CO₂ Dependence of Gas Exchanges in Rice Leaves as Affected by Temperature, Light and Vapor Saturation Deficit

Abstract

Rice plants were grown in a natural light phytotron. When plants had 12-13 leaves on the main stem, they were transferred into an artificially illuminated phytotron and acclimated a whole day to measurement conditions of net photosynthesis and transpiration. The gas exchange rates of attached, uppermost fully-expanded leaves were measured under combinations of ambient CO₂ concentration (5-500μmol mol^-1), leaf temperature (20, 25, 35°C), light intensity (400, 800μmol m^-2 s^-1 PPFD) and vapor saturation deficit (1.1-1.5, 1.6-2.5 kPa) . The maximum net photosynthetic rate with elevating CO₂ was obtained under conditions of high temperature, high light and low vapor saturation deficit. The water use efficiency of leaf expressed as the ratio of net photosynthetic rate to transpiration rate increased with elevating CO₂ concentration and the maximum values were obtained at 25°C under low vapor saturation deficit and at 20°C under high vapor saturation deficit, respectively, irrespective of light levels. The double-reciprocal plots of net photosynthetic rate and intercellular CO₂ concentration showed that the highest temperature (35°C) was effective to promote CO₂ fixation by mesophyll cells under high light conditions. Possible interactions among temperature, light and vapor saturation deficit on the gas exchanges of rice leaves under elevated CO₂ conditions were discussed.