Differential responses of plasma membrane aquaporins in mediating water transport of cucumber seedlings under osmotic and salt stresses

It has long been recognized that inhibition of plant water transport by either osmotic stress or salinity is mediated by aquaporins (AQPs), but the function and regulation of AQPs are highly variable among distinct isoforms and across different species. In this study, cucumber seedlings were subjected to polyethylene glycol (PEG) or NaCl stress for duration of 2 h or 24 h. The 2 h treatment with PEG or NaCl had non‐significant effect on the expression of plasma membrane AQP (CsPIPs) in roots, indicating the decrease in hydraulic conductivity of roots (Lp_r) and root cells (Lp_rc) measured in these conditions were due to changes in AQP activity. After both 2 h and 24 h PEG or NaCl exposure, the decrease in hydraulic conductivity of leaves (K_leaf) and leaf cells (Lp_lc) could be attributed to a down‐regulation of the two most highly expressed isoforms, CsPIP1;2 and CsPIP2;4. In roots, both Lp_r and Lp_rc were further reduced after 24 h PEG exposure, but partially recovered after 24 h NaCl treatment, which were consistent with changes in the expression of CsPIP genes. Overall, the results demonstrated differential responses of CsPIPs in mediating water transport of cucumber seedlings, and the regulatory mechanisms differed according to applied stresses, stress durations and specific organs.     

Elevated CO2 alleviates negative effects of salinity on broccoli (Brassica oleracea L. var Italica) plants by modulating water balance through aquaporins abundance

In the global change scenario, increased CO₂ may favour water use efficiency (WUE) by plants. By contrast, in arid and semiarid areas, salinity may reduce water uptake from soils. However, an elevated WUE does not ensure a reduced water uptake and upon salinity this fact may constitute an advantage for plant tolerance. In this work, we aimed to determine the combined effects of enhanced [CO₂] and salinity on the plant water status, in relation to the regulation of PIP aquaporins, in the root and leaf tissues of broccoli plants (Brassica oleracea L. var Italica), under these two environmental factors. Thus, different salinity concentrations (0, 60 and 90 mM NaCl) were applied under ambient (380 ppm) and elevated (800 ppm) [CO₂]. Under non-salinised conditions, stomatal conductance (G_s) and transpiration rate (E) decreased with rising [CO₂] whereas water potential (Ψ_ω) was maintained stable, which caused a reduction in the root hydraulic conductance (L₀). In addition, PIP1 and PIP2 abundance in the roots was decreased compared to ambient [CO₂]. Under salinity, the greater stomatal closure observed at elevated [CO₂] – compared to that at ambient [CO₂] – caused a greater reduction in G_s and E and allowed plants to maintain their water balance. In addition, a lower decrease in L₀ under salt stress was observed at elevated [CO₂], when comparing with the decrease at ambient [CO₂]. Modifications in PIP1 and PIP2 abundance or their functionality in the roots is discussed. In fact, an improved water status of the broccoli plants treated with 90 mM NaCl and elevated [CO₂], evidenced by a higher Ψ_ω, was observed together with higher photosynthetic rate and water use efficiency. These factors conferred on the salinised broccoli plants greater leaf area and biomass at elevated [CO₂], in comparison with ambient [CO₂]. We can conclude that, under elevated [CO₂] and salt stress, the water flow is influenced by the tight control of the aquaporins in the roots and leaves of broccoli plants and that increased PIP1 and PIP2 abundance in these organs provides a mechanism of tolerance that maintains the plant water status.     

