Responses Of Leaf Conductance Research Articles

Simulating nectarine tree transpiration and dynamic water storage from responses of leaf conductance to light and sap flow to stem water potential and vapor pressure deficit.

For isohydric trees mid-day water uptake is stable and depends on soil water status, reflected in pre-dawn leaf water potential (Ψpd) and mid-day stem water potential (Ψmd), tree hydraulic conductance and a more-or-less constant leaf water potential (Ψl) for much of the day, maintained by the stomata. Stabilization of Ψl can be represented by a linear relationship between canopy resistance (Rc) and vapor pressure deficit (D), and the slope (BD) is proportional to the steady-state water uptake. By analyzing sap flow (SF), meteorological and Ψmd measurements during a series of wetting and drying (D/W) cycles in a nectarine orchard, we found that for the range of Ψmd relevant for irrigated orchards the slope of the relationship of Rc to D, BD is a linear function of Ψmd. Rc was simulated using the above relationships, and its changes in the morning and evening were simulated using a rectangular hyperbolic relationship between leaf conductance and photosynthetic irradiance, fitted to leaf-level measurements. The latter was integrated with one-leaf, two-leaf and integrative radiation models, and the latter gave the best results. Simulated Rc was used in the Penman-Monteith equation to simulate tree transpiration, which was validated by comparing with SF from a separate data set. The model gave accurate estimates of diurnal and daily total tree transpiration for the range of Ψmds used in regular and deficit irrigation. Diurnal changes in tree water content were determined from the difference between simulated transpiration and measured SF. Changes in water content caused a time lag of 90-105 min between transpiration and SF for Ψmd between -0.8 and -1.55 MPa, and water depletion reached 3 l h(-1) before noon. Estimated mean diurnal changes in water content were 5.5 l day(-1) tree(-1) at Ψmd of -0.9 MPa and increased to 12.5 l day(-1) tree(-1) at -1.45 MPa, equivalent to 6.5 and 16.5% of daily tree water use, respectively. Sixteen percent of the dynamic water volume was in the leaves. Inversion of the model shows that Ψmd can be predicted from D and Rc, which may have some importance for irrigation management to maintain target values of Ψmd. That relationship will be explored in future research.

Effects of enhanced hydraulic supply for foliage on stomatal responses in little-leaf linden (Tilia cordata Mill.)

Responses of leaf conductance (gL) to variation in environmental and plant hydraulic factors were examined on intact and detached shoots of little-leaf linden (Tilia cordata Mill.) with respect to branch position in the crown. Using detached shoots, we manipulated leaf water supply and light availability in order to separate the effects of insufficient hydraulic supply and low irradiance. The intact upper-crown leaves demonstrated 2.0–2.2 times higher (P < 0.001) daily maxima of gL compared to the lower-crown leaves growing in the shadow of upper branches. Mean soil-to-leaf conductance (GT) was 1.9 times higher (P < 0.001) for the upper-crown foliage compared to that of the lower crown. The total hydraulic resistance was distributed: soil to distal shoots—41–51%, 25-cm distal shoots—10–15% and leaves—39–44%. In lower branches, gL was constrained by both low light availability and limited water supply; in upper branches, only by irradiance. Artificial reduction of hydraulic constraints raised bulk leaf water potential (ΨL) and made gL less sensitive to changes in both atmospheric and plant factors. Stomatal responses to leaf-to-air vapour pressure difference (VPD) were significantly modified by leaf water status: high ΨL seemingly inverted the gL versus VPD relationship. Enhanced water supply increased gL and transpiration rate (E) in the lower-crown foliage, but not in the upper-crown foliage. The results support the idea that leaves in the lower canopy are hydraulically more constrained than those in the upper canopy.

