Salt tolerance and salinity effects on plants: a review.

The environmental stress is a major area of scientific concern because it constraints plant as well as crop productivity. This situation has been further worsened by anthropogenic activities. Therefore, there is a much scientific saddle on researchers to enhance crop productivity under environmental stress in order to cope with the increasing food demands. The abiotic stresses such as salinity, drought, cold, and heat negatively influence the survival, biomass production and yield of staple food crops. According to an estimate of FAO, over 6 % of the world’s land is affected by salinity. Thus, salinity stress appears to be a major constraint to plant and crop productivity. Here, we review our understanding of salinity impact on various aspects of plant metabolism and its tolerance strategies in plants.

Effect of salinity stress on plants and its tolerance strategies: a review.

The leaf economics spectrum (LES) represents a suite of intercorrelated leaf traits concerning construction costs per unit leaf area, nutrient concentrations, and rates of carbon fixation and tissue turnover. Although broad trade-offs among leaf structural and physiological traits have been demonstrated, we still do not have a comprehensive view of the fundamental constraints underlying the LES trade-offs. Here, we investigated physiological and structural mechanisms underpinning the LES by analysing a novel data compilation incorporating rarely considered traits such as the dry mass fraction in cell walls, nitrogen allocation, mesophyll CO2 diffusion and associated anatomical traits for hundreds of species covering major growth forms. The analysis demonstrates that cell wall constituents are major components of leaf dry mass (18-70%), especially in leaves with high leaf mass per unit area (LMA) and long lifespan. A greater fraction of leaf mass in cell walls is typically associated with a lower fraction of leaf nitrogen (N) invested in photosynthetic proteins; and lower within-leaf CO2 diffusion rates, as a result of thicker mesophyll cell walls. The costs associated with greater investments in cell walls underpin the LES: long leaf lifespans are achieved via higher LMA and in turn by higher cell wall mass fraction, but this inevitably reduces the efficiency of photosynthesis.

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Physiological and structural tradeoffs underlying the leaf economics spectrum.

Effect of inoculation with plant growth-promoting bacteria (PGPB) on amelioration of saline stress in maize (Zea mays)

The increasing problem of global land salinisation and associated multibillion dollar losses in crop production require a better understanding of key physiological mechanisms conferring salinity tolerance in crops. The effective way of gaining such knowledge comes from studying halophytes. Halophytes have always attracted the attention of plant physiologists, due to their remarkable ability to tolerate and even benefit from salt concentrations that kill most other plant species. At the very least, halophytes may provide genes that allow transgenic conference of salinity tolerance to crops. In addition, some halophytes have already been tested as vegetable, forage and oilseed crops in agronomic field trials, whilst others already show good potential to be developed as crops. Surprisingly, our knowledge of fundamental ionic and molecular mechanisms conferring salinity tolerance in halophytes is rather limited and, at best, is restricted to several model species. This chapter summarises the current knowledge of physiological mechanisms regulating ion uptake and sequestration in halophytes. The following topics are covered: specific anatomical and morphological features of halophytes; tissue- and organ-specific ion compartmentation; mechanisms of osmotic adjustment in halophytes; radial ion transport in halophytes roots; mechanisms of Na+ and K+ loading into the xylem; Na+ sequestration in vacuoles in roots and leaf cells; ion transport in guard cells; control of ion fluxes into salt glands and bladders; and oxidative signalling and damage repair in halophytes. It is concluded that more in-depth electrophysiological and molecular studies are needed to reveal the identity of membrane transport systems and better understand what appears to be the complex and highly orchestrated regulation of ion transport and sequestration in halophytes.

Ion Transport in Halophytes

The effects of a range of salinity (0, 100, 200 and 400 mM NaCl) on growth, ion accumulation, photosynthesis and anatomical changes of leaves were studied in the mangrove, Bruguiera parviflora of the family Rhizophoraceae under hydroponically cultured conditions. The growth rates measured in terms of plant height, fresh and dry weight and leaf area were maximal in culture treated with 100 mM NaCl and decreased at higher concentrations. A significant increase of Na+ content of leaves from 46.01 mmol m-2 in the absence of NaCl to 140.55 mmol m-2 in plants treated with 400 mM NaCl was recorded. The corresponding Cl- contents were 26.92 mmol m-2 and 97.89 mmol m-2. There was no significant alteration of the endogenous level of K+ and Fe2+ in leaves. A drop of Ca2+ and Mg2+ content of leaves upon salt accumulation suggests increasing membrane stability and decreased chlorophyll content respectively. Total chlorophyll content decreased from 83.44 μg cm-2 in untreated plants to 46.56 μg cm-2 in plants treated with 400 mM NaCl, suggesting that NaCl has a limiting effect on photochemistry that ultimately affects photosynthesis by inhibiting chlorophyll synthesis (ca. 50% loss in chlorophyll). Light-saturated rates of photosynthesis decreased by 22% in plants treated with 400 mM NaCl compared with untreated plants. Both mesophyll and stomatal conductance by CO2 diffusion decreased linearly in leaves with increasing salt concentration. Stomatal and mesophyll conductance decreased by 49% and 52% respectively after 45 days in 400 mM NaCl compared with conductance in the absence of NaCl. Scanning electron microscope study revealed a decreased stomatal pore area (63%) in plants treated with 400 mM NaCl compared with untreated plants, which might be responsible for decreased stomatal conductance. Epidermal and mesophyll thickness and intercellular spaces decreased significantly in leaves after treatment with 400 mM NaCl compared with untreated leaves. These changes in mesophyll anatomy might have accounted for the decreased mesophyll conductance. We conclude that high salinity reduces photosynthesis in leaves of B. parviflora, primarily by reducing diffusion of CO2 to the chloroplast, both by stomatal closure and by changes in mesophyll structure, which decreased the conductance to CO2 within the leaf, as well as by affecting the photochemistry of the leaves.

