Influence of salt stress on C4 photosynthesis in Miscanthus sinensis Anderss.

• Miscanthus sinensis Anderss. is a good candidate for C₄ bioenergy crop development for marginal lands. As one of the characteristics of marginal lands, salinization is a major limitation to agricultural production. The present work aimed to investigate the possible factors involved in the tolerance of M. sinensis C₄ photosynthesis to salinity stress.
• Seedlings of two accessions (salt‐tolerant ‘JM0119’ and salt‐sensitive ‘JM0099’) were subjected to 0 mm NaCl (control) or 250 mm NaCl (salt stress treatment) for 2 weeks. The chlorophyll content, parameters of photosynthesis and chlorophyll a fluorescence, activity of C₄ enzymes and expression of C₄ genes were measured.
• The results showed that photosynthesis rate, transpiration rate, chlorophyll content, PSII operating efficiency, coefficient of photochemical quenching, activity of phosphoenolpyruvate carboxylase (PEPC) and pyruvate, orthophosphate dikinase (PPDK) and gene expression of PEPC and PPDK under salinity were higher after long‐term salinity exposure in ‘JM0119’ than in ‘JM0099’, while activity of NADP‐malate dehydrogenase (NADP‐MDH) and NADP‐malic enzyme (NADP‐ME), together with expression of NADP‐MDH and NADP‐ME, were much higher in ‘JM0099’ than in ‘JM0119’.
• In conclusion, the increased photosynthetic capacity under long‐term salt stress in the salt‐tolerant relative to the salt‐sensitive M. sinensis accession was mainly associated with non‐stomatal factors, such as reduced chlorophyll loss, higher PSII operating efficiency, enhanced activity of PEPC and PPDK and relatively lower activity of NADP‐ME.

     

Piriformospora indica inoculation alleviates the adverse effect of NaCl stress on growth,gas exchange and chlorophyll fluorescence in tomato (Solanum lycopersicum L.)

• Salinity is now an increasingly serious environmental issue that affects the growth and yield of many plants.
• In the present work, the influence of inoculation with the symbiotic fungus, Piriformospora indica, on gas exchange, water potential, osmolyte content, Na/K ratio and chlorophyll fluorescence of tomato plants under three salinity levels (0, 50, 100 and 150 mm NaCl) and three time periods (5, 10 and 15 days after exposure to salt) was investigated.
• Results indicate that P. indica inoculation improved growth parameters of tomato under salinity stress. This symbiotic fungus significantly increased photosynthetic pigment content under salinity, and more proline and glycine betaine accumulated in inoculated roots than in non‐inoculated roots. P. indica further significantly improved K⁺ content and reduced Na⁺ level under salinity treatment. After inoculation with the endophytic fungus, leaf physiological parameters, such as water potential, net photosynthesis, stomatal conductance and transpiration, were all higher under the salt concentrations and durations compared with controls without P. indica. With increasing salt level and salt treatment duration, values of F₀ and qP increased but F_m, F_v/F_m, F′_v/F′_m and NPQ declined in the controls, while inoculation with P. indica improved these values.
• The results indicate that the negative effects of NaCl on tomato plants were alleviated after P. indica inoculation, probably by improving physiological parameters such as water status and photosynthesis.

     

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

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

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.     

Photosystem II photochemistry and physiological parameters of three fodder shrubs, <Emphasis Type="Italic">Nitraria retusa</Emphasis>, <Emphasis Type="Italic">Atriplex halimus</Emphasis> and <Emphasis Type="Italic">Medicago arborea</Emphasis> under salt stress

Nitraria retusa and Atriplex halimus (xero-halophytes) plants were grown in the range 0–800 mM NaCl while Medicago arborea (glycophyte) in 0–300 mM NaCl. Salt stress caused a marked decrease in osmotic potential and a significant accumulation of Na⁺ and Cl⁻ in leaves of both species. Moderate salinity had a stimulating effect on growth rate, net CO₂ assimilation, transpiration and stomatal conductance for the xero-halophytic species. At higher salinities, these physiological parameters decreased significantly, and their percentages of reduction were higher in A. halimus than in N. retusa whereas, in M. arborea they decreased linearly with salinity. Nitraria retusa PSII photochemistry and carotenoid content were unaffected by salinity, but a reduction in chlorophyll content was observed at 800 mM NaCl. Similar results were found in A. halimus, but with a decrease in the efficiency of PSII (F′v/F′m) occurred at 800 mM. Conversely, in M. arborea plants we observed a significant reduction in pigment concentrations and chlorophyll fluorescence parameters. The marked toxic effect of Na⁺ and/or Cl⁻ observed in M. arborea indicates that salt damage effect could be attributed to ions’ toxicity, and that the reduction in photosynthesis is most probably due to damages in the photosynthetic apparatus rather than factors affecting stomatal closure. For the two halophyte species, it appears that there is occurrence of co-limitation of photosynthesis by stomatal and non-stomatal factors. Our results suggest that both N. retusa and A. halimus show high tolerance to both high salinity and photoinhibition while M. arborea was considered as a slightly salt tolerant species.     

