Salt sensitivity in chickpea (Cicer arietinum L.): ions in reproductive tissues and yield components in contrasting genotypes

An insight into effect of soil salinity on vegetation dynamics in the exposed seafloor of the Aral Sea

The desiccation of the Aral Sea has precipitated significant ecological degradation, resulting in the progressive development of vegetation on the exposed seafloor. Soil salinity emerges as a pivotal determinant in this ecological succession process. Employing a comprehensive methodology integrating multi-source datasets spanning from 1986 to 2023, this study elucidates the temporal changes in vegetation dynamics and soil salinity levels. Satellite imagery (Landsat-4/5/7/8), field soil samplings, hydrological and topographic data were analyzed to understand their interactions with regression analysis. The results reveal a consistent increasing trend in the Normalized Difference Vegetation Index (NDVI) across the exposed seabed since 1986. However, NDVI demonstrates a non-linear relationship with elevation in the North Aral Sea region. Interestingly, NDVI levels near an elevation of 42 m on the exposed seabed approximate those observed during the pre-recession period in the 1960s. Conversely, in the South Aral Sea region, NDVI demonstrates a linear upward trend with increasing elevation. Furthermore, the spatial distribution of soil salinity on the exposed seabed was delineated with linear regression analysis. It revealed water salinity levels at the time of sea recession can serve as a proxy for soil salinity in cases where direct soil data is unavailable. Through establishing a robust correlation between NDVI and soil salinity, the range of stable NDVI values on the exposed seabed was delineated. Lastly, three hypothetical scenarios of rising water levels were considered to evaluate changes in stable NDVI across different elevation gradients. If the water level returns to 45 m, the salt-desert area would decrease by 4.5 × 104 km2, accounting for 23 % of the total area in 1960. At this water level, it is anticipated that lake hydrological conditions and ecological environments may restore to those observed in 1981. This study provides a long-term perspective on environmental changes in the Aral Sea region by integrating multiple data sources and analytical methods. The predictive insights from the scenario analysis offer valuable guidance for future water management and ecological restoration efforts. Compared with previous studies, it presents a detailed and comprehensive picture of the interplay between vegetation dynamics and soil salinity, highlighting the critical impact of water level changes on the region's ecosystem.

Salt tolerance in mungbean is associated with controlling Na and Cl transport across roots, regulating Na and Cl accumulation in chloroplasts and maintaining high K in root and leaf mesophyll cells

Salinity tolerance requires coordinated responses encompassing salt exclusion in roots and tissue/cellular compartmentation of salt in leaves. We investigated the possible control points for salt ions transport in roots and tissue tolerance to Na+ and Cl- in leaves of two contrasting mungbean genotypes, salt-tolerant Jade AU and salt-sensitive BARI Mung-6, grown in nonsaline and saline (75 mM NaCl) soil. Cryo-SEM X-ray microanalysis was used to determine concentrations of Na, Cl, K, Ca, Mg, P, and S in various cell types in roots related to the development of apoplastic barriers, and in leaves related to photosynthetic performance. Jade AU exhibited superior salt exclusion by accumulating higher [Na] in the inner cortex, endodermis, and pericycle with reduced [Na] in xylem vessels and accumulating [Cl] in cortical cell vacuoles compared to BARI Mung-6. Jade AU maintained higher [K] in root cells than BARI Mung-6. In leaves, Jade AU maintained lower [Na] and [Cl] in chloroplasts and preferentially accumulated [K] in mesophyll cells than BARI Mung-6, resulting in higher photosynthetic efficiency. Salinity tolerance in Jade AU was associated with shoot Na and Cl exclusion, effective regulation of Na and Cl accumulation in chloroplasts, and maintenance of high K in root and leaf mesophyll cells.

Growth and Physiological Responses of Magnoliaceae to NaCl Stress

The growth and physiological characteristics of four Magnoliaceae plants (Yulania biondii, Yulania denudata, and two varieties of Magnolia wufengensis (Jiaohong 1 and Jiaohong 2)) were investigated. Four Magnoliaceae plants were subjected to various concentrations of NaCl for 60 days: 0 mM, 60 mM, 120 mM, 180 mM, and 240 mM. The leaf water content (LWC), relative growth rate of plant height and stem diameter, photosynthetic pigments, and photosynthetic rate (Pn) decreased during the NaCl treatments, indicating slowed growth and photosynthesis. Malondialdehyde (MDA), Na+, superoxide dismutase (SOD) activity, peroxidase (POD) activity, ascorbic acid (AsA) content, and soluble sugar content all increased while K+ decreased. Ascorbate peroxidase (APX) activity, glutathione (GSH), soluble protein, and proline first increased after decreasing with increasing NaCl concentration. Principal component 1 (PC1) had larger loading values for growth and photosynthesis indices, while principal component 2 (PC2) exhibited larger loading values for antioxidant substances and osmotic adjustment substances; the correlation analysis showed that PC1 and PC2 had negative correlations. The four Magnoliaceae plants exhibited largely variable growth and physiological activities in response to NaCl. Yulania denudata exhibited greater reductions in growth and photosynthesis and greater decreases in antioxidant enzyme activities and osmotic adjustment substances, which indicated poor tolerance to salt stress. Among the four Magnoliaceae plants, Jiaohong 1 exhibited the greatest salt tolerance, followed by Jiaohong 2, Yulania biondii, and Yulania denudata.

Plant growth promoting rhizobacterium Bacillus sp. BSE01 alleviates salt toxicity in chickpea (Cicer arietinum L.) by conserving ionic, osmotic, redox and hormonal homeostasis

Soil salinity leading to sodium toxicity is developing into a massive challenge for agricultural productivity globally, inducing osmotic, ionic, and redox imbalances in plants. Considering the predicted increase in salinization risk with the ongoing climate change, applying plant growth-promoting rhizobacteria (PGPR) is an environmentally safe method for augmenting plant salinity tolerance. The present study examined the role of halotolerant Bacillus sp. BSE01 as a promising biostimulant for improving salt stress endurance in chickpea. Application of PGPR significantly increased the plant height, relative water content, and chlorophyll content of chickpea under both non-stressed and salt stress conditions. The PGPR-mediated tolerance towards salt stress was accomplished by the modulation of hormonal signaling and conservation of cellular ionic, osmotic, redox homeostasis. With salinity stress, the PGPR-treated plants significantly increased the indole-3-acetic acid and gibberellic acid contents more than the non-treated plants. Furthermore, the PGPR-inoculated plants maintained lower 1-aminocyclopropane-1-carboxylic acid and abscisic acid contents under salt treatment. The PGPR-inoculated chickpea plants also exhibited a decreased NADPH oxidase activity with reduced production of reactive oxygen species compared to the non-inoculated plants. Additionally, PGPR treatment led to increased antioxidant enzyme activities in chickpea under saline conditions, facilitating the reactive nitrogen and oxygen species detoxification, thereby limiting the nitro-oxidative damage. Following salinity stress, enhanced K+ /Na+ ratio and proline content were noted in the PGPR-inoculated chickpea plants. Therefore, Bacillus sp. BSE01, being an effective PGPR and salinity stress reducer, can further be considered to develop a bioinoculant for sustainable chickpea production under saline environments.

