Increasing Photon Flux Research Articles

Relationships between nitrogen uptake and carbon assimilation in whole plants of tall fescue

Abstract. The present study investigates the relationships between nitrogen uptake, transpiration, and carbon assimilation. Plants growing on nutrient solution were enclosed for 10–16 d in a growth chamber, where temperature, photon flux density, vapour saturation deficit and CO2 concentration were controlled. One of these factors was modified every 4 to 5 d. Shoot photosynthesis and root and shoot respiration were recorded every half‐hour. Nitrogen uptake from the root medium and plant transpiration were measured daily. In most cases, an increase in photon flux density led to increases in transpiration, net daily carbon assimilation, and nitrogen uptake. By modifying transpiration rate without changing photosynthesis (varying vapour saturation deficit), or by modifying transpiration and carbon assimilation in opposite ways (varying CO2 air concentration), it was shown that nitrogen uptake does not follow transpiration, but is linked to the carbon uptake of the plant. When light was increased from low to intermediate levels, the N uptake/C assimilation ratio remained constant. At higher photon flux density, this ratio declined markedly. It is proposed that in the first case, growth is limited by carbohydrate availability, thus any increase in carbon assimilation leads to a proportional increase in nitrogen uptake, in contrast to the second situation where carbohydrates may accumulate in the plant without further nitrogen requirement.

Photosynthetic characteristics of a tropical forest understory herb, Alocasia macrorrhiza, and a related crop species, Colocasia esculenta grown in contrasting light environments.

Photosynthetic acclimation to 5 light environments ranging from 2 to 60% full sun was determined in Alocasia macrorrhiza, a shade tolerant species from tropical forest understories, and Colocasia esculenta, a cultivated species which occurs naturally in open marshy areas. Photosynthetic capacities of both species increased nearly 3 fold with increased photon flux density (PFD). In a given environment, however, photosynthetic capacities of C. esculenta were double those of A. macrorrhiza. Stomatal limitations explained only a small part of this difference. Respiration rates and estimated biochemical capacities increased in parallel to photosynthetic capacity. No differences were observed either between species or environments in the ratio of RuBP regeneration capacity to carboxylation capacity as determined from the CO2 dependence response of photosynthesis. Quantum yields of both species decreased only slightly with increasing growth PFD, providing little evidence for photoinhibition at high PFD. The results are discussed in terms of the mechanisms of and limitations on acclimation in these two species.

Rate Limitation of Non-Steady-State Photosynthesis by Ribulose-1,5-Bisphosphate Carboxylase in Spinach

A mathematical framework was developed to analyse rate limitation of non-steady-state photosynthesis following an increase in photon flux density (PFD). This analysis was employed to resolve an exponential phase of the photosynthetic response of Spinacia oleracea L. to a step increase from darkness to moderate PFD. This phase had a relaxation time of approximately 5 min, similar to the relaxation time for the activation of ribulose-1,5-bisphosphate carboxylase (Rubisco) as determined by freeze-clamp experiments following the same change in PFD. Furthermore, as the time in darkness prior to illumination was increased, the exponential phase contributed more to the overall trajectory of photosynthesis following the increase in light. The relaxation time for the increase in the contribution of this phase was 24 min. Freeze-clamp studies showed a relaxation time of 28 min for Rubisco deactivation in the dark. These results, together with measurements of RuP2 levels, suggest that the exponential phase resolved from gas exchange experiments was limited by activation of Rubisco and that Rubisco deactivation in the dark was the reason that this phase contributed more to the overall photosynthetic trajectory as time in darkness increased. A quantitative expression for the amount of extra photosynthesis that could have been obtained had Rubisco activated instantly following an increase in PFD was derived and discussed in relation to optimal functioning of the system under different environmental conditions.

Ozone flux to Picea sitchensis (Bong) Carr and Picea abies (L) Karst during short episodes and the effects of these on transpiration and photosynthesis

Sitka spruce and Norway spruce were grown in controlled environments and then exposed to ozone (O 3) for short periods as in mid-afternoon episodes experienced in the forest. For concentrations of between 20 and 300 nl litre −1 there were linear relationships between exposure concentration and O 3 uptake rates. Increasing photon flux densities increased rates of photosynthesis and transpiration, the increases being larger in actively growing than dormant seedlings. Physiological condition (dormancy or active growth), species and photon flux density were found to influence O 3 flux via their effects on stomatal conductance. Exposure to 80 nl litre −1 O 3 resulted in consistent increases of stomatal conductance and there were also indications that water-use efficiency was decreased.

