Photosynthetic and Stomatal Responses of the Grey Mangrove, Avicennia marina, to Transient Salinity Conditions

Measurements of gas exchange characteristics were made on intact, attached leaves of hydroponically grown seedlings of Avicennia marina (Forstk.) Vierh. var australasica (Walp.) Moldenke as the NaCl concentration of the culture solution was varied by step changes of 50 millimolar NaCl every 2nd day from 50 to 500 to 50 millimolar NaCl. The CO₂ assimilation rate, stomatal conductance, intercellular CO₂ concentration, and evaporation rate decreased at salinities above 250 millimolar NaCl and recovered substantially upon return to the original salinity.

The assimilation rate was measured as a function of the intercellular CO₂ concentration [A(c_i) curve]. The lower linear portion of this curve was insensitive to variation in salinity, whereas the upper nonlinear portion declined with increasing salinity, indicating a reduction in the capacity for CO₂ assimilation which recovered upon return to the original salinity. Stomatal conductance changed such that the intercellular CO₂ concentration measured under normal atmospheric conditions occurred in the transition between the lower, linear and upper nonlinear portions of the A(c_i) curve. Thus, stomatal conductance and photosynthetic capacity together co-limited the assimilation rate. The changes in gas exchange characteristics were such that water loss was minimal relative to carbon gain.

     

Effects of NaCl and CaCl(2) on Cell Enlargement and Cell Production in Cotton Roots

In many crop species, supplemental Ca²⁺ alleviates the inhibition of growth typical of exposure to salt stress. In hydroponically grown cotton seedlings (Gossypium hirsutum L. cv Acala SJ-2), both length and weight of the primary root were enhanced by moderate salinities (25 to 100 millimolar NaCl) in the presence of 10 millimolar Ca²⁺, but the roots became thinner. Anatomical analysis showed that the cortical cells of these roots were longer and narrower than those of the control plants, while cortical cells of roots grown at the same salinities but in the presence of only 0.4 millimolar Ca²⁺ became shorter and more nearly isodiametrical. Cell volume, however, was not affected by salinities up to 200 millimolar NaCl at either 0.4 or 10 millimolar Ca²⁺. Our observations suggest Ca²⁺-dependent effects of salinity on the cytoskeleton. The rate of cell production declined with increasing salinity at 0.4 millimolar Ca²⁺ but at 10 millimolar Ca²⁺ was not affected by salinities up to 150 millimolar NaCl.     

Salinity Effects on Water Potential Components and Bulk Elastic Modulus of Alternanthera philoxeroides (Mart.) Griseb

Pressure volume curves for Alternanthera philoxeroides (Mart.) Griseb. (alligator weed) grown in 0 to 400 millimolar NaCl were used to determine water potential (Ψ), osmotic potential (ψ_s), turgor potential (ψ_p) and the bulk elastic modulus (ε) of shoots at different tissue water contents. Values of ψ_s decreased with increasing salinity and tissue Ψ was always lower than rhizosphere Ψ. The relationship between ψ_p and tissue water content changed because ε increased with salinity. As a result, salt-stressed plants had larger ranges of positive turgor but smaller ranges of tissue water content over which ψ_p was positive. To our knowledge, this is the first report of such a salinity effect on ε in higher plants. These increases in ε with salinity provided a mechanism by which a large difference between plant Ψ and rhizosphere Ψ, the driving force for water uptake, could be produced with relatively little water loss by the plant. A time-course study of response after salinization to 400 millimolar NaCl showed Ψ was constant within 1 day, ψ_s and ψ_p continued to change for 2 to 4 days, and ε continued to change for 4 to 12 days. Changes in ε modified the capacity of alligator weed to maintain a positive water balance and consideration of such changes in other species of higher plants should improve our understanding of salt stress.     

