Effect of Zinc Nutritional Status on Growth, Protein Metabolism and Levels of Indole-3-acetic Acid and other Phytohormones in Bean (Phaseolus vulgarisL.)

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Abstract
Bean (Phaseolus vulgaris L. var. Prelude) plants were grown for 17 d under controlled environmental conditions with varied Zn supply in the nutrient solution. The concentrations of amino acids; indole-3-acetic acid, IAA; abscisic acid, ABA; isopentenyl adenine, I-Ade; isopentenyl adenosine, I-Ado; zeatin, Z; and zeatin riboside, ZR were determined in various shoot fractions. The growth of plants, especially shoot growth, was severely depressed under conditions of Zn deficiency. Simultaneously, concentrations of soluble protein and chlorophyll decreased, whereas amino acid concentrations increased several-fold. In the Zn-deficient plants, the level of IAA in the shoot tips and young leaves decreased to about 50% of that in Zn-sufficient plants. A similar decrease occurred in the ABA levels of shoot tips. In contrast, Zn deficiency was without effect on cytokinin levels in the leaves. Re-supply of Zn to the deficient plants for up to 96 h significantly increased shoot growth, soluble protein, and IAA levels up to the values of Zn-sufficient plants. Simultaneously, the concentration of amino acids dropped to low levels. The effect of Zn nutritional status on the tryptophan level was parallel to that of most of the other amino acids. The results confirm the role of Zn in protein synthesis and demonstrate that the decrease in IAA level in Zn-deficient plants is not brought about by impaired synthesis of tryptophan. It is also unlikely that in Zn-deficient plants the conversion of tryptophan to IAA is specifically inhibited.