Mechanism of the inhibition of phloem loading by sodium sulfite: Effect of the pollutant on the transmembrane potential difference

Sodium sulfite (Na₂SO₃) decreased uptake from 1 m M sucrose by the parenchyma and by the veins of leaves of broadbean ( Vicia faba L. cv. Aguadulce). The decrease depended on the concentration of the pollutant and the duration of pretreatment. The inhibition was non-competitive. Sulfite affected the transmembrane potential difference (PD) of the leaf tissues. The short-term response obeyed an 'all or nothing' law. At 0.1 m M and above, sulfite led to a quick depolarization of one-third of the initial potential after a lag phase of about 5 min; for concentrations lower than 0.1 m M , sulfite did not affect the potential. By contrast, the long-term effect of Na₂SO₃ on the transmembrane PD strongly depended on its concentration. After 2–12 h of pretreatmemt there was no effect at 10 μ M , a weak effect at 0.1 m M , and then increasing depolarization as the pollutant concentration increased. The inhibitory effect of Na₂SO₃ on sucrose uptake is thus, at least partly, due to its effect on a component of the proton-motive force. ΔΨ. However, the lack of correlation noticed with 0.1 m M Na₂SO₃ between the effect on sucrose uptake and the long-term effect on transmembrane PD suggests numerous sites of sulfite action.     

Importance of oxidative electron transport over oxidative phosphorylation in optimizing photosynthesis in mesophyll protoplasts of pea (Pisum sativum L.)

The role of mitochondrial respiration in optimizing photosynthesis was assessed in mesophyll protoplasts of pea ( Pisum sativum L., cv. Arkel) by using low concentrations of oligomycin (an inhibitor of oxidative phosphorylation), antimycin A (inhibits cytochrome pathway of electron transport) and salicylhydroxamic acid (SHAM, an inhibitor of alternative oxidase). All three compounds decreased the rate of photosynthetic O₂ evolution in mesophyll protoplasts, but did not affect chloroplast photosynthesis. The inhibition of photosynthesis by these mitochondrial inhibitors was stronger at optimal CO₂ (1.0 m M NaHCO₃) than that at limiting CO₂ (0.1 m M NaHCO₃). We conclude that mitochondrial metabolism through both cytochrome and alternative pathways is essential for optimizing photosynthesis at limiting as well as at optimal CO₂. The ratios of ATP to ADP in whole protoplast extracts were hardly affected, despite the marked decrease in their photosynthetic rates by SHAM. Similarly, the decrease in the ATP/ADP ratio by oligomycin or antimycin A was more pronounced at limiting CO₂ than at optimal CO₂. The mitochondrial oxidative electron transport, through both cytochrome and alternative pathways, therefore akppears to be more important than oxidative phosphorylation in optimizing photosynthesis, particularly at limiting CO₂ (when ATP demand is expected to be low). Our results also confirm that the alternative pathway has a significant role in contributing to the cellular ATP, when the cytochrome pathway is limited.     

Photosynthesis, biomass partitioning and peroxisomicine A1 production of Karwinskia species in response to nitrogen supply

Species of the genus Karwinskia are medicinal woody plants producing secondary metabolites (anthracenones, peroxisomicine A₁) with antitumour effects. We investigated the photosynthetic and growth characteristics of Karwinskia parvifolia Zucc. and Karwinskia humboldtiana Rose plants grown at two rates of nitrogen supply. There were significant differences in growth, CO₂ exchange (photosynthesis, respiration), chlorophyll and nitrogen concentration between plants grown at high nitrogen supply (HN) and low nitrogen supply (LN). At HN, the plants of both species grew faster than those cultivated at LN. Rates of photosynthesis, leaf respiration and root respiration, quantum yield, the concentration of chlorophylls ( a, b, a + b ), specific leaf area, leaf mass ratio and peroxisomicine A₁ concentration were higher in plants grown at HN. The species-specific difference in the daily carbon budget was mainly due to variation in rates of carbon fixation. K. parvifolia responded more strongly to the nitrogen treatment than K. humboldtiana , in terms of growth, as well as with respect to photosynthesis and peroxisomicine A₁ concentration.     