Effects of light quality, daylength and periodic temperature variation on the growth of lettuce and radish plants.

Effects of light qualities under continuous lighting, daylengths at a constant temperature, and periodic temperature variations under continuous lighting on the vegetative growth of lettuce and radish plants were studied to find the suitable environmental conditions for plant production in controlled environment rooms. Among the four light qualities, most growth parameters increased with increasing photon flux of the red component and the red/blue ratio in the light spectra, except when the red/farred ratio was extremely high. The plant growth was promoted with the increase of daylength up to 24 h, whereas the lighting effiency in terms of dry-matter production per lighting duration was greatest in 20 h daylength. Daily periodic temperature variations showed some effects on the plant growth and the greatest effect was found in temperature regimes of 16 H/8 L for lettuce and 20 H/4 L or 16 H/8 L for radish. The increased dry matter was distributed to both the top and roots in lettuce, but exclusively to the storage root in radish.

Photosynthetic characteristics of Spirulina platensis on solid support. Chlorophyll fluorescence kinetics

SUMMARYSeveral aspects of photosynthesis in vivo (CO2 and O2 exchange, chlorophyll fluorescence and its quenching components) were measured in Spirulina platensis after filtering suspensions of this cyanobacteria through fibre glass filters. This simple procedure allowed the use of methods which previously could be applied only to leaves. Fluorescence characteristics displayed by Spirulina in suspension or collected on the solid support were similar, indicating that filtration and assay did not by themselves affect the photosynthetic process. When oxygen evolution and carbon dioxide uptake were followed simultaneously their kinetics differed, and the rate of O2 evolution was often higher than the rate of CO2 uptake. Slow kinetics and other fluorescence characteristics were strongly modified in aged cultures or when nutrient supply during growth was not optimal, or after exposure to photoinhibitory photon flux densities. Photoinhibition and recovery were studied in two strains differing in their sensitivity to high photon flux densities. During illumination using high photon flux densities two phases could be determined. After a few minutes variable fluorescence decreased and ‘dark fluorescence’(Fo) was unchanged, but after prolonged illumination Fo increased. The latter phase appeared earlier in the more sensitive strain and could be brought forward by increasing photon flux densities or, in some circumstances, by increasing [O2].Results are discussed in terms of the underlying relationships between electron transport and photosynthetic carbon assimilation and of the use of chlorophyll fluorescence as a diagnostic probe. It is suggested that measurement of chlorophyll fluorescence (by itself or in conjunction with quantum yield) on cyanobacteria or algae collected on solid support could provide a convenient screening procedure to assess sensitivity to photoinhibition and to other kinds of stress likely to be encountered in large‐scale culture.

The effects of illumination sequence, CO2 concentration, temperature and acclimation on the convexity of the photosynthetic light response curve

It was shown previously that the convexity (curvature or rate of bending) of the photosynthetic light response curve was strongly correlated with chlorophyll content in shade acclimated conifer needles (Leverenz 1987, Physiol. Plant. 71: 20–29), in agreement with an hypothesis that gradients of light within leaves affect the convexity. In the present study it is shown that the convexity at any given chlorophyll content can be altered when leaves of Pinus sylvestris L. Picea glauca (Muench), Picea mariana (M.II.) BS.P. and Picea abies (L.) Karst pre‐treated with less shade. This probably induced a differential acclimation of cells on the top and bottom sides of the leaves to their local light environment. Leaves were illuminated on i) their top surface, ii) their bottom surface, or iii) uniformly in a light integrating sphere during measurements of photosynthesis. After shoots had been transferred from their growth environment to a new measuring environment, the convexity increased from the first to the second day towards a maximum of 0.97. The rate of increase towards this maximum was 55 to 62% per day and probably is the result of re‐acclimation of cells within the leaves. The data shown that the act of measuring photosynthesis induces a significant alteration in the experimental material when measurements are made for more than one day.The convexity of the light response curve of photosynthesis, was independent of whether the steady state measurements were made beginning in the dark and sequentially increasing photon flux density or beginning at high light and sequentially lowering photon flux density. Neither variation of CO2 concentration from 35 to 200 Pa, nor of temperature from 5° to 32°C affected the convexity.

Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Alocasia macrorrhiza in Response to Step Changes in Irradiance

The regulation of ribulose-1,5-bisphosphate (RuBP) carboxylase (Rubisco) activity and pool sizes of RuBP and P-glycerate were examined in the tropical understory species Alocasia macrorrhiza following step changes in photon flux density (PFD). Previous gas exchange analysis of this species following a step increase in PFD from 10 to 500 micromoles quanta per square meter per second suggested that the increase in photosynthetic rate was limited by the rate of increase of Rubisco activity for the first 5 to 10 minutes. We demonstrate here that the increase in photosynthetic rate was correlated with an increase in both the activation state of Rubisco and the total k(cat) (fully activated specific activity) of the enzyme. Evidence presented here suggests that a change in the pool size of the naturally occurring tight binding inhibitor of Rubisco activity, 2-carboxyarabinitol 1-phosphate, was responsible for the PFD-dependent change in the total k(cat) of the enzyme. RuBP pool size transiently increased after the increase in PFD, indicating that photosynthesis was limited by the capacity for carboxylation. After 5 to 10 minutes, RuBP pool size was again similar to the pool size at low PFD, presumably because of the increased activity of Rubisco. Following a step decrease in PFD from 500 to 10 micromoles quanta per square meter per second, Rubisco activity declined but at a much slower rate than it had increased in response to a step increase in PFD. This slower rate of activity decline than increase was apparently due to the slower rate of 2-carboxyarabinitol 1-phosphate synthesis than degradation and, to a lesser degree, to slower deactivation than activation. RuBP pool size initially declined following the decrease in PFD, indicating that RuBP regeneration was limiting photosynthesis. As Rubisco activity decreased, RuBP slowly increased to its original level at high PFD. The slow rate of activity loss by Rubisco in this species suggests a biochemical basis for the increased efficiency for CO(2) assimilation of successive lightfleck use by species such as A. macrorrhiza.

Laboratory studies of some environmental variables controlling sulfur emissions from plants

Several trace sulfur gases that can have a significant influence on atmospheric chemistry are emitted from biological systems. In order to begin to address biological questions on the mechnisms of production of such gases, laboratory-scale experiments have been developed that reproduce such emissions under controlled conditions. Using a flux chamber technique, flats containing soil, or soil plus plants were sampled for the net fluxes of sulfur gases. The major sulfur gas emitted from all the plants tested (corn, alfalfa, and wheat) was dimethyl sulfide (DMS). Alfalfa and wheat also emitted lesser amounts of methanethiol, variable amounls of hydrogen sulfide, and in some experiments wheat emitted carbon disulfide. The use of a plant incubator allowed a systematic study of the effects of variables such as temperature, photon flux, and carbon dioxide levels, on these emissions. Fluxes of all the emitted sulfur gases increased exponentially with increasing air temperature, and increased with increasing photon flux up to a saturation level of \~300 μE/m−2 sec-1. Three to four-fold changes in DMS flux were observed during light to dark or dark to light transitions. By varying the CO2 content of the chamber flush gas, it was shown that the observed sulfur fluxes from corn and alfalfa were not related to the CO2 concentration. Growing these crop plants through holes in a Teflon soil-covering film allowed a separate determination of soil and foliage emissions and substantiation of the light dependent uptake of COS by growing vegetation observed in previous field studies.

Gas Exchange Analysis of the Relative Importance of Stomatal and Biochemical Factors in Photosynthetic Induction in Alocasia macrorrhiza

When leaves of Alocasia macrorrhiza adapted to 10 micromole quanta per square meter per second were transferred to 500 micromole quanta per square meter per second, the rate of photosynthetic CO(2) assimilation increased for over 45 minutes. For the first 10 to 15 minutes, increases in both stomatal conductance and the leaf's photosynthetic capacity were responsible for the increase in assimilation rate. Thereafter, continuing increases in stomatal conductance were almost entirely responsible for further increases in assimilation rate. When conductances were initially high, assimilation rates 1 minute after the increase in photon flux density could be more than six times as high as for similar leaves with initially low conductance. Further increases in assimilation rate in these leaves with high conductance were predominantly due to increases in the induction state at the biochemical level and followed an exponential time course. When stomatal conductances were initially low, then increases in conductance were predominantly responsible for the increases in assimilation rate, with both following a sigmoidal time course. In these leaves, it was important to also consider the effect of cuticular water loss on the calculation of the intracellular partial pressure of CO(2), and an assessment of the relative importance of stomatal conductance differed considerably from one that did not include cuticular water loss.

