Hormonal Regulation of Elongation in Floating Rice During Submergence

Enhanced ethylene-producing capacity of young seedlings of floating and deepwater rice (Oryza sativa L.) during submergence has been related to increased seedling elongation via leaf sheath/leaf blade. In this study we present evidence for modulation of elongation by hormones. Floating rice cuitivars ‘TCA 4’, ‘TCA 177’, and ‘TCA 212’ were used. Effects of various hormones and chemicals on elongation and ethylene-producing capacity were determined during complete submergence of seedlings in 122- ✕ 6-cm glass cylinders. Enhancement in ethylene-producing capacity by submergence was completely suppressed by 0.05 mM aminoethoxyvinylglycine (AVG), indicating that the enhancement depended on the synthesis of 1- aminocyclopropane-l-carboxylic acid (ACC). Submergence-induced elongation was partially inhibited by 0.05 mM AVG in 24 h and reversed to some extent by 0.1 mM ACC or 0.05 mM ethephon (2-chlorethane phosphonic acid). Increase in ethylene-producing capacity during submergence was also inhibited by 1 mM Co²⁺, which had no effect on seedling elongation. Gibberellin (GA) biosynthesis inhibitor, tetcyclacis [5-(4-chlorophenyl)-3,4,5,9,10-pentaazatetracyclo-5,4,10².⁶, 0⁸.¹¹ dodeca-3, 9-diene] (TCY), strongly inhibited the increase in seedling elongation of TCA 177 without affecting ethylene- producing capacity. The effect of TCY on elongation was reversed by 0.1 mM GA₄₊₇ (gibberellin A4 + A7). Abscisic acid (ABA) inhibited the ethylene-producing capacity and the effect was reversed by kinetin. Application of GA₄₊₇ plus kinetin caused a 60% reversal of ABA inhibited elongation. Nonsubmerged floating (TCA 177) and deepwater (‘BR 118-3B-17’) rice elongated to a greater extent than the nonelongating types when sprayed with 0.1 mM GA₄₊₇. Thus, GA may influence elongation more directly than other hormones. Contribution from the IRRI, Los Baños, Philippines.