Bulk Leaf Water Potential Research Articles

Sensitivity of olive leaf turgor to air vapour pressure deficit correlates with diurnal maximum stomatal conductance

Effective study and management of crops and forests would benefit greatly from useful plant-based indicators of the biological controls on evapotranspiration, and particularly stomatal conductance (gs). Given the strong influence of gs on bulk leaf water potential and turgor pressure (P), in vivo measurement of P may provide useful information about diurnal or seasonal dynamics of gs. Moderate plant water stress affects the diurnal dynamics of P as leaf-to-air vapour pressure deficit (D) varies, and these dynamics correlate to gs. Here, we explored relative changes in P in response to changes in D under mild drought conditions, and how these changes are linked to stomatal behaviour, and specifically to diurnal maximum gs (gs,max), one of the best indicators of plant water stress. We monitored ecophysiological and environmental variables, as well as a relative proxy for P, during three consecutive seasons in a hedgerow olive orchard where trees were supplied with different irrigation treatments to create well-watered and moderately water-stressed conditions. Our results demonstrated that the sensitivity of P to D correlated well with gs,max reached by the trees within a range in which variations in gs are the main diffusional limitation to photosynthesis. We further showed that this correlation held under a wide range of meteorological conditions and soil water availability. This turgor proxy measurement, which is much easier to measure than gs, can facilitate the use of gs,max as an indicator of plant water stress and evapotranspiration in agriculture and plant science research.

Hydrostatic constraints on morphological exploitation of light in tall Sequoia sempervirens trees

We studied changes in morphological and physiological characteristics of leaves and shoots along a height gradient in Sequoia sempervirens, the tallest tree species on Earth, to investigate whether morphological and physiological acclimation to the vertical light gradient was constrained by hydrostatic limitation in the upper crown. Bulk leaf water potential (Psi) decreased linearly and light availability increased exponentially with increasing height in the crown. During the wet season, Psi was lower in the outer than inner crown. C isotope composition of leaves (delta(13)C) increased with increasing height indicating greater photosynthetic water use efficiency in the upper crown. Leaf and shoot morphology changed continuously with height. In contrast, their relationships with light availability were discontinuous: morphological characteristics did not correspond to increasing light availability above 55-85 m. Mass-based chlorophyll concentration (chl) decreased with increasing height and increasing light availability. In contrast, area-based chl remained constant or increased with increasing height. Mass-based maximum rate of net photosynthesis (P (max)) decreased with increasing height, whereas area-based P (max) reached maximum at 78.4 m and decreased with increasing height thereafter. Mass-based P (max) increased with increasing shoot mass per area (SMA), whereas area-based P (max) was not correlated with SMA in the upper crown. Our results suggest that hydrostatic limitation of morphological development constrains exploitation of light in the upper crown and contributes to reduced photosynthetic rates and, ultimately, reduced height growth at the tops of tall S. sempervirens trees.

Heterogeneity of gas exchange rates over the leaf surface in tobacco: an effect of hydraulic architecture?

Spatial heterogeneity of gas exchange rates in the leaves of Nicotiana tabacum L. (tobacco) was investigated. Leaf conductance to water vapour was higher (by about 18%) at the apical regions of leaves than at the basal ones. Local, small-scale measurements of pressure-volume (PV) parameters and water status (performed with a dewpoint hygrometer) revealed that bulk leaf water potential, osmotic potential, turgor pressure and bulk modulus of elasticity were not significantly different in the leaf apex or base. Hydraulic measurements showed that the apical regions of the leaf blade were about 30% more conductive than the basal regions. Such differences were explained by analogous differences in terms of venation patterns. In fact, vein density turned out to be higher (by about 13%) near the leaf apex with respect to the leaf base. On the contrary, stomatal density was the same both in the apical and basal leaf portions. Our data suggest that spatial stomatal heterogeneity may arise from heterogenous distribution of local hydraulic resistances and would be addressed to maintaining local water potential above critical values, possibly triggering vein cavitation.

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.

