Regulation of growth in stem sections of deep-water rice

Submergence in water greatly stimulates internodal elongation in excised stem sections of deep-water rice (Oryza sativa L. cv. Habiganj Aman II) and inhibits growth of leaf blades and leaf sheaths. The highest rates of internodal growth have been observed in continuous light. Very little growth occurs in submerged sections kept in darkness or incubated under N₂ in the light. The effect of submergence on the growth of deep-water rice is, at least in part, mediated by C₂H₄, which accumulates in the air spaces of submerged sections. This accumulation results from increased C₂H₄ synthesis in the internodes of submerged sections and reduced diffusion of C₂H₄ from the tissue into the water. Increased C₂H₄ levels accelerate internodal elongation and inhibit the growth of leaves. Compounds capable of changing the rate of C₂H₄ synthesis, namely aminoethoxyvinylglycine, an inhibitor of C₂H₄ synthesis, and 1-aminocyclopropane-1-carboxylic acid, the immediate, precursor of C₂H₄, have opposite effects on growth of internodes and leaves. The enhancement of internodal elongation by C₂H₄ is particularly pronounced in an atmosphere of high CO₂ and low O₂. The increase in C₂H₄ synthesis in internodes of submerged sections is primarily triggered by reduced atmospheric concentrations of O₂. The rate of C₂H₄ evolution by internodes isolated from stem sections and incubated in an atmosphere of low O₂ is up to four times greater than that of isolated internodes incubated in air. In contrast, C₂H₄ evolution from the leaves is reduced under hypoxic conditions. The effect of submergence on growth of stem sections of deep-water rice can be mimicked by exposing non-submerged sections to a gas mixture which is similar to the gaseous atmosphere in the internodal lacunae of submerged sections, namely 3% O₂, 6% CO₂, 91% N₂ (by vol.) and 1 l l^-1 C₂H₄. Our results indicate that growth responses obtained with isolated rice stem sections are similar to those of intact deep-water rice plants.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - AVG aminoethoxyvinylglycine