Comparative transcriptome analysis reveals molecular regulation of salt tolerance in two contrasting chickpea genotypes

Salinity is a major abiotic stress that causes substantial agricultural losses worldwide. Chickpea (Cicer arietinum L.) is an important legume crop but is salt-sensitive. Previous physiological and genetic studies revealed the contrasting response of two desi chickpea varieties, salt-sensitive Rupali and salt-tolerant Genesis836, to salt stress. To understand the complex molecular regulation of salt tolerance mechanisms in these two chickpea genotypes, we examined the leaf transcriptome repertoire of Rupali and Genesis836 in control and salt-stressed conditions. Using linear models, we identified categories of differentially expressed genes (DEGs) describing the genotypic differences: salt-responsive DEGs in Rupali (1,604) and Genesis836 (1,751) with 907 and 1,054 DEGs unique to Rupali and Genesis836, respectively, salt responsive DEGs (3,376), genotype-dependent DEGs (4,170), and genotype-dependent salt-responsive DEGs (122). Functional DEG annotation revealed that the salt treatment affected genes involved in ion transport, osmotic adjustment, photosynthesis, energy generation, stress and hormone signalling, and regulatory pathways. Our results showed that while Genesis836 and Rupali have similar primary salt response mechanisms (common salt-responsive DEGs), their contrasting salt response is attributed to the differential expression of genes primarily involved in ion transport and photosynthesis. Interestingly, variant calling between the two genotypes identified SNPs/InDels in 768 Genesis836 and 701 Rupali salt-responsive DEGs with 1,741 variants identified in Genesis836 and 1,449 variants identified in Rupali. In addition, the presence of premature stop codons was detected in 35 genes in Rupali. This study provides valuable insights into the molecular regulation underpinning the physiological basis of salt tolerance in two chickpea genotypes and offers potential candidate genes for the improvement of salt tolerance in chickpeas.

Metal-Resistant PGPR Strain Azospirillum brasilense EMCC1454 Enhances Growth and Chromium Stress Tolerance of Chickpea (Cicer arietinum L.) by Modulating Redox Potential, Osmolytes, Antioxidants, and Stress-Related Gene Expression

Heavy metal stress, including from chromium, has detrimental effects on crop growth and yields worldwide. Plant growth-promoting rhizobacteria (PGPR) have demonstrated great efficiency in mitigating these adverse effects. The present study investigated the potential of the PGPR strain Azospirillum brasilense EMCC1454 as a useful bio-inoculant for boosting the growth, performance and chromium stress tolerance of chickpea (Cicer arietinum L.) plants exposed to varying levels of chromium stress (0, 130 and 260 µM K₂Cr₂O₇). The results revealed that A. brasilense EMCC1454 could tolerate chromium stress up to 260 µM and exhibited various plant growth-promoting (PGP) activities, including nitrogen fixation, phosphate solubilization, and generation of siderophore, trehalose, exopolysaccharide, ACC deaminase, indole acetic acid, and hydrolytic enzymes. Chromium stress doses induced the formation of PGP substances and antioxidants in A. brasilense EMCC1454. In addition, plant growth experiments showed that chromium stress significantly inhibited the growth, minerals acquisition, leaf relative water content, biosynthesis of photosynthetic pigments, gas exchange traits, and levels of phenolics and flavonoids of chickpea plants. Contrarily, it increased the concentrations of proline, glycine betaine, soluble sugars, proteins, oxidative stress markers, and enzymatic (CAT, APX, SOD, and POD) and non-enzymatic (ascorbic acid and glutathione) antioxidants in plants. On the other hand, A. brasilense EMCC1454 application alleviated oxidative stress markers and significantly boosted the growth traits, gas exchange characteristics, nutrient acquisition, osmolyte formation, and enzymatic and non-enzymatic antioxidants in chromium-stressed plants. Moreover, this bacterial inoculation upregulated the expression of genes related to stress tolerance (CAT, SOD, APX, CHS, DREB2A, CHI, and PAL). Overall, the current study demonstrated the effectiveness of A. brasilense EMCC1454 in enhancing plant growth and mitigating chromium toxicity impacts on chickpea plants grown under chromium stress circumstances by modulating the antioxidant machinery, photosynthesis, osmolyte production, and stress-related gene expression.

Seed priming with NaCl helps to improve tissue tolerance, potassium retention ability of plants, and protects the photosynthetic ability in two different legumes, chickpea and lentil, under salt stress

MAIN CONCLUSION

Seed priming with NaCl mimicked the conditions of natural priming to improve the tissue tolerance nature of sensitive legumes, which helps to maintain survivability and yield in mildly saline areas. Seed priming with NaCl is a seed invigoration technique that helps to improve plant growth by altering Na⁺ and K⁺ content under salt stress. Legumes are overall sensitive to salt and salinity hampers their growth and yield. Therefore, a priming (50 mM NaCl) experiment was performed with two different legume members [Cicer arietinum cv. Anuradha and Lens culinaris cv. Ranjan] and different morpho-physiological, biochemical responses at 50 mM, 100 mM, and 150 mM NaCl and molecular responses at 150 mM NaCl were studied in hydroponically grown nonprimed and primed members. Similarly, a pot experiment was performed at 80 mM Na⁺, to check the yield. Tissue Na⁺ and K⁺ content suggested NaCl-priming did not significantly alter the accumulation of Na⁺ among nonprimed and primed members but retained more K⁺ in cells, thus maintaining a lower cellular Na⁺/K⁺ ratio. Low osmolyte content (e.g., proline) in primed members suggested priming could minimize their overall osmolytic requirement. Altogether, these implied tissue tolerance (TT) nature might have improved in case of NaCl-priming as was also reflected by a better TT score (LC₅₀ value). An improved TT nature enabled the primed plants to maintain a significantly higher photosynthetic rate through better stomatal conductance. Along with this, a higher level of chlorophyll content and competent functioning of the photosynthetic subunits improved photosynthetic performance that ensured yield under stress. Overall, this study explores the potential of NaCl-priming and creates possibilities for considerably sensitive members; those in their nonprimed forms have no prospect in mildly saline agriculture.

Epigenomics as Potential Tools for Enhancing Magnitude of Breeding Approaches for Developing Climate Resilient Chickpea

Epigenomics has become a significant research interest at a time when rapid environmental changes are occurring. Epigenetic mechanisms mainly result from systems like DNA methylation, histone modification, and RNA interference. Epigenetic mechanisms are gaining importance in classical genetics, developmental biology, molecular biology, cancer biology, epidemiology, and evolution. Epigenetic mechanisms play important role in the action and interaction of plant genes during development, and also have an impact on classical plant breeding programs, inclusive of novel variation, single plant heritability, hybrid vigor, plant-environment interactions, stress tolerance, and performance stability. The epigenetics and epigenomics may be significant for crop adaptability and pliability to ambient alterations, directing to the creation of stout climate-resilient elegant crop cultivars. In this review, we have summarized recent progress made in understanding the epigenetic mechanisms in plant responses to biotic and abiotic stresses and have also tried to provide the ways for the efficient utilization of epigenomic mechanisms in developing climate-resilient crop cultivars, especially in chickpea, and other legume crops.

Influence of Nonsteroidal Anti-Inflammatory Drugs (NSAIDs) on Photosynthetic Parameters and Secondary Metabolites of Plants from Fabaceae Family

Nonsteroidal anti-inflammatory medications (NSAIDs) are commonly used painkillers, anti-inflammatory agents, and fever reducers. They arrive in the environment from municipal wastewater and/or agriculture waste, affecting growing plants. In our study, the impact of NSAIDs, namely, diclofenac, indomethacin, naproxen, and paracetamol, on four plant species from the Fabaceae family (Cicer arietinum, Pisum sativum, Lens culinaris, and Vicia faba) was tested. The assimilation rate and stomatal conductance decreased for all plants treated with NSAIDs. Chlorophyll and carotenoid contents in the leaves of plants under stress declined by more than 15% compared with the control plants, while the flavonoids and total phenols decreased to a lesser extent. In contrast, the plants treated with NSAIDs emit terpenes and green leaf were volatile, at a level of some nmol m−2 s−1, which could influence the atmospheric reaction and ozone formation.

