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Response of two important tropical tree species of Meliaceae (Azadirachta indica -neem and Melia dubiamelia) to elevated levels of CO2 (600 and 900 ppm) under simulated temperature and moisture regimes in Automated Open Top Chambers was studied. Growth, biochemical changes, antioxidant property and gas exchange parameters were estimated. The results indicate that A. indica is expected to acclimatize under elevated CO2 concentrations whereas M. dubia was observed to be a species sensitive to elevated CO2 concentrations, affecting the photosynthetic machinery, stomatal conductance and transpiration and a subsequent decrease in carbohydrates, proteins, sugars, amino acids and phenolics. The short-term and long-term responses with respect to stomatal conductance and transpiration rates were higher in neem than melia. Thus, a positive response of neem to increased CO2 concentrations is a good indication for its future establishment in potentially changed climatic conditions.

Global climate change the looming environmental threat is mainly due to the increase in atmospheric CO 2 levels, which was increasing earlier by about 1.55 ppm per year and currently by 2.76 ppm per year. Thus, CO 2 concentration has reached 400.16 ppm in 2015. To understand the response of various tropical tree species to such an elevated CO 2 , experiments were conducted in Automated Open Top Chambers (AOTC) facility at Institute of Forest Genetics and Tree Breeding (IFGTB), Coimbatore (India). The results of initial studies indicated the scope for exploring intraspecific variation in response of tropical trees to elevated CO 2 . Subsequently, experiments were carried out to assess intra-specific variation in Neem (Azadirachta indica). The selected phenotypes of Neem (varieties or clones) were exposed to four treatments viz., i) CO 2 of 600 ppm, ii) CO 2 of 900 ppm, iii) chamber control-without any CO 2 enrichment and iv) ambient conditions. The parameters studied were shoot length, root length, dry matter accumulation in shoot and root.