Differential sensitivity of C3 and C4 turfgrass species to increasing atmospheric vapor pressure deficit

Temperature and vapor pressure deficit (VPD) effects on turfgrass growth are almost always confounded in experiments because VPD commonly is substantially increased in elevated-temperature treatments. The objective of this study as to examine specifically the influence of VPD on transpiration response of four ‘warm-season’ (C₄) and four ‘cool-season’ (C₃) turfgrasses to increasing VPD at a stable temperature (29.3 ± 1.5 °C). Although transpiration rates were noticeably lower in C₄ grasses, transpiration rates increased linearly in response to increasing VPD across the range of 0.8–3.0 kPa. In contrast, transpiration rates of C₃ increased sharply with increasing VPD across the range of low VPDs, but became constrained at higher VPDs (>1.35 kPa). Restricted transpiration rate at elevated VPD was most evident in Agrostis palustris and Lolium perenne. Assuming restricted transpiration rates reflect a limitation on leaf CO₂ uptake, these results indicate that the commonly observed decline in growth of C₃ (and success of C₄) grasses at elevated temperature may include a sensitivity to elevated VPD.     

Photosynthetic photon flux density,carbon dioxide concentration,and vapor pressure deficit effects on photosynthesis in cacao seedlings

Independent short-term effects of photosynthetic photon flux density (PPFD) of 50–400 μmol m⁻² s⁻¹, external CO₂ concentration (C _a) of 85–850 cm³ m⁻³, and vapor pressure deficit (VPD) of 0.9–2.2 kPa on net photosynthetic rate (P _N), stomatal conductance (g _s), leaf internal CO₂ concentration (C _i), and transpiration rates (E) were investigated in three cacao genotypes. In all these genotypes, increasing PPFD from 50 to 400 μmol m⁻² s⁻¹ increased P _N by about 50 %, but further increases in PPFD up to 1 500 μmol m⁻² s⁻¹ had no effect on P _N. Increasing C _a significantly increased P _N and C _i while g _s and E decreased more strongly than in most trees that have been studied. In all genotypes, increasing VPD reduced P _N, but the slight decrease in g _s and the slight increase in C _i with increasing VPD were non-significant. Increasing VPD significantly increased E and this may have caused the reduction in P _N. The unusually small response of g _s to VPD could limit the ability of cacao to grow where VPD is high. There were no significant differences in gas exchange characteristics (g _s, C _i, E) among the three cacao genotypes under any measurement conditions.     

A Method for Separating Cuticular and Stomatal Components of Gas Exchange by Amphistomatous Leaves: II. EXPERIMENTAL SOLUTION

The apparent cuticular component of transpiration of stomatabearing leaf epidermis was estimated by restricting stomataldiffusion by mass flow of air in the opposite direction. Thiswas achieved by applying an air pressure gradient across theamphistomatous leaf. Some assumptions of the previously suggestedmethod (antrcek and Slav?k, 1990) were experimentally verifiedusing maize leaves. The technique makes possible a quantitativeestimation of cuticular water loss including that of the externalperistomatal (i.e. vapour not passing through the pores) andthe respective conductance when the stomata are partially open. In addition to the fact that the cuticular portion of the totalleaf vapour loss (i.e. relative cuticular transpiration) dependson stomatal opening, even the absolute value of apparent cuticulartranspiration was (1) increased by lower vapour pressure deficitand (2) decreased with closing stomata. These changes, inducedby variations in a vapour pressure deficit of 2.45?0.35 kPa,ranged between 0.66?0.14µg cm ^–2 s^–1. Theabsolute value of apparent cuticular transpiration changed onaverage by a factor of 2.3 due to stomata opening or closingwhich was induced by turning the light on or by exogenous ABAapplication. Possible interference by residual vapour diffusingthrough the stomatal pore was evaluated by the model application.An attempt was also made to assess the cuticular component ofCO₂-uptake rate. Experimental results are discussed in contextwith the feedforward response of stomata to air humidity. Key words: Cuticular transpiration, cuticular CO₂-uptake, feedforward response, maize     