Stomatal conductance and leaf water potential responses to hydraulic conductance variation in Pinus pinaster seedlings

In this study, tree hydraulic conductance (K tree) was experimentally manipulated to study effects on short-term regulation of stomatal conductance (g s), net photosynthesis (A) and bulk leaf water potential (Ψleaf) in well watered 5–6 years old and 1.2 m tall maritime pine seedlings (Pinus pinaster Ait.). K tree was decreased by notching the stem and increased by progressively excising the root system and stem. Gas exchange was measured in a chamber at constant irradiance, vapour pressure deficit, leaf temperature and ambient CO2 concentration. As expected, we found a strong and positive relationship between g s and K tree (r = 0.92, P = 0.0001) and between A and K tree (r = 0.9, P = 0.0001). In contrast, however, we found that the response of Ψleaf to K tree depended on the direction of change in K tree: increases in K tree caused Ψleaf to decrease from around −1.0 to −0.6 MPa, but reductions in K tree were accompanied by homeostasis in Ψleaf (at −1 MPa). Both of these observations could be explained by an adaptative feedback loop between g s and Ψleaf, with Ψleaf prevented from declining below the cavitation threshold by stomatal closure. Our results are consistent with the hypothesis that the observed stomatal responses were mediated by leaf water status, but they also suggest that the stomatal sensitivity to water status increased dramatically as Ψleaf approached −1 MPa.

Dynamics of stomatal water relations following leaf excision

We examined the stomatal response to leaf excision in an evergreen woody shrub, Photinia x fraseri, using a novel combination of gas exchange, traditional water relations and modelling. Plants were kept outdoors in mild winter conditions (average daily temperature range: -1 to 12 degrees C) before being transferred to a glasshouse (temperature range: 20-30 degrees C) and allowed to acclimate for different periods before experiments. 'Glasshouse plants' were acclimated for at least 9 d, and 'outdoor plants' were acclimated for fewer than 3 d before laboratory gas exchange experiments. The transient stomatal opening response to leaf excision was roughly twice as long in outdoor plants as in glasshouse plants. To elucidate the reason for this difference, we inferred variables of stomatal water relations (epidermal and guard cell turgor pressures and guard cell osmotic pressure: Pe, Pg and pi g, respectively) from stomatal conductance (gs) and bulk leaf water potential (psi l), using a hydromechanical model of gs. psi l was calculated from cumulative post-excision transpirational water loss using empirical relationships between psi l and relative water content obtained on similar leaves. Inferred Pg and Pe both declined immediately after leaf excision. Inferred pi g also declined after a lag period. The kinetics of pi g adjustment after the lag were similar in outdoors and glasshouse plants, but the lag period was much longer in outdoor plants. This suggests that the longer transient opening response in outdoor plants resulted from slower induction, not slower execution, of guard cell osmoregulation. We discuss the implications of our results for the mechanism of short-term stomatal responses to hydraulic perturbations, for dynamic modelling of gs and for leaf water status regulation.

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.

Air humidity as an ecological factor for woodland herbs: leaf water status, nutrient uptake, leaf anatomy, and productivity of eight species grown at low or high vpd levels

Summary Ecologists have widely neglected the long term impact of vpd (the water vapor saturation deficit of the air) on plant growth and development. This paper tests the hypothesis that the prevailing vpd level during growth has a significant influence on the ecology of temperate woodland herbs, independently of soil moisture status. Eight herbaceous species ( Scrophularia nodosa, Digitalis purpurea, Campanula trachelium, Rumex sanguineus, Geum urbanum, Hieracium sylvaticum, Hordelymus europaeus, Brachypodium sylvaticum ) were grown in climate chambers at four different saturation deficits (vpd = 120, 580, 930 or 1210 Pa) with unlimited water supply in solution cultures in order to experimentally analyse long-term effects of vpd on shoot water relations, plant morphology and plant growth rate. In their natural forest floor environment these species are exposed to vpd levels of 200 to 600 Pa on summer days. After 50 to 90 days of cultivation I compared the four vpd treatments with respect to dry matter production, leaf anatomy, leaf conductance, transpiration rate, bulk leaf water potential (χ l ), leaf water content (θ l ), and shoot nutrient concentrations. Productivity was significantly higher at 120 than at 580, 930 or 1210 Pa in seven of eight species. Plants grown at low vpd had larger leaves and leaf epidermal cells, and a lower stomatal frequency than plants of high-vpd treatments which indicates a stimulation of leaf expansion by high air humidities. Low saturation deficits also affected leaf anatomy (less compact leaf mesophyll with larger fractional air space) but did not alter mesophyll cell dimensions. Leaf water potential and leaf water content were more favorable at 120 Pa than at higher vpd levels in all eight species although a clear dependence of leaf water status (χ l and θ l ) on transpiration rate did not exist. Shoot nutrient concentrations generally were lower at low than high vpd which reflects a nutrient dilution effect caused by an increased carbon assimilation of the low vpd plants. In contrast, no correlation was found between transpiration rate and shoot concentrations of Ca or other mineral nutrients. I conclude that vpd acts as a soil water-independent growth-controlling factor for hygromorphic woodland herbs. Environments with a vpd of 100 to 500 Pa improve leaf water status and stimulate leaf expansion of these species which leads to enhanced growth. Other growth-relevant parameters (leaf conductance, transpiration rate and nutrient uptake), which were also affected by vpd, had only a minor or no influence on productivity.