Effects of Salinity on Water Transport of Excised Maize (Zea mays L.) Roots

The root pressure probe was used to determine the effects of salinity on the hydraulic properties of primary roots of maize (Zea mays L. cv Halamish). Maize seedlings were grown in nutrient solutions modified by additions of NaCl and/or extra CaCl₂ so that the seedlings received one of four treatments: Control, plus 100 millimolar NaCl, plus 10 millimolar CaCl₂, plus 100 millimolar NaCl plus 10 millimolar CaCl₂. The hydraulic conductivities (Lp_r) of primary root segments were determined by applying gradients of hydrostatic and osmotic pressure across the root cylinder. Exosmotic hydrostatic Lp_r for the different treatments were 2.8, 1.7, 2.8, and 3.4·10⁻⁷ meters per second per megapascals and the endosmotic hydrostatic Lp_r were 2.4, 1.5, 2.7, and 2.3·10⁻⁷ meters per second per megapascals, respectively. Exosmotic Lp_r of the osmotic experiments were 0.55, 0.38, 0.68, and 0.60·10⁻⁷ meters per second per megapascals and the endosmotic Lp_r were 0.53, 0.21, 0.56, and 0.54·10⁻⁷ meters per second per megapascals, respectively. The osmotic Lp_r was significantly smaller (4-5 times) than hydrostatic Lp_r. However, both hydrostatic and osmotic Lp_r experiments showed that salinization of the growth media at regular (0.5 millimolar) calcium levels decreased the Lp_r significantly (30-60%). Addition of extra calcium (10 millimolar) to the salinized media caused ameliorative effects on Lp_r. The low Lp_r values may partially explain the reduction in root growth rates caused by salinity. High calcium levels in the salinized media increased the relative availability of water needed for growth. The mean reflection coefficients of the roots using NaCl were between 0.64 and 0.73 and were not significantly different for the different treatments. The mean values of the root permeability coefficients to NaCl of the different treatments were between 2.2 and 3.5·10⁻⁹ meters per second and were significantly different only in one of four treatments. Cutting the roots successively from the tip and measuring the changes in the hydraulic resistance of the root as well as staining of root cross-sections obtained at various distances from the root tip revealed that salinized roots had mature xylem elements closer to the tip (5-10 millimeters) compared with the controls (30 millimeters). Our results demonstrate that salinity has adverse effects on water transport and that extra calcium can, in part, compensate for these effects.     

Changes in plasma membrane lipids, aquaporins and proton pump of broccoli roots, as an adaptation mechanism to salinity

Salinity stress is known to modify the plasma membrane lipid and protein composition of plant cells. In this work, we determined the effects of salt stress on the lipid composition of broccoli root plasma membrane vesicles and investigated how these changes could affect water transport via aquaporins. Brassica oleracea L. var. Italica plants treated with different levels of NaCl (0, 40 or 80 mM) showed significant differences in sterol and fatty acid levels. Salinity increased linoleic (18:2) and linolenic (18:3) acids and stigmasterol, but decreased palmitoleic (16:1) and oleic (18:1) acids and sitosterol. Also, the unsaturation index increased with salinity. Salinity increased the expression of aquaporins of the PIP1 and PIP2 subfamilies and the activity of the plasma membrane H⁺-ATPase. However, there was no effect of NaCl on water permeability (P_f) values of root plasma membrane vesicles, as determined by stopped-flow light scattering. The counteracting changes in lipid composition and aquaporin expression observed in NaCl-treated plants could allow to maintain the membrane permeability to water and a higher H⁺-ATPase activity, thereby helping to reduce partially the Na⁺ concentration in the cytoplasm of the cell while maintaining water uptake via cell-to-cell pathways. We propose that the modification of lipid composition could affect membrane stability and the abundance or activity of plasma membrane proteins such as aquaporins or H⁺-ATPase. This would provide a mechanism for controlling water permeability and for acclimation to salinity stress.     

Are Root Hydraulic Conductivity Responses to Salinity Controlled by Aquaporins in Broccoli Plants?

