Hevea brasiliensis trees grown in two distinct agroclimatic regions (one experiencing drought and high temperature stress in summer and the other experiencing cold stresss in winter, both concomitant with high PPDF) showed severe inhibition of photosynthesis during the unfavourable seasons. The degree of inhibition was so large at high light intensity that the upper canopy leaves of the stressed plants exposed to high PPFD fixed little carbon for most of the day. Photosynthesis rates were higher at lower PPFD, suggesting that the shaded leaves present inside the canopy were important in the overall carbon budget of the stressed plants. Inhibition of photosynthesis due to high PPFD was also evident in the decreased quantum yield of CO2 assimilation and in vivo PSII activity was higher in the drought and cold stressed leaves than in their respective control leaves as indicated by the reductions in the maximum potential quantum yield (measured as the ration of variable to maximum chlorophyll fluorescence, Fv/Fm in the dark adapted state) and the effective quantum yield of PSII,(PSII, measured at a PPFD of 500mmol/m2/s). This corroborated the high PPFD-dependent inhibition of photosynthesis observed in mature Hevea trees experiencing drought and cold stresses in the field. The activities of several anti-oxidant enzymes such as superoxide dismutase, peroxidase, catalase, ascorbate peroxidase and concentrations of antioxidants like total glutathione in the leaves indicated possible oxidative stress in mature Hevea trees experiencing drought and cold temperature stresses concominant with high PPFD in the field.