Daytime and Nighttime Carbon Balance and Assimilate Export in Soybean Leaves at Different Photon Flux Densities

To evaluate daytime and nighttime carbon balance and assimilate export in soybean (Glycine max [L.] Merrill) leaves at different photon flux densities, rates of CO(2) exchange, specific leaf weights, and concentrations of sucrose and starch were measured at intervals in leaves of pod-bearing ;Amsoy 71' and ;Wells II' plants grown in a controlled environment room. Assimilate export was estimated from CO(2) exchange and change in specific leaf weight. Total diurnal assimilate export was similar for both cultivars. Large cultivar differences existed, however, in the partitioning of carbon into starch reserves and the relative amounts of assimilate exported during the day and the night. Total amounts of both daytime and nighttime export increased with increasing photon flux density, as did sucrose and starch concentrations, specific leaf weight, and rate of respiratory carbohydrate loss at night. Cultivar differences in nighttime rate of export were more closely related to the differences in amount of assimilate available at the end of the day than to differences in daytime rate of net CO(2) assimilation. Daytime rates of export, however, were closely related to daytime rates of net CO(2) assimilation within each cultivar. The total amount of starch depleted during the 10-hour night increased as starch concentration at the beginning of the night increased.

Concurrent measurements of oxygen- and carbon-dioxide exchange during lightflecks inAlocasia macrorrhiza (L.) G. Don

Oxygen and CO2 exchange were measured concurrently in leaves of shade-grownAlocasia macrorrhiza (L.) G. Don during lightflecks consisting of short periods of high photon flux density (PFD) superimposed on a low-PFD background illumination. Oxygen exchange was measured with a zirconium-oxide ceramic cell in an atmosphere containing 1 600 μbar O2 and 350 μbar CO2. Following an increase in PFD from 10 to 500 μmol photons·m(-2)·s(-1), O2 evolution immediately increased to a maximum rate that was about twice as high as the highest CO2-exchange rates that were observed. Oxygen evolution then decreased over the next 5-10 s to rates equal to the much more slowly increasing rates of CO2 uptake. When the PFD was decreased at the end of a lightfleck, O2 evolution decreased nearly instantaneously to the low-PFD rate while CO2 fixation continued at an elevated rate for about 20 s. When PFD during the lightfleck was at a level that was limiting for steady-state CO2 exchange, then the O2-evolution rate was constant during the lightfleck. This observed pattern of O2 evolution during lightflecks indicated that the maximum rate of electron transport exceeded the maximum rate of CO2 fixation in these leaves. In noninduced leaves, rates of O2 evolution for the first fraction of a second were about as high as rates in fully induced leaves, indicating that O2 evolution and the electron-transport chain are not directly affected by the leaf's induction state. Severalfold differences between induced and noninduced leaves in O2 evolution during a lightfleck were seen for lightflecks longer than a few seconds where the rate of O2 evolution appeared to be limited by the utilization of reducing power in CO2 fixation.

Photoinhibition and Zeaxanthin Formation in Intact Leaves

Comparative studies of chlorophyll a fluorescence, measured with a pulse amplitude modulated fluorometer, and of the pigment composition of leaves, suggest a specific role of zeaxanthin, a carotenoid formed in the xanthophyll cycle, in protecting the photosynthetic apparatus against the adverse effects of excessive light. This conclusion is based on the following findings: (a) exposure of leaves of Populus balsamifera, Hedera helix, and Monstera deliciosa to excess excitation energy (high light, air; weak light, 2% O(2), 0% CO(2)) led to massive formation of zeaxanthin and a decrease in violaxanthin. Over a wide range of conditions, there was a linear relationship between either variable, F(v), or maximum fluorescence, F(m), and the zeaxanthin content of leaves. (b) When exposed to photoinhibitory light levels in air, shade leaves of H. helix had a higher capacity for zeaxanthin formation, at the expense of beta-carotene, than shade leaves of M. deliciosa. Changes in fluorescence characteristics suggested that, in H. helix, the predominant response to high light was an increase in the rate of nonradiative energy dissipation, whereas, in M. deliciosa, photoinhibitory damage to photosystem II reaction centers was the prevailing effect. (c) Exposure of a sun leaf of P. balsamifera to increasing photon flux densities in 2% O(2) and 0% CO(2) resulted initially in increasing levels of zeaxanthin (matched by decreases in violaxanthin) and was accompanied by fluorescence changes indicative of increased nonradiative energy dissipation. Above the light level at which no further increase in zeaxanthin content was observed, fluorescence characteristics indicated photoinhibitory damage. (d) A linear relationship was obtained between the ratio of variable to maximum fluorescence, F(v)/F(m), determined with the modulated fluorescence technique at room temperature, and the photon yield of O(2) evolution, similar to previous findings (O Björkman, B Demmig 1987 Planta 170: 489-504) on chlorophyll fluorescence characteristics at 77 K and the photon yield of photosynthesis.

