Differences in efficient metabolite management and nutrient metabolic regulation between wild and cultivated barley grown at high salinity

Physiological and biochemical responses of Hordeum maritimum and H. vulgare to salt stress were studied over a 60‐h period. Growth at increasing salinity levels (0, 100, 200 and 300 mM NaCl) was assessed in hydroponic culture. H. maritimum was shown to be a true halophyte via its typical behaviour at high salinity. Shoot growth of cultivated barley was gradually reduced with increasing salinity, whereas that of wild barley was enhanced at 100 and 200 mm NaCl then slightly reduced at 300 mM NaCl. The higher salt tolerance of H. maritimum as compared to H. vulgare was due to its higher capacity to maintain cell turgor under severe salinity. Furthermore, H. maritimum exhibited fine regulation of Na⁺ transport from roots to shoots and, unlike H. vulgare, it accumulated less Na⁺ in shoots than in roots. In addition, H. maritimum can accumulate more Na⁺ than K⁺ in both roots and shoots without the appearance of toxicity symptoms, indicating that Na⁺ was well compartmentalized within cells and substituted K⁺ in osmotic adjustment. The higher degree of salt tolerance of H. maritimum is further demonstrated by its economic strategy: at moderate salt treatment (100 mm NaCl), it used inorganic solutes (such as Na⁺) for osmotic adjustment and kept organic solutes and a large part of the K⁺ for metabolic activities. Indeed, K⁺ use efficiency in H. maritimum was about twofold that in H. vulgare; the former started to use organic solutes as osmotica only at high salinity (200 and 300 mm NaCl). These results suggest that the differences in salt tolerance between H. maritimum and H. vulgare are partly due to (i) differences in control of Na⁺ transport from roots to shoots, and (ii) H. maritimum uses Na⁺ as an osmoticum instead of K⁺ and organic solutes. These factors are differently reflected in growth.     

Influence of NaCl-salinity on growth,photosynthesis, water relations and solute accumulation in <Emphasis Type="Italic">Phragmites australis</Emphasis>

The aim of this study was to investigate the effects of NaCl-salinity on the physiological attributes in common reed, Phragmites australis (Cav.) Trin. ex Steudel. Plants grew optimally under salinity treatment with standard nutrient solution without added salt and at NaCl concentrations up to 100 mM. Applied for 21 days, NaCl-salinity (300 and 500 mM) caused a significant reduction in growth allocation of all different tissues of P. australis. Shoot growth of reed plants displayed a highly significant correlation with plant–water relations and photosynthetic parameters. The net photosynthetic rate and stomatal conductance of reed plants treated with NaCl-salinity at varying osmotic potential (ψ_π) of nutrient solutions were positively correlated, and the former variable also had a strong positive relationship with transpiration rate. Leaf water potential and ψ_π followed similar trends and declined significantly as ψ_π of watering solutions was lowered. The increase in total inorganic nutrients resulting from increased Na⁺ and Cl⁻ in all tissues and K⁺, Ca²⁺ and Mg²⁺ concentrations were maintained even at the most extreme salt concentration. Common reed exhibited high K⁺/Na⁺ and Ca²⁺/Na⁺ selectivity ratios over a wide range of salinities under NaCl-salinity. These findings suggest that reed plants were able to adapt well to high salinities by lowering their leaf ψ_π and the adjustment of osmotically active solutes in the leaves.     

Effect of sodium chloride on the response of the halophyte species <Emphasis Type="Italic">Sesuvium portulacastrum</Emphasis> grown in mannitol-induced water stress

