Low temperature stress modifies the photochemical efficiency of a tropical tree species Hevea brasiliensis:: effects of varying concentration of CO2 and photon flux density

Two clones of Hevea brasiliensis (RRII 105 and PB 235) were grown for one year in two distinct agroclimatic locations (warmer and colder, W and C) in peninsular India. We simultaneously measured gas exchange and chlorophyll (Chl) fluorescence on fully mature intact leaves at different photosynthetic photon flux densities (PPFDs) and ambient CO2 concentrations (C-a) and at constant ambient O-2 concentration (21 %). Net photosynthetic rate (P-N), apparent quantum yield for CO2 assimilation (Phi(c)), in vivo carboxylation efficiency (CE), and photosystem 2 quantum yield (Phi(PS2)) were low in plants grown in C climate and these reductions were more predominant in RRII 105 than in PB 235 which was also reflected in their growth. We estimated in these clones the partitioning of photosynthetic electrons between CO2 reduction (J(A)) and processes other than CO2 reduction (J*) at low and high PPFDs and C-a. At high C-a (700 mu mol mol(-1)) most of the photosynthetic electrons were used for CO2 assimilation and negligible amount went for other processes when PPFD was low (200-300 mu mol m(-2) s(-1)) both in the C and W climates. But at high PPFD (900-1 100 mu mol m(-2) s(-1)), J* was appreciably high even at a high C-a. Hence at normal ambient C-a and high irradiance, electrons can be generated in the photosynthetic apparatus far in excess of what can be safely utilised for photosynthetic CO2 reduction. However, at high C-a there was increased diversion of electrons to photosynthetic CO2 reduction which resulted in improved photosynthetic parameters even in plants grown in C climate.