Life history and physiology of the red alga, Gelidium coulteri, in unialgal culture

Gelidium coulteri Harv., an agarophyte of commercial importance, was grown in unialgal culture in natural seawater under various environmental conditions. Growth rate, reproductive status, morphology and agar production for whole plants under each condition were determined over 2-month periods. Reproduction could be induced in these plants by (1) limiting them for nutrients, then relieving the limitation with addition of Provasoli's enrichment, or (2) increasing photon flux densities (PFD) from <50 μE m −2 s −1 to 150 μE m −2 s −1. The sexual life history was completed in 2 months. Red light inhibited tetraspore germination vs. white light, but blue and green light did not. Vegetative tetrasporophytes and gametophytes showed no differences in their growth responses to environmental changes. Growth rates increased with increasing PFD and were optimal at 150–250 μE m −2 s −1. Growth inhibition and bleaching were seen at 400 μE m −2 s −1. Agar yield on a dry weight basis increased with increasing PFD to 32–35% at 150 μE m −2 s −1 and remained constant at higher PFDs. Plants grown at PFD<50 μE m −2 s −1 were linear with little branching. Increasing PFD to 150 μE m −2 s −1 stimulated branching within 1 week. Growth rate increased with increasing temperature to a maximum at 20–27°C. Temperatures above 31°C were lethal. Agar yield was constant at 32–35% dry weight with respect to temperature to 31°C, at which agar yields increased to 40%. Growth was optimal at pH 7.5–8.5. Tissue necrosis and cell death occurred at pH below 7.0. Plants formed superficial carbonate deposits at pH above 9.0. Agar yields were constant from pH 6.0 to 9.0. Plants grown in unfortified seawater showed declines in chlorophyll content and growth rate to nearly zero after 3 weeks. Phycobiliprotein levels decreased to zero after 12 days. Agar yields of these plants were 40–60% higher than in plants grown with nutrient supplementation. Subsequent 0.36 mM nitrate addition led to a recovery of chlorophyll and phycobiliprotein in 2–3 days.

Investigation of the CO2 Dependence of Quantum Yield and Respiration in Eucalyptus pauciflora

In leaves of C(3) plants, the rate of nonphotorespiratory respiration appears to be higher in darkness than in the light. This change from a high to a low rate of carbon loss with increasing photon flux density leads to an increase in the apparent quantum yield of photosynthetic CO(2) assimilation at low photon flux densities (Kok effect). The mechanism of this suppression of nonphotorespiratory respiration is not understood, but biochemical evidence and the observation that a Kok effect is often not observed under low O(2), has led to the suggestion that photorespiration might be involved in some way. This hypothesis was tested with snowgum (Eucalyptus pauciflora Sieb. ex Spreng.) using gas exchange methods. The test was based on the assumption that if photorespiration were involved, then it would be expected that the intercellular partial pressure of CO(2) would also have an influence on the Kok effect. Under normal atmospheric levels of CO(2) and O(2), a Kok effect was found. Changing the intercellular partial pressure of CO(2), however, did not affect the estimate of nonphotorespiratory respiraton, and it was concluded that its decrease with increasing photon flux density did not involve photorespiration. Concurrent measurements showed that the quantum yield of net assimilation of CO(2) increased with increasing intercellular partial pressure of CO(2), and this increase agreed closely with predictions based on recent models of photosynthesis.

Effects of CO2enrichment at varying photon flux density on the growth ofpicea abies(L) karst. seedlings

Seedlings of Norway spruce (Picea abies (L.)) were grown at 335 and 1000 μl CO2 1−1 for 118 days in growth rooms at different irradiance levels. Photon flux density ranging from 8.6 to 34.6 mol m−2 day−t (PAR) was given either as constant light or as alternating levels in intervals of two or six hours. CO2 enrichment increased the plant dry weight from 36% to 105% by increasing photon flux density from 8.6 to 25.9 mol m−2 day−1. At constant light the dry weight apparently reached its maximum at a photon flux density of 25.9 mol m−2 day−t. At the lower radiation levels alternating in CO2 enriched air gave slightly higher dry weights compared to constant light levels. At the highest radiations the effect on dry weight was the opposite. High CO2 concentration and 300 μmol m−2 s−1 constant light (25.9 mol m−2 day−1) gave the best growth and quality of plants. Top, root, stem and foliage weight were proportionally affected. Shoot length was enhanced by CO2 enrichment. Shoot weight per cm was substantially incr...