Sesuvium portulacastrum is a halophytic species well adapted to salinity and drought. In order to evaluate the physiological impact of salt on water deficit-induced stress response, we cultivated seedlings for 12 days, in the presence or absence of 100 mmol l⁻¹ NaCl, on a nutrient solution containing either 0 mmol l⁻¹ or 25 mmol l⁻¹ mannitol. Mannitol-induced water stress reduced growth, increased the root/shoot ratio, and led to a significant decrease in water potential and leaf relative water content, whereas leaf Na⁺ and K⁺ concentrations remained unchanged. The addition of 100 mmol l⁻¹ NaCl to 25 mmol l⁻¹ mannitol-containing medium mitigated the deleterious impact of water stress on growth of S. portulacastrum, improved the relative water content, induced a significant decrease in leaf water potential and, concomitantly, resulted in enhancement of overall plant photosynthetic activity (i.e. CO₂ net assimilation rate, stomatal conductance). Presence of NaCl in the culture medium, together with mannitol, significantly increased the level of Na⁺ and proline in the leaves, but it had no effect on leaf soluble sugar content. These findings suggest that the ability of NaCl to improve plant performance under mannitol-induced water stress may be due to its effect on osmotic adjustment through Na⁺ and proline accumulation, which is coupled with an improvement in photosynthetic activity. A striking recovery in relative water content and growth of the seedlings was also recorded in the presence of NaCl on release of the water stress induced by mannitol.     

Effects of NaCl or Na<Subscript>2</Subscript>SO<Subscript>4</Subscript> salinity on plant growth,ion content and photosynthetic activity in <Emphasis Type="Italic">Ocimum basilicum</Emphasis> L.

Basil (Ocimum basilicum L., cultivar Genovese) plants were grown in Hoagland solution with or without 50 mM NaCl or 25 mM Na₂SO₄. After 15 days of treatment, Na₂SO₄ slowed growth of plants as indicated by root, stem and leaf dry weight, root length, shoot height and leaf area, and the effects were major of those induced by NaCl. Photosynthetic response was decreased more by chloride salinity than by sulphate. No effects in both treatments on leaf chlorophyll content, maximal efficiency of PSII photochemistry (F _v/F _m) and electron transport rate (ETR) were recorded. Therefore, an excess of energy following the limitation to CO₂ photoassimilation and a down regulation of PSII photochemistry was monitored under NaCl, which displays mechanisms that play a role in avoiding PSII photodamage able to dissipate this excess energy. Ionic composition (Na⁺, K⁺, Ca²⁺, and Mg²⁺) was affected to the same extent under both types of salinity, thus together with an increase in leaves Cl⁻, and roots SO₄ ²⁻ in NaCl and Na₂SO₄-treated plants, respectively, may have resulted in the observed growth retardation (for Na₂SO₄ treatment) and photosynthesis activity inhibition (for NaCl treatment), suggesting that those effects seem to have been due to the anionic component of the salts.     

Effect of high salinity on Atriplex portulacoides: Growth,leaf water relations and solute accumulation in relation with osmotic adjustment

Atriplex (Halimione) portulacoides is a halophyte with potential interest for saline soil reclamation and phytoremediation. Here, we assess the impact of salinity reaching up to two-fold seawater concentration (0–1000 mM NaCl) on the plant growth, leaf water status and ion uptake and we evaluate the contribution of inorganic and organic solutes to the osmotic adjustment process. A. portulacoides growth was optimal at 200 mM NaCl but higher salinities (especially 800 and 1000 mM NaCl) significantly reduced plant growth. Na⁺ and Cl⁻ contents increased upon salt exposure especially in the leaves compared to the roots. Interestingly, no salt-induced toxicity symptoms were observed and leaf water content was maintained even at the highest salinity level. Furthermore, leaf succulence and high instantaneous water use efficiency (WUE_i) under high salinity significantly contributed to maintain leaf water status of this species. Leaf pressure–volume curves showed that salt-challenged plants adjusted osmotically by lowering osmotic potential at full turgor (Ψ_π¹⁰⁰) along with a decrease in leaf cell elasticity (values of volumetric modulus elasticity (ε) increased). As a whole, our findings indicate that A. portulacoides is characterized by a high plasticity in terms of salt-response. Preserving leaf hydration and efficiently using Na⁺ for the osmotic adjustment especially at high salinities (800–1000 mM NaCl), likely through its compartmentalization in leaf vacuoles, are key determinants of such a performance. The selective absorption of K⁺ over Na⁺ in concomitance with an increase in the K⁺ use efficiency also accounted for the overall plant salt tolerance.     

Can elevated CO(2) improve salt tolerance in olive trees?