Broccoli (Brassica oleracea L. var. Italica) is a recognised health-promoting vegetable, which is moderately sensitive to salinity. In this study, the primary response of broccoli plants (cv. Marathon) to salinity has been characterised. For this, leaf water relations, nutrient composition, root hydraulic conductivity (L ₀) and the effect of mercury (an aquaporin blocker) on L ₀ were determined for plants grown with 0, 20, 40, 60, 80 or 100 mM NaCl for 2 weeks. During the 2 weeks of treatment, the plants showed a two-phase growth response to salinity. During the first phase (1 week), growth reduction was high, probably related to water stress as no osmotic adjustment occurred and reductions of L ₀, the mercury effect and Gs were observed. After 2 weeks, the growth reduction could have resulted from internal injury caused by Na⁺ or Cl⁻, since osmotic adjustment was achieved and water relations plus the mercury effect were re-established to a high degree, indicating high aquaporin functionality. The fact that aquaporin functionality fits well with the overall water relations response is very relevant, since the two-phase adaptation to salinity may imply two types of aquaporin regulation.     

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.     

Responses of ethylene biosynthesis to saline stress in seedlings of eight plant species

The effect of salinity (100 mM NaCl) on ethylene metabolism in the early phase of vegetative development of several plant species has been investigated. The effects of saline treatment on shoot and root growth, ranged in sensitivity with respect to species: pepper (Capsicum annum L. cv Pairal) > tomato (Lycopersicon esculentum Mill. cv Malpica) > broccoli (Brassica oleraceae L. var. Italica Plenk. cv Marathon F1) ≅ lettuce (Lactuca sativa var. longifolia Lam. cv Inverna) ≅ melon (Cucumis melo L. cv Ruano F1, Roche type) > bean (Phaseolus vulgaris L. cv. Gator Green 15) ≅ spinach (Spinacia oleracea L. cv Boeing) > beetroot (Beta vulgaris L. var. crassa (Alef.) J. Helm. cv Detroit). After saline treatment, ethylene production increased 4.2-fold in pepper shoots. Significant increases were also found in shoots of tomato, broccoli and bean. In contrast, salinity decreased shoot ethylene production rate in melon, spinach, and beetroot. In roots, the general effect of salinity was a decrease in ethylene production, especially in broccoli and bean, except in tomato root, in which a sharp increase in ethylene production occurred. In general, saline treatment increased total ACC concentration in both shoot and root in most of the plant species examined, which was related to plant sensitivity to salinity. For example, pepper shoot was the most sensitive to saline treatment, showing the highest fresh weight inhibition and the highest increase in total ACC concentration (8.5-fold), while, beetroot was less affected by salinity and showed no effect on total ACC concentration in response to saline treatment.     

Whole-plant gas exchange responses of Spartina alterniflora (Poaceae) to a range of constant and transient salinities

A two-chamber-system was used to study whole-plant gas exchange responses of Spartina alterniflora to long-term and transient salinity treatments over the range of 5 to 40 ppt NaCl. Lower photosynthetic rates, leaf water vapor conductances, belowground respiration rates, and higher aboveground respiration rates in plants adapted to 40 ppt NaCl were observed. Area-specific leaf weight increased with salinity, although the salt content of leaf tissues did not. A reduced rate of gross photosynthesis and higher aboveground respiration rate in 40-ppt NaCl plants significantly lowered the net whole-plant CO₂ gain below that of 5-ppt NaCl plants, while the net CO₂ gain of 25-ppt NaCl plants was intermediate. Within 6 hr of increasing the salinity of 5- and 25-ppt NaCl plants by 20 and 15 ppt NaCl, S. alterniflora responded by reducing leaf water vapor conductance, which in turn reduced the photosynthetic rate. This response was reversed by returning the plants to their original salinity, which indicates that S. alterniflora adjusts water loss and gas exchange in response to transient salinity stress by regulating stomatal aperture. On the other hand, decreasing salinity of the growth media of plants cultured at 25 and 40 ppt NaCl had little or no effect on gas exchange characteristics. This suggests that S. alterniflora adapts to constant salinity through fixed, salinity-dependent structural modifications, such as stomatal density.     