Humidity response of cuticular conductance of beech (Fagus sylvatica L.) leaf discs maintained at high relative water content

Cuticular conductance of adaxial (astomatous) and abaxial (stomatous)surfaces ofFagus sylvatica L. leaves was measured under varyingvapour pressure deficits (D). Conductance was determined fromgravimetric measurements of water flux made using a leaf discenvelope specially designed to maintain leaf relative watercontent and minimize reduction in cuticular hydration. The adaxialsurface provided a determination of ‘true’ cuticularconductance (g_c) and transpiration (E_c). The abaxial surfacewas used to estimate minimum leaf surface conductance (g^MIN_sur)and transpiration (E^MIN_sur). In experiment I, leaf discs wereplaced under one of a range of water vapour pressure deficits(0.4-2.0 kPa). Both g_c and g^MIN_sur decreased approximately 2-foldwith an increase in D between 0.4-2.0 kPa. The decrease in g_cwas linear, but g^MIN_sur declined more steeply at D between 0.4-0.95kPa than at D between 0.95-2.0 kPa. In experiment II, leaf discswere exposed to a stepwise change in D. After a period of acclimationto D of 0.95 kPa, responses of g_c and g^MIN_sur to an increaseor decrease in D were recorded. The response time of g_c to increasingor decreasing D were similar (<60 min). By contrast, g^MIN_surresponded more slowly to increasing than to decreasing D. Thesesignificant responses of g_c and g^MIN_sur to increasing and decreasingD are discussed in relation to hydration state of the cuticleand current knowledge of cuticle structure. Key words: Cuticle, cuticular conductance, cuticular membrane, Fagus sylvatica, humidity, vapour pressure deficit     

Temperature-Dependent Gas Exchange and Stomatal/Non-Stomatal Limitation to CO2 Assimilation of Quercus Liaotungensis under Midday High Irradiance

The effects of varying leaf temperature (T ₁) on some ecophysiological characteristics of photosynthesis for Quercus liaotungensis Koiz. under ambient radiation stress around midday on clear summer days were investigated using an IRGA equipped with a temperature-controlled cuvette. Net photosynthetic rate (P _N) decreased as T ₁ increased from 30 to 35 °C as a result of stomatal closure, whereas non-stomatal limitation led to decreased P _N in the T ₁ range of 35–45 °C. Decreased transpiration rate (E) and stomatal conductance (g _s) at leaf temperatures above 30 °C were interpreted as a combined feedward effect as a result of enhanced leaf-air vapour pressure deficit (VPD) and stomatal closure. Changes in E from T ₁ 30 to 20 °C depended on VPD when g _s was maintained constant. Water use efficiency (WUE) varied inversely with T ₁ by following a hyperbola. A decrease in intercellular CO₂ concentration (C _i) occurred as a result of stomatal closure and a relatively high carboxylation capacity, whereas inactivation of mesophyll carboxylation in combination with photorespiration might be associated with the observed increase in C _i in the T ₁ range of 40 to 45 °C.     

High vapour pressure deficit exacerbates xylem cavitation and photoinhibition in shade-grown Piper auritum H.B. & K. during prolonged sunflecks

Water relations dynamics during simulated sunflecks at high (36°C) and medium (27°C) temperatures and high and low vapour pressure deficits beween leaf and air (VPD) were studied on shade-grown Piper auritum H.B. & K. plants, a pioneer tree, common in gaps and clearings of tropical rain forests. The leaves of P. auritum wilt rapidly when exposed to high light. Exposure to high VPD and high light caused substantial and rapid dehydration of leaves. Dehydration could be prevented under high humidity irrespective of temperature. Water stored in leaf cells served as initial source for transpiration upon high light exposure. This effect increased with increasing VPD and temperature. The pronounced decrease in leaf water content over time in high light caused a rapid decrease in leaf water potential (Ψ_l) and a concomitant increase in water potential gradient (ΔΨ/Δx) between trunk and leaf, yet the high leaf elasticity (small bulk elastic modulus, ε) allowed turgor maintenance under most conditions. Under high VPD and high temperature, stomata remained open and ΔΨ/Δx frequently exceeded 0.95 MPa · m⁻¹, the cavitation-inducing threshold (ΔΨ/Δx _cav) causing high rates of acoustic emissions from stems and leaf petioles and leading to concomitant losses in hydraulic conductance per leaf area (k _l). At medium temperature (high VPD), stomatal closure contained xylem embolism by keeping ΔΨ/Δx at or below this threshold. We argue that wilting substantially contributes to creating a sufficient driving force for water uptake from the soil, and reducing the VPD (through a decrease in radiation load and thus leaf temperature) to avoid excessive dehydration. Received: 3 March 1996 / Accepted: 10 November 1996     