Elevated CO<Subscript>2</Subscript> reduces stomatal and metabolic limitations on photosynthesis caused by salinity in <Emphasis Type="Italic">Hordeum vulgare</Emphasis>

The future environment may be altered by high concentrations of salt in the soil and elevated [CO₂] in the atmosphere. These have opposite effects on photosynthesis. Generally, salt stress inhibits photosynthesis by stomatal and non-stomatal mechanisms; in contrast, elevated [CO₂] stimulates photosynthesis by increasing CO₂ availability in the Rubisco carboxylating site and by reducing photorespiration. However, few studies have focused on the interactive effects of these factors on photosynthesis. To elucidate this knowledge gap, we grew the barley plant, Hordeum vulgare (cv. Iranis), with and without salt stress at either ambient or elevated atmospheric [CO₂] (350 or 700 μmol mol⁻¹ CO₂, respectively). We measured growth, several photosynthetic and fluorescence parameters, and carbohydrate content. Under saline conditions, the photosynthetic rate decreased, mostly because of stomatal limitations. Increasing salinity progressively increased metabolic (photochemical and biochemical) limitation; this included an increase in non-photochemical quenching and a reduction in the PSII quantum yield. When salinity was combined with elevated CO₂, the rate of CO₂ diffusion to the carboxylating site increased, despite lower stomatal and internal conductance. The greater CO₂ availability increased the electron sink capacity, which alleviated the salt-induced metabolic limitations on the photosynthetic rate. Consequently, elevated CO₂ partially mitigated the saline effects on photosynthesis by maintaining favorable biochemistry and photochemistry in barley leaves.     

Effects of salt stress on photosynthesis,PSII photochemistry and thermal energy dissipation in leaves of two corn (<Emphasis Type="Italic">Zea mays</Emphasis> L.) varieties

The effect of four different NaCl concentrations (from 0 to 102 mM NaCl) on seedlings leaves of two corn (Zea mays L.) varieties (Aristo and Arper) was investigated through chlorophyll (Chl) a fluorescence parameters, photosynthesis, stomatal conductance, photosynthetic pigments concentration, tissue hydration and ionic accumulation. Salinity treatments showed a decrease in maximal efficiency of PSII photochemistry (F_v/F_m) in dark-adapted leaves. Moreover, the actual PSII efficiency (ϕ_PSII), photochemical quenching coefficient (q_p), proportion of PSII centers effectively reoxidized, and the fraction of light used in PSII photochemistry (%P) were also dropped with increasing salinity in light-adapted leaves. Reductions in these parameters were greater in Aristo than in Arper. The tissue hydration decreased in salt-treated leaves as did the photosynthesis, stomatal conductance (g _s) and photosynthetic pigments concentration essentially at 68 and 102 mM NaCl. In both varieties the reduction of photosynthesis was mainly due to stomatal closure and partially to PSII photoinhibition. The differences between the two varieties indicate that Aristo was more susceptible to salt-stress damage than Arper which revealed a moderate regulation of the leaf ionic accumulation.     