Field investigations of photosynthetic activity, gas exchange and water potential at different leaf ages in Welwitschia mirabilis during a severe drought

Drought stress amplified the age-dependent gradient in photosynthetic activity in leaves of Welwitschia mirabilis which has been previously demonstrated in well-watered plants. However, neither predawan nor daytime leaf water potential changed with leaf age under conditions of limited soil water availability. During the course of a day the combination of increasing leaf water deficit, high temperature, high water vapour deficit of the air and high radiation load seriously disrupted carbon metabolism. This led to CO 2 loss and, thus, a negative daily carbon balance with largest losses in the older part of the leaf. Stomatal conductance was greatly diminished most of the day except in the early light period. Reduction of photosynthetic efficiency was preceded by photoinhibition as indicated by a decrease in the photochemical efficiency of photo system II at the highest leaf temperature. Again, this reduction was more pronounced in the older leaf sections. Additionally, it was demonstrated that the estimation of bulk leaf water potential using leaf slices of Welwitschia mirabilis with the pressure chamber gave meaningful results; this was the most useful method for field investigations ofleaf water potential on such a difficult material.

Influence of leaf water status on stomatal response to humidity, hydraulic conductance, and soil drought in Betula occidentalis

Whole-canopy measurements of water flux were used to calculate stomatal conductance (g s ) and transpiration (E) for seedlings of western water birch (Betula occidentalis Hook.) under various soil-plant hydraulic conductances (k), evaporative driving forces (ΔN; difference in leaf-to-air molar fraction of water vapor), and soil water potentials (Ψs). As expected, g s dropped in response to decreased k or ΨS, or increased ΔN(> 0.025). Field data showed a decrease in mid-day g s with decreasing k from soil-to-petiole, with sapling and adult plants having lower values of both parameters than juveniles. Stomatal closure prevented E and Ψ from inducing xylem cavitation except during extreme soil drought when cavitation occurred in the main stem and probably roots as well. Although all decreases in g s were associated with approximately constant bulk leaf water potential (ψl), this does not logically exclude a feedback response between ΨL and g s . To test the influence of leaf versus root water status on g s , we manipulated water status of the leaf independently of the root by using a pressure chamber enclosing the seedling root system; pressurizing the chamber alters cell turgor and volume only in the shoot cells outside the chamber. Stomatal closure in response to increased ΔN, decreased k, and decreased ΨS was fully or partially reversed within 5 min of pressurizing the soil. Bulk ΨL remained constant before and after soil pressurizing because of the increase in E associated with stomatal opening. When ΔN was low (i.e., < 0.025), pressurizing the soil either had no effect on g s , or caused it to decline; and bulk ΨL increased. Increased Ψl may have caused stomatal closure via increased backpressure on the stomatal apparatus from elevated epidermal turgor. The stomatal response to soil pressurizing indicated a central role of leaf cells in sensing water stress caused by high ΔN, low k, and low ΨS. Invoking a prominent role for feedforward signalling in short-term stomatal control may be premature.

Seasonal variation in competitive effect on water stress and pine responses

Plant mixtures were established that differed in both proportion and density of loblolly pine (Pinustaeda L.), sweetgum (Liquidambarstyraciflua L.), and broomsedge (Andropogonvirginicus L.). Soil moisture availability to the pine seedlings was quantified every 2 weeks by measuring predawn xylem pressure potentials. Temporal variation in pine water potential was accounted for by a water stress integral approach. Cumulative water stress integral values were calculated over four overlapping periods, from May to June, May to July, May to August, and May to September and compared with the mean seedling stem volume index at each period to determine competitive responses at the whole plant scale. Diurnal measures of stomatal conductance were taken each month to compare competitive responses at the leaf scale. In addition, environmental and plant responses that may control stomatal behavior were quantified. The pine water stress integral was strongly influenced by competing vegetation after the onset of a period of drought in early summer. The correlation between the water stress integral and pine growth increased after a significant drying period, accounting for more than half of the variation in stem volume index at the end of the first growing season. Stomatal conductance was also influenced by competition, with competitive effects more evident during times of drought. Conductance was most often related to bulk leaf water potential, which in turn was related to competitive effects on soil moisture availability. Vapor pressure deficit also influenced stomatal conductance, but this was largely unrelated to competitive effects.

