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Due to its readily soluble nature in groundwater and surface water, an excess accumulation of soluble salts in the soil, especially sodium chloride (NaCl), could pose serious threats to crop growth and development by disrupting physiological and biochemical processes in plants. The excess of soluble salt could originate from natural processes, such as weathering (physical or chemical) of parent rock materials, wind, and rainfall, and is thus termed as primary soil salinization. Besides, various anthropogenic activities, such as irrigation with brackish or saline water, poor land and water management, overuse of synthetic fertilizers, overextraction of groundwater, and saltwater intrusion into coastal aquifers as a result of sea level rise, could also create a salinity problem, which is known as secondary soil salinization. The salinity problem in soil and land of arid and semi-arid regions is even more severe. The concentration of salt is referred to as the sum of cation and anion or only cation or anion and is expressed as mg L-1. However, it is commonly measured as electrical conductivity (EC) of saturated soil extract (ECe) for scientific and analytical purposes and is expressed with a unit of decisiemens per meter (dS m-1). Salinity is a multi-dimensional stress for plants, impairing plant growth and development through cytotoxicity caused by excessive uptake of Na+ and Cl- ions, water stress, and nutritional imbalance. In addition, salinity is generally accompanied by oxidative stress because of the overproduction of reactive oxygen species. The water scarcity phase of salinity results in substantial yield loss through rapid inhibition of growth and development, while the ionic toxicity phase causes rapid leaf senescence through a variety of adverse impacts on cellular components simultaneously. Moreover, prooxidants generation at higher levels of salt stress threatens standard metabolic processes and poses a risk of irreversible damage on biomembranes, cellular organelles, nucleic acids, and proteins. Such constraints make cropping a great challenge in salt-affected areas; however, naturally salt-tolerant plants, also known as halophytes or semi-halophytes, can easily withstand comparatively higher salt concentrations because of their adaptive morpho-physiological and molecular mechanisms. Unlike halophytic or semi-halophytic plants, most of the commonly cultivated crops fail to tolerate salt stress as they fall under glycophyte or salt-susceptible category. However, the process of natural selection coupled with developing agricultural technologies has played a substantial role in imparting greater tolerance of crops to salt stress over the past few decades. Nevertheless, rapid sea level rise driven by anthropogenic climate change affecting the salinity of surface and groundwater in coastal areas and persistent spreading of salinity both in terms of land area and concentration call for a combined approach of compatible cultivar development and adoption of suitable management strategies. This necessitates a systematic and in-depth understanding of existing adaptations, tolerance mechanisms, and cultivation methods for stepping forward in strategizing future crop yield improvement programs. Therefore, the present review focuses on highlighting (i) the factors intensifying salinity problem in soils, (ii) salinity dynamics in plants including its uptake, transport, storage, and development of stress, (iii) salinity sensing mechanism in plants, (iv) naturally occurring adaptations, (v) genetic variabilities, and (vi) tolerance mechanisms available within cultivable crop germplasms with specific emphasis on morphological and anatomical tolerance, physiochemical tolerance, and role of genetic resources in enhancing tolerance against salinity. A number of agronomic, cultural, and technological management options to alleviate the adverse impacts of salt stress on crop plants are also discussed in this review, which could potentially offer invaluable information for the development of salt-tolerant crop improvement programs and optimized land and water management practices. [ABSTRACT FROM AUTHOR]

Iron (Fe) toxicity, generated from excess reduced ferrous Fe (Fe2+) ion formation within the soil under submerged condition, is a potent environmental stress that limits lowland rice production. Total 11 diverse Thai rice genotypes, including a recognized tolerant genotype Azucena and a susceptible genotype IR64, were evaluated against 5 Fe2+ levels [0 (control), 150, 300, 600 and 900 mg/L] to screen the tested genotypes for their Fe-toxicity tolerance and to classify them as a sensitive/tolerant category. The evaluation was conducted by a germination study, followed by a polyhouse study on growth, yield and physiochemical performances. Results showed significant variations in Fe2+-tolerance across genotypes. Increasing Fe2+ level beyond 300 mg/L was detrimental for germination and growth of all the tested genotypes, although germination responses were negatively affected at Fe2+ ≥ 300 mg/L. Physiochemical responses in the form of leaf greenness, net photosynthetic rate, membrane stability index and Fe contents in leaf and root were the most representative of Fe2+-toxicity-mediated impairments on overall growth and yield. Difference in physiochemical responses was effectively correlated with the contrasting ability of the genotypes on lowering excess Fe2+ in tissues. Analysis of average tolerance and stress tolerance index unveiled that the genotypes RD85 and RD31 were the closest to the tolerant check Azucena and the sensitive check IR64, respectively. The unweighted pair group method with arithmetic means clustering revealed three major clusters, with cluster II (four genotypes) being Fe2+ tolerant and cluster I (four genotypes) being Fe2+ sensitive. Principal component (PC) analysis and genotype by trait-biplot analysis showed that the first two components explained 90.5% of the total variation, with PC1 accounting for 56.6% and PC2 for 33.9% of the total variation. The identified tolerant rice genotypes show potentials for cultivation in Fe2+-toxic lowlands for increased productivity. The findings contribute to the present understanding on Fe2+-toxicity response and provide a basis for future genotype selection or rice crop improvement programs against Fe2+-toxicity. [ABSTRACT FROM AUTHOR]