Comparative Study of the Interactive Effects of Salinity and Phosphorus Availability in Wild (Hordeum maritimum) and Cultivated Barley (H. vulgare)

The present study aimed to compare the effects of phosphorus (P) deficiency applied only or combined with salinity on root response, P partitioning, acid phosphatase activity, and phenolic compounds in wild (Hordeum maritimum) and cultivated (H. vulgare) barley species. Seedlings were grown hydroponically under low or sufficient P supply, with or without 100 mM NaCl for 55 days. Results showed that, when individually applied, P deficiency and salinity restricted the whole plant relative growth rate in both species of barley, with a more pronounced impact of the former stress. These depressive effects were more pronounced in H. vulgare than in H. maritimum. The combined effects of P deficiency and salinity were not additive neither on whole plant RGR nor on root response parameters in both species. The root area, root/shoot P content, root and leaf acid phosphatase activities, and shoot flavonoids contents increased under P deficiency conditions with and without salt in both species. Overall, the relatively better tolerance of H. maritimum plants to P deficiency applied only or combined with salinity could be explained by the capacity of this species to maintain higher P acquisition efficiency in concomitance with a larger root system, a higher root/shoot DW ratio, a higher root/shoot P content, a greater root and leaf acid phosphatase activities, and a higher flavonoid content and antioxidant capacity under combined effects of both stresses. Thus, H. maritimum constitutes a promising model to ameliorate the tolerance of the cultivated barley species under low-P soils and/or saline regions.     

Salt tolerance, salt accumulation, and ionic homeostasis in an epidermal bladder-cell-less mutant of the common ice plant Mesembryanthemum crystallinum

The aerial surfaces of the common or crystalline ice plant Mesembryanthemum crystallinum L., a halophytic, facultative crassulacean acid metabolism species, are covered with specialized trichome cells called epidermal bladder cells (EBCs). EBCs are thought to serve as a peripheral salinity and/or water storage organ to improve survival under high salinity or water deficit stress conditions. However, the exact contribution of EBCs to salt tolerance in the ice plant remains poorly understood. An M. crystallinum mutant lacking EBCs was isolated from plant collections mutagenized by fast neutron irradiation. Light and electron microscopy revealed that mutant plants lacked EBCs on all surfaces of leaves and stems. Dry weight gain of aerial parts of the mutant was almost half that of wild-type plants after 3 weeks of growth at 400 mM NaCl. The EBC mutant also showed reduced leaf succulence and leaf and stem water contents compared with wild-type plants. Aerial tissues of wild-type plants had approximately 1.5-fold higher Na(+) and Cl(-) content than the mutant grown under 400 mM NaCl for 2 weeks. Na(+) and Cl(-) partitioning into EBCs of wild-type plants resulted in lower concentrations of these ions in photosynthetically active leaf tissues than in leaves of the EBC-less mutant, particularly under conditions of high salt stress. Potassium, nitrate, and phosphate ion content decreased with incorporation of NaCl into tissues in both the wild type and the mutant, but the ratios of Na(+)/K(+) and Cl(-)/NO(3)(-)content were maintained only in the leaf and stem tissues of wild-type plants. The EBC mutant showed significant impairment in plant productivity under salt stress as evaluated by seed pod and seed number and average seed weight. These results clearly show that EBCs contribute to succulence by serving as a water storage reservoir and to salt tolerance by maintaining ion sequestration and homeostasis within photosynthetically active tissues of M. crystallinum.     

Alleviation of phosphorus deficiency stress by moderate salinity in the halophyte <Emphasis Type="Italic">Hordeum maritimum</Emphasis> L.