Daily Changes in CO(2) and Water Vapor Exchange, Chlorophyll Fluorescence, and Leaf Water Relations in the Halophyte Mesembryanthemum crystallinum during the Induction of Crassulacean Acid Metabolism in Response to High NaCl Salinity

Simultaneous measurements of net CO₂ exchange, water vapor exchange, and leaf water relations were performed in Mesembryanthemum crystallinum during the development of crassulacean acid metabolism (CAM) in response to high NaCl salinity in the rooting medium. Determinations of chlorophyll a fluorescence were used to estimate relative changes in electron transport rate. Alterations in leaf mass per unit area, which—on a short-term basis—largely reflect changes in water content, were recorded continuously with a beta-gauge. Turgor pressure of mesophyll cells was determined with a pressure probe. As reported previously (K Winter, DJ von Willert [1972] Z Pflanzenphysiol 67: 166-170), recently expanded leaves of plants grown under nonsaline conditions showed gas-exchange characteristics of a C₃ plant. Although these plants were not exposed to any particular stress treatment, water content and turgor pressure regularly decreased toward the end of the 12 hour light periods and recovered during the following 12 hours of darkness. When the NaCl concentration of the rooting medium was raised to 400 millimolar, in increments of 100 millimolar given at the onset of the photoperiods for 4 consecutive days, leaf water content and turgor pressure decreased by as much as 30 and 60%, respectively, during the course of the photoperiods. These transient decreases probably triggered the induction of the biochemical machinery which is required for CAM to operate. After several days at 400 millimolar NaCl, when leaves showed features typical of CAM, overall turgor pressure and leaf mass per unit area had increased above the levels before onset of the salt treatment, and diurnal alterations in leaf water content were reduced. Net carbon gain during photoperiods and average intercellular CO₂ partial pressures at which net CO₂ uptake occurred, progressively decreased upon salinization. Reversible diurnal depressions in leaf conductance and net CO₂ uptake, with minima recorded in the middle of the photoperiods, preceded the occurrence of nocturnal net CO₂ uptake. During these reductions, intercellular CO₂ partial pressure and rates of photosynthetic electron transport decreased. With advancing age, leaves of plants grown under nonsaline conditions exhibited progressively greater diurnal reductions in turgor pressure and developed a low degree of CAM activity.     

Growth and Osmotic Adjustment of Cultured Suspension Cells from Alternanthera philoxeroides (Mart.) Griseb After an Abrupt Increase in Salinity

We have developed a cell suspension culture from alligator weed(Alternanthera philoxeroides [Mart.] Griseb), a C₃ member ofthe Amaranthaceae. Intact plants of alligator weed can growat 400 mol m^–3 NaCl. Growth of alligator weed suspensionswas compared to growth of tobacco (Nicotiana tabacum L. cv.Wisconsin 38) suspensions after subculture to 200 mol m^–3NaCl. Fresh weight and cell density of salt-treated alligatorweed suspensions more than doubled by 7 d after subculture,but the fresh weight of salt-treated tobacco suspensions didnot double during the 21 d experiment. Correspondingly, cellviability dropped from about 90% to 77% in alligator weed andto 41% in tobacco, at 1 d after subculture to 200 mol m^–3NaCl. The symplastic volume of alligator weed cells declined36% by 2 h after subculture to 200 mol m^–3 NaCl, but cellcontents became iso-osmotic with the media at this point. Between2 h and 6 h there was a further decrease in osmotic potential,an increase in turgor potential and a partial recovery of symplasticvolume. Turgor potential was similar to that in control cellsby 24 h, indicating significant osmotic adjustment. Turgor potentialsremained similar in both treatments from 24 h through 21 d butthe average symplastic volume of salt-treated cells was 11 %less than in control cells. Therefore, alligator weed suspensioncells exhibit a rapid recovery of water balance and cell growthafter an abrupt and substantial increase in salinity. Key words: Cell culture, growth, osmotic adjustment, salinity, turgor potential     

