Effect of leaf age and position on light-saturated CO2 assimilation rate, photosynthetic capacity, and stomatal conductance in rubber trees

Shoots of the tropical latex-producing tree Hevea brasiliensis (rubber tree) grow according to a periodic pattern, producing four to five whorls of leaves per year. All leaves in the same whorl were considered to be in the same leaf-age class, in order to assess the evolution of photosynthesis with leaf age in three clones of rubber trees, in a plantation in eastern Thailand. Light-saturated CO2 assimilation rate (A (max)) decreased more with leaf age than did photosynthetic capacity (maximal rate of carboxylation, V (cmax) , and maximum rate of electron transport, J (max)), which was estimated by fitting a biochemical photosynthesis model to the CO2-response curves. Nitrogen-use efficiency (A (max)/N-a, N-a is nitrogen content per leaf area) decreased also with leaf age, whereas J (max) and V (cmax) did not correlate with N (a). Although measurements were performed during the rainy season, the leaf gas exchange parameter that showed the best correlation with A (max) was stomatal conductance (g (s)). An asymptotic function was fitted to the A (max)-g (s) relationship, with R (2) = 0.85. A (max), V (cmax), J (max) and g (s) varied more among different whorls in the same clone than among different clones in the same whorl. We concluded that leaf whorl was an appropriate parameter to characterize leaves for the purpose of modelling canopy photosynthesis in field-grown rubber trees, and that stomatal conductance was the most important variable explaining changes in A (max) with leaf age in rubber trees.