Changes in leaf nitrogen and carbohydrates underlie temperature and CO2 acclimation of dark respiration in five boreal tree species

We tested the hypothesis that acclimation of foliar dark respiration to CO₂ concentration and temperature is associated with adjustments in leaf structure and chemistry. Populus tremuloides Michx. , Betula papyrifera Marsh. , Larix laricina (Du Roi) K. Koch , Pinus banksiana Lamb., and Picea mariana (Mill.) B.S.P. were grown from seed in combined CO₂ (370 or 580 μ mol mol^–1) and temperature treatments (18/12, 24/18, or 30/24 °C). Temperature and CO₂ effects were predominately independent. Specific respiration rates partially acclimated to warmer thermal environments through downward adjustment in the intercept, but not Q ₁₀ of the temperature–response functions. Temperature acclimation of respiration was larger for conifers than broad-leaved species and was associated with pronounced reductions in leaf nitrogen concentrations in conifers at higher growth temperatures. Short-term increases in CO₂ concentration did not inhibit respiration. Growth in the elevated CO₂ concentration reduced leaf nitrogen and increased non-structural carbohydrate concentrations. However, for a given nitrogen concentration, respiration was higher in leaves grown in the elevated CO₂ concentration, as rates increased with increasing carbohydrates. Across species and treatments, respiration rates were a function of both leaf nitrogen and carbohydrate concentrations ( R ² = 0·71, P < 0·0001). Long-term acclimation of foliar dark respiration to temperature and CO₂ concentration is largely associated with changes in nitrogen and carbohydrate concentrations.     

Role of sodium in the ABA‐mediated long‐term growth response of bean to salt stress

Long‐term salt effects on plant growth have often been related to direct ion toxicity due to the accumulation of high ion concentrations in plant tissue. This work examines the relative importance of endogenous ABA, as well as Na⁺ and Cl⁻ toxicity, in the inhibition of leaf growth and photosynthesis, in bean plants grown at 1, 25, 50 and 75 m M NaCl until the fruit‐bearing stage. All salt‐treated plants showed very high leaf Cl⁻ concentrations, with little difference between plants exposed to 50 or 75 m M NaCl. The 25 and 50 mM salt‐treated plants were able to successfully exclude Na⁺ from their leaves, and only suffered an initial decline in the rate of leaf growth. Plants exposed to 75 m M NaCl showed an increase in Na⁺ leaf concentrations with an accompanying decrease in growth and photosynthesis as salt exposure progressed. A high correlation was found between leaf Na⁺ and leaf growth. Leaf ABA significantly increased with salt supply, and was highly correlated with both leaf Na⁺ and leaf growth. Our results suggest that in bean plants under long‐term salt stress, leaf ABA may participate in the regulation of leaf growth, and leaf Na⁺ would be at least partly responsible for increased ABA levels.     

Interaction of elevated CO2 and O3 on growth, photosynthesis and respiration of three perennial species grown in low and high nitrogen

Seedlings of three species native to central North America, a C₃ tree, Populus tremuloides Michx., a C₃ grass, Agropyron smithii Rybd., and a C₄ grass, Bouteloua curtipendula Michx., were grown in all eight combinations of two levels each of CO₂, O₃ and nitrogen (N) for 58 days in a controlled environment. Treatment levels consisted of 360 or 674 μmol mol^-1 CO₂, 3 or 92 nmol mol^-1 O₃, and 0.5 or 6.0 m M N. In situ photosynthesis and relative growth rate (RGR) and its determinants were obtained at each of three sequential harvests, and leaf dark respiration was measured at the second and third harvests. In all three species, plants grown in high N had significantly greater whole-plant mass, RGR and photosynthesis than plants grown in low N. Within a N treatment, elevated CO₂ did not significantly enhance any of these parameters nor did it affect leaf respiration. However, plants of all three species grown in elevated CO₂ had lower stomatal conductance compared to ambient CO₂-exposed plants. Seedlings of P. tremuloides (in both N treatments) and B. curtipendula (in high N) had significant ozone-induced reductions in whole-plant mass and RGR in ambient but not under elevated CO_2. This negative O₃ impact on RGR in ambient CO₂ was related to increased leaf dark respiration, decreased photosynthesis and/or decreased leaf area ratio, none of which were noted in high O₃ treatments in the elevated CO₂ environment. In contrast, A. smithii was marginally negatively affected by high O_3.     