Variation in leaf conductance of silver birch: effects of irradiance, vapour pressure deficit, leaf water status and position within a crown

Responses of leaf conductance ( g L) to variation in photosynthetic photon flux density ( Q P), leaf-to-air vapour pressure difference (VPD), bulk leaf water potential ( Ψ x) and soil to leaf hydraulic conductance ( G T) were studied in silver birch ( Betula pendula Roth) foliage with respect to leaf position within the canopy. The upper canopy leaves demonstrated 2.0–2.4 times higher ( P < 0.001) daily maxima of g L as compared to the lower-canopy leaves growing in the shadow of upper branches. Functional acclimation of the shade foliage occurred in the form of both a steeper initial slope of the light-response curve and a lower light-saturation point of g L. Leaf conductance decreased if Ψ x fell below certain values after the noon, while the sun foliage experienced greater negative water potentials than the shade foliage. In a diurnal scale the influence of bulk leaf water potential on g L altogether was rather weak. The lower-canopy foliage exhibited more conservative water-use behaviour, having lower maximum stomatal conductance and greater sensitivity to Ψ x bringing about a smaller responsiveness to VPD than the upper canopy foliage. The mean G T was 1.7–1.8 times bigger ( P < 0.001) for the upper canopy, compared to the lower canopy; the shade foliage responded more sensitively to changes in G T; there were steeper water potential gradients between the soil and lower-canopy leaves; g L in the lower-canopy foliage was more strictly controlled by leaf water status—the evidence suggesting that the shade foliage may be hydraulically more constrained. We set up a hypothesis that stomatal conductance at the base of the live crown is limited not only by low light availability but also by plant's inner hydraulic constraints.

Effects of light availability versus hydraulic constraints on stomatal responses within a crown of silver birch

Responses of leaf conductance (gL) to variation in photosynthetic photon flux density (QP), leaf-to-air vapour pressure difference (VPD), bulk leaf water potential (Psi(x)), and total hydraulic conductance (GT) were examined in silver birch (Betula pendula Roth) with respect to leaf position in the crown. To reduce limitations caused by insufficient water supply or low light availability, experiments were also performed with branchlets cut from two different canopy layers. The intact upper-canopy leaves demonstrated 1.8-2.0 times higher (P<0.001) daily maxima of gL compared with the lower-canopy leaves growing in the shadow of upper branches. In the morning, gL in the shade foliage was primarily constrained by low light availability, in the afternoon, by limited water supply. Leaf conductance decreased when Psi(x) fell below certain values around midday, while the sun foliage experienced greater negative water potentials than the shade foliage. Midday stomatal openness was controlled by leaf water status and temperature, rather than by transpiration rate (E) via the feedforward mechanism. Mean GT was 1.7 times higher (P<0.001) for the upper-canopy foliage compared to that of the lower canopy. At least 34-39% of the total resistance to the water flow from soil up to the shade foliage, and 54% up to the sun foliage, resided in 30-cm distal parts of the branches. Artificial reduction of hydraulic constraints raised Psi(x) and made gL less sensitive to changes in both atmospheric and plant factors. Improved water supply increased gL and E in the lower-canopy foliage, but not in the upper-canopy foliage. The results support the idea that leaves in the lower canopy are hydraulically more constrained than in the upper canopy.

Within-crown variation in leaf conductance of Norway spruce: effects of irradiance, vapour pressure deficit, leaf water status and plant hydraulic constraints

Responses of leaf conductance (gL) to variation in photosynthetic photon flux density, leaf-to-air vapour pressure difference, shoot water potential and soil-to-leaf hydraulic conductance (GT) were studied in Picea abies (L.) Karst. foliage with respect to shoot age and position within the canopy. The upper canopy shoots demonstrated on average 1.6 times higher daily maximum gL as compared to the lower canopy shoots growing in the shadow of upper branches. Functional acclimation of the shade foliage occurred in the form of both a steeper initial slope of the light-response curve and a lower light-saturation point of gL. The mean GT was 1.6-1.8 times bigger for the upper canopy compared to the lower canopy. We set up an hypothesis that stomatal conductance at the base of the live crown is constrained not only by low light availability but also by plant's inner hydraulic limitations.