Comparative Flower Transcriptome Network Analysis Reveals DEGs Involved in Chickpea Reproductive Success during Salinity

Salinity is increasingly becoming a significant problem for the most important yet intrinsically salt-sensitive grain legume chickpea. Chickpea is extremely sensitive to salinity during the reproductive phase. Therefore, it is essential to understand the molecular mechanisms by comparing the transcriptomic dynamics between the two contrasting genotypes in response to salt stress. Chickpea exhibits considerable genetic variation amongst improved cultivars, which show better yields in saline conditions but still need to be enhanced for sustainable crop production. Based on previous extensive multi-location physiological screening, two identified genotypes, JG11 (salt-tolerant) and ICCV2 (salt-sensitive), were subjected to salt stress to evaluate their phenological and transcriptional responses. RNA-Sequencing is a revolutionary tool that allows for comprehensive transcriptome profiling to identify genes and alleles associated with stress tolerance and sensitivity. After the first flowering, the whole flower from stress-tolerant and sensitive genotypes was collected. A total of ~300 million RNA-Seq reads were sequenced, resulting in 2022 differentially expressed genes (DEGs) in response to salt stress. Genes involved in flowering time such as FLOWERING LOCUS T (FT) and pollen development such as ABORTED MICROSPORES (AMS), rho-GTPase, and pollen-receptor kinase were significantly differentially regulated, suggesting their role in salt tolerance. In addition to this, we identify a suite of essential genes such as MYB proteins, MADS-box, and chloride ion channel genes, which are crucial regulators of transcriptional responses to salinity tolerance. The gene set enrichment analysis and functional annotation of these genes in flower development suggest that they can be potential candidates for chickpea crop improvement for salt tolerance.

Salt tolerance at vegetative stage is partially associated with changes in grain quality and starch physicochemical properties of rice exposed to salinity stress at reproductive stage

BACKGROUND

Rice yield and grain quality are highly sensitive to soil salinity. Distinct rice genotypes respond to salinity stress differently. To explore the variation in grain yield and grain trait adaptation to moderate, reproductive-stage salinity stress (4 dS/m electrical conductivity), four rice cultivars differing in degrees of vegetative salt tolerance, including Pokkali (salt-tolerant), RD15 (moderately salt-tolerant), KDML105 (moderately salt-susceptible) and IR29 (salt-susceptible), were examined.

RESULTS

Grain fertility and 100-grain weight of RD15, KDML105 and IR29, as well as grain morphology of KDML105 and IR29, were significantly disturbed. Interestingly, grain starch accumulation in RD15 and KDML105 was enhanced under stress. However, only RD15 showed changes in starch physicochemical properties, including increased granule diameter, decreased gelatinization peak temperature (T_p ) and decreased retrogradation onset temperature (T_o ). Notably, Pokkali maintained productivity, grain quality, and starch properties, while the grain quality of IR29 remained unchanged under salinity stress. Multivariate analysis displayed clear separation of productivity, grain morphology, and starch variables of RD15 in the salt-treated group relative to the control group, suggesting that it was the cultivar most impacted by salt stress despite its moderate salt-tolerance at vegetative stage.

CONCLUSION

Our results demonstrate specific salinity responses among the rice genotypes, and suggest discrepancies between degrees of salt tolerance at vegetative stage versus the ability to maintain both grain quality and starch properties in response to salinity stress imposed at reproductive stage. © 2021 Society of Chemical Industry.

Biochemical, physiological, and growth evaluation of different chickpea genotypes under varying salinity regimes

Abstract Biochemical and physiological parameters, growth, and yield of field crops especially salt sensitive crops like chickpea are affected adversely by salinity in arid to semi-arid regions. To investigate the effect of different salinity levels on growth, biochemical and physiological parameters of chickpea genotypes, a pot experiment following CRD, two factor factorial design, was conducted in the glasshouse at the Institute of Biotechnology and Genetic Engineering, The University of Agriculture, Peshawar, Pakistan. Ten (10) kg of soil was filled in each pot and salinity levels were maintained @ S0= 0 mM NaCl, S1= 50 mM NaCl, S2= 100 mM NaCl and S3= 150 mM by applying NaCl and 5 genotypes of chickpea (KK-2, Bhakkar-2011, Bittle-98, Punjab-2008, and CM-98) were used. At crop maturity, growth parameters, physiological, biochemical, and ionic parameters were measured using standard analysis procedures. Salinity reduced the growth and yield of all genotypes, but the rate of decrease was different among the genotypes tested. From the results, a decrease in K concentration, K/Na ratio, transpiration rate, stomatal conductance, N, and P was observed in all genotypes with the increase in salinity. An increase in salinity level increased the proline content (35.45%), crude protein (42%), H2O2 (19%), lipid peroxidation (62%), carbohydrates (23.22%), and Na+ concentration (137%). The highest level of salinity, 150 mM NaCl has exhibited the highest salinity stress in all parameters. Genotype KK-2 and Bhakkar-11 showed a lower rate of relative decrease in yield (4.5 and 12%), K+/Na+ ratio (23.34 and 11.47%), and K+ concentration (7.9 and 11%), respectively, and the lowest relative increase in Na+ accumulation (20.3 and 0.48%), @ 50 mM salinity compared to control. Genotype KK-2 and Bhakkar-11 proved better @ 50mM salinity. The findings suggest that the critical level of the salinity must be kept in mind and the salt-tolerant genotypes should be cultivated in salt affected soils.

Response of Mungbean (cvs. Celera II-AU and Jade-AU) and Blackgram (cv. Onyx-AU) to Transient Waterlogging

Mungbean [Vigna radiata (L.) Wilczek] and blackgram [Vigna mungo (L.) Hepper] are important crops for smallholder farmers in tropical and subtropical regions. Production of both crops is affected by unexpected and increasingly frequent extreme precipitation events, which result in transient soil waterlogging. This study aimed to compare the waterlogging tolerance of mungbean and blackgram genotypes under the varying duration of waterlogging stress at germination and seedling stages. We evaluated the responses to different durations of transient waterlogging in a sandy clay loam under temperature-controlled glasshouse conditions. Waterlogging durations were 0, 1, 2, 3, 4, 5, 6, 7, and 8 days during germination and 0, 2, 4, 8, and 16 days during the seedling stage. We used two mungbean genotypes (green testa), Celera II-AU (small-seeded), and Jade-AU (large-seeded), contrasting in seed size and hypocotyl pigmentation, and a blackgram genotype (black testa), Onyx-AU. Waterlogging reduced soil redox potential, delayed or even prevented germination, decreased seedling establishment, and affected shoot and root development. In the seedlings waterlogged (WL) at 15 days after sowing (DAS), adventitious root formation and crown nodulation varied between the genotypes, and 16 days of waterlogging substantially reduced growth but did not result in plant death. Plants in soil with waterlogging for 8-16 days followed by drainage and sampling at 39 DAS had reduced shoot and root dry mass by 60-65% in mungbean and 40% in blackgram compared with continuously drained controls, due at least in part to fewer lateral roots. Soil plant analysis development (SPAD) chlorophyll content was also reduced. Onyx-AU, a blackgram genotype, was more tolerant to transient waterlogging than Jade-AU and Celera II-AU in both growth stages. Of the two mungbean genotypes, Celera II-AU had a greater seedling establishment than Jade-AU post waterlogging imposed at sowing. In contrast, Jade-AU had more plant biomass and greater recovery growth than Celera II-AU after waterlogging and recovery during the seedling stage. Both species were delayed in emergence in response to the shorter periods of transient waterlogging at germination, and with the longer waterlogging germination and emergence failed, whereas at the seedling stage both showed adaptation by the formation of adventitious roots.

Tolerance of four grain legume species to waterlogging, hypoxia and anoxia at germination and recovery

Legume seeds, when relay sown following rice, may suffer from soil waterlogging and the associated hypoxia or even anoxia. This study evaluated the tolerance of grain legume species, grass pea (three genotypes), lentil (two genotypes), faba bean (two genotypes) and field pea (one genotype), to soil waterlogging in a glasshouse, to anoxia and hypoxia in temperature-controlled room at germination and seedling stages. Changes in oxygen in the surface layers of soil, with time after waterlogging, were measured by microelectrode profiling. The soil profiling showed that soil oxygen declined and then stabilized by the fourth day after waterlogging and oxygen was not detected at 8 mm below the soil surface. Germination of seeds under waterlogging for up to 12 days and seedling survival after the soil was drained for up to 36 days were measured in pot experiments. Seed germination and/or survival in anoxia (N₂-flushed solutions) and hypoxia (1.0 and 2.5 kPa oxygen) were evaluated, and so were post-anoxia or post-hypoxia recoveries, all in comparison with aerated controls. Lentil had higher seedling emergence (55 %) than the other species during soil waterlogging. However, lentil had lower seedling survival (9 %) than grass pea (28 %) during recovery following soil drainage. Grass pea seeds were more tolerant of anoxia and of hypoxia than the seeds of the three other species. In conclusion, grass pea, with higher percent germination and seedling survival during recovery, is more tolerant to waterlogging and subsequent soil drainage than the three other grain legume species. Grass pea was also more tolerant of hypoxia and of anoxia at the seed germination stage. These findings demonstrate the superior waterlogging tolerance of grass pea in relay sowing, as compared with the other grain legumes.

