Effects of Rubisco kinetics and Rubisco activation state on the temperature dependence of the photosynthetic rate in spinach leaves from contrasting growth temperatures

Recently, several studies reported that the optimum temperature for the initial slope [IS(C-i )] of the light-saturated photosynthetic rate (A) versus intercellular CO2 concentration (C-i) curve changed, depending on the growth temperature. However, few studies compare IS(C-i) with ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) properties. Here, we assessed Rubisco activation state and in vitro Rubisco kinetics, the main determinants of IS(C-i), in spinach leaves grown at 30/25 [high temperature (HT)] and 15/10 degrees C [low temperature (LT)]. We measured Rubisco activation state and A at a CO2 concentration of 360 mu L L-1 (A(360)) at various temperatures. In both HT and LT leaves, the Rubisco activation state decreased with increasing temperatures above the optimum temperatures for A(360), while the activation state remained high at lower temperatures. To compare Rubisco characteristics, temperature dependences of the maximum rate of ribulose 1,5-bisphosphate (RuBP) carboxylation (V-cmax), specificity factor (S-c/o) and thermal stability were examined. We also examined V-cmax and thermal stability in the leaves that were transferred from HT to LT conditions and were subsequently kept under LT conditions for 2 weeks (HL). Rubisco purified from HT, LT and HL leaves are called HT, LT and HL Rubisco, respectively. Thermal stabilities of LT and HL Rubisco were similar and lower than that of HT Rubisco. Both V-cmax and S-c/o in LT Rubisco were higher than those of HT Rubisco at low temperatures, while these were lower at high temperatures. V-cmax in HL Rubisco were similar to those of LT Rubisco at low temperatures, and to those of HT Rubisco at high temperatures. The predicted photosynthetic rates, taking account of the Rubisco kinetics and the Rubisco activation state, agreed well with A(360) in both HT and LT leaves. This study suggests that photosynthetic performance is largely determined by the Rubisco kinetics at low temperature and by Rubisco Kinetics and the Rubisco activation state at high temperature.