Hordeum maritimum (Poacea) is a facultative halophyte potentially useful for forage production in saline zones. Here, we assessed whether moderate NaCl-salinity can modify the plant response to phosphorus (P) shortage. Plants were cultivated for 55 days under low or sufficient P supply (5 or 60 μmol plant⁻¹ week⁻¹ KH₂PO₄, respectively), with or without 100 mM NaCl. When individually applied, salinity and P deficiency significantly restricted whole-plant growth, with a more marked effect of the latter stress. Plants subjected to P deficiency showed a significant increase in root growth (as length and dry weight) and root/shoot DW ratio. Enhanced root growth and elongation presumably correspond to the well-known root adaptive response to mineral deficiency. However, leaf relative water content, leaf P concentration, and leaf gas exchange parameters were significantly restricted. The interactive effects of salinity and P deficiency were not added one to another neither on whole plant biomass nor on plant nutrient uptake. Indeed, 100 mM NaCl-addition to P-deficient plants significantly restored the plant growth and improved CO₂ assimilation rate, root growth, K⁺/Na⁺ ratio and leaf proline and soluble sugar concentrations. It also significantly enhanced leaf total antioxidant capacity and leaf anthocyanin concentration. This was associated with significantly lower leaf osmotic potential, leaf Na⁺ and malondialdehyde (MDA) concentration. Taken together, these results suggest that mild salinity may mitigate the adverse effects of phosphorus deficiency on H. maritimum by notably improving the plant photosynthetic activity, the osmotic adjustment capacity, the selective absorption of K⁺ over Na⁺ and antioxidant defence.     

Transport, synthesis and catabolism of abscisic acid (ABA) in intact plants of castor bean (Ricinus communis L.) under phosphate deficiency and moderate salinity

Flows of abscisic acid (ABA) were investigated in whole plantsof castor bean (Ricinus communis) grown in sand culture undereither phosphate deficiency or moderate salinity. Xylem transportof ABA in P-deficient plants was stimulated by a factor of 6whereas phloem transport was affected only very slightly. ABAdeposition into leaves of P-deficient plants was not appreciablydifferent from the controls because of strong net degradationin leaves. Since conjugation of ABA was strongly reduced inall organs of P-deficient plants ABA was presumably metabolizedmainly to phaseic acid and dihydrophaseic acid. The increasedimport of ABA occurred predominantly into fully differentiatedbut not senescent leaves and showed a good correlation withthe inhibition of leaf conductance under P deficiency. As with low-P-plants salt stress increased ABA synthesis inroots and associated transport in the xylem. However, salinitycaused a distinctly greater accumulation of ABA in the leaves,stem segments and the apex than in P-deficient plants. As opposedto P deficiency, ABA export in the phloem from the leaves wasstimulated by salinity. Modelling of ABA flows within an individualleaf over its life cycle showed that young growing leaves importedABA from both phloem and xylem, whereas the adult non-senescentleaves were a source of ABA and thus provided a potential shoot-to-rootstress signal as well as an acceptor for reciprocal signalsfrom root to shoot. In senescing leaves ABA flows and accumulationwere somewhat retarded and ABA was lost in net terms by exportfrom the leaf. Key words: Abscisic acid, phosphorus deficiency, salt stress, phloem and xylem transport     

A root's ability to retain K+ correlates with salt tolerance in wheat

Most work on wheat breeding for salt tolerance has focused mainly on excluding Na(+) from uptake and transport to the shoot. However, some recent findings have reported no apparent correlation between leaf Na(+) content and wheat salt tolerance. Thus, it appears that excluding Na(+) by itself is not always sufficient to increase plant salt tolerance and other physiological traits should also be considered. In this work, it was investigated whether a root's ability to retain K(+) may be such a trait, and whether our previous findings for barley can be extrapolated to species following a 'salt exclusion' strategy. NaCl-induced kinetics of K(+) flux from roots of two bread and two durum wheat genotypes, contrasting in their salt tolerance, were measured under laboratory conditions using non-invasive ion flux measuring (the MIFE) technique. These measurements were compared with whole-plant physiological characteristics and yield responses from plants grown under greenhouse conditions. The results show that K(+) flux from the root surface of 6-d-old wheat seedlings in response to salt treatment was highly correlated with major plant physiological characteristics and yield of greenhouse-grown plants. This emphasizes the critical role of K(+) homeostasis in plant salt tolerance and suggests that using NaCl-induced K(+) flux measurements as a physiological 'marker' for salt tolerance may benefit wheat-breeding programmes.     