Effects of environment during growth on the sensitivity of leaf conductance to changes in humidity

AbstractSoybeans (Glycine max) and grain amaranth (Amaranthus hypochondriacus) were grown at a range of temperatures, carbon dioxide concentrations and light conditions in controlled environment chambers, and the response of leaf conductance to water vapour to changes in humidity was then measured under a standard set of conditions. The sensitivity of conductance was analysed in terms of (i) the absolute sensitivity of conductance to changes in leaf to air water vapour pressure difference (LAVPD), (ii) the sensitivity of conductance relative to the absolute value of conductance, and (iii) the slope of the relationship between conductance and an index incorporating assimilation rate, carbon dioxide concentration and relative humidity. The sensitivity of conductance varied substantially with growth conditions for all three analyses in both species. The growth temperature of 25 °C increased the sensitivity of conductance by all three measures compared with growth at 20 or 30 °C in amaranth, with little difference between 25 and 30 °C in soybean. Growth at elevated carbon dioxide decreased sensitivity in amaranth by all three measures, and decreased the absolute but not the relative sensitivity in soybean. Growth at reduced photon flux density and growth at high stand density reduced sensitivity in amaranth by all three measures. In soybean, growth at high stand density reduced sensitivity by all three measures, but growth at low photon flux density increased the relative sensitivity. The sensitivity of leaf conductance to changes in humidity varied by a factor of two or more with growth environment by all measures of sensitivity in both the C3 and the C4 species.

Differential response of leaf conductance, carbon isotope discrimination and water‐use efficiency to nitrogen deficiency in maritime pine and pedunculate oak plants

SUMMARYGrowth, CO2 assimilation rate (A), leaf conductance (g), transpiration efficiency (W= ratio biomass production/plant water use) and carbon isotope discrimination (Δ) were assessed in maritime pine (Pinus pinaster Ait.) and pedunculate oak (Quercus robur L.) grown on a sand–peat mixture with three levels of fertilization: F100, optimal complete fertilization; F25, 25% of the optimal fertilizer supply; F0, no fertilization. Leaf phosphorus (P) and potassium (K) concentrations were affected little by the diminishing nutrient availability. Reduced fertilization decreased plant nitrogen (N) concentration in both species but leaf N concentration was less affected in oak than in pine. In pine W was markedly reduced in response to reduced leaf or whole plant N concentration, which was consistent with the sharp decrease also observed for plant intrinsic water‐use efficiency (ratio A/g) both at the instantaneous (gas exchange data) and time‐integrated (A/g derived from Δ measurements) levels. In this species, lowered W in the N deficient conditions was primarily associated with enhanced values of g. The existence of such a stomatal response pattern, confirmed by the increase in plant transpiration between F100 and F25, has not been reported before. In oak, both A and g were decreased in F25 and F0 as compared with F100. W was not affected – and instantaneous as well as time‐integrated A/g values were only slightly decreased – in relation to decreasing plant N concentration. For F100, no difference in W was noticed between pine and oak though the Δ values were 2·6‰ lower in oak. We speculate that this discrepancy was linked with higher plant‐carbon losses through processes like respiration, fine‐root mortality or root exudation in oak. The isotopic approach proved useful for assessing the effects of nutritional status on W, but has to be used with caution when comparing different species.

Inheritance of temperature adaptation in intra- and inter-specific Populus crosses

