Leaf-air Vapour Pressure Difference Research Articles

Optimal stomatal theory predicts CO2 responses of stomatal conductance in both gymnosperm and angiosperm trees.

Optimal stomatal theory predicts that stomata operate to maximise photosynthesis (Anet ) and minimise transpirational water loss to achieve optimal intrinsic water-use efficiency (iWUE). We tested whether this theory can predict stomatal responses to elevated atmospheric CO2 (eCO2 ), and whether it can capture differences in responsiveness among woody plant functional types (PFTs). We conducted a meta-analysis of tree studies of the effect of eCO2 on iWUE and its components Anet and stomatal conductance (gs ). We compared three PFTs, using the unified stomatal optimisation (USO) model to account for confounding effects of leaf-air vapour pressure difference (D). We expected smaller gs , but greater Anet , responses to eCO2 in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO2 in all PFTs, and that increases in Anet had stronger effects than reductions in gs . The USO model correctly captured stomatal behaviour with eCO2 across most datasets. The chief difference among PFTs was a lower stomatal slope parameter (g1 ) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO2 conditions.

Osmotic adjustment and hormonal regulation of stomatal responses to vapour pressure deficit in sunflower.

Dynamic variation of the stomatal pore in response to changes in leaf–air vapour pressure difference (VPD) constitutes a critical regulation of daytime gas exchange. The stomatal response to VPD has been associated with both foliage abscisic acid (ABA) and leaf water potential (Ψ l); however, causation remains a matter of debate. Here, we seek to separate hydraulic and hormonal control of stomatal aperture by manipulating the osmotic potential of sunflower leaves. In addition, we test whether stomatal responses to VPD in an ABA-deficient mutant (w-1) of sunflower are similar to the wild type. Stomatal apertures during VPD transitions were closely linked with foliage ABA levels in sunflower plants with contrasting osmotic potentials. In addition, we observed that the inability to synthesize ABA at high VPD in w-1 plants was associated with no dynamic or steady-state stomatal response to VPD. These results for sunflower are consistent with a hormonal, ABA-mediated stomatal responses to VPD rather than a hydraulic-driven stomatal response to VPD.

Hydraulic conductance as well as nitrogen accumulation plays a role in the higher rate of leaf photosynthesis of the most productive variety of rice in Japan

An indica variety Takanari is known as one of the most productive rice varieties in Japan and consistently produces 20–30% heavier dry matter during ripening than Japanese commercial varieties in the field. The higher rate of photosynthesis of individual leaves during ripening has been recognized in Takanari. By using pot-grown plants under conditions of minimal mutual shading, it was confirmed that the higher rate of leaf photosynthesis is responsible for the higher dry matter production after heading in Takanari as compared with a japonica variety, Koshihikari. The rate of leaf photosynthesis and shoot dry weight became larger in Takanari after the panicle formation and heading stages, respectively, than in Koshihikari. Roots grew rapidly in the panicle formation stage until heading in Takanari compared with Koshihikari. The higher rate of leaf photosynthesis in Takanari resulted not only from the higher content of leaf nitrogen, which was caused by its elevated capacity for nitrogen accumulation, but also from higher stomatal conductance. When measured under light-saturated conditions, stomatal conductance was already decreased due to the reduction in leaf water potential in Koshihikari even under conditions of a relatively small difference in leaf–air vapour pressure difference. In contrast, the higher stomatal conductance was supported by the maintenance of higher leaf water potential through the higher hydraulic conductance in Takanari with the larger area of root surface. However, no increase in root hydraulic conductivity was expected in Takanari. The larger root surface area of Takanari might be a target trait in future rice breeding for increasing dry matter production.