We compared growth, leaf gas exchange characteristics, water relations, chlorophyll fluorescence, and Na⁺ and Cl⁻ concentration of two cultivars (‘Koroneiki’ and ‘Picual’) of olive (Olea europaea L.) trees in response to high salinity (NaCl 100 mM) and elevated CO₂ (eCO₂) concentration (700 μL L⁻¹). The cultivar ‘Koroneiki’ is considered to be more salt sensitive than the relatively salt-tolerant ‘Picual’. After 3 months of treatment, the 9-month-old cuttings of ‘Koroneiki’ had significantly greater shoot growth, and net CO₂ assimilation (A_CO2) at eCO₂ than at ambient CO₂, but this difference disappeared under salt stress. Growth and A_CO2 of ‘Picual’ did not respond to eCO₂ regardless of salinity treatment. Stomatal conductance (g_s) and leaf transpiration were decreased at eCO₂ such that leaf water use efficiency (WUE) increased in both cultivars regardless of saline treatment. Salt stress increased leaf Na⁺ and Cl⁻ concentration, reduced growth and leaf osmotic potential, but increased leaf turgor compared with non-salinized control plants of both cultivars. Salinity decreased A_CO2, g_s, and WUE, but internal CO₂ concentrations in the mesophyll were not affected. eCO₂ increased the sensitivity of PSII and chlorophyll concentration to salinity. eCO₂ did not affect leaf or root Na⁺ or Cl⁻ concentrations in salt-tolerant ‘Picual’, but eCO₂ decreased leaf and root Na⁺ concentration and root Cl⁻ concentration in the more salt-sensitive ‘Koroneiki’. Na⁺ and Cl⁻ accumulation was associated with the lower water use in ‘Koroneiki’ but not in ‘Picual’. Although eCO₂ increased WUE in salinized leaves and decreased salt ion uptake in the relatively salt-tolerant ‘Koroneiki’, growth of these young olive trees was not affected by eCO₂.     

Na<Superscript>+</Superscript>/H<Superscript>+</Superscript> exchange activity in the alkaliphile halotolerant cyanobacterium <Emphasis Type="Italic">Aphanothece halophytica</Emphasis>

The activity of Na⁺/H⁺ exchanger to remove toxic Na⁺ is important for growth of organisms under high salinity. In this study, the halotolerant cyanobacterium Aphanothece halophytica was shown to possess Na⁺/H⁺ exchange activity since exogenously added Na⁺ could dissipate a pre-formed pH gradient, and decrease extracellular pH. Kinetic analysis yielded apparent K _m (Na⁺) and V _max of 20.7 ± 3.1 mM and 3,333 ± 370 nmol H⁺ min⁻¹ mg⁻¹, respectively. For cells grown under salt-stress condition, the apparent K _m (Na⁺) and V _max was 18.3 ± 3.5 mM and 3,703 ± 350 nmol H⁺ min⁻¹ mg⁻¹, respectively. Three cations with decreasing efficiency namely Li⁺, Ca²⁺, and K⁺ were also able to dissipate pH gradient. Only marginal exchange activity was observed for Mg²⁺. The exchange activity was strongly inhibited by Na⁺-gradient dissipators, monensin, and sodium ionophore as well as by CCCP, a protonophore. A. halophytica showed high Na⁺/H⁺ exchange activity at neutral and alkaline pH up to pH 10. Cells grown at pH 7.6 under high salinity exhibited higher Na⁺/H⁺ exchange activity than those grown under low salinity during 15 days of growth suggesting a role of Na⁺/H⁺ exchanger for salt tolerance in A. halophytica. Cells grown at alkaline pH of 9.0 also exhibited a progressive increase of Na⁺/H⁺ exchange activity during 15 days of growth.     

Influence of salinity on Na+ and K+ accumulation, and gas exchange in Avicennia germinans

We analysed plant growth, ion accumulation, leaf water relations, and gas exchange of Avicennia germinans (L.) L. subjected to a long-term, controlled salinity gradient from 0 to 55 ‰. Growth and leaf area were affected by salinity higher than 10 ‰. As salinity increased, the predawn leaf water potential (Ψ_w) and leaf osmotic potential (Ψ_s) decreased. Leaf Ψ_w was at least −0.32 MPa lower than the Ψ_w of solution. Na⁺ and K⁺ ions explained about 78 % of decrease in Ψ_s. K⁺ tissue water concentration decreased by more than 60 % in all salinity treatments as compared with those grown at 0 ‰. Inversely, Na⁺ concentration in tissue water increased with nutrient solution salinity. The maximum net photosynthetic rate (P _N) and stomatal conductance (g _s) decreased by 68 and 82 %, respectively, as salinity increased from 0 to 55 ‰; the intercellular CO₂ concentration (C _i) followed the same trend. The P _N as a function of C _i showed that both the initial linear slope and upper plateau of the P _N vs. C _i curve were markedly affected by high salinity (40 and 55 ‰).     