Response of transpiration to rain pulses for two tree species in a semiarid plantation

Responses of transpiration (E _c) to rain pulses are presented for two semiarid tree species in a stand of Pinus tabulaeformis and Robinia pseudoacacia. Our objectives are to investigate (1) the environmental control over the stand transpiration after rainfall by analyzing the effect of vapor pressure deficit (VPD), soil water condition, and rainfall on the post-rainfall E _c development and recovery rate, and (2) the species responses to rain pulses and implications on vegetation coverage under a changing rainfall regime. Results showed that the sensitivity of canopy conductance (G _c) to VPD varied under different incident radiation and soil water conditions, and the two species exhibited the same hydraulic control (?dG _c/dlnVPD to G _cref ratio) over transpiration. Strengthened physiological control and low sapwood area of the stand contributed to low E _c. VPD after rainfall significantly influenced the magnitude and time series of post-rainfall stand E _c. The fluctuation of post-rainfall VPD in comparison with the pre-rainfall influenced the E _c recovery. Further, the stand E _c was significantly related to monthly rainfall, but the recovery was independent of the rainfall event size. E _c enhanced with cumulative soil moisture change (ΔVWC) within each dry–wet cycle, yet still was limited in large rainfall months. The two species had different response patterns of post-rainfall E _c recovery. E _c recovery of P. tabulaeformis was influenced by the pre- and post-rainfall VPD differences and the duration of rainless interval. R. pseudoacacia showed a larger immediate post-rainfall E _c increase than P. tabulaeformis did. We, therefore, concluded that concentrated rainfall events do not trigger significant increase of transpiration unless large events penetrate the deep soil and the species differences of E _c in response to pulses of rain may shape the composition of semiarid woodlands under future rainfall regimes.     

Transpiration efficiency of the grapevine cv. Semillon is tied to VPD in warm climates

Water‐use efficiency in grapevines is dependent on the aerial and below‐ground environment of the plant. Specifically, transpiration efficiency, the ratio of net carbon fixation to water loss, may be influenced by soil moisture and the leaf‐to‐air vapour pressure deficit (VPD) in the soil–plant–atmosphere continuum. The interactive effect of these abiotic parameters, however, has not been suitably investigated in field‐grown grapevines. Accordingly, gas exchange of an anisohydric variety, Semillon, was assessed across a number of vineyards in two warm grape‐growing regions of New South Wales (NSW) to ascertain how soil moisture and VPD interact to affect transpiration efficiency at the leaf level. Leaf gas exchange measurements demonstrated that the rate of transpiration (E) was driven by VPD, particularly under high soil moisture. Both high VPD and low soil moisture decreased photosynthesis (A) and instantaneous leaf transpiration efficiency (A/E). Increased intrinsic leaf transpiration efficiency (A/g) in response to drying soil was limited to vines growing in a non‐irrigated vineyard. In this site, A/g was negatively related to vine water status. VPD did not have a substantial influence on A/g in any vineyard. While VPD is the main driver for A/E, soil moisture is an important determinant of A/g. Under high VPD, stomatal closure in Semillon leaves was not substantial enough to suitably curtail transpiration, and as a consequence A/E declined. These data indicate that in warm climates, irrigation scheduling of anisohydric varieties must take into account both VPD and soil moisture so that vine water status can be maintained.     