Diurnal and seasonal variation in photosynthesis of peach palms grown under subtropical conditions

The Amazonian peach palm (Bactris gasipaes Kunth) has been grown for heart-of-palm production under subtropical conditions. As we did not see any substantial study on its photosynthesis under Amazonian or subtropical conditions, we carried out an investigation on the diurnal and seasonal variations in photosynthesis of peach palms until the first heart-of-palm harvest, considering their relationship with key environmental factors. Spineless peach palms were grown in 80-L plastic pots, under irrigation. Gas exchange and chlorophyll fluorescence emission measurements were taken in late winter, mid spring, mid summer and early autumn, from 7:00 to 18:00 h, with an additional chlorophyll fluorescence measurement at 6:00 h. The highest net CO2 assimilation (P N), observed in mid summer, reached about 15 μmol m−2 s−1, which was about 20% higher than the maximum values found in autumn and spring, and 60% higher than that in winter The same pattern of diurnal course for P N was observed in all seasons, showing higher values from 8:00 to 9:00 h and declining gradually from 11:00 h toward late afternoon. The diurnal course of stomatal conductance (g s) followed the same pattern of P N, with the highest value of 0.6 mol m−2 s−1 being observed in February and the lowest one (0.23 mol m−2 s−1) in September. The maximal quantum yield of photosystem II (Fv/Fm) was above 0.75 in the early morning in all the months. The reversible decrease was observed around midday in September and October, suggesting the occurrence of dynamic photoinhibition. A significant negative correlation between the leaf-air vapour pressure difference (VPDleaf-air) and P N and a positive correlation between P N and g s were observed. The photosynthesis of peach palm was likely modulated mainly by the stomatal control that was quite sensible to atmospheric environmental conditions. Under subtropical conditions, air temperature (T air) and VPDleaf-air impose more significant effects over P N of peach palm than an excessive photosynthetic photon flux density (PPFD). The occurrence of dynamic photoinhibition indicates that under irrigation, peach palms appeared to be acclimated to the full-sunlight conditions under which they have been grown.

The stomatal response to evaporative demand persists at night in Ricinus communis plants with high nocturnal conductance

Evidence is building that stomatal conductance to water vapour (g(s)) can be quite high in the dark in some species. However, it is unclear whether nocturnal opening reflects a mechanistic limitation (i.e. an inability to close at night) or an adaptive response (i.e. promoting water loss for reasons unrelated to carbon gain). Further, it is unclear if stomatal responses to leaf-air vapour pressure difference (D) persist in the dark. We investigated nocturnal stomatal behaviour in castor bean (Ricinus communis L.) by measuring gas exchange and stomatal responses to D in the light and in the dark. Results were compared among eight growth environments [two levels for each of three treatment variables: air saturation deficit (D(a)), light and water availability]. In most plants, stomata remained open and sensitive to D at night. g(s) was typically lower at night than in the day, whereas leaf osmotic pressure (Pi) was higher at night. In well-watered plants grown at low D(a), stomata were less sensitive to D in the dark than in the light, but the reverse was found for plants grown at high D(a). Stomata of droughted plants were less sensitive to D in the dark than in the light regardless of growth D(a). Drought also reduced g(s) and elevated Pi in both the light and the dark, but had variable effects on stomatal sensitivity to D. These results are interpreted with the aid of models of stomatal conductance.

Stomatal conductance in a tropical xerophilous shrubland at a lava substratum.

Diurnal variation in leaf stomatal conductance (gs) of three xerophilous species (Buddleia cordata, Senecio praecox and Dodonaea viscosa) was measured over a 10-month period during the dry and wet seasons in a shrubland that is developing in a lava substratum in Mexico. Averaged stomatal conductances were 147 and 60.2 (B. cordata), 145 and 24.8 (D. viscosa) and 142.8 and 14.1 mmol m(-2) s(-1) (S. praecox) during the wet and dry season respectively. Leaf water potential (Psi) varied in a range of -0.6 to -1.2 (S. praecox), -0.6 to -1.8 (B. cordata) and -0.9 to -3.4 MPa (D. viscosa) during the same measurement periods. Stomata were more sensitive to changes in irradiance, air temperature and leaf-air vapour pressure difference in the rainy season than the dry season. Although stomatal responses to Psi were difficult to distinguish in any season (dry or rainy), data for the entire period of measurement showed a positive correlation, stomata tending to open as Psi increased, but there is strong evidence of isohydric behaviour in S. praecox and B. cordata. A multiplicative model relating gs to environmental variables and to Psi accounted for 79%-83% of the variation of gs in three sites (pooled data); however, the performance of the model was poorer (60%-76%) for individual species from other sites not included in the pooled data.