Overexpression of <Emphasis Type="Italic">OsVP1</Emphasis> and <Emphasis Type="Italic">OsNHX1</Emphasis> Increases Tolerance to Drought and Salinity in Rice

Drought and salinity are major abiotic stresses affecting rice production. To improve plant tolerance to salinity and drought, we overexpressed rice Na⁺/H⁺ exchangers (OsNHX1) and H⁺-pyrophosphatase in tonoplasts (OsVP1) in a japonica elite rice cultivar, Zhonghua 11. Compared with our wild-type control, transgenic plants overexpressing both genes incurred less damage when exposed to long-term treatment with 100 mM NaCl or water deprivation. Under high-saline conditions, the transformants accumulated less Na⁺ and malondialdehyde in the leaves, thereby allowing the plants to maintain a low level of leaf water potential and reduce stress-induced damage. Those transgenics also had higher photosynthetic activity during the stress period. Under those conditions, they also showed an increase in root biomass, which enabled more water uptake. These results suggest that OsVP1 and OsNHX1 improve the tolerance of rice crops against drought and salt by employing multiple strategies in addition to osmotic regulation.     

Comparative effectiveness of <Emphasis Type="Italic">Pseudomonas</Emphasis> and <Emphasis Type="Italic">Serratia</Emphasis> sp. containing ACC-deaminase for improving growth and yield of wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) under salt-stressed conditions

Ethylene synthesis is accelerated in response to various environmental stresses like salinity. Ten rhizobacterial strains isolated from wheat rhizosphere taken from different salt affected areas were screened for growth promotion of wheat under axenic conditions at 1, 5, 10 and 15 dS m⁻¹. Three strains, i.e., Pseudomonas putida (N21), Pseudomonas aeruginosa (N39) and Serratia proteamaculans (M35) showing promising performance under axenic conditions were selected for a pot trial at 1.63 (original), 5, 10 and 15 dS m⁻¹. Results showed that inoculation was effective even in the presence of higher salinity levels. P. putida was the most efficient strain compared to the other strains and significantly increased the plant height, root length, grain yield, 100-grain weight and straw yield up to 52, 60, 76, 19 and 67%, respectively, over uninoculated control at 15 dS m⁻¹. Similarly, chlorophyll content and K⁺/Na⁺ of leaves also increased by P. putida over control. It is highly likely that under salinity stress, 1-aminocyclopropane-1-carboxylic acid-deaminase activity of these microbial strains might have caused reduction in the synthesis of stress (salt)-induced inhibitory levels of ethylene. The results suggested that these strains could be employed for salinity tolerance in wheat; however, P. putida may have better prospects in stress alleviation/reduction.     

Photosynthetic limitations of a halophyte sea aster (<Emphasis Type="Italic">Aster tripolium</Emphasis> L) under water stress and NaCl stress

To understand the mechanisms of salt tolerance in a halophyte, sea aster (Aster tripolium L.), we studied the changes of water relation and the factors of photosynthetic limitation under water stress and 300 mM NaCl stress. The contents of Na⁺ and Cl^- were highest in NaCl-stressed leaves. Leaf osmotic potentials (Ψ _s) were decreased by both stress treatments, whereas leaf turgor pressure (Ψ _t) was maintained under NaCl stress. Decrease inΨ _s without any loss ofΨ _t accounted for osmotic adjustment using Na⁺ and Cl^- accumulated under NaCl stress. Stress treatments affected photosynthesis, and stomatal limitation was higher under water stress than under NaCl stress. Additionally, maximum CO₂ fixation rate and O₂ evolution rate decreased only under water stress, indicating irreversible damage to photosynthetic systems, mainly by dehydration. Water stress severely affected the water relation and photosynthetic capacity. On the other hand, turgid leaves under NaCl stress have dehydration tolerance due to maintenance of Ψ _t and photosynthetic activity. These results show that sea aster might not suffer from tissue dehydration in highly salinized environments. We conclude that the adaptation of sea aster to salinity may be accomplished by osmotic adjustment using accumulated Na⁺ and Cl^-, and that this plant has typical halophyte characteristics, but not drought tolerance. Electronic Publication     