Na+ and/or Cl- Toxicities Determine Salt Sensitivity in Soybean (Glycine max (L.) Merr.), Mungbean (Vigna radiata (L.) R. Wilczek), Cowpea (Vigna unguiculata (L.) Walp.), and Common Bean (Phaseolus vulgaris L.)

Grain legumes are important crops, but they are salt sensitive. This research dissected the responses of four (sub)tropical grain legumes to ionic components (Na+ and/or Cl-) of salt stress. Soybean, mungbean, cowpea, and common bean were subjected to NaCl, Na+ salts (without Cl-), Cl- salts (without Na+), and a "high cation" negative control for 57 days. Growth, leaf gas exchange, and tissue ion concentrations were assessed at different growing stages. For soybean, NaCl and Na+ salts impaired seed dry mass (30% of control), more so than Cl- salts (60% of control). All treatments impaired mungbean growth, with NaCl and Cl- salt treatments affecting seed dry mass the most (2% of control). For cowpea, NaCl had the greatest adverse impact on seed dry mass (20% of control), while Na+ salts and Cl- salts had similar intermediate effects (~45% of control). For common bean, NaCl had the greatest adverse effect on seed dry mass (4% of control), while Na+ salts and Cl- salts impaired seed dry mass to a lesser extent (~45% of control). NaCl and Na+ salts (without Cl-) affected the photosynthesis (Pn) of soybean more than Cl- salts (without Na+) (50% of control), while the reverse was true for mungbean. Na+ salts (without Cl-), Cl- salts (without Na+), and NaCl had similar adverse effects on Pn of cowpea and common bean (~70% of control). In conclusion, salt sensitivity is predominantly determined by Na+ toxicity in soybean, Cl- toxicity in mungbean, and both Na+ and Cl- toxicity in cowpea and common bean.

Evaluating the Contribution of Growth, Physiological, and Ionic Components Towards Salinity and Drought Stress Tolerance in Jatropha curcas

Salinity and drought stress, singly or in combination, are major environmental menaces. Jatropha curcas L. is a biodiesel plant that can tolerate long periods of drought. However, the growth performance and stress tolerance based on physical, chemical, and physiological attributes of this plant have not yet been studied. To address this question, J. curcas seedlings were grown in a completely randomized design in plastic pots filled with soil to evaluate the effects of salinity and drought stresses on growth, ionic composition, and physiological attributes. The experiment consisted of six treatments: control (without salinity and drought stress), salinity alone (7.5 dS m⁻¹, 15 dS m⁻¹), drought, and a combination of salinity and drought (7.5 dS m⁻¹+ Drought, 15 dS m⁻¹+Drought). Our results revealed that, compared with the control, both plant height (PH) and stem diameter (SD) were reduced by (83%, 80%, and 77%) and (69%, 56%, and 55%) under salinity and drought combination (15 dS m⁻¹+Drought) after three, six, and nine months, respectively. There was 93% more leaf Na⁺ found in plants treated with 15 dS m⁻¹+Drought compared with the control. The highest significant average membrane stability index (MSI) and relative water content (RWC) values (81% and 85%, respectively) were found in the control. The MSI and RWC were not influenced by 7.5 dS m⁻¹ and drought treatments and mostly contributed towards stress tolerance. Our findings imply that J. curcas is moderately tolerant to salinity and drought. The Na⁺ toxicity and disturbance in K⁺: Na⁺ ratio were the main contributing factors for limited growth and physiological attributes in this plant.

Exposure to High Salinity During Seed Development Markedly Enhances Seedling Emergence and Fitness of the Progeny of the Extreme Halophyte Suaeda salsa

Irrigation with 200 mM NaCl significantly increases vegetative and reproductive growth of the extreme halophyte Suaeda salsa. However, little is known about how the progeny of S. salsa plants grown under a continuous NaCl supply behave in terms of growth and seed set parameters. We investigated various plant growth and reproductive parameters of the progeny that germinated from seeds harvested from mother plants grown under 0 or 200 mM NaCl over three generations. Seedling emergence, plant height, stem diameter, total branch length, flowering branch length, flowering branch ratio, and seed production were all significantly enhanced in the progeny produced by mother plants grown with 200 mM NaCl compared to progeny of mother plants grown on low salinity conditions. Therefore, irrigation with 200 mM of NaCl is beneficial to seed development in the halophyte S. salsa and possibly contributes to population establishment in high salinity environments. Likewise, the prolonged absence of NaCl in the growth environment inhibits seed development, results in lower seed quality, and thus limits seedling growth of the progeny, thereby restricting S. salsa to a high salinity ecological niche.

A SNP-Based Genome-Wide Association Study to Mine Genetic Loci Associated to Salinity Tolerance in Mungbean (Vigna radiata L.)

Mungbean (Vigna radiata (L.) R. Wilzeck var. radiata) is a protein-rich short-duration legume that fits well as a rotation crop into major cereal production systems of East and South-East Asia. Salinity stress in arid areas affects mungbean, being more of a glycophyte than cereals. A significant portion of the global arable land is either salt or sodium affected. Thus, studies to understand and improve salt-stress tolerance are imminent. Here, we conducted a genome-wide association study (GWAS) to mine genomic loci underlying salt-stress tolerance during seed germination of mungbean. The World Vegetable Center (WorldVeg) mungbean minicore collection representing the diversity of mungbean germplasm was utilized as the study panel and variation for salt stress tolerance was found in this germplasm collection. The germplasm panel was classed into two agro-climatic groups and showed significant differences in their germination abilities under salt stress. A total of 5288 SNP markers obtained through genotyping-by-sequencing (GBS) were used to mine alleles associated with salt stress tolerance. Associated SNPs were identified on chromosomes 7 and 9. The associated region at chromosome 7 (position 2,696,072 to 2,809,200 bp) contains the gene Vradi07g01630, which was annotated as the ammonium transport protein (AMT). The associated region in chromosome 9 (position 19,390,227 bp to 20,321,817 bp) contained the genes Vradi09g09510 and Vradi09g09600, annotated as OsGrx_S16-glutaredoxin subgroup II and dnaJ domain proteins respectively. These proteins were reported to have functions related to salt-stress tolerance.

NaCl improves reproduction by enhancing starch accumulation in the ovules of the euhalophyte Suaeda salsa

BACKGROUND

Halophytes show optimal reproduction under high-salinity conditions. However, the role of NaCl in reproduction and its possible mechanisms in the euhalophyte Suaeda salsa remain to be elucidated.

RESULTS

We performed transcript profiling of S. salsa flowers and measured starch accumulation in ovules, sugar contents in flowers, and photosynthetic parameters in the leaves of plants supplied with 0 and 200 mM NaCl. Starch accumulation in ovules, sugar contents in flowers and ovules, and net photosynthetic rate and photochemical efficiency in leaves were significantly higher in NaCl-treated plants vs. the control. We identified 14,348 differentially expressed genes in flowers of NaCl-treated vs. control plants. Many of these genes were predicted to be associated with photosynthesis, carbon utilization, and sugar and starch metabolism. These genes are crucial for maintaining photosystem structure, regulating electron transport, and improving photosynthetic efficiency in NaCl-treated plants. In addition, genes encoding fructokinase and sucrose phosphate synthase were upregulated in flowers of NaCl-treated plants.

CONCLUSIONS

The higher starch and sugar contents in the ovules and flowers of S. salsa in response to NaCl treatment are likely due to the upregulation of genes involved in photosynthesis and carbohydrate metabolism, which increase photosynthetic efficiency and accumulation of photosynthetic products under these conditions.