Characteristics of gas exchange and morphology of a spring ephemeral, Erythronium japonicum , in comparison with a sun plant, Glycine max

Morphological characteristics and responses of gas exchanges to light intensity were examined in a typical vernal species, Erythronium japonicum Decne (E. japonicum), grown (i) on the floor of a deciduous broad-leaved Quercus mongolica forest (one of its native habitats, the Q. mongolica stand); (ii) bare land left undisturbed for 9 years after forest clearing (the bare stand); and (iii) in a sun crop, soybean, grown for 110 days in an experimental field and for 17 days in pots, in order to evaluate the adaptability of the photosynthetic process of this vernal species to its shady native habitats. The daytime solar radiation, ai and leaf temperatures and leaf–air vapor pressure difference (VPD) were significantly higher at the bare stand than at the Q. mongolica stand. When environmental factors observed at the Q. mongolica and bare stands were reproduced in an assimilation chamber, leaf temperatures of E. japonicum plants increased markedly with increased radiation, whereas those of soybean plants differed little from the respective air temperatures. The photosynthetic and transpiration rates and stomatal conductance in the former plants placed under conditions at the Q. mongolica stand increased with radiation and reached respective steady state values at maximum radiation at the site; whereas, under the conditions at the bare stand, they also increased and reached respective steady state values, but then continuously decreased to be lower than the respective value at the Q. mongolica stand. However, both rates and the conductance in the soybean plants under both conditions increased significantly with radiation and reached much higher respective values at the respective maximum radiations. Water use efficiency for E. japonicum plants was much higher under conditions at the Q. mongolica stand than at the bare stand and was practically equal to those for soybean plants under both conditions. Water potential in the leaves of E. japonicum at maximum radiation at the bare stand was one-third that of those at the Q. mongolica stand. The potential in soybean leaves differed little between both conditions and was roughly equal to the low value in E. japonicum leaves at the bare stand conditions. The stomatal densities on upper and lower leaf surfaces and the ratio of root weight to leaf area (R : L) differed little between E. japonicum plants grown at both stands as well as between young and adult soybean plants. However, the densities on the upper and lower surfaces of E. japonicum were 25% and 60% of the respective values of both soybean plants. The ratios of R : L of the E. japonicum plants were only one-quarter that of the young and adult soybean plants. The cooperation between these morphological and gas exchange characteristics in E. japonicum plants is discussed in relation to adaptation to the environment in native habitats.

Role of climate, crown position, tree age and altitude in calculated ozone flux into needles of Picea abies and Pinus cembra: a synthesis

Ozone (O3) flux into Norway spruce (Picea abies) and cembran pine (Pinus cembra) needles was estimated under ambient conditions at six rural sites between 580 and 1950 m a.s.l. We also assessed age-related differences in O3 flux by examining changes in leaf conductance across the life span of Norway spruce. At the leaf level O3 flux into the needles was effectively controlled by stomatal conductance and, hence by factors such as temperature, irradiance and humidity, which control stomatal conductance. Seasonal variations in O3 flux were mainly attributed to the course of the prevailing temperature. During the growing season, however, data have emphasised leaf–air vapour pressure difference as the environmental factor most likely to control stomatal conductance and O3 flux into the needles. In the sun crown stomatal conductance averaged over the growing season decreased with increasing tree age from 42.0±3.5 mmol O3 m−2 s−1 in 17-year-old trees to 7.1±1.0 mmol O3 m−2 s−1 in 216-year-old trees, indicating that O3 concentration in the substomatal cavities is higher in young than in old trees. Independent from tree age stomatal conductance and O3 flux were approximately 50% lower in shade needles as compared to sun-exposed needles. Stomatal conductance was also greater in the current flush (24±5.6 mmol O3 m−2 s−1) and in 1-year old needles (16±4 mmol O3 m−2 s−1) than in older needle age classes (12±1 mmol O3 m−2 s−1, averaged across the four older needle age classes). In trees similar in age (60–65 years old) average O3 flux into sun needles increased from 0.55±0.36 nmol m−2 s−1 at the valley floor to 0.9 nmol m−2 s−1 in 1950 m a.s.l. Cumulative O3 uptake during the vegetation period increased from 11.4±1.7 mol m−2 in the valley to 14 mol m−2 at the alpine timberline. Although stomatal conductance provides the principal limiting factor for O3 flux, additional field research is necessary in order to improve our understanding concerning the quantitative ‘physiological threshold dose’ which internally can be active and can have adverse effects of O3 on forest trees.