Populustrichocarpa Torr. &amp; Gray from Alaska was crossed with the same species from a Montana population, and the resulting intraspecific hybrid was crossed with Populusdeltoides Bartr. ex Marsh, from Minnesota. Ramets from the five parents and hybrids were tested for the response of leaf conductance to temperature of the prior night. Leaf stomatal density and size were determined and net photosynthesis was measured between 15 and 40 °C for all clones. Alaska and Montana sources of P. trichocarpa and their hybrid showed mildly sensitive leaf conductance responses to 10 °C nights and comparable responses to 4 °C nights. The Alaska parent had an optimum rate of net photosynthesis at 20 °C, 10 °C lower than that of its hybrid, but the Montana parent's temperature response curve was less clearly defined. Although the P. trichocarpa hybrid was intermediate or similar to both parents in temperature adaptation, in stomatal density, size, and Q-ratio it was more similar to the Alaska parent. The P. deltoides parent and its interspecific hybrid exhibited a sensitive pattern of leaf conductance response to cool nights. The hybrid had a net photosynthetic temperature response curve that closely paralleled that of the P. deltoides parent, yet rates were lower, between 20 and 35 °C, in both compared with the P. trichocarpa parent. The interspecific hybrid more closely resembled the P. deltoides than the P. trichocarpa parent in overall temperature adaptation and had morning leaf conductance higher than that of both. Stomatal density and size were more unlike the P. deltoides than the P. trichocarpa parent, and the Q-ratio of the hybrid was intermediate to that of the parents.

Leaf conductance of four glasshouse vegetable crops as affected by air humidity

Porometer measurements were conducted on eggplant, cucumber, sweet pepper and tomato in a glasshouse during day and night conditions at different levels of air vapour pressure deficit. The response of leaf conductance was described as an empirical non-linear function of vapour pressure deficit at leaf surface ( D 0 ) and solar radiation. Leaf conductance at night clearly responded to D 0 . Highest conductance was observed with tomato and cucumber. It is argued that effects of humidity on cuticular conductance may contribute to the increased leaf conductance at low D 0 but also that stomata respond to D 0 at night. If both day and night measurements are combined into one model, the relative response of leaf conductance to vapour pressure deficits is equal for the four species.

Do stomata respond to relative humidity?

Abstract. The objective of the work reported was to answer the following questions: (1) Do stomata respond to both humidity and temperature? (2) Do these responses interact in such a way that relative humidity at the leaf surface is a more appropriate variable than water vapour saturation deficit at the leaf surface and yields a simpler description of the compound response? To answer these questions, we measured the response of leaf conductance to humidity under constant leaf temperature, and the response to increasing leaf temperature under constant relative humidity and under constant water vapour saturation deficit. We found that, in Hedera helix subsp. canariensis (Willd.) Coutinho, there was a reversible response to humidity under constant temperature, and that there was also a response to temperature under constant relative humidity. The relationship between leaf conductance and relative humidity was different when measured at the same temperature rather than at different temperatures. An inversely proportional response was consistently obtained when stomatal conductance was expressed in relation to water vapour saturation deficit. The interaction between the effects of leaf temperature and water vapour saturation deficit was not compatible with a mechanism of response to humidity and temperature based solely on relative humidity. From these data, we conclude that water vapour saturation deficit is a more appropriate variable for describing stomatal responses to humidity.

Water relations of loblolly pine trees in southeastern Oklahoma following precommercial thinning

Xylem pressure potential, leaf conductance, transpiration, and soil moisture were measured during three summers following precommercial thinning of a 10-year-old stand of loblolly pine (Pinustaeda L.) in southeastern Oklahoma. The stand was thinned to three target basal-area levels: 5.8, 11.5, and 23 m2•ha−1 (control). Soil water potential increased significantly in response to thinning during the summer of each year studied. However, plant water relations were relatively unaffected by the treatments. Significant thinning effects on diurnal xylem pressure potential were observed on only 7 of 55 measurement periods. Treatment differences in conductance and transpiration observed during the first year of the study appeared to be related to differences in light interception and crown exposure. Regression analysis indicated response of leaf conductance and transpiration to predawn xylem pressure potential and vapor pressure deficit was not affected by the thinning treatments. Overall, the results of this study are consistent with a hypothesis in which transpiration, leaf area, and water potential interact to form a homeostatic relationship.

