Background
The authors write that "excess light limits photosynthesis by photoinhibition, resulting in reduced carbon gain and also causing photo-damage (Oquist and Huner, 1993; Pastenes et al., 2003; Allakhverdiev and Murata, 2004; Nishiyama et al., 2006)," and they say that "plants grown in tropical climates usually experience significantly high irradiance leading to the strong midday depression of photosynthesis (Hymus et al., 2001)."

What was done
Utilizing two open-top chambers in the Botanical Gardens of the University of Hyderabad, India -- each of which contained four six-month-old specimens of the fast-growing tropical Gmelina arborea tree, which they maintained at optimum moisture and nutrient levels -- Rasineni et al. measured several plant physiological properties and processes related to leaf photosynthesis and photosystem II (PSII) photochemistry and photoinhibition at both ambient and elevated CO2 concentrations (360 and 460 ppm, respectively), working with "well-expanded and light-exposed leaves randomly chosen from the upper half of the plant canopy."

What was learned
The three Indian scientists found that there were no significant differences in CO2 assimilation rates between the ambient and elevated CO2 grown plants during early morning hours; but they discovered that, thereafter, "photosynthesis typically maximized between 0900 hours and 1000 hours in both ambient and elevated CO2-grown plants," which experienced net photosynthetic rates of 20 and 32.5 µmol/m²/s, respectively, for a stunning CO2-induced enhancement of 62%, which for the more standard CO2 enrichment of 300 ppm would be roughly equivalent to an enhancement of 180%. Subsequently, during the following midday period of 1100-1300 hours, the net photosynthesis rate was still significantly enhanced by about 37% (roughly equivalent to a 300-ppm induced increase of more than 100%) in the elevated CO2 treatment, after which the difference between the net photosynthetic rates of the two CO2 treatments once again became insignificant.

What it means
Noting that the "elevated CO2 treatment mitigated PSII-photoinhibition through enhanced electron transport rates and through efficient biochemical reactions in leaves of G. arborea," Rasineni et al. conclude that their data "demonstrate that future increases in atmospheric CO2 may have positive effects on photochemical efficiency in fast growing tropical tree species," allowing them to take great advantage of the high-light midday period of potential maximum growth in earth's tropical regions.

References
Allakhverdiev, S.I. and Murata, N. 2004. Environmental stress inhibits the synthesis de novo of proteins involved in the photodamage-repair cycle of photosystem II in Synechocystis sp. PCC 6803. Biochimica et Biophysica Acta 1657: 23-32.

Hymus, G.J., Baker, N.R. and Long, S.P. 2001. Growth in elevated CO2 can both increase and decrease photochemistry and photoinhibition of photosynthesis in a predictable manner. Dactylis glomerata growth in two levels of nitrogen nutrition. Plant Physiology 127: 1204-1211.

Nishiyama, Y., Allakhverdiev, S.I. and Murata, N. 2006. A new paradigm for the action of reactive oxygen species in the photoinhibition of photosystem II. Biochimica et Biophysica Acta 1757: 742-749.

Oquist, G. and Huner, N.P.A. 1993. Cold-hardening-induced resistance to photoinhibition of photosynthesis in winter rye is dependent upon an increased capacity for photosynthesis. Planta 189: 150-156.

Pastenes, C., Santa-Maria, E., Infante, R. and Franck, N. 2003. Domestication of the Chilean guava (Ugni molinae Turcz.) a forest understory shrub, must consider light intensity. Scientia Horticulturae 98: 71-84.