Effects of micro‐environmental factors on photosynthetic CO2 uptake and carbon fixation metabolism in a spring ephemeral, Erythronium japonicum, growing in native and open habitats

Microenvironmental factors and physiological parameters (such as water potential, activity of ribulose 1,5‐bisphosphate carboxylase (RuBPcase), levels of ribulose 1,5‐bisphosphate (RuBP), 3‐phosphoglyceric acid (PGA) and sucrose in leaves) affecting photosynthetic processes of the typical vernal species Erythronium japonicum Decne. were examined on the floor of a deciduous broad‐leaved Quercus mongolica forest (Q.m. stand) and on bare land left undisturbed for 7 years after forest clearing (bare stand). Daytime solar radiation and the air and leaf temperatures at the bare stand were significantly higher than those at the Q.m. stand. The relative air humidity was very low and did not differ much between the stands, whereas the leaf–air vapor pressure difference (VPD) at the bare stand was twice as high as that at the Q.m. stand. The water potential in leaves at the bare stand was lower than two times that at the Q.m. stand. Therefore, the aboveground parts of the plants at the bare stand were subjected to much more severe heat stress than those at the Q.m. stand. When these environmental factors observed at the bare stand were reproduced in an assimilation chamber, the rate of photosynthetic CO2 uptake, stomatal conductance and water potential in leaves were significantly low in comparison with those when the factors at the Q.m. stand were simulated. The internal CO2 partial pressure in leaves at the bare stand was considerably lower than that at the Q.m. stand. Consequently, the decrease in the photosynthetic rate of the plants at the bare stand was caused mainly by a decrease in stomatal conductance through a lowering of water potential due to subjection of the aboveground parts to much more severe heat stress than that at the Q.m. stand. The possibility that an inhibition of the photosynthetic carbon fixation metabolism induced by the decrease in water potential contributes to the reduction in photosynthetic CO2 uptake in the plants at the bare stand is also discussed in light of physiological characteristics such as the activity of RuBPcase and levels of PGA, RuBP and sucrose in the leaves.

El effecto del microambiente en la conductividad estomática de Buddleia cordata H. B. K., en la Reserva del Pedregal de San Ángel

La estacionalidad de la precipitación y el substrato en el Pedregal de San Ángel deben jugar un papel importante en el estado hídrico de las plantas que en él habitan. Por lo tanto, se realizaron mediciones de conductividad estomática (g), potencial hídrico foliar (? h), densidad de flujo fotónico fotosintético (Q), temperatura del aire (Ta) y la diferencia de presión de vapor entre la hoja y el aire (DPV) en Buddleia cordata H.B.K., por ser una especie perennifolia y dominante del estrato arbóreo. Las mediciones se llevaron a cabo durante las estaciones seca y húmeda en el Pedregal de San Angel que es una comunidad que se desarrolla en un substrato de lava. Las conductividades que se registraron fueron de hasta 330 mmol m-2 s-1 durante la época húmeda, mientras que en la época seca se redujeron hasta cerca de un 54%. Se realizaron dos tipos de análisis, el de la curva envolvente y el de regresión múltiple. Ambos análisis mostraron un efecto significativo de Q, Ta y DPV sobre g tanto en cada una de las épocas como anualmente y los estomas fueron más sensibles a estas variables en la época húmeda que en la seca. El efecto de ? h, no fue significativo en los análisis por época, pero sí en un análisis anual. Los resultados de este tipo de análisis pueden ser una herramienta muy efectiva en la formulación de modelos matemáticos que simulen o pronostiquen la conductividad estomática.