NaCl treatment markedly enhanced pollen viability and pollen preservation time of euhalophyte Suaeda salsa via up regulation of pollen development-related genes

Vegetable growth of halophytes has significantly increased through moderate salinity. However, little is known about the reproductive traits of euhalophytes. Male reproduction is pivotal for fertilization and seed production and sensitive to abiotic stressors. The pollen viability and pollen longevity of Suaeda salsa treated with 0 and 200 mM of NaCl were evaluated. It was revealed that the pollen size of S. salsa treated with NaCl was significantly bigger than that in controls. Furthermore, the pollen viability of S. salsa plants treated with NaCl was also significantly higher than that of control after 8 h of the pollens were collected (from 10 to 27 h). The pollen viability of NaCl-treated plants in the field could be maintained for 8 h (from 07:00 to 15:00) in sunny days, which was 1 h longer than that of control plants (from 07:00 to 14:00). Meanwhile, the pollen preservation time of NaCl-treated plants was 16 h at room temperature, which was 8 h longer than that of control plants. Genes related to pollen development, such as SsPRK3, SsPRK4, and SsLRX, exhibited high expression in the flowers of NaCl-treated plants. This indicated that NaCl markedly improved the pollen viability and preservation time via the increased expression of pollen development-related genes, and this benefits the population establishment of halophytes such as S. salsa in saline regions.

Effects of Salt Stress on Growth, Photosynthesis, and Mineral Nutrients of 18 Pomegranate (Punica granatum) Cultivars

Pomegranate (Punica granatum L.) is widely grown in arid and semiarid regions, where the salinization may have developed through irrigation. A greenhouse experiment was conducted to investigate NaCl stress on growth, photosynthesis, and nutrients of 18 pomegranate cultivars. One group was irrigated twice a week with a nutrient solution. The other group was watered twice a week with the same nutrient solution and 200 mM NaCl for five weeks. Dry weight, shoot length, new shoot number, root length and number, leaf area, leaf relative water content, and net photosynthesis of salt-treated plants were negatively impacted by salt stress, and there was a significant difference among cultivars. Few foliar damages were observed. Na content of plants significantly increased in all cultivars, while P, S, K, Ca, Mg, Si, Al, Zn content of plants decreased under salt stress. Fe, Mn, and Cu content increased in most cultivars. Pomegranate accumulated supraoptimal Na mostly in roots and transported more K and Ca to shoots, which was attributed to maintaining a higher ratio of K/Na and Ca/Na in the aerial part of plants. Ten of the 18 cultivars were considered salt-tolerant, which would offer a reference for pomegranate cultivation on saline lands.

Salinity tolerance in chickpea is associated with the ability to 'exclude' Na from leaf mesophyll cells

Salinity tolerance is associated with Na 'exclusion' from, or 'tissue tolerance' in, leaves. We investigated whether two contrasting chickpea genotypes, salt-tolerant Genesis836 and salt-sensitive Rupali, differ in leaf tissue tolerance to NaCl. We used X-ray microanalysis to evaluate cellular Na, Cl, and K concentrations in various cell types within leaflets and also in secretory trichomes of the two chickpea genotypes in relation to photosynthesis in control and saline conditions. TEM was used to assess the effects of salinity on the ultrastructure of chloroplasts. Genesis836 maintained net photosynthetic rates (A) for the 21 d of salinity treatment (60 mM NaCl), whereas A in Rupali substantially decreased after 11 d. Leaflet tissue [Na] was low in Genesis836 but had increased markedly in Rupali. In Genesis836, Na was accumulated in epidermal cells but was low in mesophyll cells, whereas in Rupali cellular [Na] was high in both cell types. The excessive accumulation of Na in mesophyll cells of Rupali corresponded to structural damage to the chloroplasts. Maintenance of photosynthesis and thus salinity tolerance in Genesis836 was associated with an ability to 'exclude' Na from leaflets and in particular from the photosynthetically active mesophyll cells, and to compartmentalize Na in epidermal cells.

Waterlogging Tolerance at Germination in Field Pea: Variability, Genetic Control, and Indirect Selection

In the Eastern Gangetic Plain of South Asia field pea (Pisum sativum L.) is often grown as a relay crop where soil waterlogging (WL) causes germination failure. To assess if selection for WL tolerance is feasible, we studied the response to WL stress at germination stage in a recombinant inbred line (RIL) population from a bi-parental cross between WL-contrasting parents and in a diversity panel to identify extreme phenotypes, understand the genetics of WL tolerance and find traits for possible use in indirect selection. The RIL population and the diversity panel were screened to test the ability of germination under both waterlogged and drained soils. A total of 50, most WL tolerant and sensitive, genotypes from each of both the RIL and the diversity panel were further evaluated to assay testa integrity/leakage in CaSO₄ solution. Morphological characterization of both populations was undertaken. A wide range of variation in the ability to germination in waterlogged soil was observed in the RIL population (6-93%) and the diversity panel (5-100%) with a high broad-sense heritability (H ² > 85%). The variation was continuously distributed indicating polygenic control. Most genotypes with a dark colored testa (90%) were WL tolerant, whereas those with a light colored testa were all WL sensitive in both the RIL population and diversity panel. Testa integrity, measured by electrical conductivity of the leakage solute, was strongly associated with WL tolerance in the RIL population (r _G = -1.00) and the diversity panel (r _G = -0.90). Therefore, testa integrity can be effectively used in indirect selection for WL tolerance. Response to selection for WL tolerance at germination is confidently predicted enabling the adaptation of the ancient model pea to extreme precipitation events at germination.

Salt-Enhanced Reproductive Development of Suaeda salsa L. Coincided With Ion Transporter Gene Upregulation in Flowers and Increased Pollen K+ Content

Halophytes are adapted to saline environments and demonstrate optimal reproductive growth under high salinity. To gain insight into the salt tolerance mechanism and effects of salinity in the halophyte Suaeda salsa, the number of flowers and seeds, seed size, anther development, ion content, and flower transcript profiles, as well as the relative expression levels of genes involved in ion transport, were analyzed in S. salsa plants treated with 0 or 200 mM NaCl. The seed size, flower number, seed number per leaf axil, and anther fertility were all significantly increased by 200 mM NaCl treatment. The Na+ and Cl- contents in the leaves, stems, and pollen of NaCl-treated plants were all markedly higher, and the K+ content in the leaves and stems was significantly lower, than those in untreated control plants. By contrast, the K+ content in pollen grains did not decrease, but rather increased, upon NaCl treatment. Genes related to Na+, K+ and, Cl- transport, such as SOS1, KEA, AKT1, NHX1, and CHX, showed increased expression in the flowers of NaCl-treated plants. These results suggest that ionic homeostasis in reproductive organs, especially in pollen grains under salt-treated conditions, involves increased expression of ion transport-related genes.

Salinity stress response and 'omics' approaches for improving salinity stress tolerance in major grain legumes

Sustaining yield gains of grain legume crops under growing salt-stressed conditions demands a thorough understanding of plant salinity response and more efficient breeding techniques that effectively integrate modern omics knowledge. Grain legume crops are important to global food security being an affordable source of dietary protein and essential mineral nutrients to human population, especially in the developing countries. The global productivity of grain legume crops is severely challenged by the salinity stress particularly in the face of changing climates coupled with injudicious use of irrigation water and improper agricultural land management. Plants adapt to sustain under salinity-challenged conditions through evoking complex molecular mechanisms. Elucidating the underlying complex mechanisms remains pivotal to our knowledge about plant salinity response. Improving salinity tolerance of plants demand enriching cultivated gene pool of grain legume crops through capitalizing on 'adaptive traits' that contribute to salinity stress tolerance. Here, we review the current progress in understanding the genetic makeup of salinity tolerance and highlight the role of germplasm resources and omics advances in improving salt tolerance of grain legumes. In parallel, scope of next generation phenotyping platforms that efficiently bridge the phenotyping-genotyping gap and latest research advances including epigenetics is also discussed in context to salt stress tolerance. Breeding salt-tolerant cultivars of grain legumes will require an integrated "omics-assisted" approach enabling accelerated improvement of salt-tolerance traits in crop breeding programs.