Transpiration of spring barley under nitrogen and phosphorus deficiency during leaf wilting

In an open gas exchange system with a thermocouple psychrometer the transpiration rate of the first leaf in 8-day plants of spring barley was measured in dependence on the water saturation deficit (ΔW _sat). The plants were cultivated in Richter’s nutrient solution, either complete, or deficient in nitrogen or phosphorus. The cuticular transpiration (as measured in the dark) was unaffected by N and P deficiency. The N deficiency reduced the transpiration rate by increasing stomatal resistance since full water saturation of the leaf (67% rate of the control variant) up to stomatal closing at Δ W_sat = 14%. The P deficiency does not affect the transpiration rate at initial phases of wilting, but the stomata close only at a higher Δ W_sat (25%) than those in the control.     

Whole-Plant Water use and Canopy Conductance of Cassava Under Iimited Available Soil Water and Varying Evaporative Demand

Cassava (Manihot esculenta Crantz), a perennial woody shrub, is known to be highly productive under favourable conditions and produce reasonably well under adverse conditions where other crops fail. Using constant heat sap flow sensors, sap flow density (F _d ) of cassava was monitored for 10 days in December 2002. Sap flow was highly correlated (R ² =0.72, P<0.05) to incoming solar radiation (R _s) than to other climatic factors. Using cross-correlation analysis, no time shift was detected between F _d and solar radiation, whereas vapour pressure deficit (VPD) lags F _d by 110 min. Solar radiation and VPD together explained 83% of diurnal variation in sap flow. Whole-plant transpiration ranged from 0.8 to 1.2 mm day⁻¹ and daily canopy conductance (g _c), computed based on the inverted Penman–Monteith model, varied between 0.7 and 2.1 mm s⁻¹ (mean = 1.4 ± 0.5 mm s⁻¹). For the measurement period, characterized by high evaporative demand coupled with low available soil water, transpiration accounted for 21% of the available energy and was only able to meet 24% of the atmospheric water demand. Average decoupling factor (Ω) of 0.05±0.02 estimated suggested that a 10% change in g _c may lead to more than 9% change in transpiration which further supports the notion that stomata play significant role in regulating cassava water use compared to other known mechanisms. Beyond light saturation (R _s >300 W m⁻²) and at higher VPD (>1.0 kPa), wind effects on the canopy transpiration under water stress condition were low, while VPD explains 94% of the observed variance in daily canopy conductance.     

Modelling Diurnal Courses of Photosynthesis and Transpiration of Leaves on the Basis of Stomatal and Non-Stomatal Responses,Including Photoinhibition

A mathematical model for photoinhibition of leaf photosynthesis was developed by formalising the assumptions that (1) the rate of photoinhibition is proportional to irradiance; and (2) the rate of recovery, derived from the formulae for a pseudo first-order process, is proportional to the extent of inhibition. The photoinhibition model to calculate initial photo yield is integrated into a photosynthesis-stomatal conductance (g _s) model that combines net photosynthetic rate (P _N), transpiration rate (E), and g _s, and also the leaf energy balance. The model was run to simulate the diurnal courses of P _N, E, g _s, photochemical efficiency, i.e., ratio of intercellular CO₂ concentration and CO₂ concentration over leaf surface (C _i/C _s), and leaf temperature (T ₁) under different irradiances, air temperature, and humidity separately with fixed time courses of others. When midday depression occurred under high temperature, g _s decreased the most and E the least. The duration of midday depression of g _s was the longest and that in E the shortest. E increased with increasing vapour pressure deficit (VPD) initially, but when VPD exceeded a certain value, it decreased with increasing VPD; this was caused by a rapid decrease in g _s. When air temperature exceeded a certain value, an increase in solar irradiance raised T ₁ and the degree of midday depression. High solar radiation caused large decrease in initial photon efficiency (). P _N, E, and g _s showed reasonable decreases under conditions causing photoinhibition compared with non-photoinhibition condition under high irradiance. The T ₁ under photoinhibition was higher than that under non-photoinhibition conditions, which was evident under high solar irradiance around noon. The decrease in C _i/C _s at midday implies that stomatal closure is a factor causing midday depression of photosynthesis.     