Effects of non-uniform root zone salinity on water use, Na+ recirculation, and Na+ and H+ flux in cotton

A new split-root system was established through grafting to study cotton response to non-uniform salinity. Each root half was treated with either uniform (100/100?mM) or non-uniform NaCl concentrations (0/200 and 50/150?mM). In contrast to uniform control, non-uniform salinity treatment improved plant growth and water use, with more water absorbed from the non- and low salinity side. Non-uniform treatments decreased Na(+) concentrations in leaves. The [Na(+)] in the '0' side roots of the 0/200 treatment was significantly higher than that in either side of the 0/0 control, but greatly decreased when the '0' side phloem was girdled, suggesting that the increased [Na(+)] in the '0' side roots was possibly due to transportation of foliar Na(+) to roots through phloem. Plants under non-uniform salinity extruded more Na(+) from the root than those under uniform salinity. Root Na(+) efflux in the low salinity side was greatly enhanced by the higher salinity side. NaCl-induced Na(+) efflux and H(+) influx were inhibited by amiloride and sodium orthovanadate, suggesting that root Na(+) extrusion was probably due to active Na(+)/H(+) antiport across the plasma membrane. Improved plant growth under non-uniform salinity was thus attributed to increased water use, reduced leaf Na(+) concentration, transport of excessive foliar Na(+) to the low salinity side, and enhanced Na(+) efflux from the low salinity root.     

Overexpression of an H+-PPase gene from Thellungiella halophila in cotton enhances salt tolerance and improves growth and photosynthetic performance

Salinity is one of the major environmental factors limiting plant growth and productivity. An H(+)-PPase gene, TsVP from Thellungiella halophila, was transferred into cotton (Gossypium hirsutum) in sense and antisense orientations under control of the cauliflower mosaic virus (CaMV) 35S promoter. Southern and Northern blotting analysis showed that the sense or antisense TsVP were integrated into the cotton genome and expressed. Transgenic plants overexpressing the vacuolar H(+)-PPase were much more resistant to 150 and 250 mM NaCl than the isogenic wild-type plants. In contrast, the plants from the antisense line (L-2), with lower H(+)-PPase activity, were more sensitive to salinity than the wild-type plants. Overexpressing TsVP in cotton improved shoot and root growth and photosynthetic performance. These transgenic plants accumulated more Na(+), K(+), Ca(2+), Cl(-) and soluble sugars in their root and leaf tissues under salinity conditions compared with the wild-type plants. The lower membrane ion leakage and malondialdehyde (MDA) level in these transgenic plants suggest that overexpression of H(+)-PPase causes the accumulation of Na(+) and Cl(-) in vacuoles instead of in the cytoplasm, thus reducing their toxic effects. On the other hand, the increased accumulation of ions and sugars decreases the solute potential in cells, and facilitates water uptake under salinity, which is an important mechanism for the increased salt tolerance in TsVP-overexpressing cotton.     

Growth, nitrogen fixation and ammonium assimilation in common bean (Phaseolus vulgaris): effect of phosphorus

