Effects of growth temperature on the responses of ribulose-1,5-bisphosphate carboxylase, electron transport components, and sucrose synthesis enzymes to leaf nitrogen in rice, and their relationships to photosynthesis

Effects of growth temperature on the photosynthetic gas-exchange rates and their underlying biochemical properties were examined in young, fully expanded leaves of rice (Oryza sativa L). The plants were grown hydroponically under day/night temperature regimes of 18/15° C, 23/18° C, and 30/23° C and all photosynthetic measurements were made at a leaf temperature of 25°C and an irradiance of 1800 μmol quanta m^-2 s^-1. Growth temperature affected the photosynthetic CO₂ response curve. The relative ratio of the initial slope to the CO₂-saturated photosynthesis increased with rising growth temperature. This was caused mainly by an increase in CO₂-limited photosynthesis for a given leaf nitrogen content with rising growth temperature. However, there was no difference in ribulose-1,5-bisphosphate carboxylase (Rubisco) content at any given leaf nitrogen content among temperature treatments. In addition, the activation state and catalytic turnover rate of Rubisco were not affected by growth temperature. The increase in CO₂-limited photosynthesis with rising growth temperature was the result of an increase in the CO₂ transfer conductance between the intercellular airspaces and the carboxylation sites. The amounts of total chlorophyll and light-harvesting chlorophyll a/b protein II increased for the same leaf nitrogen content with rising growth temperature, but the amounts of cytochrome f and coupling factor 1 and the activities of cytosolic fructose- 1,6-bisphosphatase and sucrose-phosphate synthase were the same between plants grown at 23/18°C and those grown at 30/23°C. Similarly, CO₂-saturated photosynthesis was not different for the same leaf nitrogen content between these treatments. For the 18/15°C-grown plants, a slight decrease in the amounts of cytochrome f and coupling factor 1 and an increase in the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were found, but these were not reflected in CO₂-saturated photosynthesis.