Cuticular Conductance and the Humidity Response of Stomata

Meidner, H. 1986. Cuticular conductance and the humidity responseof stomata.—J. exp. Bot. 37: 517–525. Detailed measurements of cuticular vapour loss from leaves ofseveral species showed that cuticular conductance declined froman early morning maximum of 0?02 cm s^–1 to between 0?004and 0.005 cm scm s^–1 even in the absence of stomatal transpiration.Re-establishment of the maximum conductance occurred only ina humid atmosphere and when the xylem system was under pressure(simulated mild root pressure) Cuticular vapour loss alone is,therefore, unlikely to be the underlying mechanism of the humidityresponse of Stomata. Evidence for the existence of a humidity-sensing feed-forwardmechanism is discussed and it is shown that when detailed measurementsare made the humidity response is found to have two phases.This indicates a perturbation of the fine turgor balance betweenepidermat and guard cells that exists in a transpiring leaf.It is argued that the humidity response can be accounted forby reference to hydropassive movements which initiate a metabolicadjustment of the guard cells to altered evaporative demand. Key words: Cuticle, conductance, humidity, stomata, transpiration     

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 ¹³C of plant parts sampled at mid-grain filling and maturity. The differences between treatments were maximal in mature grains, where high VPD increased ¹³C 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 (P _N) of the flag leaves decreased significantly under water stress at both VPD regimes. However, P _N 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 P _N 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.     

Apparent responses of stomata to transpiration and humidity in a hybrid poplar canopy

It has been proposed that the stomatal response to humidity relies on sensing of the transpiration rate itself rather than relative humidity or the saturation deficit per se. We used independent measurements of stomatal conductance (g_s), transpiration (E), and leaf-to-air vapour pressure difference (V) in a hybrid poplar canopy to evaluate relationships between g_s and E and between g_s and V. Relationships between E, V and total vapour phase conductance or crown conductance (g_c) were also assessed. Conductance measurements were made on exposed and partially shaded branches over a wide range of incident solar radiation. In exposed branches, g_s appeared to decline linearly with increasing E and increasing V at both high and low irradiance. However, in a partially shaded branch, a bimodal relationship between g_s and E was observed in which g_s continued to decrease after E had reached a maximum value and begun to decrease. The relationship between g_s and V for this branch was linear. Plots of g_c against E always yielded bimodal or somewhat variable relationships, whereas plots of g_c against V were invariably linear. It was not possible to derive a unique relationship between conductance and E or V because prevailing radiation partially determined the operating range for conductance. Normalization of data by radiation served to linearize responses observed within the same day or type of day, but even after normalization, data collected on partly cloudy days could not be used to predict stomatal behaviour on clear days and vice versa. An additional unidentified factor was thus also involved in determining operating ranges of conductance on days with different overall radiation regimes. We suggest that the simplest mechanism to account for the observed humidity responses is stomatal sensing of the epidermal or cuticular transpiration rate rather than the bulk leaf or stomatal transpiration rate.     

Stomatal sensitivity to carbon dioxide and humidity: a comparison of two c(3) and two c(4) grass species

The sensitivity of stomatal conductance to changes of CO₂ concentration and leaf-air vapor pressure difference (VPD) was compared between two C₃ and two C₄ grass species. There was no evidence that stomata of the C₄ species were more sensitive to CO₂ than stomata of the C₃ species. The sensitivity of stomatal conductance to CO₂ change was linearly proportional to the magnitude of stomatal conductance, as determined by the VPD, the same slope fitting the data for all four species. Similarly, the sensitivity of stomatal conductance to VPD was linearly proportional to the magnitude of stomatal conductance. At small VPD, the ratio of intercellular to ambient CO₂ concentration, C_i/C_a, was similar in all species (0.8-0.9) but declined with increasing VPD, so that, at large VPD, C_i/C_a was 0.7 and 0.5 (approximately) in C₃ and C₄ species, respectively. Transpiration efficiency (net CO₂ assimilation rate/transpiration rate) was larger in the C₄ species than in the C₃ species at current atmospheric CO₂ concentrations, but the relative increase due to high CO₂ was larger in the C₃ than in the C₄ species.     

