Gas Composition in the Internal Air Spaces of Deepwater Rice in Relation to Growth Induced by Submergence

Deepwater rice (Oryza sativa L.) responds to partial submergence by rapid elongation of the internodes. The elongation of intact plants was measured during 7 days of submergence using angular transducers. Growth rates began to increase within 3 to 4 h upon submergence and peaked after 3 days; they were higher in the light than in the dark. The composition of the gas phase in the internodal air spaces was analyzed at different times after partial submergence. In the light, the O2 concentrations in internal gas compartments of intact plants dropped initially, but recovered largely within 90 min. During longer periods of submergence, the internal O2 concentration followed a diurnal pattern with O2 levels being lower during the night than during the day. An O2 gradient was found from the apical parts of the plant near the water surface to the basal parts of the shoot. Concentrations of CO2 changed with a pattern inverse to that of the O2concentrations. The level of ethylene in the internodal lacunae increased upon submergence and reached 1μl-liter−1 after 72 h. It was higher at the end of the dark period than during the day. The gas exchange of the submerged parts of the plant seems to depend mainly on mass flow of air from the atmosphere to the root system. Mass flow through this pathway was determined at different pressure gradients and was compared to the intake of air into plants as established in earlier investigations. A significant decrease in gas flow resistance found during 7 days of submergence may constitute a long-term adaptation to flooding.