Independent effects of the environment on the leaf gas exchange of three banana ( Musa sp.) cultivars of different genomic constitution

Circumstantial evidence suggests that the Musa balbisiana (B) genome confers greater drought tolerance to bananas and plantains than the Musa acuminata (A) genome. Hence the genetic makeup of bananas and plantains may affect the response of leaf gas exchange to the environment. Field data cannot be readily used to study the independent effects of environment but laboratory studies allow independent control of environmental parameters. We examined the independent effects of photosynthetic photon flux density, I, from 0 to 1450 μmol quanta m −2 s −1, leaf temperature, T l, from 21°C to 43°C, and leaf–air vapour pressure difference, Δ e, from 1.5 to 5.7 kPa on the stomatal conductance, g s, transpiration, E t, net photosynthesis, P n, internal CO 2 concentration, C i, and instantaneous water use efficiency, E w, of three Musa cultivars: cv Williams (AAA), cv Lady Finger (AAB), and cv Bluggoe (ABB). M. balbisiana genomes reduced the sensitivity of g s and P n to Δ e more than M. acuminata genomes. Genomic composition did not affect the responses to T l. As Δ e increased, g s and P n declined linearly at the rate of approximately 10% of predicted maximum g s and P n per 1 kPa increase in Δ e. This reduced stomatal aperture reduced C i, which declined exponentially, thereby limiting P n. Optimum temperatures for g s were 35°C and 39°C when Δ e was 1.5 and 3.0 kPa respectively. Optimum temperatures for P n were about 29°C when Δ e was 1.5 kPa and 33°C when Δ e was 3.0 kPa. The predicted maximum temperature where P n=0.0 would occur was 43°C to 44°C for all responses regardless of Δ e. The Williams cultivar was least sensitive to I showing less than 70% of predicted maximum photosynthesis and less than 50% of predicted maximum stomatal conductance at 1250 μmol quanta m −2 s −1. We conclude that there are genetic differences in the response of leaf gas exchange to changing environment within banana and plantains. The mechanism underlying the response of leaf gas exchange is through an effect of Δ e and I on the stomata, rather than an effect of T l on photosynthetic activity. Increasing proportions of B genomes decrease the sensitivity of stomata to Δ e but increases the sensitivity to I, especially at low photosynthetic photon flux densities. They also increase water use efficiency at the leaf level of organisation. The lower sensitivity of g s and P n to Δ e of cultivars containing more B genomes is consistent with the view that the B genome contributes to drought tolerance in Musa sp.

Leaf δ13C in Pinus resinosa trees and understory plants: variation associated with light and CO2 gradients.

Our objective was to evaluate the relative importance of gradients in light intensity and the isotopic composition of atmospheric CO2 for variation in leaf carbon isotope ratios within a Pinus resinosa forest. In addition, we measured photosynthetic gas exchange and leaf carbon isotope ratios on four understory species (Dryopteris carthusiana, Epipactus helleborine, Hieracium floribundum, Rhamnus frangula), in order to estimate the consequence of the variation in the understory light microclimate for carbon gain in these plants. During midday, CO2 concentration was relatively constant at vertical positions ranging from 15 m to 3 m above ground. Only at positions below 3 m was CO2 concentration significantly elevated above that measured at 15 m. Based on the strong linear relationship between changes in CO2 concentration and δ13C values for air samples collected during a diurnal cycle, we calculated the expected vertical profile for the carbon isotope ratio of atmospheric CO2 within the forest. These calculations indicated that leaves at 3 m height and above were exposed to CO2 of approximately the same isotopic composition during daylight periods. There was no significant difference between the daily mean δ13C values at 15 m (-7.77‰) and 3 m (-7.89‰), but atmospheric CO2 was significantly depleted in 13C closer to the ground surface, with daily average δ13C values of -8.85‰ at 5 cm above ground. The light intensity gradient in the forest was substantial, with average photosynthetically active radiation (PAR) on the forest floor approximately 6% of that received at the top of the canopy. In contrast, there were only minor changes in air temperature, and so it is likely that the leaf-air vapour pressure difference was relatively constant from the top of the canopy to the forest floor. For red pine and elm tree samples, there was a significant correlation between leaf δ13C value and the height at which the leaf sample was collected. Leaf tissue sampled near the forest floor, on average, had lower δ13C values than samples collected near the top of the canopy. We suggest that the average light intensity gradient through the canopy was the major factor influencing vertical changes in tree leaf δ13C values. In addition, there was a wide range of variation (greater than 4‰) among the four understory plant species for average leaf δ13C values. Measurements of leaf gas exchange, under natural light conditions and with supplemental light, were used to estimate the influence of the light microclimate on the observed variation in leaf carbon isotope ratios in the understory plants. Our data suggest that one species, Epipactus helleborine, gained a substantial fraction of carbon during sunflecks.