Salinity and Nitrogen Effects on Photosynthesis, Ribulose-1,5-Bisphosphate Carboxylase and Metabolite Pool Sizes in Phaseolus vulgaris L

Salinity (100 millimolar NaCl) was found to reduce photosynthetic capacity independent of stomatal closure in Phaseolus vulgaris. This reduction was shown to be a consequence of a reduction in the efficiency of ribulose-1,5-bisphosphate (RuBP) carboxylase (RuBPCase) rather than a reduction in the leaf content of photosynthetic machinery. In control plants, photosynthesis became RuBP-limited at approximately 1.75 moles RuBP per mole 2-carboxyarabinitol bisphosphate binding sites. Salinization caused the RuBP pool size to reach this limiting value for CO₂ fixation at much lower values of intercellular CO₂. Plants grown at low nitrogen and ± NaCl became RuBP limited at similar RuBP pool sizes as the high nitrogen-grown plants. At limiting RuBP pool sizes and equal values of intercellular CO₂ photosynthetic capacity of salt-stressed plants was less than control plants. This effect of salinity on RuBPCase activity could not be explained by deactivation of the enzyme or inhibitor synthesis. Thus, salinity reduced photosynthetic capacity by reducing both the RuBP pool size by an effect on RuBP regeneration capacity and RuBPCase activity by an unknown mechanism when RuBP was limiting.     

Effect of salinity on nodule formation by soybean

A split-root growth system was employed to evaluate the effect of NaCl on nodule formation by soybean (Glycine max L. Merr. cv Davis). By applying the salinity stress and rhizobial inoculum to only one-half the root system, the effects of salinity on shoot growth were eliminated in the nodulation process. Rhizobium colonization of inoculated root surfaces was not affected by the salt treatments (0.0, 26.6, 53.2, and 79.9 millimolar NaCl). While shoot dry weight remained unaffected by the treatments, total shoot N declined from 1.26 grams N per pot at 0.0 millimolar NaCl to 0.44 grams N per pot at 79.9 millimolar NaCl. The concentration of N in the shoot decreased from 3.75% N (0.0 millimolar NaCl) to 1.26% N at 79.9 millimolar NaCl. The decrease in shoot N was attributed to a sharp reduction in nodule number and dry weight. Nodule number and weight were reduced by approximately 50% at 26.6 millimolar NaCl, and by more than 90% at 53.2 and 79.9 millimolar NaCl. Nodule development, as evidenced by the average weight of a nodule, was not as greatly affected by salt as was nodule number. Total nitrogenase activity (C₂H₂ reduction) decreased proportionally in relation to nodule number and dry weight. Specific nitrogenase activity, however, was less affected by salinity and was not depressed significantly until 79.9 millimolar NaCl. In a second experiment, isolates of Rhizobium japonicum from nodules formed at 79.9 millimolar NaCl did not increase nodulation of roots under salt stress compared to nodule isolates from normal media (0.0 millimolar NaCl). Salt was applied (53.2 millimolar NaCl) to half root systems at 0, 4, 12, and 96 hours from inoculation in a third experiment. By delaying the application of salt for 12 hours, an increase in nodule number, nodule weight, and shoot N was observed. Nodule formation in the 12- and 96-hour treatments was, however, lower than the control. The early steps in nodule initiation are, therefore, extremely sensitive to even low concentrations of NaCl. The sensitivity is not related to rhizobial survival and is probably due to the salt sensitivity of root infection sites.     