Soil salinity determines the relative abundance of C3/C4 species in Argentinean grasslands

Aim The C₄ and crassulacean acid metabolism (CAM) pathways are adaptations to compensate for high rates of photorespiration and water and carbon deficiency. This is the first attempt to compare the relative abundance of C₃ vs. C₄ + CAM species in temperate and subtropical grasslands across a latitudinal gradient in central Argentina. We predict that under the same rainfall regime, C₄ + CAM plants will have larger soil coverage in highly saline soils than in neighbouring non‐saline ones. Location Data were taken from three phytogeographical provinces in the Santa Fe province of Argentina: Chaquenian, Pampean and Espinal. Methods The salinity of the soil was estimated through aqueous solution conductivity. The proportions of species belonging to C₃/C₄ + CAM photosynthetic pathways were compared among halophyte and non‐halophyte communities with a χ² homogeneity test. The sum of cover percentages corresponding to the C₃ and C₄ + CAM photosynthetic pathways were calculated and compared using analysis of variance (ANOVA). Results The soil conductivity values were higher in the halophyte than in the non‐halophyte communities for the same phytogeographical area. The C₄ + CAM plants had much higher soil coverage values in halophyte than in non‐halophyte communities in the Pampean and Espinal phytogeographical provinces. The differences were not statistically significant in the Chaquenian province. Main conclusions Soil drought provoked by soil salinity results in a much higher soil cover by C₄ + CAM plants in regions with positive to neutral water balance (i.e. Pampean and Espinal). This differential abundance pattern in C₄ + CAM functional group is not observed in areas where a pronounced water deficit exists per se (Chaquenian region), and therefore C₄ + CAM plants predominate in all environments regardless of soil salinity. Our results suggest that one of the main environmental forces driving the upsurge of C₄ species in Argentinean grasslands might have been the strong local soil salinity gradient.     

Development and evaluation of novel salt‐tolerant Eucalyptus trees by molecular breeding using an RNA‐Binding‐Protein gene derived from common ice plant (Mesembryanthemum crystallinum L.)

The breeding of plantation forestry trees for the possible afforestation of marginal land would be one approach to addressing global warming issues. Here, we developed novel transgenic Eucalyptus trees (Eucalyptus camaldulensis Dehnh.) harbouring an RNA‐Binding‐Protein (McRBP) gene derived from a halophyte plant, common ice plant (Mesembryanthemum crystallinum L.). We conducted screened‐house trials of the transgenic Eucalyptus using two different stringency salinity stress conditions to evaluate the plants’ acute and chronic salt stress tolerances. Treatment with 400 mM NaCl, as the high‐stringency salinity stress, resulted in soil electrical conductivity (EC) levels >20 mS/cm within 4 weeks. With the 400 mM NaCl treatment, >70% of the transgenic plants were intact, whereas >40% of the non‐transgenic plants were withered. Treatment with 70 mM NaCl, as the moderate‐stringency salinity stress, resulted in soil EC levels of approx. 9 mS/cm after 2 months, and these salinity levels were maintained for the next 4 months. All plants regardless of transgenic or non‐transgenic status survived the 70 mM NaCl treatment, but after 6‐month treatment the transgenic plants showed significantly higher growth and quantum yield of photosynthesis levels compared to the non‐transgenic plants. In addition, the salt accumulation in the leaves of the transgenic plants was 30% lower than that of non‐transgenic plants after 15‐week moderate salt stress treatment. There results suggest that McRBP expression in the transgenic Eucalyptus enhances their salt tolerance both acutely and chronically.     

Antioxidative Defense Potential to Salinity in the Euhalophyte Salicornia brachiata

The antioxidative defense mechanism to salinity was assessed by monitoring the activities of some antioxidative enzymes and levels of antioxidants in an obligate halophyte, Salicornia brachiata, subjected to varying levels of NaCl (0, 200, 400, and 600 mM) under hydroponic culture. In the shoots of S. brachiata, salt treatment preferentially enhanced the activities of ascorbate peroxidase (APX), guaiacol peroxidase (POX), glutathione reductase (GR), and superoxide dismutase (SOD), whereas it induced the decrease of catalase (CAT) activity. Similarly, salinity caused an increase in total glutathione content (GSH + GSSG) and a decrease in total ascorbate content. Growth of S. brachiata was optimum at 200 mM NaCl and decreased with further increase in salinity. Salinity caused an increase in Na⁺ content and a decrease in K⁺ content of shoots. Proline levels did not change at low (0-200 mM NaCl) or moderate (400 mM NaCl) salinities, whereas a significant increase in proline level was observed at high salinity (600 mM NaCl). Accumulation of Na⁺ may have a certain role in osmotic homeostasis under low and moderate salinities in S. brachiata. Parameters of oxidative stress such as malondialdehyde (MDA), a product of lipid peroxidation, and H₂O₂ concentrations decreased at low salinity (200 mM NaCl) and increased at moderate (400 mM NaCl) and high salinities (600 mM NaCl). As a whole, our results suggest that the capacity to limit ionic and oxidative damage by the elevated levels of certain antioxidative enzymes and antioxidant molecules is important for salt tolerance of S. brachiata.     