Leaf cell membrane stability‐based mechanisms of zinc nutrition in mitigating salinity stress in rice

• Excess salt affects about 955 million ha of arable land worldwide, and 49% of agricultural land is Zn‐deficient. Soil salinity and zinc deficiency can intensify plant abiotic stress. The mechanisms by which Zn can mitigate salinity effects on plant functions are not well understood.
• We conducted an experiment to determine how Zn and salinity effects on rice plant retention of Zn, K⁺ and the salt ion Na⁺ affect chlorophyll formation, leaf cell membrane stability and grain yield. We examined the mechanisms of Zn nutrition in mitigating salinity stress by examining plant physiology and nutrition. We used native Zn‐deficient soils (control), four salinity (EC ) and Zn treatments – Zn 10 mg·kg^?1 (Zn₁₀), EC 5 dS ·m^?1 (EC ₅), Zn₁₀+EC ₅ and Zn₁₅+EC ₅, a coarse rice (KS ‐282) and a fine rice (Basmati‐515) in the study.
• Our results showed that Zn alone (Zn₁₀) significantly increased rice tolerance to salinity stress by promoting Zn/K⁺ retention, inhibiting plant Na⁺ uptake and enhancing leaf cell membrane stability and chlorophyll formation in both rice cultivars in native alkaline, Zn‐deficient soils (P  <  0.05). Further, under the salinity treatment (EC ₅), Zn inputs (10–15 mg·kg^?1) could also significantly promote rice plant Zn/K⁺ retention and reduce plant Na⁺ uptake, and thus increased leaf cell membrane stability and grain yield. Coarse rice was more salinity‐tolerant than fine rice, having significantly higher Zn/K⁺ nutrient retention.
• The mechanistic basis of Zn nutrition in mitigating salinity impacts was through promoting plant Zn/K⁺ uptake and inhibiting plant Na⁺ uptake, which could result in increased plant physiological vigour, leaf cell membrane stability and rice productivity.

     

Regeneration of plantlets of guava (<Emphasis Type="Italic">Psidium guajava</Emphasis> L.) from somatic embryos developed under salt-stress condition

The present study demonstrates the regeneration of plantlets of guava (Psidium guajava L.) from somatic embryos developed under salt-stress conditions. With increasing concentrations of NaCl in induction medium (MS + 4.52 μM 2,4-d + 5% sucrose) from 0 to 200 mM, the number of somatic embryos per responsive explant decreased. Somatic embryos induced on 0–100 mM NaCl containing medium developed into torpedo stages, whereas, the development of somatic embryos that differentiated on 150 and 200 mM NaCl-supplemented medium was arrested prior to torpedo stage and did not undergo maturation phase. Somatic embryos that developed on NaCl-containing medium, showed better germination in the presence of NaCl as compared with those developed on medium without NaCl. The effect of increasing salt-stress was also investigated on plant growth, chlorophyll and carotenoids, Na⁺ and K⁺, and proline and glycine betaine accumulation in in vitro grown plantlets. The level of Na⁺ in leaves increased with increasing concentrations of NaCl in the medium. Accumulation of free proline and glycine betaine in leaves significantly increased with increasing salinity. The results suggest that accumulation of proline and glycine betaine may be important for osmotic adjustment in guava under salinity stress.     