Change in the Respiratory Rate of Sea Urchin Spermatozoa following Sperm-egg Interaction in the Absence of Mg2+

Spermatozoa of the sea urchin, Hemicentrotus pulcherrimus (10⁸ cells/ml), preincubated with unfertilized eggs deprived of jelly coats (more than l0⁵ cells/ml) at 20°C for 20min in Mg²⁺ free artificial sea water containing 1 mM Ca²⁺ (MFASW), exhibited very low respiration, which was enhanced by 2, 4 dinitrophenol (DNP). The fertilization rate in MFASW was usually less than 5% and was about 25% at most. Preincubation with fertilized eggs (with and without a fertilization membrane) in MFASW did not reduced the respiratory rate of spermatozoa. The rate of sperm respiration was lower in MFASW than in artificial sea water (ASW), but was higher than the respiratory rate of spermatozoa preincubated in MFASW with unfertilized eggs. Sperm respiration in MFASW or in ASW was not stimulated by 2, 4 dinitrophenol. Almost complete inhibition of sperm respiration was obtained with unfertilized eggs fixed with glutaraldehyde at concentrations of above 10⁵ cells/ml in MFASW and of about l0⁴ cells/ml in ASW. The respiratory rate of spermatozoa treated with fixed eggs was enhanced by DNP. It is concluded that the respiratory rate of the spermatozoa is reduced by their interaction with unfertilized eggs before their penetration into the eggs.     

Growth and photosynthetic response of three soybean cultivars to simultaneous increases in growth temperature and CO2

Three soybean ( Glycine max L. Merr.) cultivars (Maple Glen, Clark and CNS) were exposed to three CO₂ concentrations (370, 555 and 740 μmol mol⁻¹) and three growth temperatures (20/15°, 25/20° and 31/26°C, day/night) to determine intraspecific differences in single leaf/whole plant photosynthesis, growth and partitioning, phenology and final biomass. Based on known carboxylation kinetics, a synergistic effect between temperature and CO₂ on growth and photosynthesis was predicted since elevated CO₂ increases photosynthesis by reducing photorespiration and photorespiration increases with temperature. Increasing CO₂ concentrations resulted in a stimulation of single leaf photosynthesis for 40–60 days after emergence (DAE) at 20/15°C in all cultivars and for Maple Glen and CNS at all temperatures. For Clark, however, the onset of flowering at warmer temperatures coincided with the loss of stimulation in single leaf photosynthesis at elevated CO₂ concentrations. Despite the season-long stimulation of single leaf photosynthesis, elevated CO₂ concentrations did not increase whole plant photosynthesis except at the highest growth temperature in Maple Glen and CNS, and there was no synergistic effect on final biomass. Instead, the stimulatory effect of CO₂ on growth was delayed by higher temperatures. Data from this experiment suggest that: (1) intraspecific variation could be used to select for optimum soybean cultivars with future climate change; and (2) the relationship between temperature and CO₂ concentration may be expressed differently at the leaf and whole plant levels and may not solely reflect known changes in carboxylation kinetics.     

The effect of an elevated atmospheric CO2 concentration on growth, photosynthesis and respiration of Plantago major

The effect of an elevated atmospheric CO₂ concentration on growth, photosynthesis and root respiration of Plantago major L. ssp. major L. was investigated. Plants were grown in a nutrient solution in growth chambers at 350 and 700 μl I⁻¹ CO₂ during 7 weeks. The total dry weight of the Co₂-enriched plants at the end of this period was 50% higher than that of control plants. However, the relative growth rate (RGR) was stimulated only during the first half of the growing period. The transient nature of the stimulation of the RGR was not likely to be due to end-product inhibition of photosynthesis. It is suggested that in P. major , a rosette plant, self-shading causes a decline in photosynthesis and results in an increase in the shoot: root ratio and a decrease in RGR. CO₂-enriched plants grow faster and cosequently suffer more from self-shading. Corrected for this ontogenetic drift, high CO₂ concentrations stimulated the RGR of P. major throughout the entire experiment.     