Morpho-physiological variations in response to NaCl stress during vegetative and reproductive development of rice

The complex nature of plant resistance to adverse environmental conditions, such as salinity and drought requires a better understanding of the stress-induced changes that may be involved in tolerance mechanisms. Here we investigate stress-related morpho-physiological effects during vegetative and reproductive growth in two Japonica rice cultivars (Bomba and Bahia) exposed to a range of NaCl concentrations from the seedling stage. The stress-related detrimental effects were observed either earlier or to a higher extent in cv. Bomba than in Bahia. Damages to the photosynthetic apparatus were related to loss of chlorophyll (Chl) and to a decrease of the maximum potential efficiency of PSII (F _v /F _m), affecting negatively net CO₂ assimilation rate (P _N). Stress-related leaf anatomical alterations were analysed during the vegetative and reproductive stages. The size of bulliform cells as well as dimensions related to the vascular system increased under mild stress but decreased in the longer term or under higher stress level. The pattern of the anatomical alterations observed at the reproductive stage under 20 mM NaCl was reflected in poor panicle development and yield loss, with effects more pronounced in cv. Bomba than in Bahia. In summary, our results show that some physiological and, particularly, leaf anatomical responses induced by NaCl stress are distinctive indicators of sensitivity to salt stress in rice cultivars.     

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.     

Hydraulic resistance to radial water flow in growing hypocotyl of soybean measured by a new pressure-perfusion technique

Hydraulic resistances to water flow have been determined in the cortex of hypocotyls of growing seedlings of soybean (Glycine max L. Merr. cv. Wayne). Data at the cell level (hydraulic conductivity, Lp; half-time of water exchange, T _1/2; elastic modulus, ; diffusivity for the cell-to-cell pathway, D _c) were obtained by the pressure probe, diffusivities for the tissue (D _t) by sorption experiments and the hydraulic conductivity of the entire cortex (Lp_r) by a new pressure-perfusion technique. For cortical cells in the elongating and mature regions of the hypocotyls T _1/2=0.4–15.1 s, Lp=0.2·10^-5–10.0·10^-5 cm s^-1 bar^-1 and D _c=0.1·10^-6–5.5·10^-6 cm² s^-1. Sorption kinetics yielded a tissue diffusivity D _t=0.2·10^-6–0.8·10^-6 cm² s^-1. The sorption kinetics include both cell-wall and cell-to-cell pathways for water transport. By comparing D _c and D _t, it was concluded that during swelling or shrinking of the tissue and during growth a substantial amount of water moves from cell to cell. The pressure-perfusion technique imposed hydrostatic gradients across the cortex either by manipulating the hydrostatic pressure in the xylem of hypocotyl segments or by forcing water from outside into the xylem. In segments with intact cuticle, the hydraulic conductance of the radial path (Lp_r) was a function of the rate of water flow and also of flow direction. In segments without cuticle, Lp_r was large (Lp_r=2·10^-5–20·10^-5 cm s^-1 bar^-1) and exceeded the corticla cell Lp. The results of the pressure-perfusion experiments are not compatible with a cell-to-cell transport and can only the explained by a preferred apoplasmic water movement. A tentative explanation for the differences found in the different types of experiments is that during hydrostatic perfusion the apoplasmic path dominates because of the high hydraulic conductivity of the cell wall or a preferred water movement by film flow in the intercellular space system. For shrinking and swelling experiments and during growth, the films are small and the cell-to-cell path dominates. This could lead to larger gradients in water potential in the tissue than expected from Lp_r. It is suggested that the reason for the preference of the cell-to-cell path during swelling and growth is that the solute contribution to the driving force in the apoplast is small, and tensions normally present in the wall prevent sufficiently thick water films from forming. The solute contribution is not very effective because the reflection coefficient of the cell-wall material should be very small for small solutes. The results demonstrate that in plant tissues the relative magnitude of cell-wall versus cell-to-cell transport could dependent on the physical nature of the driving forces (hydrostatic, osmotic) involved.Abbreviations and symbols D _c diffusivity of the cell-to-cell pathway - D _t diffusivity of the tissue - radial flow rate per cm² of segment surface - Lp hydraulic conductivity of plasma-membrane - Lp_r radial hydraulic conductance of the cortex - T _1/2 half-time of water exchange between cell and surroundings - volumetric elastic modulus     

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.

