Photosynthetic capacity and temperature responses of photosynthesis of rubber trees (Hevea brasiliensis Mull. Arg.) acclimate to changes in ambient temperatures

The aim of this study was to assess the temperature response of photosynthesis in rubber trees (Hevea brasiliensis Mull. Arg.) to provide data for process-based growth modeling, and to test whether photosynthetic capacity and temperature response of photosynthesis acclimates to changes in ambient temperature. Net CO2 assimilation rate (A) was measured in rubber saplings grown in a nursery or in growth chambers at 18 and 28A degrees C. The temperature response of A was measured from 9 to 45A degrees C and the data were fitted to an empirical model. Photosynthetic capacity (maximal carboxylation rate, V (cmax), and maximal light driven electron flux, J (max)) of plants acclimated to 18 and 28A degrees C were estimated by fitting a biochemical photosynthesis model to the CO2 response curves (A-C (i) curves) at six temperatures: 15, 22, 28, 32, 36 and 40A degrees C. The optimal temperature for A (T (opt)) was much lower in plants grown at 18A degrees C compared to 28A degrees C and nursery. Net CO2 assimilation rate at optimal temperature (A (opt)), V (cmax) and J (max) at a reference temperature of 25A degrees C (V (cmax25) and J (max25)) as well as activation energy of V (cmax) and J (max) (E (aV) and E (aJ)) decreased in individuals acclimated to 18A degrees C. The optimal temperature for V (cmax) and J (max) could not be clearly defined from our response curves, as they always were above 36A degrees C and not far from 40A degrees C. The ratio J (max25)/V (cmax25) was larger in plants acclimated to 18A degrees C. Less nitrogen was present and photosynthetic nitrogen use efficiency (V (cmax25)/N (a)) was smaller in leaves acclimated to 18A degrees C. These results indicate that rubber saplings acclimated their photosynthetic characteristics in response to growth temperature, and that higher temperatures resulted in an enhanced photosynthetic capacity in the leaves, as well as larger activation energy for photosynthesis.