Seasonal variation in foliar carbon exchange in Pinus radiata and Populus deltoides: respiration acclimates fully to changes in temperature but photosynthesis does not

We investigated seasonal variation in dark respiration and photosynthesis by measuring gas exchange characteristics on Pinus radiata and Populus deltoides under field conditions each month for 1 year. The field site in the South Island of New Zealand is characterized by large day-to-day and seasonal changes in air temperature. The rate of foliar respiration at a base temperature of 10 °C ( R ₁₀) in both pine and poplar was found to be greater during autumn and winter and displayed a strong downward adjustment in warmer months. The sensitivity of instantaneous leaf respiration to a 10 °C increase in temperature ( Q ₁₀) was also greater during the winter period. The net effect of this strong acclimation was that the long-term temperature response of respiration was essentially flat over a wide range of ambient temperatures. Seasonal changes in photosynthesis were sensitive to temperature but largely independent of leaf nitrogen concentration or stomatal conductance. Over the range of day time growth temperatures (5–32 °C), we did not observe strong evidence of photosynthetic acclimation to temperature, and the long-term responses of photosynthetic parameters to ambient temperature were similar to previously published instantaneous responses. The ratio of foliar respiration to photosynthetic capacity ( R _d/ A _sat) was significantly greater in winter than in spring/summer. This indicates that there is little likelihood that respiration would be stimulated significantly in either of these species with moderate increases in temperature – in fact net carbon uptake was favoured at moderately higher temperatures. Model calculations demonstrate that failing to account for strong thermal acclimation of leaf respiration influences determinations of leaf carbon exchange significantly, especially for the evergreen conifer.     

How does the C4 grass Eragrostis pilosa respond to elevated carbon dioxide and infection with the parasitic angiosperm Striga hermonthica?

Eragrostis pilosa (Linn.) P Beauv., a C₄ grass native to east Africa, was grown at both ambient (350 μmol mol⁻¹ and elevated (700 μmol mol⁻¹) CO₂ in either the presence or absence of the obligate, root hemi-parasite Striga hermonthica (Del.) Benth. Biomass of infected grasses was only 50% that of uninfected grasses at both CO₂ concentrations, with stems and reproductive tissues of infected plants being most severely affected. By contrast, CO₂ concentration had no effect on growth of E. pilosa , although rates of photosynthesis were enhanced by 30–40% at elevated CO₂. Infection with S. hermonthica did not affect either rates of photosynthesis or leaf areas of E. pilosa , but did bring about an increase in root:shoot ratio, leaf nitrogen and phosphorus concentration and a decline in leaf starch concentration at both ambient and elevated CO₂. Striga hermonthica had higher rates of photosynthesis and shoot concentrations of soluble sugars at elevated CO₂, but there was no difference in biomass relative to ambient grown plants. Both infection and growth at elevated CO₂ resulted in an increase in the Δ¹³C value of leaf tissue of E. pilosa , with the CO₂ effect being greater. The proportion of host-derived carbon in parasite tissue, as determined from δ¹³C values, was 27% and 39% in ambient and elevated CO₂ grown plants, respectively. In conclusion, infection with S. hermonthica limited growth of E. pilosa , and this limitation was not removed or alleviated by growing the association at elevated CO₂.     

Leaf area index, canopy structure and photosynthesis of red clover (Trifolium pratense L.)

Abstract. The influence of leaf age, total leaf area and its dispersion in space on canopy photosynthesis were studied using microswards of red clover ( Trifolium pratense L.) which were established in the greenhouse. Two varieties, Renova (flowering) and Molstad (non-flowering), were sown in separate plastic boxes at densities of 225, 400 and 625 plants per m².
Vertical distribution of photosynthetically active radiation (PAR), leaf area, leaf age and ¹⁴CO₂-fixation were determined periodically. Net photosynthesis and dark respiration of canopies were measured. Maximum photosynthetic capacity of individual leaves was measured on plants taken from the intact canopy or from plants where shading of the growing leaves had been prevented.
Net photosynthetic rate of canopies increased linearly with leaf area index (LAI) up to an LAI of 3.5 and then declined at higher LAI, independent of variety and sowing density. Below the optimum LAI, net photosynthesis depended mainly on interception of PAR. Decrease in canopy photosynthesis above the optimum LAI was due to a higher proportion of old leaves with decreased photosynthetic capacity, and not to an increase in respiring plant parts. It is concluded that LAI and position of leaf age categories in the canopy are more important than vertical distribution of leaf area in determining canopy photosynthesis of red clover.     