Environmental and physiological regulation of transpiration in tropical forest gap species: the influence of boundary layer and hydraulic properties.

Environmental and physiological regulation of transpiration were examined in several gap-colonizing shrub and tree species during two consecutive dry seasons in a moist, lowland tropical forest on Barro Colorado Island, Panama. Whole plant transpiration, stomatal and total vapor phase (stomatal + boundary layer) conductance, plant water potential and environmental variables were measured concurrently. This allowed control of transpiration (E) to be partitioned quantitatively between stomatal (g s) and boundary layer (g b) conductance and permitted the impact of invividual environmental and physiological variables on stomatal behavior and E to be assessed. Wind speed in treefall gap sites was often below the 0.25 m s-1 stalling speed of the anemometer used and was rarely above 0.5 m s-1, resulting in uniformly low g b (c. 200-300 mmol m-2 s-1) among all species studied regardless of leaf size. Stomatal conductance was typically equal to or somewhat greater than g b. This strongly decoupled E from control by stomata, so that in Miconia argentea a 10% change in g s when g s was near its mean value was predicted to yield only a 2.5% change in E. Porometric estimates of E, obtained as the product of g s and the leaf-bulk air vapor pressure difference (VPD) without taking g b into account, were up to 300% higher than actual E determined from sap flow measurements. Porometry was thus inadequate as a means of assessing the physiological consequences of stomatal behavior in different gap colonizing species. Stomatal responses to humidity strongly limited the increase in E with increasing evaporative demand. Stomata of all species studied appeared to respond to increasing evaporative demand in the same manner when the leaf surface was selected as the reference point for determination of external vapor pressure and when simultaneous variation of light and leaf-air VPD was taken into account. This result suggests that contrasting stomatal responses to similar leaf-bulk air VPD may be governed as much by the external boundary layer as by intrinsic physiological differences among species. Both E and g s initially increased sharply with increasing leaf area-specific total hydraulic conductance of the soil/root/leaf pathway (G t), becoming asymptotic at higher values of G t. For both E and g s a unique relationship appeared to describe the response of all species to variations in G t. The relatively weak correlation observed between g s and midday leaf water potential suggested that stomatal adjustment to variations in water availability coordinated E with water transport efficiency rather than bulk leaf water status.

Simultaneous Effects of Foliar Nitrogen, Temperature, and Humidity on Gas Exchange in Pinus radiata

Seedlings of Pinus radiata were grown in a glasshouse in large pots with sand as the potting mix. They were kept well-watered and frequently supplied with nutrient solutions which contained different amounts of nitrogen for different treatments. Carbon assimilation and diffusive conductance of the foliage were measured under steady-state conditions at saturating light in all treatments. Experimental variables were leaf-air vapour pressure difference and leaf temperature at time of measurement. Data were fitted to a non-linear regression equation to examine responses of carbon assimilation, diffusive conductance, transpiration, assimilatory nitrogen-use efficiency, and assimilatory transpiration efficiency to foliar nitrogen concentration expressed on a leaf area basis ([N]), to leaf temperature, and to leaf-air vapour pressure (D). Parameters from the regression have been used to plot three-dimensional surfaces, so that simultaneous effects of experimental variables can be easily visualised. Carbon assimilation increased linearly with foliar [N], declined exponentially as D increased, and had a broad temperature optimum between c. 14 and 38�C. Diffusive conductance increased linearly with foliar [N], was related to the reciprocal of D, and declined as temperature increased. Using climatic vapour pressure deficit and air temperature data for Canberra, ACT, and for Mount Gambier, SA, and with the functions that had been fitted to experimental data, it was found that these regional climatic differences have potential for appreciably affecting carbon gain and water loss in the regions, which have P. radiata plantations. Predicted differences in carbon gain are of the order of reported differences in stem growth in the regions. This shows the need to take into account regional variation in climatic variables that strongly affect gas exchange when investigating regional differences in productivity.