     

Wheat methionine sulfoxide reductase genes and their response to abiotic stress

The wheat cultivar Shanrong no. 3 (cv. SR3) tolerates both salinity and drought stress more effectively than does its progenitor cultivar Jinan 177 (cv. JN177). When the cultivars are subjected to stress, a number of genes encoding methionine sulfoxide reductase (MSRs) are known to be upregulated in SR3. Here, a set of 12 full length Triticum aestivum MSR (TaMSR) cDNAs have been isolated from cv. SR3. The genes were transcribed in the wheat root, stem, and leaf in plants sampled at various developmental stages. Those induced by salinity and drought harbored known stress-responsive cis elements in their promoter region. The constitutive expression in Arabidopsis thaliana of four MSRs which were induced by salt and drought in microarray assay showed that the product of one (TaMSRA2) heightened the plant’s tolerance to NaCl, methylviologen (MV), and abscisic acid, that of the second (TaMSRA5) enhanced salinity tolerance, that of the third (TaMSRB1.1) increased tolerance to salinity, MV and H₂O₂, and that of the fourth (TaMSRB5.1) increased tolerance to both salinity and mannitol. The effect of the presence in A. thaliana of TaMSRB1.1 was to suppress the accumulation of reactive oxygen species and to increase the intracellular content of soluble sugars.     

Photosynthetic Parameters at the Vegetative Stage and during Grain Development of Two Hexaploid Wheat Cultivars Differing in Salt Tolerance

Response of two spring wheat (Triticum aestivum L.) cultivars, salt tolerant SARC-I and salt sensitive Potohar, to different concentrations of NaCl was examined under glasshouse conditions. Eighteen-day-old plants of both the lines grown in sand culture were irrigated with 0 (control), 80, 160 or 240 mM NaCl in full strength Hoagland's nutrient solution. Shoot fresh and dry masses, and leaf area per plant of SARC-I at the vegetative stage, were significantly greater than those of cv. Potohar at higher salt concentrations, however, relative growth rate (RGR) of cv. Potohar was significantly higher than that of SARC-I. SARC-I had higher net photosynthetic rate (P_N), stomatal conductance (g_s) and transpiration rate (E) than cv. Potohar at the vegetative stage, but the cultivars did not differ significantly in water-use efficiency (P_N/E), intrinsic water use efficiency (P_N/g_s), and intercellular/ambient CO₂ concentration ratio. At the grain development stage, SARC-I had significantly higher P_N and g_s in the flag leaf than cv. Potohar under salinity. SARC-I was superior to cv. Potohar with respect to number of grains per spike, number of grains per spikelet, mean grain mass, and grain yield per plant at all NaCl concentrations.     

Water transport in maize roots : measurement of hydraulic conductivity, solute permeability, and of reflection coefficients of excised roots using the root pressure probe

A root pressure probe has been used to measure the root pressure (P_r) exerted by excised main roots of young maize plants (Zea Mays L.). Defined gradients of hydrostatic and osmotic pressure could be set up between root xylem and medium to induce radial water flows across the root cylinder in both directions. The hydraulic conductivity of the root (Lp_r) was evaluated from root pressure relaxations. When permeating solutes were added to the medium, biphasic root pressure relaxations were observed with water and solute phases and root pressure minima (maxima) which allowed the estimation of permeability (P_Sr) and reflection coefficients (σ_sr) of roots. Reflection coefficients were: ethanol, 0.27; mannitol, 0.74; sucrose, 0.54; PEG 1000, 0.82; NaCl, 0.64; KNO₃, 0.67, and permeability coefficients (in 10⁻⁸ meters per second): ethanol, 4.7; sucrose, 1.6; and NaCl, 5.7. Lp_r was very different for osmotic and hydrostatic gradients. For hydrostatic gradients Lp_r was 1·10⁻⁷ meters per second per megapascal, whereas in osmotic experiments the hydraulic conductivity was found to be an order of magnitude lower. For hydrostatic gradients, the exosmotic Lp_r was about 15% larger than the endosmotic, whereas in osmotic experiments the polarity in the water movement was reversed. These results either suggest effects of unstirred layers at the osmotic barrier in the root, an asymmetrical barrier, and/or mechanical effects. Measurements of the hydraulic conductivity of individual root cortex cells revealed an Lp similar to Lp_r (hydrostatic). It is concluded that, in the presence of external hydrostatic gradients, water moves primarily in the apoplast, whereas in the presence of osmotic gradients this component is much smaller in relation to the cell-to-cell component (symplasmic plus transcellular transport).     

