Stress-relaxation Analysis of Submerged and Air-grown Rice Coleoptiles: Correlations with Cell Wall Biosynthesis and Growth

The growth rate of rice coleoptiles grown under water was decreased by exposure to air. In parameters of the stress-relaxation of cell walls, water-grown coleoptiles had a larger pre-extension time (Tp) and a smaller stress-relaxation rate (b) as compared with those exposed to air. Tp and b were significantly correlated with the growth rate of coleoptiles grown under different conditions. The amounts of cell wall components, such as hemicellulose, cellulose, diferulic acid (DFA) and ferulic acid (FA), in air-exposed coleoptiles increased rapidly as compared with those in submerged ones. When 7-h air-exposed seedlings were submerged again, the growth rate of re-submerged coleoptiles was still lower as compared with that of continuously submerged ones during the subsequent incubation under water. The decrease in Tp, the increase in b, and the increases in the amounts of those wall components by air-exposure were not completely recovered even when coleoptiles were re-submerged after 7h. There were close correlations between the amounts of cell wall components per unit length of coleoptile and Tp and b. These results indicate that Tp and b can represent the ability of rice coleoptiles to grow under different conditions and that those wall components are involved in determining Tp and b of cell walls.