The impact of phosphorous nutrition on plant growth, symbiotic N₂ fixation, ammonium assimilation, carbohydrate and amino-acid accumulation, as well as on nitrogen, phosphorus and ATP content in tissues in common bean ( Phaseolus vulgaris ) plants was investigated. Plants inoculated with Rhizobium tropici CIAT899 were grown in Leonard jars under controlled conditions, with P-deficient (0 and 0.1 m M ), P-medium (0.5, 1 and 1.5 m M ) and P-high (2 m M ) conditions in a N-free nutrient solution. The P application, increased leaf area, whole plant DW (67%), nodule biomass (4-fold), and shoot and root P content (4- and 6-fold, respectively) in plant harvested at the onset of flowering (28-days-old). However, P treatments decreased the total soluble sugar and amino acid content in vegetative organs (leaf, root and nodules). The root growth proved less sensitive to P deficiency than did shoot growth, and the leaf area was significantly reduced at low P-application. The absence of a relationship between shoot N content, and P levels in the growth medium could indicate that nitrogen fixation requires more P than does plant growth. The optimal amount for the P. vulgaris – R. tropici CIAT899 symbiosis was 1.5 m M P, this treatment augmented nodule-ARA 20-fold, and ARA per plant 70-fold compared with plants without P application.     

Paxillus involutus Strains MAJ and NAU mediate K(+)/Na(+) homeostasis in ectomycorrhizal Populus x canescens under sodium chloride stress

Salt-induced fluxes of H(+), Na(+), K(+), and Ca(2+) were investigated in ectomycorrhizal (EM) associations formed by Paxillus involutus (strains MAJ and NAU) with the salt-sensitive poplar hybrid Populus × canescens. A scanning ion-selective electrode technique was used to measure flux profiles in non-EM roots and axenically grown EM cultures of the two P. involutus isolates to identify whether the major alterations detected in EM roots were promoted by the fungal partner. EM plants exhibited a more pronounced ability to maintain K(+)/Na(+) homeostasis under salt stress. The influx of Na(+) was reduced after short-term (50 mm NaCl, 24 h) and long-term (50 mm NaCl, 7 d) exposure to salt stress in mycorrhizal roots, especially in NAU associations. Flux data for P. involutus and susceptibility to Na(+)-transport inhibitors indicated that fungal colonization contributed to active Na(+) extrusion and H(+) uptake in the salinized roots of P. × canescens. Moreover, EM plants retained the ability to reduce the salt-induced K(+) efflux, especially under long-term salinity. Our study suggests that P. involutus assists in maintaining K(+) homeostasis by delivering this nutrient to host plants and slowing the loss of K(+) under salt stress. EM P. × canescens plants exhibited an enhanced Ca(2+) uptake ability, whereas short-term and long-term treatments caused a marked Ca(2+) efflux from mycorrhizal roots, especially from NAU-colonized roots. We suggest that the release of additional Ca(2+) mediated K(+)/Na(+) homeostasis in EM plants under salt stress.     

Investigation of the effects of Vesicular Arbuscular Mycorrhiza on mineral nutrition and growth of <Emphasis Type="Italic">Carthamus tinctorius</Emphasis> under salt stress conditions

Salt stress is considered as one of the most important abiotic factors limiting plant growth and yield in many areas of the world. It has been shown that Vesicular Arbuscular Mycorrhizal Fungi (AMF) can alleviate this deficiency. The effects of AMF inoculation on growth variables and mineral nutrition of Carthamus tinctorius L. under salt stress condition were studied. Plants were grown in a sterilized, low-P sandy soil with Glomus etunicatum inoculum (10–12 spore/g soil) in a greenhouse. RLC (Root Length Colonized) percent was higher in control plants than treated ones with different salt concentrations. Shoot and root weights, height, the number of leaves, the number of lateral branches, and also leaf area of mycorrhizal (M) plants were higher than nonmycorrhizal (NM) ones in both controlled and salt-treated plants. P, Zn, Fe, Ca, K, Cu, and N contents in M plants were higher than in NM plants in control, low and medium salinity conditions, but Na content was lower in aerial parts of the M plants. The results showed a higher tolerance of inoculated M plants toward salt stress and their better growth.     