Effects of salinity and illumination on photosynthesis and water balance of Spartina alterniflora Loisel

Summary Plants of the salt marsh grass Spartina alterniflora Loisel were collected from North Carolina and grown under controlled nutrient, temperature, and photoperiod conditions. Plants were grown at two different illumination levels; substrate salinity was varied, and leaf photosynthesis, transpiration, total chlorophyll, leaf xylem pressure, and specific leaf weight were measured. Conditions were controlled so that gaseous and liquid phase resistances to CO₂ diffusion could be calculated. Growth at low illumination and high salinity (30 ppt) resulted in a 50% reduction in photosynthesis. The reduction in photosynthesis of plants grown at low illumination was correlated with an increase in gaseous resistance. Photosynthetic rates of plants grown at high salinity and high illumination were reduced only slightly compared to rates of plants grown, in 10 ppt and Hoagland's solution. Both high salinity and high illumination were correlated with increases in specific leaf weight. Chlorophyll data indicate that specific leaf weight differences were the result of increases in leaf thickness. It is therefore hypothesized that photosynthetic response can be strongly influenced by salinity-induced changes in leaf structure. Similarities in photosynthetic rate on an area basis at high, illumination were apparently the result, of increases in leaf thickness at high salinity. Photosynthetic rates were generally quite high, even at salinities close to open ocean water, and it is concluded that salinity rarely limits photosynthesis in S. alterniflora.     

PHOTOSYNTHETIC RATE AND MESOPHYLL SURFACE AREA IN EXPANDING LEAVES OF ALTERNANTHERA PHILOXEROIDES GROWN AT TWO LIGHT LEVELS

Leaves of Alternanthera philoxeroides, alligator weed, developed at a photosynthetic photon flux density (PPFD, light energy at wavelengths of 400 to 700 nm) of 790 μmol sec⁻¹ m⁻² (High Light) had less surface area, were thicker, had a higher maximum P_n (net rate of CO₂ uptake), and required a higher PPFD for saturation of P_n, than leaves developed at 160 μmol sec⁻¹ m⁻² (Low Light). Mesophyll thickness at Low Light was within 19% of maximum 2 days after emergence but at High Light, thickness increased 79% between 2 and 16 days after leaf emergence. The ratio of mesophyll surface area to leaf surface area decreased during development in both light treatments; the ratio, however, was over 70% greater in fully expanded High Light leaves than in Low Light leaves. Maximum P_n expressed on a leaf surface area basis was 158% greater in High Light leaves than in Low Light leaves, but P_n was only 58% greater when expressed on a mesophyll surface area basis. It was estimated that fully expanded High Light leaves fixed 72% more CO₂ per leaf (P_n expressed per unit surface area times the total surface area per leaf) than fully expanded Low Light leaves when P_n was measured at the PPFD leaves expanded under. Both High and Low Light leaves would fix about the same amount of CO₂ per leaf when P_n was measured at 160 μmol sec⁻¹ m⁻² because the larger surface area of the Low Light leaves offset small differences in P_n.     

Intracellular Localization of Enzymes of Carbon Metabolism in Mesembryanthemum crystallinum Exhibiting C(3) Photosynthetic Characteristics or Performing Crassulacean Acid Metabolism

Mesembryanthemum crystallinum, a halophilic, inducible Crassulacean acid metabolism (CAM) species, was grown at NaCl concentrations of 20 and 400 millimolar in the rooting medium. Plants from the low salinity treatment showed exclusively C₃-photosynthetic net CO₂ fixation, whereas plants exposed to the high salinity level exhibited net CO₂ dark fixation involving CAM. Mesophyll protoplasts, isolated from both tissues, were gently ruptured, and the intracellular localization of enzymes was studied following differential centrifugation and Percoll density gradient centrifugation of protoplast extracts. Both centrifugation techniques resulted in the separation of intact chloroplasts, with up to 90% yield, from other organelles and the nonparticulate fraction of cells. Enzymes were identified by determination of activity and by sodium dodecyl sulfate gel electrophoresis of enzyme protein.     