Leaf illumination and root cooling inhibit bean leaf expansion by decreasing turgor pressure

Phaseolus vulgaris plants with expanding primary leaves were subjected to dark-light or light-dark transition at a root temperature of 25 o C, or to root cooling to 10 o C. Illumination or darkening caused rapid changes in water flux through the plants and in epidermal turgor pressure when analysed by pressure probe. However, these were not concurrent with variations in bulk leaf water potential and turgor pressure as determined by the pressure chamber method. In addition, the turgor pressure of epidermis measured with the pressure probe was invariably 0.05 to 0.15 MPa lower than that measured in bulk tissue with the pressure chamber. Cooling roots to 10 o C induced water stress and wilting

Responses of Soybean Leaf Angle, Photosynthesis and Stomatal Conductance to Leaf and Soil Water Potential

The hypothesis that soil watrer potential is better correlated to heliotropic leaf orientation, photosaturated photosynthetic CO 2 assimilation and stomatal conductance during periods of limited water availability than is bulk leaf water potential was examined in greenhouse-grown soybean (Glycine max) plants, submitted to a progressive drought. Paired plants were exposed to either 1000 or 100 μmol m −2 s −1 photon flux debsities for 45-60 mins.

Ecophysiological Responses in Mesic versus Xeric Hardwood Species to an Early-Season Drought in Central Pennsylvania

Abstract Field and laboratory studies were used to evaluate net photosynthesis and plant and tissue water relations in saplings of co-occurring xeric (Quercus prinus and Q. ilicifolia) versus mesic (Q. rubra and Castanea dentata) hardwood species during a droughty summer. Mean daily net photosynthesis, leaf conductance, leaf water potential, and water use efficiency were reduced in all four species during the peak drought period. Throughout the study, net photosynthesis and leaf conductance were on average high in Q. ilicifolia and Q. prinus, intermediate in Q. rubra, and low in C. dentata. Leaf water potential was generally highest in C. dentata and not significantly different among the Quercus species. There were few consistent patterns in the tissue water relations data, including a lack of osmotic adjustment during peak drought, a lack of correlation between osmotic potentials and gas exchange rates, decreases in osmotic potentials and tissue elasticity after the peak drought period ended, and bulk leaf water potential values below the osmotic potential at zero turgor, despite relatively high gas exchange in those plants. Thus, the tissue water relations data provided little insight to mechanisms of drought tolerance in these species and suggest some methodology problems using standard pressure-volume procedures. For. Sci. 36(4):970-981.

Cellular Processes Limiting Leaf Growth in Plants under Hypoxic Root Stress

The effects of root hypoxia on leaf growth of a Populus trichocarpa x deltoides hybrid have been assessed. Clonal plants were subjected to hypoxic root conditions in pot culture by flooding and in solution culture by gassing with nitrogen. The rate of leaf expansion declined within 8 h and was suppressed for the duration of the treatment. Final leaf size was reduced by 35% to 60% compared to aerated plants. Final epidermal cell size and number depended both on the developmental stage of the leaf at the onset of stress and on the duration of the treatment. No differences in bulk leaf water potential were measured between the hypoxic and aerated plants. Cell wall extensibility was lower, leaf solute potential was more negative and turgor potential was higher in leaves of hypoxia-treated plants than of aerated plants. These data suggest that leaf growth of hypoxia-stressed plants is limited by cell wall extensibility. The mechanism by which the root stress induces changes in leaf cell wall characteristics is not known.

The effects of root treatments on growth of the primary leaves of Phaseolus vulgaris L.: General features.

Seedlings of Phaseolus vulgaris, grown at 22.5 °C in moist vermiculite for 7 days, were transferred to nutrient solution held either at ambient temperature or at 10 °C, or subjected to a root-pruning treatment which removed all lateral roots and part of the main root. The effect of both treatments was to reduce primary leaf growth rate, within 3-6 h in the one case and 12 h in the other. The lower growth rate was maintained for the whole of the expansion period so that the final size of primary leaves on treated plants was only 35-40% of the controls. The smaller leaf size was caused by a reduction in average cell size judged from epidermal cell surface area and anatomical studies. Cell number per leaf was not affected by treatment. Root cooling caused an ephemeral initial fall in bulk leaf water potential, estimated by pressure chamber, but water stress was not thought to be the cause of the inhibition of leaf growth.

