Effects of O2 Concentration on Rice Seedlings

Abstract

The ability of rice, wheat [Triticum sativum cv. MEC] and oat [Avena sativa] seedlings to germinate and grow as the O2 concentration was lowered to zero was compared. The germination of rice was completely unaffected by O2 supply, whereas that of oats and wheat was strongly retarded at levels < 5% O2. In contrast to the coleoptiles of oats and wheat and to roots of all 3 spp. where growth was progressively diminished as the O2 concentration was lowered, that of the rice coleoptile was progressively increased. The dry weight and content of protein, sugars and cellulose were all depressed in the rice coleoptile in anoxia and the levels of several respiratory enzymes, particularly those of mitochondria, were also much lower than those of the coleoptiles grown in air. In 1% O2, the growth of the rice coleoptile was similar to that in air. The effect of ethanol concentration on germination and growth of rice was measured. Coleoptile growth was reduced when the ethanol concentration exceeded 40 mM and root growth was somewhat more sensitive. Coleoptiles of all 3 spp. grown in air were transferred to N2 and chemical accumulation was measured over 24 h. The rate of ethanol accumulation in oats was close to that in rice and in all 3 spp. the amounts of ethanol lost to the surrounding medium were those expected from simple diffusion from the tissue. The ability of the rice coleoptile to grow in anoxia is apparently not due to a particularly low rate of ethanol formation or to unusual ethanol tolerance. Any explanation of the success of rice in anoxia must encompass the much lower rate of ATP synthesis than that in air and account for the biochemical deficiencies of the coleoptile.