Influence of Applied NaCl on Crassulacean Acid Metabolism and Ionic Levels in a Cactus, Cereus validus

To determine possible physiological responses to salinity, seedlings of Cereus validus Haworth, a cactus from Salinas Grandes, Argentina, were treated with up to 600 millimolar NaCl for up to 16 days when they were about 9 months old and 100 millimeters tall. Salt stress decreased stem biomass, e.g. it was 19.7 grams for controls and 11.4 grams for plants treated with 400 millimolar NaCl for 14 days. Nocturnal CO₂ uptake in these obligate Crassulacean acid metabolism (CAM) plants was inhibited 67% upon treatment with 400 millimolar NaCl for 14 days (controls, 181 millimoles CO₂ per square meter), while nocturnal accumulation of malate was inhibited 49% (controls, 230 millimoles malate per square meter). The larger accumulation of malate as compared to uptake of atmospheric CO₂ suggests that internal CO₂ recycling occurred during the dark period. Such recycling was lower in the controls (~20%) than in the NaCl-treated plants (~50%). The nocturnal increase in malate and titratable acidity depended on the total daily photosynthetically active radiation available; measurements suggest a quantum requirment of 26 photons per malate. As NaCl in the medium was increased to 600 millimolar in daily increments of 50 millimolar, Na and Cl concentrations in the roots increased from about 7 to 100 millimolar, but K concentration in the cell sap remained near 26 millimolar. Concomitantly, concentrations of Na and Cl in the shoots increased from 8 to 17 millimolar and from 1 to 7 millimolar, respectively, while the K concentration increased about 16 to 60 millimolar. In plants maintained for 14 days at 500 millimolar NaCl, the root levels of Na and Cl increased to 260 millimolar, the shoot levels were about 60 millimolar, and the stem bases began to become necrotic. Such Na retention in the roots together with the special possibilities of carbon reutilization given by CAM are apparently survival mechanisms for the temporarily saline conditions experienced in its natural habitat.     

Carbon and nitrogen limitations on soybean seedling development

Carbon and nitrogen limitations on symbiotically grown soybean seedlings (Glycine max [L.] Merr.) were assessed by providing 0.0, 1.0, or 8.0 millimolar NH₄NO₃ and 320 or 1,000 microliters CO₂/liter for 22 days after planting. Maximum development of the Rhizobium-soybean symbiosis, as determined by acetylene reduction, was measured in the presence of 1.0 millimolar NH₄NO₃ under both levels of CO₂. Raising NH₄NO₃ from 0.0 to 8.0 millimolar under 320 microliters CO₂/liter increased plant dry weight by 251% and Kjeldahl N content by 287% at 22 days after planting. Increasing NH₄NO₃ from 1.0 to 8.0 millimolar under 320 microliters CO₂/liter increased total dry weight and Kjeldahl N by 100 and 168%, respectively, on day 22. Raising CO₂ from 320 to 1,000 microliters CO₂/liter during the same period had no significant effect on Kjeldahl N content of plants grown with 0.0 or 1.0 millimolar NH₄NO₃. The maximum CO₂ treatment effects were observed in plants supplied with 8.0 millimolar NH₄NO₃, where dry weight and Kjeldahl N content were increased 64% and 20%, respectively. An increase in shoot CO₂-exchange rate associated with the CO₂-enrichment treatment was reflected in a significant increase in leaf dry weight and starch content for plants grown with 1,000 microliters CO₂/liter under all combined N treatments. These data show directly that seedling growth in symbiotically grown soybeans was limited primarily by N availability. The failure of the CO₂-enrichment treatment to increase total plant N significantly in Rhizobium-dependent plants indicates that root nodule development and functioning in such plants was not limited by photosynthate production.     

