Employing the non-invasive techniques of infra-red gas analysis and pulse amplitude modulated chlorophyll fluorometry, we determined the partitioning of photosynthetic electrons between photosynthetic carbon reduction and other reductive processes resulting in the formation of active oxygen species (AOS) in intact green leaves. This we studied in plant species that are adapted to two different agro-climatic conditions, namely the warm plains (76°36'E, 9°32'N) and the cool mountains (1 600 m a.s.l.) in the south Indian state of Kerala. Ground frost and low temperature were more harmful to those species adapted to the warm plains than the ones adapted to the cool mountains. Exposure to low temperature decreased leaf photosynthetic carbon assimilation rates and quantum yield of photochemical activity in species naturally adapted to the warm plains. High irradiances further aggravated the harmful effects of low temperature stress possibly by overproducing AOS. This resulted in severe peroxidative damage as inferred by the accumulation of malondialdehyde (MDA) in the leaves. and B. Alam, J. Jacob.