Leaf water relations and net gas exchange responses of salinized Carrizo citrange seedlings during drought stress and recovery

BACKGROUND AND AIMS: Since salinity and drought stress can occur together, an assessment was made of their interacting effects on leaf water relations, osmotic adjustment and net gas exchange in seedlings of the relatively chloride-sensitive Carrizo citrange, Citrus sinensis x Poncirus trifoliata. METHODS: Plants were fertilized with nutrient solution with or without additional 100 mm NaCl (salt and no-salt treatments). After 7 d, half of the plants were drought stressed by withholding irrigation water for 10 d. Thus, there were four treatments: salinized and non-salinized plants under drought-stress or well-watered conditions. After the drought period, plants from all stressed treatments were re-watered with nutrient solution without salt for 8 d to study recovery. Leaf water relations, gas exchange parameters, chlorophyll fluorescence, proline, quaternary ammonium compounds and leaf and root concentrations of Cl(-) and Na(+) were measured. KEY RESULTS: Salinity increased leaf Cl(-) and Na(+) concentrations and decreased osmotic potential (Psi(pi)) such that leaf relative water content (RWC) was maintained during drought stress. However, in non-salinized drought-stressed plants, osmotic adjustment did not occur and RWC decreased. The salinity-induced osmotic adjustment was not related to any accumulation of proline, quaternary ammonium compounds or soluble sugars. Net CO(2) assimilation rate (A(CO2)) was reduced in leaves from all stressed treatments but the mechanisms were different. In non-salinized drought-stressed plants, lower A(CO2) was related to low RWC, whereas in salinized plants decreased A(CO2) was related to high levels of leaf Cl(-) and Na(+). A(CO2) recovered after irrigation in all the treatments except in previously salinized drought-stressed leaves which had lower RWC and less chlorophyll but maintained high levels of Cl(-), Na(+) and quaternary ammonium compounds after recovery. High leaf levels of Cl(-) and Na(+) after recovery apparently came from the roots. CONCLUSIONS: Plants preconditioned by salinity stress maintained a better leaf water status during drought stress due to osmotic adjustment and the accumulation of Cl(-) and Na(+). However, high levels of salt ions impeded recovery of leaf water status and photosynthesis after re-irrigation with non-saline water.     

Heritability of,and relationships between phosphorus and nitrogen concentration in shoot,stolon and root of white clover (Trifolium repens L.)

Ninety eight white clover genotypes were cloned and grown in pots at two levels of phosphorus (P) supply in soil. After harvest the nitrogen (N) and P content of shoot (leaf, petiole and unrooted stolon), stolon and root tissue was determined. Broad sense heritabilities for %N, %P, and proportion of total N or P in each tissue type were calculated. Heritabilities ranged from 0.22 to 0.68. They were generally higher for %P than %N; and higher in shoot and stolon tissue than root tissue for %P, %N, and proportion of N or P. Level of P in which plants were grown had little effect on heritability values. Genotypes from bred cultivars differed from those collected from hill country pastures for plant size, and partitioning of N and P to shoot, stolon and root. Relationships between plant characters were examined to determine the consequences of selection.     

Influence of saline irrigation on growth,ion accumulation and partitioning,and leaf gas exchange of carrot (Daucus carota L.)