Acclimation to humidity modifies the link between leaf size and the density of veins and stomata

The coordination of veins and stomata during leaf acclimation to sun and shade can be facilitated by differential epidermal cell expansion so large leaves with low vein and stomatal densities grow in shade, effectively balancing liquid‐ and vapour‐phase conductances. As the difference in vapour pressure between leaf and atmosphere (VPD) determines transpiration at any given stomatal density, we predict that plants grown under high VPD will modify the balance between veins and stomata to accommodate greater maximum transpiration. Thus, we examined the developmental responses of these traits to contrasting VPD in a woody angiosperm (Toona ciliata M. Roem.) and tested whether the relationship between them was altered. High VPD leaves were one‐third the size of low VPD leaves with only marginally greater vein and stomatal density. Transpirational homeostasis was thus maintained by reducing stomatal conductance. VPD acclimation changed leaf size by modifying cell number. Hence, plasticity in vein and stomatal density appears to be generated by plasticity in cell size rather than cell number. Thus, VPD affects cell number and leaf size without changing the relationship between liquid‐ and vapour‐phase conductances. This results in inefficient acclimation to VPD as stomata remain partially closed under high VPD.     

Ontogenetic and seasonal development of wax composition and cuticular transpiration of ivy (Hedera helix L.) sun and shade leaves

The ontogenetic and seasonal development of wax composition and cuticular transpiration of sun and shade leaves of ivy (Hedera helix L.) was analysed by investigating leaves varying in age between 4 and 202 d. It was discovered that the total amount of solvent-extractable wax was composed of two distinct fractions, separable by column chromatography: (i) a less polar or apolar monomeric wax fraction consisting of the typical linear, long-chain aliphatics usually described as cuticular wax components and (ii) a polar, oligomeric wax fraction consisting of primary alcohols and acids mostly esterified to C₁₂-, C₁₄- and C₁₆-ω-hydroxyfatty acids. The apolar wax fraction, which could be analysed directly by gas chromatography coupled with mass spectrometry (GC-MS), exhibited pronounced seasonal changes in composition. Wax amounts in the apolar fraction reached a maximum after about 30 d and gradually decreased again during the remaining period of the season investigated. In contrast, the polar wax fraction, which was analysable by GC-MS only after transesterification, rapidly increased early in the season, reaching a plateau after 40 d, and then remained constant during the rest of the season. Thus, total amounts of solvent-extractable cuticular waxes, which can be determined gravimetrically, will only be detected by GC-MS after fractionation and transesterification, a methodological approach rarely applied in the past in cuticular wax analysis. Additionally, investigation of the cutin polymer matrix after depolymerisation through transesterification, revealed that only those primary alcohols and acids forming an essential part of the apolar and the polar wax fractions were esterified during the investigated season and incorporated in increasing amounts into the cutin polymer matrix (matrix-bound wax fraction). Thus, it can be concluded that a complete analysis of cuticular wax of ivy and its seasonal development can only be achieved if all the relevant fractions (i) the less polar or apolar, (ii) the polar and (iii) the wax fraction bound to the cutin polymer matrix are investigated. Cuticular transpiration rapidly decreased within the first 30 d and essentially remained constant during the rest of the season. Thus, changes in cuticular water permeability were closely correlated with the most prominent changes in wax amounts and composition occurring during the first 30 d of ontogenetic leaf development. However, during the remainder of the year, up to 202 d, cuticular transport properties remained constant, although significant quantitative and qualitative changes in cuticular wax composition continued to occur. Thus, our study clearly demonstrated that there will be no simple relationship between chemical composition of cuticular waxes and transport properties of isolated ivy leaf cuticles. Received: 2 March 1998 / Accepted: 26 June 1998     

Control of Photosynthesis and Stomatal Conductance in Ricinus communis L. (Castor Bean) by Leaf to Air Vapor Pressure Deficit

