Unique responses of three highly salt-tolerant wild <i>Vigna</i> species against salt stress

The genus <i>Vigna</i> contains important crops such as cowpea and mungbean. Wild <i>Vigna</i> showing higher salt tolerance than <i>Vigna</i> crops were screened and their tolerance mechanisms are discussed. Primary screening using 7 <i>Vigna</i> crops and 23 wild <i>Vigna</i> under 300 mM NaCl selected <i>V. luteola, V. marina</i> and <i>V. vexillata</i>. A study under different salt concentrations revealed the highest survival ability of <i>V. marina</i>. Diversity of salt tolerance in each species was revealed using a total of 230 accessions. Growth and physiological responses under 150 mM NaCl were then compared using two selected accessions from each species. The pattern of Na⁺ accumulation in roots, stems and leaves suggested that <i>V. vexillata</i> (V1) and <i>V. luteola</i> (L8, L9) are ‘Na⁺ excluder’ type, while <i>V. marina</i> (M1, M4) is ‘Na⁺ includer’ type. <i>V. luteola</i> (L8, L9) showed the highest dry matter production under control condition and well-maintained shoot dry weight under salt stress. Interestingly, <i>V. luteola</i> (L9) accumulated the highest Na⁺ in roots (3000 μM g⁻¹) and increased root dry weight under salt stress, which might work as Na⁺ reservoir restricting Na⁺ transition to the leaves, leading to the increased photosynthetic rate. <i>V. luteola</i> has great potential in areas where moderate salt damage occurs. <i>V. marina</i> (M1, M4) accumulated Na⁺ at high level in roots, stem, and leaves. Under salt stress, they increased stomatal conductance, transpiration rate, and photosynthetic rate, which suggested the adaptational regulation of aquaporin gene expression. <i>V. marina</i> will be useful as food, pasture and phytoremediation legumes in highly salt-damaged areas.