Diel O2 Dynamics in Partially and Completely Submerged Deepwater Rice: Leaf Gas Films Enhance Internodal O2 Status, Influence Gene Expression and Accelerate Stem Elongation for 'Snorkelling' during Submergence

Deepwater rice has a remarkable shoot elongation response to partial submergence. Shoot elongation to maintain air-contact enables 'snorkelling' of O-2 to submerged organs. Previous research has focused on partial submergence of deepwater rice. We tested the hypothesis that leaf gas films enhance internode O-2 status and stem elongation of deepwater rice when completely submerged. Diel patterns of O-2 partial pressure (pO(2)) were measured in internodes of deepwater rice when partially or completely submerged, and with or without gas films on leaves, for the completely submerged plants. We also took measurements for paddy rice. Deepwater rice elongated during complete submergence and the shoot tops emerged. Leaf gas films improved O-2 entry during the night, preventing anoxia in stems, which is of importance for elongation of the submerged shoots. Expressions of O-2 deprivation inducible genes were up-regulated in completely submerged plants during the night, and more so when gas films were removed from the leaves. Diel O-2 dynamics showed similar patterns in paddy and deepwater rice. We demonstrated that shoot tops in air enabled 'snorkelling' and increased O-2 in internodes of both rice ecotypes; however, 'snorkelling' was achieved only by rapid shoot elongation by deepwater rice, but not by paddy rice.