductance were reduced under conditions having high evapotranspiration. Representative air and dew point temperature input data for a particular application are necessary for satisfactory results, whereas irradiation may be less well characterized for applications with high atmospheric vapor-pressure deficit. The effects of a general rise in atmospheric temperature on forest water budgets are discussed.