Interaction between Light Intensity and NaCl Salinity and Their Effects on Growth, CO(2) Assimilation, and Photosynthate Conversion in Young Broad Beans

Seedings of Vicia faba were grown for four weeks at two different light intensities (55 and 105 watts per square meter) in a saline (50 millimolar NaCl) and nonsaline nutrient solution. NaCl salinity depressed growth and restricted protein formation, CO₂ assimilation, and especially the incorporation of photosynthates into the lipid fraction. Conversion of photosynthates in leaves was much more affected by salinity than was photosynthate turnover in roots. The detrimental effect of NaCl salinity on growth, protein formation, and CO₂ assimilation was greater under low than under high light conditions. Plants of the high light intensity treatment were more capable of excluding Na⁺ and Cl⁻ and accumulating nutrient cation species (Ca²⁺, K⁺, Mg²⁺) than plants grown under low light intensity. It is suggested that the improved ionic status provided better conditions for protein synthesis, CO₂ assimilation, and especially for the conversion of photosynthates into lipids.     

Photosynthetic activity and leaf antioxidative responses of Atriplex portulacoides subjected to extreme salinity

Responses of Atriplex portulacoides upon 40-day-long exposure to salinity (0?C1,000?mM NaCl) were investigated. Mother plants originated from a sabkha located in a semi-arid region of Tunisia. The plant relative growth rate and leaf expansion increased significantly at 200?mM NaCl but decreased at higher salinities. Interestingly, the plants survived salinity as high as 1,000?mM NaCl without displaying salt-induced toxicity symptoms. Despite significant increase in leaf Na⁺ and Cl^? concentrations upon salt treatment, no significant effect on leaf relative water content was registered. Chlorophyll contents and the gas exchange parameters showed a significant stimulation at the optimal salinity (200?mM NaCl) followed by a decline at higher salinities. Extreme salinity hardly impacted the maximal efficiency of photosystem II photochemistry (F _v/F _m), but a marked decrease in the relative quantum yield of photosystem II (??_PSII) was observed, along with a significant increase in non-photochemical quenching (NPQ). Leaf malondialdehyde and carotenoid contents were generally unaffected following salt exposure, whereas those of anthocyanins, polyphenols, and proline increased significantly, being maximal at 1,000?mM NaCl. Leaf superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11), and glutathione reductase (EC 1.6.4.2) activities were significantly stimulated by salinity, whereas catalase (EC 1.11.1.6) activity was maximal in the 0?C400?mM NaCl range. As a whole, protecting the photosynthetic machinery from salt-induced photodamage together with the sustained antioxidant activity may account for the performance of A. portulacoides under high salinity.     

Salinity Induced Limitations on Photosynthesis in Prunus salicina, a Deciduous Tree Species

The response of photosynthetic CO₂ assimilation to salinization in 19 year old Prunus salicina was evaluated under field conditions for a 3 year period. The observed decline in CO₂ assimilation capacity was apparently related to increasing leaf chloride (Cl⁻) content, and independent of changes in leaf carbohydrate status. The response of net CO₂ assimilation (A) to leaf intercellular CO₂ partial pressure (C_i) indicated that the reduction in the capacity for A with Cl⁻ was not the result of decreased stomatal conductance but a consequence of nonstomatal inhibition. The nonstomatal limitations to CO₂ assimilation capacity, as determined by the response of A to C_i and biochemical assay, were related to a decline in the activity of ribulose 1,5-bisphosphate carboxylase (Rubpcase) and the pool size of triose phosphate, ribulose 1,5-bisphosphate (Rubp) and phosphoglycerate with increasing salinity. Lack of agreement between the initial slope of the A to C_i response curve and Rubpcase activity suggests the occurrence of heterogeneous stomatal apertures with the high salinity treatment (28 millimolar). Prolonged exposure to chloride salts appeared to increase the Rubp or Pi regeneration limitation, decrease Rubpcase activity and reduce leaf chlorophyll content. Observed changes in the biochemical components of CO₂ fixation may, in turn, affect total leaf carbohydrates, which also declined with time and salinity. The reduction in Rubpcase activity was apparently a consequence of a reduced Rubpcase protein level rather than either a regulatory or inhibitory effect.     

