Effects of internal conductance and Rubisco on the optimum temperature for leaf photosynthesis in Fallopia japonica growing at different altitudes

Abstract

AbstractTo investigate mechanisms of adjustment of the optimum temperature for leaf photosynthesis in alpine plants, we compared the temperature responses of photosynthesis, internal conductance (gi), and the amounts of activated Rubisco (ε) in two Fallopia japonica populations growing at elevations of 100 m (K0100), and 2250 m (F2250). There was an obvious difference in photosynthesis at high temperatures between the two populations, although there was no significant difference in the CO2/O2 specificity of Rubisco. Optimum temperatures for photosynthesis were 25 and 30 °C in F2250 and K0100, respectively. The temperature response of ε was similar to that of photosynthesis. The mean values of ε decreased 25 % (F2250) and 24 % (K0100), for temperatures 5 °C above the optimum for photosynthesis. In contrast, gi exponentially increased with increasing temperature in both populations. There was no significant difference in gi between populations for any given temperature. In both populations, there were no changes in CO2 concentrations at the Rubisco active site, when temperatures were above the photosynthetic optimum temperature. This clearly shows that photosynthetic optimum temperatures were not affected by photosynthetic limitation of CO2 diffusing from intercellular air spaces to Rubisco. Furthermore, the atmospheric pressure had a minor effect on the temperature response of photosynthesis. Thus, the decrease in ε in response to elevated temperatures reduced the photosynthetic optimum temperature in highland population of F. japonica, which was adjusted to the habitat.