Effects of salt on growth, ion accumulation, photosynthesis and leaf anatomy of the mangrove, Bruguiera parviflora

Oil analysis and antimicrobial activity from seeds of elite genotype of Pongamia pinnata was carried out in the current study. The highest oil yield (33%) from seeds was recovered in n-Hexane. Physico-chemical properties of crude oil established suitability of P. pinnata for its use as a potential biofuel crop. The total mono unsaturated fatty acid (oleic acid 46%) present in seed oil was more in comparison to polyunsaturated fatty acid (33%) as analyzed by GC–MS. Seed oil also showed inhibition against the tested fungal and bacterial cultures. However, the efficacy of antimicrobial activity of the seed oil at four concentration levels (50%, 80%, 90% and 100%) against various pathogenic indicators was found to be concentration-dependent. The obtained results confirmed the use of seed oil from well characterized elite genotype of Pongamia as diesel fuel and in pharmaceuticals.

Physico-chemical characterization and antimicrobial activity from seed oil of Pongamia pinnata, a potential biofuel crop.

Molecular genetic fingerprints of nine Curcuma species from Northeast India were developed using PCR-based markers. The aim involves elucidating there intra- and inter-specific genetic diversity important for utilization, management, and conservation. Twelve random amplified polymorphic DNA (RAPD), 19 Inter simple sequence repeats (ISSRs), and four amplified fragment length polymorphism (AFLP) primers produced 266 polymorphic fragments. ISSR confirmed maximum polymorphism of 98.55% whereas RAPD and AFLP showed 93.22 and 97.27%, respectively. Marker index and polymorphic information content varied in the range of 8.64–48.1, 19.75–48.14, and 25–28 and 0.17–0.48, 0.19–0.48, and 0.25–0.29 for RAPD, ISSR, and AFLP markers, respectively. The average value of number of observed alleles, number of effective alleles, mean Nei’s gene diversity, and Shannon’s information index were 1.93–1.98, 1.37–1.62, 0.23–0.36, and 0.38–0.50, respectively, for three DNA markers used. Dendrograms based on three molecular data using unweighted pair group method with arithmetic mean (UPGMA) was congruent and classified the Curcuma species into two major clusters. Cophenetic correlation coefficient between dendrogram and original similarity matrix were significant for RAPD (r = 0.96), ISSR (r = 0.94), and AFLP (r = 0.97). Clustering was further supported by principle coordinate analysis. High genetic polymorphism documented is significant for conservation and further improvement of Curcuma species.

Genetic Relationship of Curcuma Species from Northeast India Using PCR-Based Markers

An efficient plant regeneration protocol was developed from rhizomes of two Curcuma species C. longa and C. amada. Response was highly dependent on the season, with above 69 % of culture developing adventitious shoots during spring. Greatest regeneration and multiplication was observed in modified Murashige and Skoog (MS) medium supplemented with 13.31 μM benzyladenine and 2.68 μM α-naphthalene acetic acid (NAA) in C. longa or 2.46 μM indolebutyric acid in C. amada. Effect of sugars and agar at different concentrations were also studied and 2 % maltose and 0.7 % agar were found optimum for shoot multiplication and regeneration. Most plantlets developed roots simultaneously but others formed roots when subcultured in ½ MS medium supplemented with 2.68 μM NAA. Plants were successfully hardened in greenhouse with 80 % survival. The genetic purity of micropropagated plantlets was analyzed using RAPD and protein profiles.

Plant regeneration in Curcuma species and assessment of genetic stability of regenerated plants

An improved micropropagation protocol was developed for Zingiber moran and Z. zerumbet, two wild species of the genus Zingiber, found in Northeast India. The effects of growth regulators, sugar concentrations, and nutrients were tested on the rate of shoot initiation and multiplication. An increase in proliferation and multiplication occurred in modified Murashige and Skoog (MS) medium supplemented with benzyladenine and kinetin. About 2 % sucrose and 0.7 % agar were found to be the optimum for shoot multiplication and regeneration. Naphthalene acetic acid at 0.5 mg/L produced the best rooting response for both the species. Regenerated plantlets were acclimatized successfully and cytogenetic stability was confirmed by RAPD profiling and ploidy checks.

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Archana Das

Papers

Effects of salt on growth, ion accumulation, photosynthesis and leaf anatomy of the mangrove, Bruguiera parviflora

Physico-chemical characterization and antimicrobial activity from seed oil of Pongamia pinnata, a potential biofuel crop.

Genetic Relationship of Curcuma Species from Northeast India Using PCR-Based Markers

Plant regeneration in Curcuma species and assessment of genetic stability of regenerated plants

Micropropagation and cytogenetic assessment of Zingiber species of Northeast India