What was done
The authors grew eight-month-old saplings of two clones (PB 235 and RRII 105) of Hevea brasiliensis, a tree species indigenous to humid tropical climates, in 0.75-m³ polybags filled with garden soil during the two coldest months of the year in warm and cool regions of peninsular India having mean minimum temperatures of 22.0 and 10.0°C, respectively, while they measured a number of photosynthetic properties of the trees' leaves.

What was learned
Alam et al. report that "irrespective of the differences in growth environment, higher CO2 in the ambient air during measurements improved the photochemical efficiency of the plants as reflected in higher net photosynthetic rate, effective photosystem 2 quantum efficiency, and photochemical quenching."

What it means
The Indian scientists say their results imply that (1) "inhibition in photochemical efficiency due to sub-optimal temperatures could be improved considerably with higher CO2 concentration by making more CO2 available to photosynthesis," (2) "plants grown under elevated CO2 will have decreased intrinsic oxidative stress," and (3) "higher CO2 may lead to an increased metabolic flexibility to encounter such stress," citing, in this regard, the complementary work of Polle et al. (1996), Schwanz et al. (1996), Devakumar and Jacob (1997), Azevedo et al. (1998), Morison and Lawlor (1999), and Schwanz and Polle (2001).

References
Azevedo, R.A., Alas, R.M., Smith, R.J. and Lea, P.J.  1998.  Response of antioxidant enzymes to transfer from elevated carbon dioxide to air and ozone fumigation, in leaves and roots of wild-type and a catalase-deficient mutant of barley.  Physiologia Plantarum 104: 280-292.

Devakumar, A.S. and Jacob, J.  1997.  Growth and development of plants in a CO2-enriched world.  Agro's Annual Review of Plant Physiology, pp. 281-317.

Morison, J.I.L. and Lawlor, D.W.  1999.  Interactions between increasing CO2 concentration and temperature on plant growth.  Plant, Cell and Environment 22: 659-682.

Polle, A., Eiblmeir, M., Sheppard, L. and Murray, M.  1996.  Responses of antioxidant enzymes to elevated CO2 in leaves of beech (Fagus sylvatica L.) seedlings grown under a range of nutrient regimes.  Plant, Cell and Environment 20: 1317-1321.

Schwanz, P., Kimball, B.A., Idso, S.B., Hendrix, D.L. and Polle, A.  1996.  Antioxidants in sun and shade leaves of sour orange trees (Citrus aurantium) after long-term acclimation to elevated CO2.  Journal of Experimental Botany 47: 1941-1950.

Schwanz, P. and Polle, A.  2001.  Differential stress responses of antioxidative systems to drought in pendunculate oak (Quercus robur) and maritime pine (Pinus pinaster) grown under high CO2 concentrations.  Journal of Experimental Botany 52: 133-143.