ELONGATION GROWTH IN THE ABSENCE OF OXYGEN - THE RICE COLEOPTILE

Rice, one of the few plant species adapted to growth in wetland condit ions, is able to germinate in waterlogged soils promoting only the gro wth of a white coleoptile in order to reach the surface of the water, contact the atmosphere, and transfer oxygen to the seed, allowing subs equent growth of the radicle and leaf. In the anoxic cells of rice col eoptiles, an efficient alcoholic fermentation allows an elevated energ y charge to be maintained. Significant RNA and protein syntheses inclu ding phosphorylation and glycosylation occur too. The cytoplasmic pH i s maintained at a level far from acidosis. The anoxic growth of rice c oleoptiles, essentially an elongation growth, is sustained by a high t urgor pressure, with free amino acids and potassium as main components . Among the metabolic processes involved in the regulation of the elon gation of rice coleoptiles, a crucial role is played by amino acid met abolism and the accumulation of putrescine, which is able to stimulate plasmalemma ATPase activity. Anaerobic elongation is also stimulated in the presence of 20% CO2 in the growth medium, inhibited by light an d abscisic acid, unaffected by ethylene, and slightly promoted by auxi n. The role of both metabolites and hormones along with environmental factors in maintaining cellular homeostasis and coleoptile elongation are reconsidered and discussed in Light of new data.