Osmotic adjustment,water relations and growth attributes of the xero-halophyte <Emphasis Type="Italic">Reaumuria vermiculata</Emphasis> L. (Tamaricaceae) in response to salt stress

Reaumuria vermiculata (L.), a perennial dwarf shrub in the family of Tamaricaceae, is a salt-secreting xero-halophyte found widely in arid areas of Tunisia. In the present study, physiological attributes of R. vermiculata were investigated under salt stress. Four-month-old plants were subjected to various salinity levels (0, 100, 200, 300, 400 or 600 mM NaCl) for 30 days under greenhouse conditions. Results showed that plants grew optimally when treated with standard nutrient solution without NaCl supply. However, increasing osmolality of nutrient solutions caused a significant reduction in biomass production and relative growth rate. This reduction was more pronounced in roots than in shoots. In addition, this species was able to maintain its shoot water content at 30% of the control even when subjected to the highest salt level, whereas root water content seemed to be unaffected by salt. Shoot water potential declined significantly as osmotic potential of watering solutions was lowered and the more negative values were reached at 600 mM NaCl (−3.4 MPa). Concentrations of Na⁺ and Cl⁻ in the shoots of R. vermiculata were markedly increased with increasing osmolality of nutrient solutions, whereas concentration of K⁺ was not affected by NaCl supply. Salt excretion is an efficient mechanism of Na⁺ exclusion from the shoots of this species exhibiting high K⁺/Na⁺ selectivity ratio over a wide range of NaCl salinity. Proline accumulation in shoots was significantly increased with increase in salt level and may play a role in osmoregulation.     

Leaf gas exchange,water relations,nutrient content and growth in citrus and olive seedlings under salinity

The effects of salinity on growth, leaf nutrient content, water relations, gas exchange parameters and chlorophyll fluorescence were studied in six-month-old seedlings of citrus (Citrus limonia Osbeck) and rooted cuttings of olive (Olea europaea L. cv. Arbequina). Citrus and olive were grown in a greenhouse and watered with half strength Hoagland’s solution plus 0 or 50 mM NaCl for citrus, or plus 0 or 100 mM NaCl for olive. Salinity increased Cl⁻ and Na⁺ content in leaves and roots in both species and reduced total plant dry mass, net photosynthetic rate and stomatal conductance. Decreased growth and gas exchange was apparently due to a toxic effect of Cl⁻ and/or Na⁺ and not due to osmotic stress since both species were able to osmotically adjust to maintain pressure potential higher than in non-salinized leaves. Internal CO₂ concentration in the mesophyll was not reduced in either species. Salinity decreased leaf chlorophyll a content only in citrus.     

Regulated alterations in redox and energetic status are the key mediators of salinity tolerance in the halophyte <Emphasis Type="Italic">Sesuvium portulacastrum</Emphasis> (L.) L

The present work addresses the importance of antioxidant, redox and energetic parameters in regulating salt-tolerance in Sesuvium portulacastrum. Experiments were conducted on 45 days old plants subjected to 250 and 1,000 mM NaCl stress for 2–8 days. Plants showed no significant change in growth parameters (shoot length, dry weight, and water content) at 250 mM NaCl as compared to control. However, growth of plants was significantly affected at 1,000 mM NaCl. The differential growth behaviour could be attributed to a greater decline in the energetic parameters (in terms of ratios of NADP/NADPH and ATP/ADP) at 1,000 mM NaCl than at 250 mM NaCl. The osmotic stress imposed to plants at 250 mM NaCl was presumably balanced by the accumulation of sodium ions (Na⁺), an energetically favorable process, and did not require an increased synthesis of proline. In contrast, to counter osmotic stress at 1,000 mM NaCl, plants accumulated Na⁺ as well as proline and were, therefore, energetically stressed. Further, the response of enzymatic and molecular antioxidants at 1,000 mM was either close to or even lower than that at 250 mM, which resulted in oxidative damage at 1,000 mM, particularly on longer durations. In conclusion, it is suggested that altered redox and energetic status of the plants could play a key role in mediating the tolerance of Sesuvium under salinity stress.     

Alleviation of salinity stress in broccoli using foliar urea or methyl-jasmonate: analysis of growth,gas exchange,and isotope composition

We studied the effects of foliar application of urea or methyl-jasmonate (MeJA) on the salinity tolerance of broccoli plants (Brassisca oleracea L. var. italica). Plant dry weight, leaf CO₂ assimilation, and root respiration were reduced significantly under moderate saline stress (40 mM NaCl) but application of either urea or MeJA maintained growth, gas exchange parameters, and leaf N–NO₃ ⁻ concentrations at values similar to those of non-salinized plants. Additionally, when these two foliar treatments were applied leaf Na⁺ concentration was reduced compared with control plants grown at 40 mM NaCl. However, at a higher salt concentration (120 mM NaCl), no effect of the foliar applications was found on these parameters. Salinity also decreased leaf δ¹⁵N but increased δ¹³C. Our study shows the feasibility of using foliar urea or MeJA to improve tolerance under moderate saline stress.     