Deposition pattern of hydrogen peroxide in the leaf sheaths of rice under salt stress

Deposition pattern of hydrogen peroxide (H₂O₂) under salt stress (100 mM NaCl) was examined cytochemically in rice (Oryza sativa L. cv. Pokkali) through the reaction of H₂O₂ with cerium chloride (CeCl₃) to produce electron dense precipitates of cerium perhydroxide. The distribution pattern of cerium perhydroxide precipitates in leaf sheath was considerably different from other parts of rice under salinity stress. Cerium perhydroxide precipitates were mainly accumulated on the tonoplast of leaf sheath under salinity, although they were localized on the cell wall and plasma membrane in all other tissues such as leaf blade and root.     

Effects of short- and long-term salinity on leaf water relations, gas exchange, and growth in Ipomoea pes-caprae

Ipomoea pes-caprae is widespread in pantropical coastal areas along the beach. The aim of this study was to investigate the salinity tolerance level and physiological mechanisms that allow I. pes-caprae to endure abrupt increases in salinity under brief or prolonged exposure to salinity variations. Xylem sap osmolality (X_osm), leaf water relations, gas exchange, and number of produced and dead leaves were measured at short- (1-7 d) and long- (22-46 d) term after a sudden increase in soil salinity of 0, 85, 170, and 255 mM NaCl. In the short-term, X_osm was not affected by salinity, but in the long-term there was a significant increase in plants grown in presence of salt compared with control plants. After salt addition, the plants showed osmotic stress with temporal cell turgor loss. However, the water potential gradient for water uptake was re-established at 4, 7 and 22 d after salt addition, at 85, 170 and 255 mM NaCl, respectively. In the short-term I. pes-caprae was able to tolerate salinities of up to 255 mM NaCl without significant reduction in carbon assimilation or growth. With the duration of stress, leaf ion concentration continued to increase and reached toxic levels at high salinity with a progressive decrease in photosynthetic rate, reduced leaf formation and accelerated senescence. Then, if high levels of soil salts from tidal inundation occur for short periods, the survival of I. pes-caprae is possible, but prolonged exposure to salinity may induce metabolic damage and reduce drastically the plant growth.     

Diffusional conductance to CO2 is the key limitation to photosynthesis in salt‐stressed leaves of rice (Oryza sativa)

Salinity significantly limits leaf photosynthesis but the factors causing the limitation in salt‐stressed leaves remain unclear. In the present work, photosynthetic and biochemical traits were investigated in four rice genotypes under two NaCl concentration (0 and 150 mM) to assess the stomatal, mesophyll and biochemical contributions to reduced photosynthetic rate (A) in salt‐stressed leaves. Our results indicated that salinity led to a decrease in A, leaf osmotic potential, electron transport rate and CO₂ concentrations in the chloroplasts (C_c) of rice leaves. Decreased A in salt‐stressed leaves was mainly attributable to low C_c, which was determined by stomatal and mesophyll conductance. The increased stomatal limitation was mainly related to the low leaf osmotic potential caused by soil salinity. However, the increased mesophyll limitation in salt‐stressed leaves was related to both osmotic stress and ion stress. These findings highlight the importance of considering mesophyll conductance when developing salinity‐tolerant rice cultivars.