Photosynthetic Characteristics of Sun Versus Shade Plants of Encelia farinosa as Affected by Photosynthetic Photon Flux Density, Intercellular CO2 Concentration, Leaf Water Potential, and Leaf Temperature

Limitations to photosynthesis were examined for Encelia farinosa Toney et A.Gray, a common C3 sub-shrub in arid regions of south-westem United States, for plants grown in full sunlight and those shaded to 40% of full sunlight. The initial slopes of CO2 assimilation (A) versus intercellular CO2 concentration curves were similar for sun and shade plants at low photosynthetic photon flux density (PPFD) but higher for sun plants as the PPFD increased, indicating a greater limitation by carboxylation capacity in shade plants. Sun plants had higher electron transport rates but a lower ratio of electron transport capacity to carboxylation capacity (Vmax); the ratio was inversely proportional to mesophyll conductance for both sun and shade plants. Dark respiration decreased with decreasing leaf water potential (Ψ1) in sun plants but remained unchanged in shade plants; day respiration was little affected by PPFD for both sun and shade plants. Stomatal conductance (gs) was similar for sun and shade plants under high soil-moisture conditions but higher in sun plants as Ψ1 decreased; for all data considered together, changes in the leaf-air vapour pressure difference accounted for 71% of the variation in gs. The lower A for shade plants of E. farinosa apparently resulted from a lower Vmax, as well as a lower gs when plants were under water stress.

Stomatal responses to changing irradiance in Phaseolus vulgaris L.

The effects of air temperature (T a ), leaf-air vapour pressure differences (VPD) and water deficit on stomatal responses to changing irradiance were studied in Phaseolus vulgaris L. Responses were approximately sigmoidal, with rates of closure being faster than the rates of opening. The mean half-time for closure was 5.4 min and for the opening 9.2 min. Under water deficit, both stomatal opening and closing were faster than in well-watered conditions. Stomata were more sensitive to VPD and water stress than to Ta. The higher the VPD or Ta the more rapid was the stomatal response, except in stressed plants where there was no significant effect of Ta

Gas Exchange of Oil Palm in Relation to Light, Vapour Pressure Deficit, Temperature and Leaf Age

Gas exchange in leaves of oil palm (Elaeis guineensis) was measured in the field over a range of photon fluxes, leaf temperatures and leaf-air vapour pressure differences (VPD). There were no differences in leaflet photosynthesis within one frond, but a reduction depending on frond age. For fronds of the same age, between-tree variation was low. The light saturated rate of net CO 2 assimilation was 20 μmol m −2 s −1 when photosynthetically active radiation (PAR) exceeded 1100 μmol m −2 s −1 . Apparent quantum yield averaged 0.051 mol mol −1 . When leaf temperature increased from 30°C to 38°C, net CO 2 uptake showed no variation as transpiration rate andstomatal conductance increased slightly. A small increase in VPD induced rapid stomatal closure and consequently a reduction in the transpiration rate (...)

Comparison of Modeled and Observed Environmental Influences on the Stable Oxygen and Hydrogen Isotope Composition of Leaf Water in Phaseolus vulgaris L.