Castor bean (Ricinus communis L.) has a high photosynthetic capacity under high humidity and a pronounced sensitivity of photosynthesis to high water vapor pressure deficit (VPD). The sensitivity of photosynthesis to varying VPD was analyzed by measuring CO₂ assimilation, stomatal conductance (g_s), quantum yield of photosystem II (_II), and nonphotochemical quenching of chlorophyll fluorescence (q_N) under different VPD. Under both medium (1000) and high (1800 micromoles quanta per square meter per second) light intensities, CO₂ assimilation decreased as the VPD between the leaf and the air around the leaf increased. The g_s initially dropped rapidly with increasing VPD and then showed a slower decrease above a VPD of 10 to 20 millibars. Over a temperature range from 20 to 40°C, CO₂ assimilation and g_s were inhibited by high VPD (20 millibars). However, the rate of transpiration increased with increasing temperature at either low or high VPD due to an increase in g_s. The relative inhibition of photosynthesis under photorespiring (atmospheric levels of CO₂ and O₂) versus nonphotorespiring (700 microbars CO₂ and 2% O₂) conditions was greater under high VPD (30 millibars) than under low VPD (3 millibars). Also, with increasing light intensity the relative inhibition of photosynthesis by O₂ increased under high VPD, but decreased under low VPD. The effect of high VPD on photosynthesis under various conditions could not be totally accounted for by the decrease in the intercellular CO₂ in the leaf (C_i) where C_i was estimated from gas exchange measurements. However, estimates of C_i from measurements of _II and q_N suggest that the decrease in photosynthesis and increase in photorespiration under high VPD can be totally accounted for by stomatal closure and a decrease in C_i. The results also suggest that nonuniform closure of stomata may occur in well-watered plants under high VPD, causing overestimates in the calculation of C_i from gas exchange measurements. Under low VPD, 30°C, high light, and saturating CO₂, castor bean (C₃ tropical shrub) has a rate of photosynthesis (61 micromoles CO₂ per square meter per second) that is about 50% higher than that of tobacco (C₃) or maize (C₄) under the same conditions. The chlorophyll content, total soluble protein, and ribulose-1,5-bisphosphate carboxylase/oxygenase level on a leaf area basis were much higher in castor bean than in maize or tobacco, which accounts for its high rates of photosynthesis under low VPD.     

Alteration of transpiration rate, by changing air vapour pressure deficit, influences leaf extension rate transiently in Miscanthus

A controlled environment chamber for whole plants is described in which vapour pressure deficit (VPD) and temperature can be controlled independently. Plant responses to changes in VPD at constant temperature were measured in terms of leaf extension and plant transpiration rates. Manipulation of VPD independently of temperature was shown to be capable of altering the leaf extension rates of the C4 grass Miscanthus x giganteus grown in hydroponics. The effects of VPD on leaf extension are attributed to changes in transpiration rate and hence leaf water status. It was found that, at a temperature of 20C, the influence of a fixed change in VPD was proportionally less than those observed at temperatures which are close to the threshold for growth (between 6 and 10C). These responses are discussed in relation to our current understanding of the mechanisms of cell growth. The fact that the VPD effects on leaf expansion rates were largely transient suggest that simple models driven by temperature alone are adequate to predict leaf expansion within the temperature range 6-20°C, for this genotype of Miscanthus, in the field.Key words: Leaf growth, Miscanthus x giganteus, temperature, vapour pressure deficit, C4 plants.      

Vapour Pressure Deficit Response of Cuticular Conductance in Intact Leaves of Fagus sylvatica L.

The cuticular conductance (g_c) of the astomatous adaxial surfaceof Fagus sylvatica L. leaves was determined under varying vapourpressure deficits at a constant temperature of 20 °C. Cuticularconductance was determined from the weight loss of detachedleaves after both the stomatous abaxial surface and the cutend of the petiole had been sealed using low melting-point paraffinwax. Cuticular conductance was found to decrease as the watervapour pressure was increased in steps. No response was observedwhen vapour pressure deficit was decreased from an initiallyhigh value. It is concluded that these results are consistentwith the hypothesis that cuticular conductance is influencedby the water content of the cuticle and that the apparent hysteresisis likely to be a result of the long time-constant for the processof cuticle rehydration in intact leaves. Key words: Cuticle, cuticular conductance, Fagus sylvatica L., Fagaceae, vapour pressure deficit, VPD