Like those of many horticultural crop species, the growth and leaf gas exchange responses of carrot (Daucus carota L.) to salinity are poorly understood. In this study ion accumulation in root tissues (periderm, xylem and phloem tissues) and in leaves of different ages was assessed for carrot plants grown in the field with a low level of salinity (5.8 mM Na(+) and 7.5 mM Cl(-)) and in a glasshouse with salinity ranging from 1-80 mM. At low levels of salinity (1-7.5 mM), in both the field and glasshouse, carrot leaves accumulated high concentrations of Cl(-) (140-200 mM); these appear to be the result of a high affinity for Cl(-) uptake and a low retention of Cl(-) in the root system. However, Cl(-) uptake is under tight control, with an 80-fold increase in external salinity resulting in only a 1.5-fold change in the Cl(-) concentration of the shoot and no increase in the Cl(-) concentration of the root xylem tissue. In contrast to Cl(-), shoot Na(+) concentrations were comparatively low (30-40 mM) but increased by seven-fold when salinity was increased by 80-fold. Growth over the 56-d treatment period in the glasshouse was insensitive to salinity less than 20 mM, but at higher concentrations the yield of carrot tap roots declined by 7 % for each 10 mM increase in salinity. At low levels of salinity the accumulation of high concentrations of Cl(-) (150 mM) in carrot laminae did not appear to limit leaf gas exchange. However, photosynthesis and stomatal conductance were reduced by 38 and 53 %, respectively, for plants grown at a salinity of 80 mM compared with those grown at 1 mM. Salinity-induced reductions in both p(i) and carbon isotope discrimination (delta) were small (2.5 Pa and 1.4 per thousand, respectively, at 80 mM) indicating that the reduction in photosynthesis was only marginally influenced by CO(2) supply. At a salinity of 80 mM the photosynthetic capacity was reduced, with a 30 % reduction in the CO(2)-saturated rate of photosynthesis (A(max)) and a 40 % reduction in both the apparent rate of RuBP-carboxylase-limited CO(2) fixation (V(cmax)) and the electron transport rate limiting RuBP regeneration (J(max)). This study has shown that carrot growth and leaf gas exchange are insensitive to the high leaf Cl(-) concentrations that occur at low levels (1-7 mM) of salinity. However, growth is limited at salinity levels above 20 mM and leaf gas exchange is limited at salinity levels above 8 mM.     

Growth,nutrient dynamics,and efficiency responses to carbon dioxide and phosphorus nutrition in soybean

Plant mineral nutrients such as phosphorus may exert major control on crop responses to the rising atmospheric carbon dioxide (CO₂) concentrations. To evaluate the growth, nutrient dynamics, and efficiency responses to CO₂ and phosphorus nutrition, soybean (Glycine max (L.) Merr.) was grown in controlled environment growth chambers with sufficient (0.50 mM) and deficient (0.10 and 0.01 mM) phosphate (Pi) supply under ambient and elevated CO₂ (aCO₂, 400 and eCO₂, 800 µmol mol^?1, respectively). The CO₂ × Pi interaction was detected for leaf area, leaf and stem dry weight, and total plant biomass. The severe decrease in plant biomass in Pi-deficient plants (10–76%) was associated with reduced leaf area and photosynthesis (P_net). The degree of growth stimulation (0–55% total biomass) by eCO₂ was dependent upon the severity of Pi deficiency and was closely associated with the increased phosphorus utilization efficiency. With the exception of leaf and root biomass, Pi deficiency decreased the biomass partitioning to other plant organs with the maximum decrease observed in seed weight (8–42%) across CO₂ levels. The increased tissue nitrogen (N) concentration in Pi-deficient plants was accredited to the lower biomass and increased nutrient uptake due to the larger root to shoot ratio. The tissue P and N concentration tended to be lower at eCO₂ versus aCO₂ and did not appear to be the main cause of the lack of CO₂ response of growth and P_net under severe Pi deficiency. The leaf N/P ratio of >16 was detrimental to soybean growth. The tissue P concentration needed to attain the maximum productivity for biomass and seed yield tended to be higher at eCO₂ versus aCO₂. Therefore, the eCO₂ is likely to increase the leaf critical P concentration for maximum biomass productivity and yield in soybean.     

Salinity tolerance of 'Valencia' orange trees on rootstocks with contrasting salt tolerance is not improved by moderate shade

