Leaf growth and K⁺/Na⁺ ratio as an indication of the salt tolerance of three sorghum cultivars grown under salinity stress and IAA treatment

The salt tolerance of three sorghum (<i>Sorghum bicolor</i> L.) cultivars (Dorado, Hagen Shandawil and Giza 113) and their responses to shoot spraying with 25 ppm IAA were studied. Salinity stress induced substantial differences between the three sorghum cultivars in the leaf area, dry mass, relative water content and tolerance index of the leaves. Dorado and Hagen Shandawil tolerated salinity up to 88 and 44 mM NaCl, respectively, but above this level, and at all salinity levels in Giza 113, a significant reduction in these parameters was recorded. The rate of reduction was lower in Dorado than in Hagen Shandawil and Giza 113, allowing the sequence Dorado ? Hagen Shandawil ? Giza 113 to be established for the tolerance of these cultivars to salinity. The differences in the tolerance of the sorghum cultivars were associated with large differences in K⁺ rather than in Na⁺, which was found to be similar in the whole plant. The youngest leaf was able to maintain a higher K⁺content than the oldest leaf. Consequently the K⁺/Na⁺ratios were higher in the most salt-tolerant cultivar Dorado than in the other sorghum cultivars, and in the youngest than in the oldest leaf. In conformity with this mechanism, the stimulatory effect of the exogenous application of IAA was mostly associated with a higher K⁺/Na⁺ ratio. Shoot spraying with IAA partially alleviated the inhibitory effect of salinity on leaf growth and on the K⁺ and Ca²⁺ contents, especially at low and moderate levels of salinity, while it markedly retarded the accumulation of Na⁺ in the different organs of sorghum cultivars. Abbreviations: LA: Leaf area, DM: Dry mass, IAA: Indole acetic acid, RWC: Relative water content,TI: Tolerance index