Leaf conductance of four species growing in a subarctic marsh

This paper describes measurements of leaf conductance made with a diffusion porometer on four wetland species growing in a coastal marsh on James Bay. The measurement period covered most of the growing season. In general, leaf conductance for the sedge, Carex paleacea, was larger than for three woody species, Alnus rugosa, Salix bebbianna, and Salix discolor. No distinct pattern was observed in maximum conductance throughout the growing season; however, leaf conductance demonstrated distinct diurnal trends for all species. The response of leaf conductance to various environmental variables was examined and it was found that conductance of all species was most strongly related to the atmospheric vapour pressure deficit. Two models for predicting leaf conductance from environmental variables showed moderate success.

Stomatal responses of Populus to leaf water potential

The leaf conductance responses of clones of Populus trichocarpa, P. deltoides, and hybrids to changing leaf water potential were examined. Unlike the other species, P. trichocarpa not acclimated to water stress showed an inability to control water loss at low leaf water potential. It is suggested that stomata of P. trichocarpa plants grown under well-watered conditions remain open in spite of a loss of turgor in the leaf. Preconditioning through water stress of developing leaves, however, leads to greater stomatal control in foliage following rewatering. Leaf conductance of stressed and rewatered P. deltoides and hybrids remained near zero, while the leaf conductance of P. trichocarpa was partially reduced.

Morphological and physiological changes in black alder induced by water stress

Abstract. Black alder seedlings were exposed to 12 weeks of sublethal water stress by watering only when visibly wilted. Control seedlings were watered regularly throughout the treatment period. Stressed seedlings exhibited significant osmotic adjustment of over 0.4 MPa. The water stress treatment also significantly reduced leaf size, increased epicuticular wax content, and increased the root shoot ratio. The response of leaf conductance to decreasing leaf water potential was influenced by the previous water stress treatment Stressed seedlings had a much lower initial leaf conductance, but showed a gradual drop in leaf conductance as leaf water potential decreased; whereas, control seedling leaf conductance fell rapidly. These morphological and physiological modifications in response to moisture stress have the potential for significantly improving black alder drought tolerance.

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content : III. In the sclerophyllous woody species Nerium oleander.

The responses of leaf conductance, leaf water potential and rates of transpiration and net photosynthesis at different vapour pressure deficits ranging from 10 to 30 Pa kPa-1 were followed in the sclerophyllous woody shrub Nerium oleander L. as the extractable soil water content decreased. When the vapour pressure deficit around a plant was kept constant at 25 Pa kPa-1 as the soil water content decreased, the leaf conductance and transpiration rate showed a marked closing response to leaf water potential at-1.1 to-1.2 MPa, whereas when the vapour pressure deficit around the plant was kept constant at 10 Pa kPa-1, leaf conductance decreased almost linearly from-0.4 to-1.1 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps, decreased leaf conductance at all exchangeable soil water contents. Changing the leaf water potential in a single leaf by exposing the remainder of the plant to a high rate of transpiration decreased the water potential of that leaf, but did not influence leaf conductance when the soil water content was high. As the soil water content was decreased, leaf conductances and photosynthetic rates were higher at equal levels of water potential when the decrease in potential was caused by short-term increases in transpiration than when the potential was decreased by soil drying.As the soil dried and the stomata closed, the rate of photosynthesis decreased with a decrease in the internal carbon dioxide partial pressure, but neither the net photosynthetic rate nor the internal CO2 partial pressure were affected by low water potentials resulting from short-term increases in the rate of transpiration. Leaf conductance, transpiration rate and net photosynthetic rate showed no unique relationship to leaf water potential, but in all experiments the leaf gas exchange decreased when about one half of the extractable soil water had been utilized. We conclude that soil water status rather than leaf water status controls leaf gas exchange in N. oleander.