Changes in gene expression during germination reveal pea genotypes with either "quiescence" or "escape" mechanisms of waterlogging tolerance

Waterlogging causes germination failure in pea (Pisum sativum L.). Three genotypes (BARI Motorshuti-3, Natore local-2 [NL-2], and Kaspa) contrasting in ability to germinate in waterlogged soil were exposed to different durations of waterlogging. Whole genome RNAseq was employed to capture differentially expressing genes. The ability to germinate in waterlogged soil was associated with testa colour and testa membrane integrity as confirmed by electrical conductivity measurements. Genotypes Kaspa and NL-2 displayed different mechanisms of tolerance. In Kaspa, an energy conserving strategy was indicated by a strong upregulation of tyrosine protein kinsase and down regulation of linoleate 9S-lipoxygenase 5, a fat metabolism gene. In contrast, a faster energy utilization strategy was suggested in NL-2 by the marked upregulation of a subtilase family protein and peroxisomal adenine nucleotide carrier 2, a fat metabolizing gene. Waterlogging susceptibility in germinating seeds of genotype BARI Motorshuti-3 was linked to upregulation of a kunitz-type trypsin/protease inhibitor that blocks protein metabolism and may lead to excessive lipid metabolism and the membrane leakage associated with waterlogging damage. Pathway analyses based on gene ontologies showed seed storage protein metabolism as upregulated in tolerant genotypes and downregulated in the sensitive genotype. Understanding the tolerance mechanism provides a platform to breed for adaptation to waterlogging stress at germination in pea.

Differential expression of flowering genes in Arabidopsis thaliana under chronic and acute ionizing radiation

PURPOSE

Chronic and acute irradiations have drastic effects on flowering stage that plays an important role in further seed development and can determine seed yield. The expression of the key flowering genes, AP1, CO, GI, FT, FLC, and LFY, sensitive to irradiation repair gene RAD51 and the proliferation gene PCNA2 were studied in the wild-type Arabidopsis thaliana (Columbia ecotype) under chronic and acute irradiations.

MATERIALS AND METHODS

Chronic irradiation was performed using the radioactive isotope ¹³⁷СsCl in two total doses of 3 cGy and 17 cGy, with the dose rate of 10^-7 cGy/s and 6.8 10^-6 cGy/s, respectively. The plants were grown under chronic irradiation during 6 weeks, from seeds till the 6.3 stage of flowering. For acute exposure, the plants were X-ray irradiated one time at the 5.0 development stage (20 days old) by a total dose of 15 Gy with the dose rate of 89 cGy/s.

RESULTS

After chronic irradiation with the 3 cGy dose the irradiated plants demonstrated 8 ± 2.8 days earlier flowering than in the control group. However, at the 17 cGy chronic and at the 15 Gy acute doses plants showed 14 ± 3.7 and 2 ± 1.4 days later flowering, respectively. The 3 cGy chronic exposure significantly increased the expression of the CO gene by a factor of 1.152 (1.087-1.217 95% C.I.) and decreased the expression of the FT gene by a factor of 0.128 (0.021-0.396 95% C.I.). The 17 cGy chronic exposure decreased expression of the AP1 gene by a factor of 0.872 (0.803-0.940 95% C.I.) and the LFY gene by a factor of 0.471 (0.306-0.687 95% C.I.). The 15 Gy acute exposure decreased the expression of the AP1 gene by a factor of 0.104 (0.074-0.144 95% C.I.) and the PCNA2 gene by a factor of 0.346 (0.238-0.488 95% C.I.).

CONCLUSIONS

The increased expression of the CO gene and decreased expression of the AP1 and FT genes under the lower dose of chronic exposure were associated with earlier flowering. The acute exposure increased the expression of the PCNA2 gene and decreased the expression of the flowering genes, except AP1. The flowering was delayed under both the higher dose of chronic exposure and under acute exposure, but it was less affected by the latter. Presumably, it was related to the activation of DNA repair under the 3 cGy chronic and 15 Gy acute irradiations.

Elucidating the role of osmotic, ionic and major salt responsive transcript components towards salinity tolerance in contrasting chickpea (Cicer arietinum L.) genotypes

The growth of chickpea (Cicer arietinum L.) is extremely hampered by salt stress. Understanding of physio-biochemical and molecular attributes along with morphological traits contributing to the salinity tolerance is important for developing salt tolerant chickpea varieties. To explore these facts, two genotypes CSG8962 and HC5 with contrasting salt tolerance were evaluated in the salinity stress (Control and 120 mM NaCl) conditions. CSG8962 maintained lower Na/K ratio in root and shoot, trammeled Na translocation to the shoots from roots compared to HC5 which ascribed to better exclusion of salt from its roots and compartmentation in the shoot. In chickpea, salt stress specifically induced genes/sequences involved at several levels in the salt stress signaling pathway. Higher induction of trehalose 6 phosphate synthase and protein kinase genes pertaining to the osmotic and signaling modules, respectively, were evident in CSG8962 compared to HC5. Further transcripts of late embryogenesis abundant, non-specific lipid transfer protein, HI and 219 genes/sequences were also highly induced in CSG8962 compared to HC5 which emphasizes the better protection of cellular membranous network and membrane-bound macromolecules under salt stress. This further suppressed the stress enhanced electrolyte leakage, loss of turgidity, promoted the higher compatible solute accumulation and maintained better cellular ion homoeostasis in CSG8962 compared to HC5. Our study further adds to the importance of these genes in salt tolerance by comparing their behavior in contrasting chickpea genotypes.

Differential Regulation of Genes Involved in Root Morphogenesis and Cell Wall Modification is Associated with Salinity Tolerance in Chickpea

Salinity is a major constraint for intrinsically salt sensitive grain legume chickpea. Chickpea exhibits large genetic variation amongst cultivars, which show better yields in saline conditions but still need to be improved further for sustainable crop production. Based on previous multi-location physiological screening, JG 11 (salt tolerant) and ICCV 2 (salt sensitive) were subjected to salt stress to evaluate their physiological and transcriptional responses. A total of ~480 million RNA-Seq reads were sequenced from root tissues which resulted in identification of 3,053 differentially expressed genes (DEGs) in response to salt stress. Reproductive stage shows high number of DEGs suggesting major transcriptional reorganization in response to salt to enable tolerance. Importantly, cationic peroxidase, Aspartic ase, NRT1/PTR, phosphatidylinositol phosphate kinase, DREB1E and ERF genes were significantly up-regulated in tolerant genotype. In addition, we identified a suite of important genes involved in cell wall modification and root morphogenesis such as dirigent proteins, expansin and casparian strip membrane proteins that could potentially confer salt tolerance. Further, phytohormonal cross-talk between ERF and PIN-FORMED genes which modulate the root growth was observed. The gene set enrichment analysis and functional annotation of these genes suggests they may be utilised as potential candidates for improving chickpea salt tolerance.

NaCl markedly improved the reproductive capacity of the euhalophyte Suaeda salsa

The effect of NaCl on reproductive development was investigated in euhalophyte Suaeda salsa L. under controlled conditions. Results showed that NaCl promoted the reproductive growth of S. salsa and 200mM NaCl was optimal. This was reflected in the increases of seed yield, seed number, flower number per plant and leaf axil, 1000 seed weight, as well as a decrease in flower abortion percentage with supply of NaCl. NaCl reduced the flower abortion percentage by increasing stigma receptivity instead of pollen viability. The Na+ and Cl- concentration in petals, stems and leaves were increased significantly but slightly in stamen and pistil. In contrast, the K+ concentration decreased markedly in leaves, stems and petals but a little in stamen and pistil. The Na+ and Cl- concentrations also increased significantly in seed from mother plants exposed to NaCl, whereas K+ decreased. However, seed quality was not influenced. Our results showed that high concentration of NaCl markedly increases the seed number and quality of S. salsa primarily via increasing flower number and fertility and S. salsa develops strategy to maintain ion homeostasis in reproductive organs for the generation. These factors play a pivotal role in setting up plant populations in saline environment.