The effects of shading in combination with salinity treatments were studied in citrus trees on two rootstocks with contrasting salt tolerance to determine if shading could reduce the negative effects of salinity stress. Well-nourished 2-year-old 'Valencia' orange trees grafted on Cleopatra mandarin (Cleo, relatively salt tolerant) or Carrizo citrange (Carr, relatively salt sensitive), were grown either under a 50% shade cloth or left unshaded in full sunlight. Half the trees received no salinity treatment and half were salinized with 50 mM Cl- during two 9 week salinity periods in the spring and autumn interrupted by an 11 week rainy period. The shade treatment reduced midday leaf temperature and leaf-to-air vapour pressure deficit regardless of salinity treatments. In non-salinized trees, shade increased midday CO2 assimilation rate (A(CO2)) and stomatal conductance, but had no effect on leaf transpiration (E(lf)). Shade also increased leaf chlorophyll and photosynthetic water use efficiency (A(CO2)/E(lf)) in leaves on both rootstocks and increased total plant dry weight in Cleo. The salinity treatment reduced leaf growth and leaf gas exchange parameters. Shade decreased Cl- concentrations in leaves of salinized Carr trees, but had no effect on leaf or root Cl- of trees on Cleo. There were no significant differences in leaf gas exchange parameters of shaded and unshaded salinized plants but the growth reduction from salinity stress was actually greater for shaded than for unshaded trees. Shaded trees on both rootstocks had higher leaf Na+ than unshaded trees after the first salinity period, and this shade-induced elevated leaf Na+ persisted after the second salinity period in trees on Carr. Thus, shading did not alleviate the negative effects of salinity on growth and Na+ accumulation.     

Solute balance of a maize (Zea mays L.) source leaf as affected by salt treatment with special emphasis on phloem retranslocation and ion leaching

Strategies for avoiding ion accumulation in leaves of plants grown at high concentration of NaCl (100 mol m(-3)) in the rooting media, i.e. retranslocation via the phloem and leaching from the leaf surface, were quantified for fully developed leaves of maize plants cultivated hydroponically with or without salt, and with or without sprinkling (to induce leaching). Phloem sap, apoplastic fluid, xylem sap, solutes from leaf and root tissues, and the leachate were analysed for carbohydrates, amino acids, malate, and inorganic ions. In spite of a reduced growth rate Na(+) and Cl(-) concentrations in the leaf apoplast remained relatively low (about 4-5 mol m(-3)) under salt treatment. Concentrations of Na(+) and Cl(-) in the phloem sap of salt-treated maize did not exceed 12 and 32 mol m(-3), respectively, and thus remained lower than described for other species. However, phloem transport rates of these ions were higher than reported for other species. The relatively high translocation rate of ions found in maize may be due to the higher carbon translocation rate observed for C(4) plants as opposed to C(3) plants. Approximately 13-36% of the Na(+) and Cl(-) imported into the leaves through the xylem were exported by the phloem. It is concluded that phloem transport plays an important role in controlling the NaCl content of the leaf in maize. Surprisingly, leaching by artificial rain did not affect plant growth. Ion concentrations in the leachate were lower than reported for other plants but increased with NaCl treatment.     

Interactive effects of salinity and iron deficiency in Medicago ciliaris

In calcareous salt-affected soils, iron availability to plants is subjected to the effects of both sodium and bicarbonate ions. Our aim was to study interactive effects of salinity and iron deficiency on iron acquisition and root acidification induced by iron deficiency in Medicago ciliaris L., a species commonly found in saline ecosystems. Four treatments were used: C, control treatment, complete medium (CM) containing 30 microM Fe; S, salt treatment, CM with 75 mM NaCl; D, deficient treatment, CM containing only 1 microM Fe; DS, interactive treatment, CM containing 1 microM Fe with 75 mM NaCl. Our study showed that plant growth and chlorophyll content were much more affected by the interactive treatment than by iron deficiency or by the salt treatment, indicating an additive effect of these constraints in DS plants. These results could be partially explained by Na accumulation in shoots as well as a limitation of nutrient uptake such as Fe and K under salt stress, under iron deficiency, and especially under their combined effect. The study also showed that root acidification was deeply diminished when iron deficiency was associated with salinity. This probably explained the decrease of Fe uptake and suggested that root proton pump activity would be inhibited by salinity.     

