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

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Abstract
To investigate mechanisms of adjustment of the optimum temperature for leaf photosynthesis in alpine plants, we compared the temperature responses of photosynthesis, internal conductance ( g i ), 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 CO 2 /O 2 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, g i exponentially increased with increasing temperature in both populations. There was no significant difference in g i between populations for any given temperature. In both populations, there were no changes in CO 2 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 CO 2 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.

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