Interactive effects of atmospheric CO2 enrichment,salinity and flooding on growth of C3 (Elymus athericus) and C4 (Spartina anglica) salt marsh species

The growth response of Dutch salt marsh species (C₃ and C₄) to atmospheric CO₂ enrichment was investigated. Tillers of the C₃ speciesElymus athericus were grown in combinations of 380 and 720 11^-1 CO₂ and low (O) and high (300 mM NaCl) soil salinity. CO₂ enrichment increased dry matter production and leaf area development while both parameters were reduced at high salinity. The relative growth response to CO₂ enrichment was higher under saline conditions. Growth increase at elevated CO₂ was higher after 34 than 71 days. A lower response to CO₂ enrichment after 71 days was associated with a decreased specific leaf area (SLA). In two other experiments the effect of CO₂ (380 and 720 11^-1) on growth of the C₄ speciesSpartina anglica was studied. In the first experiment total plant dry weight was reduced by 20% at elevated CO₂. SLA also decreased at high CO₂. The effect of elevated CO₂ was also studied in combination with soil salinity (50 and 400 mM NaCl) and flooding. Again plant weight was reduced (10%) at elevated CO₂, except under the combined treatment high salinity/non-flooded. But these effects were not significant. High salinity reduced total plant weight while flooding had no effect. Causes of the salinity-dependent effect of CO₂ enrichment on growth and consequences of elevated CO₂ for competition between C₃ and C₄ species are discussed.     

Photosynthetic and Stomatal Responses of Two Mangrove Species, Aegiceras corniculatum and Avicennia marina, to Long Term Salinity and Humidity Conditions

Gas exchange characteristics were studied in two mangrove species, Aegiceras corniculatum (L.) Blanco and Avicennia marina (Forstk.) Vierh. var australasica (Walp.) Moldenke, grown under a variety of salinity and humidity conditions. The assimilation rate was measured as a function of the intercellular CO₂ concentration [A(c_i) curve]. The photosynthetic capacity decreased with increase in salinity from 50 to 500 millimolar NaCl, as shown by decline in both the initial linear slope and the upper plateau of the A(c_i) curve, with A. corniculatum being the more sensitive species. The decline in photosynthetic capacity was enhanced by increase in the leaf to air vapor pressure difference from 6 to 24 millibars, but this treatment caused a decrease in only the upper plateau of the A(c_i) curve. Stomatal conductance was such that the intercellular CO₂ concentration obtaining under normal atmospheric conditions occurred near the transition between the lower linear and upper plateau portions of the A(c_i) curves. Thus, stomatal conductance and photosynthetic capacity together co-limited the assimilation rate, which declined with increasing salinity and decreasing humidity. The marginal water cost of carbon assimilation was similar in most treatments, despite variation in the water loss/carbon gain ratio.     

Effects of the interaction between vapor-pressure deficit and salinity on growth and photosynthesis of <Emphasis Type="Italic">Cucumis sativus</Emphasis> seedlings under different CO<Subscript>2</Subscript> concentrations

We studied growth and photosynthesis of cucumber (Cucumis sativus) seedlings under two vapor-pressure deficit levels (VPD; 0.4 and 3.0 kPa), two salinity levels (0 mM and 34 mM NaCl), and two CO₂ concentrations ([CO₂]; 400 and 1,000 μmol mol^–1). Relative growth rate (RGR) decreased with increasing VPD, but the causal factor differed between salinity levels and CO₂ concentrations. Under ambient [CO₂], RGR decreased with increasing VPD at low salinity mainly due to decreased leaf area ratio (LAR), and decreased net assimilation rate (NAR) at high salinity. The decrease in intercellular [CO₂] (C_i) with decreasing stomatal conductance caused by high VPD did not significantly limit net photosynthetic rate (P_N) at low salinity, but P_N was potentially limited by C_i at high salinity. At high [CO₂], high VPD reduced LAR, but did not affect NAR. This is because the decrease in C_i occurred where slope of P_N–C_i curve was almost flat.     