An Empirical Model of Net Photosynthesis and Leaf Conductance for the Simulation of Diurnal Courses of CO2 and H2O Exchange

An empirical model of CO2 uptake and water loss of leaves is established using steady-state responses of gas exchange to climatic factors as input. From the model the response surface of net CO2 assimilation and leaf conductance to climate can be derived. The model consists of two submodels, one describing the response of CO2 uptake to light and temperature, the other describing the response of leaf conductance to temperature and humidity. Both submodels are joined via the linear relationship between CO2 uptake and leaf conductance at short-term (minutes) variation of irradiance. From the humidity response of leaf conductance and the 'demand function' (Raschke 1979) of CO2 uptake in the mesophyll, the effect of stomata on the diffusion of CO2 between leaf and air is determined. The model is tested by comparing measured and calculated diurnal courses of gas exchange for two plants of Pinus silvestris, differing in photosynthetic capacity due to different levels of magnesium nutrition. Applications and limitations of the model are discussed.

Effect of boundary layer conductance on the response of stomata to humidity

Abstract. Leaf conductance responses to leaf to air water vapour partial pressure difference (VPD) have been measured at air speeds of 0.5 and 3.0 ms−1 in single attached leaves of three species in order to test the hypothesis that leaf conductance response to VPD is controlled by evaporation from the outer surface of the epidermis, rather than by evaporation through stomata. Total conductance decreased linearly with increassing VPD at both air speeds, but was decreased 1.6 3.0 times as much as by a given incrase in VPD at high than at low air speed. depending on species. In all species the relationship between leaf conductance and the gradient for evaporation from the epidermis was the same at both values of boundary layer conductance, supporting the hypothesis that direct epidermal evaporation controls stomatal guard cell behaviour in responses of stomata to VPD in these species.

Leaf conductance response of phaseolus vulgaris to ozone flux density

The effect of ozone flux density on leaf conductance to ozone in Phaseolus vulgaris was examined. The change in conductance was measured within the first two hours of fumigation for mature, fruiting 6-week-old plants of an ozone sensitive cultivar (Seafarer); for young, 14-day-old plants of the same cultivar; and for an ozone resistant cultivar (Gold Crop). Young Seafarer plants showed no change in conductance to ozone over a wide range of ozone flux densities. Gold Crop showed a decrease in conductance of −3.1 % /(mgO 3 m −2 h −1) whereas mature Seafarer plants exhibited a stronger decrease of −7.7% /(mgO 3 m −2 h −1). Diffusion porometer measurements taken on fruiting Seafarer plants in the field illustrated that a decrease in leaf diffusive conductance to water is related to visual ozone injury.

Field Studies of Leaf Conductance Response to Environmental Variables in Citrus

(1) The relationship between leaf conductance and environmental variables was examined during two summer periods in 22-year-old Shamouti orange trees. Soil water potential in the upper 0.6 m of the root zone was reduced from -0-01 to -0.87 MPa by withholding water for a period of 45 days. (2) Leaf conductance was dominantly controlled by soil water potentials lower than -0-02 MPa. At high soil water potentials, hourly midday conductance was negatively correlated with leaf-to-air vapour pressure difference. Response of leaf conductance to irradiance was evident only at low photon flux densities. (3) A model which has been used successfully in other species to relate leaf conductance to atmospheric variables, i.e. irradiance, leaf-to-air vapour pressure difference, and leaf temperature, was applied. In contrast to work in other species, only 1 1% of the variations in the measurements were accounted for by the model. (4) Average midday leaf conductance was linearly related to leaf water potential. Although hourly leaf water potential was not linearly related to leaf conductance, daily courses of the two variables showed evidence of feedback control of conductance. (5) The hourly rate of change in leaf water potential showed several distinct oscillations during the day and was highly correlated with leaf conductance.

Rapid stomatal responses to humidity

The response of leaf conductance in apple to rapid changes in atmospheric humidity was studied using a continuous flow porometer. Leaf-air vapour pressure difference was changed by adjusting the humidity of the inlet air or by altering the flow rate of the air through the chamber. The time course of the response of leaf conductance to leaf-air vapour pressure difference was monitored for periods up to 10 min using a chart-recorder. There were significant changes in leaf conductance within seconds of changing humidity. These were attributed to alterations in stomatal aperture.