Changes in the thermal dissipation and the electron flow in the water-water cycle in rice grown under conditions of physiologically low temperature

Effects of low temperature on chlorophyll (Chl) fluorescence, gas exchange rate, the amounts of xanthophyll cycle pigments (Xp) and the activities of several antioxidant enzymes were examined in the 8th leaf of two rice (Oryza sativa L.) cultivars (japonica and indica types) and rbcS antisense rice. All plants were grown hydroponically at 25/20°C (day/night), and then exposed to 20/17°C (day/ night) after full expansion of the 8th leaf, or exposed to either 20/17°C or 15/13°C (day/night) during the expansion of the 8th leaf. All plants exposed to low temperatures showed a decrease in CO₂ assimilation rate without photoinhibition, and increases in the fraction of thermal dissipation in PSII, and in the electron flux through the water-water cycle (WWC) were observed. Although the increase of thermal dissipation was associated with increases in the ratio of carotenoids to Chl, the ratio of Xp to carotenoids and the de-epoxidation state of Xp, the increase of the electron flux of WWC was not accompanied by an increase in the activities of antioxidant enzymes. Such photoprotective responses did not differ between during and after full expansion of the leaf, and did not differ among the three genotypes. Quantitative analyses on the dissipation of excess light energy showed that thermal dissipation makes a larger contribution than WWC. Thus, although low temperature led to a decrease in CO₂ assimilation, rice potentially coped with the excess light energy by increasing the thermal dissipation and the electron flux of WWC under low temperature irrespective of leaf development and genotypes.