Anoxia promotes gravitropic curvature in rice pulvini but inhibits it in wheat and oat pulvini

Gravitropic curvature of pulvini of wheat and oat stem segments gradually declined with decreasing atmospheric O₂ concentration and was almost completely blocked under anoxia, whereas that of rice stem segments was enhanced under hypoxia and anoxia. Anoxia substantially increased the ethanol content in pulvini of gravistimulated stem segments in rice, wheat and oat, but the ethanol content showed no marked difference between rice pulvini and wheat and oat pulvini. The concentrations of exogenous ethanol and acetaldehyde required to inhibit the gravitropic curvature of pulvini were significantly higher in rice segments than in wheat and oat segments. However, in all three species, the concentrations of ethanol and acetaldehyde required to completely inhibit curvature were several-fold higher than the endogenous levels that accumulated in pulvini gravistimulated in N₂. The pulvini of rice segments gravistimulated in N₂ did not contain much more ATP than those of wheat or oat segments gravistimulated in N₂. When applied unilaterally to the pulvini of vertically oriented stem segments incubated in N₂, indole-3-acetic acid induced bending in rice stem segments but not in wheat and oat stem segments. Transference of graviresponsive pulvini of rice, as well as those of wheat and oat, from aerobic conditions to anaerobic conditions led to cessation of gravitropic curvature within several minutes, but subsequently only gravitropic curvature of anoxic rice pulvini was completely recovered within 2 h. A large portion of this recovery was blocked by cordycepin, a transcription inhibitor. These results suggested that anoxia-induced expression of any gene or genes enables rice pulvini to respond to gravistimulation under anaerobic conditions, and that such a gene or genes might be unrelated to ethanol fermentation and ATP production in anaerobic conditions.