Imperative roles of halotolerant plant growth-promoting rhizobacteria and kinetin in improving salt tolerance and growth of black gram (Phaseolus mungo)

The salinity stress causes a major threat for plant growth, yield, and biomass production. The present study was designed to assess the effect of exogenously applied kinetin and halotolerant plant growth-promoting rhizobacteria (H-PGPR) on alleviation of salt stress in black gram (Phaseolus mungo). A total of 15 rhizobacterial isolates obtained from a salt-affected area were analyzed for their capability to improve growth of P. mungo plants growing in greenhouse conditions. Out of the tested rhizobacteria, the two bacterial isolates which exhibited maximum growth potential were screened and their growth-promoting attributes were evaluated. The role of screened H-PGPR and/or kinetin (8 and 10 μM) was evaluated in P. mungo plants irrigated with three levels of brackish water (S1 = 3, S2 = 5, and S3 = 7 dSm^-1) under field condition. Salt stress reduced transpiration rate, stomatal conductance, salt tolerance index, growth, leaf area, photosynthetic pigments, leaf relative water content (LRWC), biomass production, and seed yield in subjected plants. Conversely, the salinized plants treated with kinetin and/or H-PGPR exhibited improved levels of chlorophyll contents, LRWC, root growth, shoot growth, biomass production, and seed yield. The H-PGPR and/or kinetin supplementation also reduced electrolyte leakage in salt-stressed plants. Overall, the present findings will be of great value to recognize the mechanism of salt stress alleviation in P. mungo plants under the influence of H-PGPR and/or kinetin.

Improving Salt Tolerance of Chickpea Using Modern Genomics Tools and Molecular Breeding

INTRODUCTION

The high protein value, essential minerals, dietary fibre and notable ability to fix atmospheric nitrogen make chickpea a highly remunerative crop, particularly in low-input food production systems. Of the variety of constraints challenging chickpea productivity worldwide, salinity remains of prime concern owing to the intrinsic sensitivity of the crop. In view of the projected expansion of chickpea into arable and salt-stressed land by 2050, increasing attention is being placed on improving the salt tolerance of this crop. Considerable effort is currently underway to address salinity stress and substantial breeding progress is being made despite the seemingly highly-complex and environment-dependent nature of the tolerance trait.

CONCLUSION

This review aims to provide a holistic view of recent advances in breeding chickpea for salt tolerance. Initially, we focus on the identification of novel genetic resources for salt tolerance via extensive germplasm screening. We then expand on the use of genome-wide and cost-effective techniques to gain new insights into the genetic control of salt tolerance, including the responsive genes/QTL(s), gene(s) networks/cross talk and intricate signalling cascades.

Effects, tolerance mechanisms and management of salt stress in grain legumes

Salt stress is an ever-present threat to crop yields, especially in countries with irrigated agriculture. Efforts to improve salt tolerance in crop plants are vital for sustainable crop production on marginal lands to ensure future food supplies. Grain legumes are a fascinating group of plants due to their high grain protein contents and ability to fix biological nitrogen. However, the accumulation of excessive salts in soil and the use of saline groundwater are threatening legume production worldwide. Salt stress disturbs photosynthesis and hormonal regulation and causes nutritional imbalance, specific ion toxicity and osmotic effects in legumes to reduce grain yield and quality. Understanding the responses of grain legumes to salt stress and the associated tolerance mechanisms, as well as assessing management options, may help in the development of strategies to improve the performance of grain legumes under salt stress. In this manuscript, we discuss the effects, tolerance mechanisms and management of salt stress in grain legumes. The principal inferences of the review are: (i) salt stress reduces seed germination (by up to more than 50%) either by inhibiting water uptake and/or the toxic effect of ions in the embryo, (ii) salt stress reduces growth (by more than 70%), mineral uptake, and yield (by 12-100%) due to ion toxicity and reduced photosynthesis, (iii) apoplastic acidification is a good indicator of salt stress tolerance, (iv) tolerance to salt stress in grain legumes may develop through excretion and/or compartmentalization of toxic ions, increased antioxidant capacity, accumulation of compatible osmolytes, and/or hormonal regulation, (v) seed priming and nutrient management may improve salt tolerance in grain legumes, (vi) plant growth promoting rhizobacteria and arbuscular mycorrhizal fungi may help to improve salt tolerance due to better plant nutrient availability, and (vii) the integration of screening, innovative breeding, and the development of transgenics and crop management strategies may enhance salt tolerance and yield in grain legumes on salt-affected soils.

Vegetative and reproductive growth of salt-stressed chickpea are carbon-limited: sucrose infusion at the reproductive stage improves salt tolerance

Reproductive processes of chickpea (Cicer arietinum L.) are particularly sensitive to salinity. We tested whether limited photoassimilate availability contributes to reproductive failure in salt-stressed chickpea. Rupali, a salt-sensitive genotype, was grown in aerated nutrient solution, either with non-saline (control) or 30mM NaCl treatment. At flowering, stems were either infused with sucrose solution (0.44M), water only or maintained without any infusion, for 75 d. The sucrose and water infusion treatments of non-saline plants had no effect on growth or yield, but photosynthesis declined in response to sucrose infusion. Salt stress reduced photosynthesis, decreased tissue sugars by 22-47%, and vegetative and reproductive growth were severely impaired. Sucrose infusion of salt-treated plants increased total sugars in stems, leaves and developing pods, to levels similar to those of non-saline plants. In salt-stressed plants, sucrose infusion increased dry mass (2.6-fold), pod numbers (3.8-fold), seed numbers (6.5-fold) and seed yield (10.4-fold), yet vegetative growth and reproductive failure were not rescued completely by sucrose infusion. Sucrose infusion partly rescued reproductive failure in chickpea by increasing vegetative growth enabling more flower production and by providing sucrose for pod and seed growth. We conclude that insufficient assimilate availability limits yield in salt-stressed chickpea.

Salt sensitivity in chickpea is determined by sodium toxicity

Salt sensitivity in chickpea is determined by Na(+) toxicity, whereas relatively high leaf tissue concentrations of Cl(-) were tolerated, and the osmotic component of 60-mM NaCl was not detrimental. Chickpea (Cicer arietinum L.) is sensitive to salinity. This study dissected the responses of chickpea to osmotic and ionic components (Na(+) and/or Cl(-)) of salt stress. Two genotypes with contrasting salt tolerances were exposed to osmotic treatments (-0.16 and -0.29 MPa), Na(+)-salts, Cl(-)-salts, or NaCl at 0, 30, or 60 mM for 42 days and growth, tissue ion concentrations and leaf gas-exchange were assessed. The osmotic treatments and Cl(-)-salts did not affect growth, whereas Na(+)-salts and NaCl treatments equally impaired growth in either genotype. Shoot Na(+) and Cl(-) concentrations had markedly increased, whereas shoot K(+) had declined in the NaCl treatments, but both genotypes had similar shoot concentrations of each of these individual ions after 14 and 28 days of treatments. Genesis836 achieved higher net photosynthetic rate (64-84 % of control) compared with Rupali (35-56 % of control) at equivalent leaf Na(+) concentrations. We conclude that (1) salt sensitivity in chickpea is determined by Na(+) toxicity, and (2) the two contrasting genotypes appear to differ in 'tissue tolerance' of high Na(+). This study provides a basis for focus on Na(+) tolerance traits for future varietal improvement programs for salinity tolerance in chickpea.