Effects of Salinity on Stomatal Conductance, Photosynthetic Capacity, and Carbon Isotope Discrimination of Salt-Tolerant (Gossypium hirsutum L.) and Salt-Sensitive (Phaseolus vulgaris L.) C(3) Non-Halophytes

The effects of salinity on growth, stomatal conductance, photosynthetic capacity, and carbon isotope discrimination (Δ) of Gossypium hirsutum L. and Phaseolus vulgaris L. were evaluated. Plants were grown at different NaCl concentrations from 10 days old until mature reproductive structures were formed. Plant growth and leaf area development were strongly reduced by salinity, in both cotton and bean. Stomatal conductance also was reduced by salinity. The Δ always declined with increasing external salinity concentration, indicating that stomatal limitation of photosynthesis was increased. In cotton plant dry matter, Δ correlated with the ratio of intercellular to atmospheric CO₂ partial pressures (p_l/p_a) calculated by gas exchange. This correlation was not clear in bean plants, although Δ showed a more pronounced salt induced decline in bean than in cotton. Possible effects of heterogeneity of stomatal aperture and consequent overestimation of p_l as determined from gas exchange could explain these results. Significant differences of Δ between leaf and seed material were observed in cotton and bean. This suggests different patterns of carbon allocation between leaves and seeds. The photon yield of O₂ evolution determined at rate-limiting photosynthetic photon flux density was insensitive to salinity in both species analyzed. The light- and CO₂-saturated rate of CO₂ uptake and O₂ evolution showed a salt induced decline in both species. Possible explanations of this observation are discussed. O₂ hypersensitivity was observed in salt stressed cotton plants. These results clearly demonstrate that the effect of salinity on assimilation rate was mostly due to the reduction of stomatal conductance, and that calculation of p_l may be overestimated in salt stressed plants, because of heterogeneity of stomatal aperture over the leaf surface.     

The effects of salinity on photosynthesis and growth of the single-cell C<Subscript>4</Subscript> species <Emphasis Type="Italic">Bienertia sinuspersici</Emphasis> (Chenopodiaceae)

Recent research on the photosynthetic mechanisms of plant species in the Chenopodiaceae family revealed that three species, including Bienertia sinuspersici, can carry out C₄ photosynthesis within individual photosynthetic cells, through the development of two cytoplasmic domains having dimorphic chloroplasts. These unusual single-cell C₄ species grow in semi-arid saline conditions and have semi-terete succulent leaves. The effects of salinity on growth and photosynthesis of B. sinuspersici were studied. The results show that NaCl is not required for development of the single-cell C₄ system. There is a large enhancement of growth in culture with 50–200 mM NaCl, while there is severe inhibition at 400 mM NaCl. With increasing salinity, the carbon isotope values (δ¹³C) of leaves increased from −17.3^o/_oo (C₄-like) without NaCl to −14.6^o/_oo (C₄) with 200 mM NaCl, possibly due to increased capture of CO₂ from the C₄ cycle by Rubisco and reduced leakiness. Compared to growth without NaCl, leaves of plants grown under saline conditions were much larger (~2 fold) and more succulent, and the leaf solute levels increased up to ~2000 mmol kg solvent⁻¹. Photosynthesis on an incident leaf area basis (CO₂ saturated rates, and carboxylation efficiency under limiting CO₂) and stomatal conductance declined with increasing salinity. On a leaf area basis, there was some decline in Rubisco content with increasing salinity up to 200 mM NaCl, but there was a marked increase in the levels of pyruvate, Pi dikinase, and phosphoenolpyruvate carboxylase (possibly in response to sensitivity of these enzymes and C₄ cycle function to increasing salinity). The decline in photosynthesis on a leaf area basis was compensated for on a per leaf basis, up to 200 mM NaCl, by the increase in leaf size. The influence of salinity on plant development and the C₄ system in Bienertia is discussed.