Arbuscular mycorrhiza influences carbon‐use efficiency and grain yield of wheat grown under pre‐ and post‐anthesis salinity stress

• Soil salinity severely affects and constrains crop production worldwide. Salinity causes osmotic and ionic stress, inhibiting gas exchange and photosynthesis, ultimately impairing plant growth and development. Arbuscular mycorrhiza (AM) have been shown to maintain light and carbon use efficiency under stress, possibly providing a tool to improve salinity tolerance of the host plants. Thus, it was hypothesized that AM will contribute to improved growth and yield under stress conditions.
• Wheat plants (Triticum aestivum L.) were grown with (AMF+) or without (AMF?) arbuscular mycorrhizal fungi (AMF) inoculation. Plants were subjected to salinity stress (200 mm NaCl) either at pre‐ or post‐anthesis or at both stages. Growth and yield components, leaf chlorophyll content as well as gas exchange parameters and AMF colonization were analysed.
• AM plants exhibited a higher rate of net photosynthesis and stomatal conductance and lower intrinsic water use efficiency. Furthermore, AM wheat plants subjected to salinity stress at both pre‐anthesis and post‐anthesis maintained higher grain yield than non‐AM salinity‐stressed plants.
• These results suggest that AMF inoculation mitigates the negative effects of salinity stress by influencing carbon use efficiency and maintaining higher grain yield under stress.

     

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     

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.     

Cyclic electron flow,NPQ and photorespiration are crucial for the establishment of young plants of Ricinus communis and Jatropha curcas exposed to drought

• Although plant physiological responses to drought have been widely studied, the interaction between photoprotection, photorespiration and antioxidant metabolism in water‐stressed plants is scarcely addressed.
• This study aimed to evaluate the physiological adjustments preserving photosynthesis and growth in two plant species with different tolerance to drought: Jatropha curcas and Ricinus communis. We measured stress indicators, gas exchange, photochemistry of PSII and PSI, antioxidant enzymes, cyclic electron flow and photorespiration.
• Physiological stress indicators associated with reduction in growth confirmed R. communis as sensitive and J. curcas as tolerant to drought. Drought induced loss of photosynthesis in R. communis, whereas J. curcas maintained higher leaf gas exchange and photochemistry under drought. In addition, J. curcas showed higher dissipation of excess energy and presented higher cyclic electron flow when exposed to drought. Although none of these mechanisms have been triggered in R. communis, this species showed increases in photorespiration. R. communis displayed loss of Rubisco content while the Rubisco relative abundance did not change in J. curcas under drought. Accordingly, the in vivo maximum Rubisco carboxylation rate (V_cmax) and the maximum photosynthetic electron transport rate driving RuBP regeneration (J_max) were less affected in J. curcas. Both species displayed an efficient antioxidant mechanism by increasing activities of ascorbate peroxidase (APX) and superoxide dismutase (SOD).
• Overall, we suggest that the modulation of different photoprotective mechanisms is crucial to mitigate the effects caused by excess energy, maintaining photosynthetic apparatus efficiency and promoting the establishment of young plants of these two species under drought.

     

Salt tolerance of Beta macrocarpa is associated with efficient osmotic adjustment and increased apoplastic water content

The chenopod Beta macrocarpa Guss (wild Swiss chard) is known for its salt tolerance, but the mechanisms involved are still debated. In order to elucidate the processes involved, we grew wild Swiss chard exposed to three salinity levels (0, 100 and 200 mm NaCl) for 45 days, and determined several physiological parameters at the end of this time. All plants survived despite reductions in growth, photosynthesis and stomatal conductance in plants exposed to salinity (100 and 200 mm NaCl). As expected, the negative effects of salinity were more pronounced at 200 mm than at 100 mm NaCl: (i) leaf apoplastic water content was maintained or increased despite a significant reduction in leaf water potential, revealing the halophytic character of B. macrocarpa; (ii) osmotic adjustment occurred, which presumably enhanced the driving force for water extraction from soil, and avoided toxic build up of Na⁺ and Cl^– in the mesophyll apoplast of leaves. Osmotic adjustment mainly occurred through accumulation of inorganic ions and to a lesser extent soluble sugars; proline was not implicated in osmotic adjustment. Overall, two important mechanisms of salt tolerance in B. macrocarpa were identified: osmotic and apoplastic water adjustment.     