Keeping a positive carbon balance under adverse conditions: responses of photosynthesis and respiration to water stress

Drought and salinity (i.e. soil water stress) are the main environmental factors limiting photosynthesis and respiration and, consequently, plant growth. This review summarizes the current status of knowledge on photosynthesis and respiration under water stress. It is shown that diffusion limitations to photosynthesis under most water stress conditions are predominant, involving decreased mesophyll conductance to CO₂, an important but often neglected process. A general failure of photochemistry and biochemistry, by contrast, can occur only when daily maximum stomatal conductance ( g _s) drops below 0.05–0.10 mol H₂O m⁻² s⁻¹. Because these changes are preceded by increased leaf antioxidant activities ( g _s below 0.15–0.20 mol H₂O m⁻² s⁻¹), it is suggested that metabolic responses to severe drought occur indirectly as a consequence of oxidative stress, rather than as a direct response to water shortage. As for respiration, it is remarkable that the electron partitioning towards the alternative respiration pathway sharply increases at the same g _s threshold, although total respiration rates are less affected. Despite the considerable improvement in the understanding of plant responses to drought, several gaps of knowledge are highlighted which should become research priorities for the near future. These include how respiration and photosynthesis interact at severe stress, what are the boundaries and mechanisms of photosynthetic acclimation to water stress and what are the factors leading to different rates of recovery after a stress period.     

Fluoride inhibits root water transport and affects leaf expansion and gas exchange in aspen (Populus tremuloides) seedlings

The effects of sodium fluoride (0.3, 5 and 10 m M NaF) on root hydraulic conductivity, and gas exchange processes were examined in aspen ( Populus tremuloides Michx.) seedlings grown in solution culture. A long-term exposure of roots to NaF significantly decreased root hydraulic conductivity ( L _p) and stomatal conductance ( g _s). Root absorbed NaF significantly affected electrolyte leakage in leaf tissues and substantially restricted leaf expansion. NaF did not significantly affect leaf chlorophyll contents but decreased net photosynthesis ( P _n). A short-term exposure of excised roots to 5 m M NaF and KF significantly decreased root water flow ( Q _v) with a concomitant decline in root respiration and reduced g _s when applied through intact roots or excised stems. The same molar concentration of NaCl also decreased Q _v and g _s in intact seedlings, but to a lesser extent than NaF or KF, and did not significantly affect root respiration. The results suggest that fluoride metabolically inhibited Q _v or L _p, probably by affecting water channel activity. We suggest that the metabolic inhibition of L _p by root-absorbed fluoride affected gas exchange and leaf expansion in aspen seedlings.     

Modelling environmental controls on ecosystem photosynthesis and the carbon isotope composition of ecosystem-respired CO2 in a coastal Douglas-fir forest

We developed and applied an ecosystem-scale model that calculated leaf CO₂ assimilation, stomatal conductance, chloroplast CO₂ concentration and the carbon isotope composition of carbohydrate formed during photosynthesis separately for sunlit and shaded leaves within multiple canopy layers. The ecosystem photosynthesis model was validated by comparison to leaf-level gas exchange measurements and estimates of ecosystem-scale photosynthesis from eddy covariance measurements made in a coastal Douglas-fir forest on Vancouver Island. A good agreement was also observed between modelled and measured δ ¹³C values of ecosystem-respired CO₂ ( δ _R). The modelled δ _R values showed strong responses to variation in photosynthetic photon flux density (PPFD), air temperature, vapour pressure deficit (VPD) and available soil moisture in a manner consistent with leaf-level studies of photosynthetic ¹³C discrimination. Sensitivity tests were conducted to evaluate the effect of (1) changes in the lag between the time of CO₂ fixation and the conversion of organic matter back to CO₂; (2) shifts in the proportion of autotrophic and heterotrophic respiration; (3) isotope fractionation during respiration; and (4) environmentally induced changes in mesophyll conductance, on modelled δ _R values. Our results indicated that δ _R is a good proxy for canopy-level C _c/ C _a and ¹³C discrimination during photosynthetic gas exchange, and therefore has several applications in ecosystem physiology.     