Nitric Oxide Mitigates Salt Stress by Regulating Levels of Osmolytes and Antioxidant Enzymes in Chickpea

This work was designed to evaluate whether external application of nitric oxide (NO) in the form of its donor S-nitroso-N-acetylpenicillamine (SNAP) could mitigate the deleterious effects of NaCl stress on chickpea (Cicer arietinum L.) plants. SNAP (50 μM) was applied to chickpea plants grown under non-saline and saline conditions (50 and 100 mM NaCl). Salt stress inhibited growth and biomass yield, leaf relative water content (LRWC) and chlorophyll content of chickpea plants. High salinity increased electrolyte leakage, carotenoid content and the levels of osmolytes (proline, glycine betaine, soluble proteins and soluble sugars), hydrogen peroxide (H2O2) and malondialdehyde (MDA), as well as the activities of antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase in chickpea plants. Expression of the representative SOD, CAT and APX genes examined was also up-regulated in chickpea plants by salt stress. On the other hand, exogenous application of NO to salinized plants enhanced the growth parameters, LRWC, photosynthetic pigment production and levels of osmolytes, as well as the activities of examined antioxidant enzymes which is correlated with up-regulation of the examined SOD, CAT and APX genes, in comparison with plants treated with NaCl only. Furthermore, electrolyte leakage, H2O2 and MDA contents showed decline in salt-stressed plants supplemented with NO as compared with those in NaCl-treated plants alone. Thus, the exogenous application of NO protected chickpea plants against salt stress-induced oxidative damage by enhancing the biosyntheses of antioxidant enzymes, thereby improving plant growth under saline stress. Taken together, our results demonstrate that NO has capability to mitigate the adverse effects of high salinity on chickpea plants by improving LRWC, photosynthetic pigment biosyntheses, osmolyte accumulation and antioxidative defense system.

Transcriptome analyses reveal genotype- and developmental stage-specific molecular responses to drought and salinity stresses in chickpea

Drought and salinity are the major factors that limit chickpea production worldwide. We performed whole transcriptome analyses of chickpea genotypes to investigate the molecular basis of drought and salinity stress response/adaptation. Phenotypic analyses confirmed the contrasting responses of the chickpea genotypes to drought or salinity stress. RNA-seq of the roots of drought and salinity related genotypes was carried out under control and stress conditions at vegetative and/or reproductive stages. Comparative analysis of the transcriptomes revealed divergent gene expression in the chickpea genotypes at different developmental stages. We identified a total of 4954 and 5545 genes exclusively regulated in drought-tolerant and salinity-tolerant genotypes, respectively. A significant fraction (~47%) of the transcription factor encoding genes showed differential expression under stress. The key enzymes involved in metabolic pathways, such as carbohydrate metabolism, photosynthesis, lipid metabolism, generation of precursor metabolites/energy, protein modification, redox homeostasis and cell wall component biogenesis, were affected by drought and/or salinity stresses. Interestingly, transcript isoforms showed expression specificity across the chickpea genotypes and/or developmental stages as illustrated by the AP2-EREBP family members. Our findings provide insights into the transcriptome dynamics and components of regulatory network associated with drought and salinity stress responses in chickpea.

GmSALT3, Which Confers Improved Soybean Salt Tolerance in the Field, Increases Leaf Cl- Exclusion Prior to Na+ Exclusion But Does Not Improve Early Vigor under Salinity

Soil salinity reduces soybean growth and yield. The recently identified GmSALT3 (Glycine max salt Tolerance-associated gene on chromosome 3) has the potential to improve soybean yields in salinized conditions. Here we evaluate the impact of GmSALT3 on soybean performance under saline or non-saline conditions. Three sets of near isogenic lines (NILs), with genetic similarity of 95.6-99.3% between each pair of NIL-T and NIL-S, were generated from a cross between two varieties 85-140 (salt-sensitive, S) and Tiefeng 8 (salt-tolerant, T) by using marker-assisted selection. Each NIL-T; 782-T, 820-T and 860-T, contained a common ~1000 kb fragment on chromosome 3 where GmSALT3 was located. We show that GmSALT3 does not contribute to an improvement in seedling emergence rate or early vigor under salt stress. However, when 12-day-old seedlings were exposed to NaCl stress, the NIL-T lines accumulated significantly less leaf Na⁺ compared with their corresponding NIL-S, while no significant difference of K⁺ concentration was observed between NIL-T and NIL-S; the magnitude of Na⁺ accumulation within each NIL-T set was influenced by the different genetic backgrounds. In addition, NIL-T lines accumulated less Cl^- in the leaf and more in the root prior to any difference in Na⁺; in the field they accumulated less pod wall Cl^- than the corresponding NIL-S lines. Under non-saline field conditions, no significant differences were observed for yield related traits within each pair of NIL-T and NIL-S lines, indicating there was no yield penalty for having the GmSALT3 gene. In contrast, under saline field conditions the NIL-T lines had significantly greater plant seed weight and 100-seed weight than the corresponding NIL-S lines, meaning GmSALT3 conferred a yield advantage to soybean plants in salinized fields. Our results indicated that GmSALT3 mediated regulation of both Na⁺ and Cl^- accumulation in soybean, and contributes to improved soybean yield through maintaining a higher seed weight under saline stress.

Oxygen deficiency and salinity affect cell-specific ion concentrations in adventitious roots of barley (Hordeum vulgare)

Oxygen deficiency associated with soil waterlogging adversely impacts root respiration and nutrient acquisition. We investigated the effects of O2 deficiency and salinity (100 mM NaCl) on radial O2 concentrations and cell-specific ion distributions in adventitious roots of barley (Hordeum vulgare). Microelectrode profiling measured O2 concentrations across roots in aerated, aerated saline, stagnant or stagnant saline media. X-ray microanalysis at two positions behind the apex determined the cell-specific elemental concentrations of potassium (K), sodium (Na) and chloride (Cl) across roots. Severe O2 deficiency occurred in the stele and apical regions of roots in stagnant solutions. O2 deficiency in the stele reduced the concentrations of K, Na and Cl in the pericycle and xylem parenchyma cells at the subapical region. Near the root apex, Na declined across the cortex in roots from the aerated saline solution but was relatively high in all cell types in roots from the stagnant saline solution. Oxygen deficiency has a substantial impact on cellular ion concentrations in roots. Both pericycle and xylem parenchyma cells are involved in energy-dependent K loading into the xylem and in controlling radial Na and Cl transport. At root tips, accumulation of Na in the outer cell layers likely contributed to reduction of Na in inner cells of the tips.

Salt sensitivity in chickpea: Growth, photosynthesis, seed yield components and tissue ion regulation in contrasting genotypes

Chickpea is a relatively salt sensitive species but shows genotypic variation for salt tolerance, measured as grain yield per plant in mild-to-moderately saline soil. This experiment was designed to evaluate some physiological responses to salinity in three contrasting genotypes. One tolerant (Genesis836), one moderately tolerant (JG11) and one sensitive (Rupali) genotype were grown for 108d in non-saline nutrient solution (controls) and two levels of salinity treatment (30 and 60mM NaCl). No plants survived to maturity in the 60mM NaCl treatment; however, Genesis836 survived longer (87d) than JG11 (67d) while Rupali died after 27d; only Genesis836 flowered, but no pods were filled. At 30mM NaCl, Genesis836 produced a few filled pods, whereas JG11 and Rupali did not. Genotypic differences in plant dry mass at the vegetative stage were evident only at 60mM NaCl, while at maturity differences were evident at 30mM NaCl. Photosynthesis was maintained to different degrees by the three genotypes (e.g. at 30mM NaCl, 35-81% of controls; highest in Genesis836); photosynthesis was restricted predominately due to non-stomatal limitations as the intercellular CO2 concentration was only modestly affected (94-99% of controls). Photosystem II damage was evident in the less tolerant genotypes (e.g. at 30mM NaCl, actual quantum efficiency of photosystem II values were 63-96% of controls). Across treatments, shoot dry mass was negatively correlated with both Na(+) and Cl(-) shoot concentrations. However, the sensitive genotype (Rupali) had equal or lower concentrations of these ions in green leaves, stems or roots compared to tolerant genotypes (JG11 and Genesis836); ion 'exclusion' does not explain variation for salt tolerance among these three chickpea genotypes. The large difference between Rupali (sensitive) and Genesis836 (tolerant) in the salt-induced reduction in net photosynthesis via non-stomatal limitations and the assessed damage to photosystem II, but with similar leaf ion concentrations, provides evidence that variation in 'tissue tolerance' of Na(+) and/or Cl(-) in leaves contributes to the differential salt tolerance of these chickpea genotypes.