In this paper we describe how a model of stable isotope fractionation processes, originally developed by H. Craig and L. I. Gordon ([1965] in E Tongiorgi, ed, Proceedings of a Conference on Stable Isotopes in Oceanographic Studies and Paleotemperature, Spoleto, Italy, pp 9-130) for evaporation of water from the ocean, can be applied to leaf transpiration. The original model was modified to account for turbulent conditions in the leaf boundary layer. Experiments were conducted to test the factors influencing the stable isotopic composition of leaf water under controlled environment conditions. At steady state, the observed leaf water isotopic composition was enriched above that of stem water with the extent of the enrichment dependent on the leaf-air vapor pressure difference (VPD) and the isotopic composition of atmospheric water vapor (AWV). The higher the VPD, the larger was the observed heavy isotope content of leaf water. At a constant VPD, leaf water was relatively depleted in heavy isotopes when exposed to AWV with a low heavy isotope composition, and leaf water was relatively enriched in heavy isotopes when exposed to AWV with a large heavy isotope composition. However, the observed heavy isotope composition of leaf water was always less than that predicted by the model. The extent of the discrepancy between the modeled and observed leaf water isotopic composition was a strong linear function of the leaf transpiration rate.

Transpiration efficiency and apparent cuticular transpiration in some c<sub>3</sub> and c<sub>4</sub> plants

Amphistomatous C3 (Nicotiana tabacum L., Datura stramonium L.) and C4 (Sorghum saccharatum Pers. and Zea mays L.) species were examined to find how (if at all) their inherent differences in water-use economy are reflected in apparent cuticular transpiration or vice versa. Transpiration efficiency (TE) was calculated from steady state photosynthesis (A) and transpiration (E) rates estimated for the upper side of the leaf after light induction of stomata opening. Apparent cuticular transpiration ('Ec) was measured as the part of transpiration which was not eliminated by convective counteraction of the air stream passing across the amphistomatous leaf: total pressure difference (AP) across the leaf was increased and the minimal value of EΔPτ0 was taken as the apparent cuticular transpiration rate ('Ec). 'Ec was treated relative to E at AP equal to zero (EGDP=0), E'cr. Measurements were carried out under two leaf-air vapour pressure differences (VPD).Er (i.e. EGDPτ0/EGDP=0) versus GDP patterns differed qualitatively between the investigated C3 and C4 plants. TE increased and 'Ecr decreased from tobacco, stramony, maize to sorghum for both VPD of air. 'Ecr and TE were approximately linearly related, the slope being dependent on VPD. The increase in VPD resulted in larger E and slightly smaller epidermal conductance (g) at GDP equal to zero. Both E'cr and E'cr decreased markedly at the same time especially, for species with high TE. The results were considered as an indirect confirmation that E'c values estimated by the technique used reflect species-specific differences in external peristomatal and cuticular vapour loss, at least in a relative sense.

Regulation of transpiration in field-grown sugarcane: Evaluation of the stomatal response to humidity with the Bowen ratio technique

Parallel time courses of net radiation, canopy conductance and evapotranspiration in a sugarcane canopy demonstrate the interaction of environmental and physiological factors in regulating water loss. The Bowen ratio-energy balance technique was used to evaluate the stomatal response to humidity in the absence of potential artifacts associated with porometer and leaf chamber measurements. Canopy conductance ( g c) responded to leaf-air vapor pressure difference ( V), supporting the validity of available physiological data showing clear stomatal responses to V in porometers and leaf chambers. Several factors were found to obscure the stomatal response to V under field conditions. The stomatal closing stimulus, V, and the opening stimulus, photon flux density ( I), were positively correlated. This caused offsetting stomatal responses to V and I, which reduced the net stomatal response when V changed with radiation-driven changes in leaf temperature. Normalization of g c by I revealed the expected hyperbolic decline of g c with increasing V. Low boundary layer conductance attenuated V imposed at the leaf surface relative to V measured with reference to humidity in the bulk atmosphere. This reduced the stimulus for stomatal response to V, below that indicated by agrometeorological measurements. Use of air saturation deficit ( D) rather than V to express evaporative demand overestimates V, minimizing apparent stomatal sensitivity to evaporative demand, when leaves are cooler than the air. These factors may result in small stomatal responses to V that escape detection by indirect measures of stomatal movement, such as those based on the temperature of exposed leaves.