On the mechanism of salt tolerance in olive (Olea europaea L.) under low- or high-Ca2+ supply

The effect of changes in Ca²⁺/Na⁺ ratios at the root zone has been reported in Olea europaea, a species mostly cultivated in calcareous soils. Plants were exposed to low (2.0 mM, low-Ca) or high-Ca²⁺ supply (9.0 mM, high-Ca) and supplied with 0 or 200 mM NaCl. Measurements were performed on water relations, gas exchange and photosynthetic performances, ion fluxes at whole-plant and leaf level, Na⁺ allocation at organismal level, the elemental and soluble carbohydrate concentration in the leaf. Most parameters were also measured during a period of relief from salinity stress, as Olea europaea suffers from fluctuating root zone NaCl concentrations over the whole growing season. High-Ca²⁺ supply decreased stomatal conductance, especially during the first two weeks of treatment. In response to salinity stress (i) leaf turgor potential was more severely depressed in high-Ca than in low-Ca plants, whereas net CO₂ assimilation rate and relative growth rate were unaffected by root zone Ca²⁺ concentrations (ii) high-Ca plants had a markedly superior ability to both exclude Na⁺ from the shoot and to selectively transport K⁺ over Na⁺ than low-Ca plants; (iii) both CO₂ carboxylation efficiency and maximal efficiency of PSII photochemistry (F_v/F_m) were significantly smaller in low-Ca than in high-Ca plants, likely as a result of a greater accumulation of toxic ions. Consistently, when osmotic stress was relieved by supplying plants with good quality water (relief period), both photosynthetic (+44%) and growth rates (+65%) recovered to a markedly superior degree in high-Ca than in low-Ca plants which had been previously treated with 200 mM NaCl. We conclude that (1) high-Ca²⁺ supply expose olive leaves to a more severe dehydration, but allowed to restrict both the entry and the allocation of potentially toxic ions to sensitive shoot organs; (2) a transient restriction of water-mass flow to the shoot during salinization may be of relatively minor significance in Olea europaea, which is very tolerant to drought; (3) overall salt tolerance in Olea europaea, as in most evergreen sclerophylls inhabiting Mediterranean areas, tightly depends upon the ability to reduce water uptake and transpiration during the dry/warm period and to recover photosynthetic and growth rates when low-salinity flood water is available. Therefore, data from the present experiment allow conclude that an increase in root zone Ca²⁺ concentration enhances tolerance to salinity stress in olive plants.     

Water relations and xylem transport of nutrients in pepper plants grown under two different salts stress regimes

Two iso-osmotic concentrations of NaCl and Na₂SO₄ were used for discriminating between the effects of specific ion toxicities of salt stress on pepper plants (Capsicum annuum L.) grown in hydroponic conditions, in a controlled-environment greenhouse. The two salts were applied to plants at different electrical conductivities, and leaf water relations, osmotic adjustment and root hydraulic conductance were measured. Leaf water potential (_w), leaf osmotic potential (_o) and leaf turgor potential (_p) decreased significantly when EC increased, but the decrease was less for NaCl- than for Na₂SO₄-treated plants. The reduction in stomatal conductance was higher for NaCl-treated plants. There were no differences in the effect of both treatments on the osmotic adjustment, and a reduction in root hydraulic conductance and the flux of solutes into the xylem was observed, except for the saline ions (Na⁺, Cl^– and SO₄ ^2–). Therefore, pepper growth decreased with increasing salinity because the plants were unable to adjust osmotically or because of the toxic effects of Cl^–, SO₄ ^2– and/or Na⁺. However, turgor of NaCl-treated plants was maintained at low EC (3 and 4 dS m^–1) probably due to the maintenance of water transport into the plant (decrease of stomatal conductance), which, together with the lower concentration of Na⁺ in the plant tissues compared with the Na₂SO₄ treatment, could be the cause of the smaller decrease in growth.