Beneficial role of acetylcholine in chlorophyll metabolism and photosynthetic gas exchange in Nicotiana benthamiana seedlings under salinity stress

• Acetylcholine (ACh) is believed to improve plant growth. However, regulation at biochemical and molecular levels is largely unknown.
• The present study investigated the impact of exogenously applied ACh (10 µm ) on growth and chlorophyll metabolism in hydroponically grown Nicotiana benthamiana under salt stress (150 mm NaCl).
• Salinity reduced root hydraulic conductivity while ACh‐treated seedlings exhibited a significant increase, resulting in increased relative water content. Salinity induced a reduction in chlorophyll biosynthetic intermediates, such as protoporphyrin‐IX, Mg‐photoporphyrin‐IX and protochlorophyllide, which were significantly ameliorated in the presence of ACh. This influence of ACh on chlorophyll synthesis was confirmed by up‐regulation of HEMA1, CHLH, CAO and POR genes. Gas exchange parameters, i.e. stomatal conductance, internal CO₂ concentration and transpiration rate, increased with ACh, thereby alleviating the salinity effects on photosynthesis. In addition, the salinity‐induced enhancement of lipid peroxidation declined after ACh treatment through modulation of the activity of the assayed antioxidant enzymes (superoxide dismutase and peroxidase). Importantly, ACh significantly reduced the uptake of Na and increased uptake of K, resulting in a decline in the Na/K ratio.
• Results of the present study indicate that ACh can be effective in ameliorating NaCl‐induced osmotic stress, altering chlorophyll metabolism and thus photosynthesis by maintaining ion homeostasis, hydraulic conductivity and water balance.

     

Morphological and biomechanical responses of floodplain willows to tidal flooding and salinity

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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.     

Physiological response of the facultative halophyte,Aeluropus littoralis,to different salt types and levels

The present study investigated the effects of NaCl, KCl and Na₂SO₄ salts on the C₄ excreting halophyte Aeluropus littoralis in relation to growth, mineral status and photosynthesis in greenhouse conditions. Plantlets were subjected to five salinity levels: 0, 200, 400, 600 and 800 mM for 30 days. Growth decreased progressively with salinity increase, its reduction might be correlated with the high sodium (and/or chloride) accumulation in plant tissues, the decrease of leaf water status and the decline of the net photosynthetic rate and the intrinsic water use efficiency. Na₂SO₄ appeared more toxic than KCl and NaCl, especially at 200 mM. At 200 mM, Na₂SO₄ reduced plant growth by 61% while for other salt forms, the reductions were less than 20%. At this salt level, stomatal conductance showed a consistent pattern with plant growth and could adequately explain the variations between the effects of the three salt types.     

Expression of the Arabidopsis vacuolar H+‐pyrophosphatase gene (AVP1) improves the shoot biomass of transgenic barley and increases grain yield in a saline field

Cereal varieties with improved salinity tolerance are needed to achieve profitable grain yields in saline soils. The expression of AVP1, an Arabidopsis gene encoding a vacuolar proton pumping pyrophosphatase (H⁺‐PPase), has been shown to improve the salinity tolerance of transgenic plants in greenhouse conditions. However, the potential for this gene to improve the grain yield of cereal crops in a saline field has yet to be evaluated. Recent advances in high‐throughput nondestructive phenotyping technologies also offer an opportunity to quantitatively evaluate the growth of transgenic plants under abiotic stress through time. In this study, the growth of transgenic barley expressing AVP1 was evaluated under saline conditions in a pot experiment using nondestructive plant imaging and in a saline field trial. Greenhouse‐grown transgenic barley expressing AVP1 produced a larger shoot biomass compared to null segregants, as determined by an increase in projected shoot area, when grown in soil with 150 mm NaCl. This increase in shoot biomass of transgenic AVP1 barley occurred from an early growth stage and also in nonsaline conditions. In a saline field, the transgenic barley expressing AVP1 also showed an increase in shoot biomass and, importantly, produced a greater grain yield per plant compared to wild‐type plants. Interestingly, the expression of AVP1 did not alter barley leaf sodium concentrations in either greenhouse‐ or field‐grown plants. This study validates our greenhouse‐based experiments and indicates that transgenic barley expressing AVP1 is a promising option for increasing cereal crop productivity in saline fields.