Dependence of Stomatal Conductance on leaf Water Potential, Turgor Potential, and Relative Water Content in Field‐Grown Soybean and Maize1

Soybean [Glycine max. (L.) Merr.] and maize (Zea mays L.) were grown in field plots on an Arredondo fine sand (loamy, siliceous hyperthermic Grossarenic Paleudult) and subjected to either five or four water management treatments in 1985 and 1986, respectively. Treatments included daily irrigation, irrigation at either 4‐ or 8‐day intervals, irrigation to maintain a slight reduction in stomatal conductance, and no irrigation. The objective of the study was to investigate the hypothesis that stomatal conductance (gs) is more closely related to leaf relative water content (RWC) than to either bulk leaf water (ψL) and xylem potential (ψx) or leaf turgor potential (ψp), of which have been frequently used to describe leaf water status. An important feature of this study was an attempt to characterize the waterelations and stomatal conductance of field‐grown crops subjected only to mild water deficits. A comparison of the coefficients of variation among the parameters used to evaluate leaf water status showed the greatest value for ψp, followed by ψL, with RWC having the smallest. When mild plant water deficits were imposed, gs was significantly decreased without decreases in either ψL, ψp, or RWC. There were no significant relationships between gs and any of the measures of leaf water status when data from only the daily irrigated and mildly stressed treatments were pooled. However, linear regressions of gs vs. ψL, ψp, ψx, and RWC were often statistically significant when data from the nonirrigated, severely stressed treatment were included in the analyses. We conclude that bulk leaf measurements of ψL, ψp, ψx, and RWC cannot be used to describe accurately the response of gs to soil dehydrations that result in only mild plant water stress in field‐grown soybean and maize.

Effect of Stem Excision Under Water on Bulk Leaf Water Potential, Leaf Conductance, CO2 Assimilation and Stemwood Water Storage in Eucalyptus behriana F. Muell

Cutting the stem, under water and above the lignotuber, of intact trees of Eucalyptus behriana caused rapid increases in the bulk leaf water potential (Ψ) in the canopy above. In eight separate experiments the mean increase in Ψ of 3.1 MPa in 1 h was associated with rapid and persistent decreases in leaf conductance (g), which have been ascribed to hydropassive changes in leaf turgor. The water content of the stemwood increased by 1.8-2.0% as the bulk water potential increased by approximately 2.5 MPa. In the first hour after cutting the stems under water, the observed rate of water uptake by the cut stem was approximately six times the estimated transpiration rate of the leaves. This discrepancy is attributed to an increase in stem water content. The specific capacitance of the stemwood was estimated to be 0.008 g g-1 (dry weight) Mpa-1. The quantity of water available from storage in the stem was estimated to be less than 0.1 mm. Continuous measurements of gas exchange on a leaf before, during and after stem-cutting showed that CO2 assimilation decreased in parallel with g, but in a manner that caused increases in intercellular CO2 concentration. This indicated that the rapid increases of bulk leaf turgor, induced by stem-cutting, affected photosynthetic capacity.

Gas Exchange, Water Relations and Ion Concentrations of Leaves of Salt Stressed 'Valencia' Orange, Citrus sinensis (L.) Osbeck

'Valencia' orange [Citrus sinensis (L.) Osbeck] scions grafted on sweet orange [C. sinensis (L.) Osbeck cv. Parramatta sweet orange] rootstock were grown in soil culture under controlled environmental conditions. Salt stress was imposed by adding NaCl to the nutrient solution in increments of 5 mol m-3 per day to a final concentration of 50 mol m-3. Leaf gas exchange, water relations and sodium, chloride and potassium concentrations were monitored until 89 days after commencement of salt treatment. Initial CO2 assimilation rates were relatively low (2.8-4.4 �mol CO2 m-2 s-1) and were stimulated by 72-86% when ambient oxygen partial pressure was reduced from 210 mbar to 21 mbar. After 14 days salt treatment, there was an increase in assimilation rate of approximately 20% associated with a decrease in osmotic potential (π) of 0.6 MPa. Reduction in � occurred without foliar ion accumulation. Assimilation rates gradually declined thereafter, averaging less than 1 �mol CO2m-2 s-1 at day 89. Lower CO2 assimilation rates were not a consequence of increased photorespiration as no change in the extent of oxygen inhibition of CO2 assimilation or CO2 compensation point occurred with salinisation. Stomatal conductance appeared less sensitive to salt treatment than intrinsic photosynthesis, resulting in higher intercellular partial pressures of CO2 in salt stressed leaves (291 cf. 259 pbar for controls at day 89). Water use efficiency was accordingly lower in salt affected leaves. Salinised leaves had consistently more negative osmotic potentials than control leaves; turgor potential was thus maintained at or above control levels for a given bulk-leaf water potential. Since leaf turgor was maintained via osmotic adjustment and uptake of sodium and chloride, lower assimilation rates were attributed to a toxic ion effect.