Effect of CO2-enrichnient on seedling physiology and growth of two tropical tree species

Seedlings of two tree species from the Atlantic lowlands of Costa Rica, Ochroma la-gopus Swartz, a fast-growing pioneer species, and Pentaclethra macroloba (Willd.) Kuntze, a slower-growing climax species, were grown under enriched atmospheric CO₂ in controlled environment chambers. Carbon dioxide concentrations were maintained at 350 and 675 μl 1⁻¹ under photosynthetic photon flux densities of 500 μol m⁻² s⁻¹ and temperatures of 26°C day and 20°C night. Total biomass of both species increased significantly in the elevated CO₂ treatment; the increase in biomass was greatest for the pioneer species, O. lagopus . Both species had greater leaf areas and specific leaf weights with increased atmospheric CO₂. However, the ratio of non-pho-tosynthetic tissue to leaf area also increased in both species leading to decreased leaf area ratios. Plants of both species grown at 675 μl 1⁻¹ CO₂ had lower chlorophyll contents and photosynthesis on a leaf area basis than those grown at 350 μl 1⁻¹. Reductions in net photosynthesis occurred despite increased internal CO₂ concentrations in the CO₂-enriched treatment. Stomatal conductances of both species decreased with CO₂-enrichment resulting in significant increases in water use efficiency.     

Gas exchange measurements required to predict the newly fixed carbon pool size in a source leaf of corn (Zea mays L.)

Abstract. Steady-state photosynthesis (P_n), post-illumination CO₂ release rates (R), sucrose-phosphate synthase (SPS) activities, and levels of starch, sucrose and hexoses were measured in the source leaf of corn ( Zea mays L.) during a 16-h photoperiod at 800 μmol m ² s ¹. P_n and SPS activity remained constant. Carbohydrate pools increased at a linear rate, except the first and last hour of the photoperiod. Both the CO₂ evolution rate at the moment of light removal (R_max) and SPS activity decreased by one half after the onset of darkness (0 60 min). Sucrose diminished during this period by 40%, whereas the starch remained constant. Thereafter, starch mobilization began, followed by a gradual decline in leaf respiration. The average dark export rate was calculated to be 60% less than that during the day. Maintenance respiration (R_m) of an attached leaf after 48 h darkness was determined. Plants were illuminated for different intervals (e.g. 5, 10 or 20 min), each followed by dark periods sufficient for respiration to decline to R_m. The ratio of C assimilated in light to that released in dark was 6:1. After the 48-h dark period, the minimal period of illumination (T_min) required to restore P_n and R_max to the original level was determined. A mathematical analysis of the kinetics involved in the recovery of P_n and R_max provided an estimate of turnover time (0.22h) and pool size 9.15 mmol m ²) for the newly fixed carbon.     

Effect of abscisic acid on CO2 exchange in Lemna gibba

The effects of abscisic acid (ABA) on photosynthesis, dark respiration, and photorespiration were studied in Lemna gibba L. plants. The initial concentration of ABA in the nutrient solution was 10⁻⁷M and in a few experiments, 10⁻⁶M. The cultures were grown in the same solution for time periods ranging from one hour to 12 days. Net photosynthesis, measured as CO₂ uptake by infrared gas analyser technique, was inhibited after four hours of ABA treatment and reached a minimum after four to seven days depending on the time of the year. After 12 days a substantial recovery of photosynthesis was observed. Dark respiration was significantly stimulated after two to seven days of ABA treatment but then returned to the control level. The transient effects of ABA on photosynthesis and dark respiration corresponded to the previously measured time course of [¹⁴C]-ABA uptake by Lemna . Photorespiration measured as oxygen inhibition of photosynthesis was not affected by ABA.     

Effects of nitrogen deficiency on leaf photosynthetic response of tall fescue to water deficit

Abstract. The objective of the present work was to study the effect of nitrogen deficiency on drought sensitivity of tall fescue plants. The authors compared photosynthetic and stomatal behaviour of plants grown at either high (8 mol m⁻³) or low (0.5 mol m⁻³) nitrogen levels during a drought cycle followed by rehydration. Other processes investigated were stomatal and non-stomatal inhibition of leaf photosynthesis, water use efficiency and leaf rolling. Plants were grown in pots in controlled conditions on expanded clay. A Wescor in situ hygrometer placed on the leaf base outside the assimilation chamber permitted, simultaneously to leaf gas exchange measurements, monitoring of leaf water potential. Drought was imposed by withholding water from the pot. CO₂ uptake and stomatal conductance decreased and leaves started to roll at a lower leaf water potential in the high-N than in the low-N grown plants. Stomatal inhibition of leaf photosynthesis seemed larger in the low-N than in the high-N plants. Water-use efficiency increased more in the high-N than in the low-N grown plants during the drought. The decrease of photosynthesis was largely reversible after rehydration in low-N but not in high-N leaves. The authors suggest that low-N plants avoid water deficit rather than tolerate it.