Effects of indole-3-acetic acid and zinc on the growth, osmotic potential and soluble carbon and nitrogen components of soybean plants growing under water deficit

Three-week old soybean (Glycine max) plants were subjected to a factorial combination of four regimes of soil matric water potential (psi(m) = - 0. 03, - 0.5, - 1.0 and - 1.5 MPa), two levels of supplementary Zn (O and 20 mgl(-1)) and two levels of foliar IAA application (O and 10 mgl(-1)). Under control conditions (no Zn, no IAA), increasing soil drying progressively retarded shoot and root growth (length and dry mass production), reduced leaf relative water content (RWC) and decreased the contents of chlorophyll (Chl) and shoot soluble sugars (SS), but increased soluble sugar content of roots and lowered osmotic water potential of shoots and roots (osmotic adjustment). Total free amino acid (TAA) content increased in shoots but decreased in roots whereas contents of soluble proteins (SP) decreased in shoots and roots. The effect of water stress was statistically significant (p < 0.05) and had a major effect las indicated by eta(2) values) on leaf RWC, shoot and root dry masses and osmo tic potential. Supplementary Zn improved root growth at all levels of stress and shoot growth under severe stress. Improvement of growth was positively correlated with the internal tissue Zn concentrations (r - 0.91 and 0.86 for shoot and 0.94 and 0.82 for root length and dry mass respectively). Exogenous IAA raised (p < 0.05) RWC, Chi, DM (slightly), root SS, and SP, whereas shoot TAA was lowered. Effects on root TAA and shoot SS were more complex: they were lowered at zero stress and raised under severe stress. IAA and Zn in combination had additive effects on Chi, growth and osmotic potential, but their combined effects on SP and TAA were more complex. It is concluded that the treatment of soybean plants grown under conditions of low soil water potentials and Zn deficiency with Zn and IAA solutions counteracted the deleterious effects of stress, especially at high stress levels, and helped stressed plants to grow successfully under these adverse unfavourable conditions. (C) 2000 Academic Press.