Additive effects of Na+ and Cl- ions on barley growth under salinity stress

Soil salinity affects large areas of the world's cultivated land, causing significant reductions in crop yield. Despite the fact that most plants accumulate both sodium (Na(+)) and chloride (Cl(-)) ions in high concentrations in their shoot tissues when grown in saline soils, most research on salt tolerance in annual plants has focused on the toxic effects of Na(+) accumulation. It has previously been suggested that Cl(-) toxicity may also be an important cause of growth reduction in barley plants. Here, the extent to which specific ion toxicities of Na(+) and Cl(-) reduce the growth of barley grown in saline soils is shown under varying salinity treatments using four barley genotypes differing in their salt tolerance in solution and soil-based systems. High Na(+), Cl(-), and NaCl separately reduced the growth of barley, however, the reductions in growth and photosynthesis were greatest under NaCl stress and were mainly additive of the effects of Na(+) and Cl(-) stress. The results demonstrated that Na(+) and Cl(-) exclusion among barley genotypes are independent mechanisms and different genotypes expressed different combinations of the two mechanisms. High concentrations of Na(+) reduced K(+) and Ca(2+) uptake and reduced photosynthesis mainly by reducing stomatal conductance. By comparison, high Cl(-) concentration reduced photosynthetic capacity due to non-stomatal effects: there was chlorophyll degradation, and a reduction in the actual quantum yield of PSII electron transport which was associated with both photochemical quenching and the efficiency of excitation energy capture. The results also showed that there are fundamental differences in salinity responses between soil and solution culture, and that the importance of the different mechanisms of salt damage varies according to the system under which the plants were grown.     

Na+ accumulation in root symplast of sunflower plants exposed to moderate salinity is transpiration-dependent

Twenty-day-old sunflower plants (Helianthus annuus L. cv Sun-Gro 380) grown hydroponically under controlled conditions were used to study the effect of transpiration on Na(+) compartmentalization in roots. The plants were exposed to low Na(+) concentrations (25mM NaCl) and different environmental humidity conditions over a short time period (8.5h). Under these conditions, Na(+) was accumulated primarily in the root, but only the Na(+) accumulated in the root symplast was dependent on transpiration, while the Na(+) accumulated in both the shoot and the root apoplast exhibited a low transpiration dependence. Moreover, Na(+) content in the root apoplast was reached quickly (0.25h) and increased little with time. These results suggest that, in sunflower plants under moderate salinity conditions, Na(+) uptake in the root symplast is mediated by a transport system whose activity is enhanced by transpiration.     

Oxygation Enhances Growth, Gas Exchange and Salt Tolerance of Vegetable Soybean and Cotton in a Saline Vertisol

Impacts of salinity become severe when the soil is deficient in oxygen. Oxygation (using aerated water for subsurface drip irrigation of crop) could minimize the impact of salinity on plants under oxygen-limiting soil environments. Pot experiments were conducted to evaluate the effects of oxygation (12% air volume/volume of water) on vegetable soybean (moderately salt tolerant) and cotton (salt tolerant) in a salinized vertisol at 2, 8, 14, 20 dS/m ECe. In vegetable soybean, oxygation increased above ground biomass yield and water use efficiency (WUE) by 13% and 22%, respectively, compared with the control. Higher yield with oxygation was accompanied by greater plant height and stem diameter and reduced specific leaf area and leaf Na+ and Cl-concentrations. In cotton, oxygation increased lint yield and WUE by 18% and 16%, respectively, compared with the control, and was accompanied by greater canopy light interception, plant height and stem diameter. Oxygation also led to a greater rate of photosynthesis, higher relative water content in the leaf, reduced crop water stress index and lower leaf water potential. It did not, however, affect leaf Na+ or Cl- concentration. Oxygation invariably increased, whereas salinity reduced the K+ : Na+ ratio in the leaves of both species. Oxygation improved yield and WUE performance of salt tolerant and moderately tolerant crops under saline soil environments, and this may have a significant impact for irrigated agriculture where saline soils pose constraints to crop production.