Photosynthesis in an Australian rainforest tree, Argyrodendron peralatum, during the rapid development and relief of water deficits in the dry season.

Rates of apparent photosynthesis were measured in situ at five positions between the upper crown and a lower branch of a 34 m tall Argyrodendron peralatum (F.M. Bailey) H.L. Edlin ex I.H. Boas tree, and on an understorey sapling of the same species growing in a northern Australian rainforest. At the end of the dry season, rapid reductions in photosynthetic rates occurred in the upper crown within three days after a rain event, but changes in the lower crown and the sapling were less marked. Complete recovery of photosynthesis followed a second rain event. At high photon flux densities, stomatal conductance to water vapour decreased in a curvilinear fashion as the vapour pressure difference between leaf and air increased. Apparent photosynthesis was linearly related to stomatal conductance on the first clear day after each rain event, but there was no relationship between these parameters at the end of a brief natural drying cycle. Under conditions of adequate water supply, stomatal conductances of both upper crown and understorey leaves increased linearly with increasing photon flux density up to about 300 μmol m-2 s-1. During water deficits, stomatal conductances in leaves from the understorey increased much more rapidly at very low photon flux densities than did conductances in leaves from the upper canopy.

Survey on sexual pheromone specificity in Laminariales (Phaeophyceae)

No abstract available.

Photoinhibition and Reactivation of Photosynthesis in the Cyanobacterium Anacystis nidulans

The susceptibility of photosynthesis to photoinhibition and its recovery were studied on cultures of the cyanobacterium Anacystis nidulans. Oxygen evolution and low temperature fluorescence kinetics were measured. Upon exposure to high light A. nidulans showed a rapid decrease in oxygen evolution followed by a quasi steady state rate of photosynthesis. This quasi steady state rate decreased with increasing photon flux density of the photoinhibitory light. Reactivation of photosynthesis in dim light after the photoinhibitory treatment was rapid: 85 to 95% recovery occurred within 2 hours. In the presence of the translation inhibitor, streptomycin (250 micrograms per milliliter), no reactivation occurred. We also found that the damage increased dramatically if the high light treatment was done with streptomycin added. A transcription inhibitor, rifampicin, did not inhibit the reactivation process. Based on these data we conclude that the photoinhibitory damage observed is the net result of a balance between the photoinhibitory process and the operation of the repairing mechanism(s).

Photosynthetic and Stomatal Responses of Spinach Leaves to Salt Stress

The gas exchange of spinach plants, salt-stressed by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar, was studied 3 weeks after starting NaCl treatment. Photosynthesis became light saturated at 1100 to 1400 micromoles per square meter per second in salt-treated plants and at approximately 2000 micromoles per square meter per second in control plants. Photosynthetic capacity of the mesophyll measured as a function of intercellular partial pressure of CO(2) at the light intensity prevailing during growth and at light saturation were both decreased in the salttreated plants. The CO(2) compensation points and relative enhancements of photosynthesis at low O(2) were not affected by salinity. The lower photosynthetic rates in salt-treated leaves at 450 micromoles per square meter per second were associated with a 70% reduction in stomatal conductance and low intercellular CO(2) (219 microbars; cf. 285 microbars for controls). Increasing photon flux density to light saturation extended the linear portions of the CO(2) response curves, increased stomatal conductances, increased intercellular CO(2) in the salt-treated plants, but lowered it in controls, and accentuated differences in photosynthetic rate (area basis) between the treatments.Leaves from salt-treated plants were thicker but contained about 73% of the chlorophyll per unit area of control plants. When photosynthetic rates were expressed on a chlorophyll basis there was no difference in initial slope of assimilation versus intercellular CO(2) between treatments. Photosynthetic rates (chlorophyll basis) at light saturation differed only by 20% which was also observed earlier with isolated, intact chloroplasts (Robinson et al. 1983 Plant Physiol 73: 238-242).Measurement of carbon isotope ratio revealed less discrimination against (13)C with salt treatment and confirmed the persistence of low intercellular partial pressures of CO(2) during plant growth. The development of a thicker leaf with less chlorophyll per unit area during salt treatment permitted stomatal conductance and intercellular partial pressure of CO(2) to decline without restricting photosynthesis and had the benefit of greatly increasing water use efficiency.