Background
The authors write that "in natural environments, almost all leaves experience large temporal variations in photosynthetic photon flux density (PFD)," citing Chazdon (1988), Tang et al. (1988), Knapp and Smith (1990) and Pearcy (2007); and because of this fact, they say "it is necessary to understand how leaf photosynthetic CO2 uptake will change in response to variations in PFD under high CO2 conditions." Sadly, however, they note that "most studies on photosynthesis in response to elevated CO2 have only focused on 'steady-state' responses to constant light intensity," and, therefore, they focus their efforts on the effects of high CO2 on photosynthetic induction.

What was done
Working with two Populus genotypes with different stomatal responses to changes in light intensity - Populus koreana x trichocarpa cv. Peace (which shows a minimal stomatal response to PFD change) and P. euramericana cv. I-55 (which has a normal stomatal response to PFD change) - Tomimatsu and Tang grew the two genotypes under three CO2 regimes (380, 700 and 1020 ppm) for a period of two months, whereupon they measured leaf photosynthetic rates during a sudden PFD increase from 20 to 800 µmol/m²/sec, in order to assess the photosynthetic induction time (IT) required for the CO2 uptake rate to reach 50% (IT50) and 90% (IT90) of the steady-state photosynthetic rate following the increase in PFD.

What was learned
In the words of the two researchers, "photosynthetic induction times, both IT50 and IT90, were significantly shorter in plants grown in high CO2 regimes in the two poplar genotypes, which is consistent with an increase in the induction state under high CO2 regimes," as suggested by the work of Tinoco-Ojanguren and Pearcy (1993), Pearcy et al. (1994) and Valladares et al. (1997).

What it means
Describing the significance of their results for us, Tomimatsu and Tang write that their study suggests that "leaf carbon gain would increase under high CO2 environments in response to light fluctuations based on the reduction in photosynthetic induction observed" in their experiment. And they say that Leakey et al. (2005) and Niinemets (2007) suggest that "the stimulation of photosynthesis under high CO2 and fluctuating light conditions may lead to increases in seedling growth and future survival in the understory." In addition, Tomimatsu and Tang write that "climate change may particularly increase CO2 levels in the understory in closed canopies, and thus the increase in leaf carbon gain of photosynthetic induction under high CO2 regimes may further add to whole canopy photosynthesis," in support of which conclusion they cite the studies of Buchmann et al. (1997), Holtum and Winter (2001), Leakey et al. (2005) and Niinemets (2007).

References
Buchmann, N., Guehl, J.M., Barigah, T.S. and Ehleringer, J.R. 1997. Interseasonal comparison of CO2 concentrations, isotopic composition, and carbon dynamics in an Amazonian rainforest (French Guiana). Oecologia 110: 120-131.

Knapp, A.K. and Smith, W.K. 1990. Stomatal and photosynthetic responses to variable sunlight. Plant Physiology 78: 160-165.

Leakey, A.D.B., Scholes, J.D. and Press, M.C. 2005. Physiological and ecological significance of sunflecks for dipterocarp seedlings. Journal of Experimental Botany 56: 469-482.

Niinemets, U. 2007. Photosynthesis and resource distribution through plant canopies. Plant, Cell, and Environment 30: 1052-1071.

Pearcy, R.W. 2007. Responses of plants to heterogeneous light environments. In: Pugnaire, F.I. and Valladares, F. (Eds.). Functional Plant Ecology. CRC Press, New York, New York, USA, pp. 213-246.

Pearcy, R.W., Chazdon, R.L., Gross, L.J. and Mott, K.A. 1994. Photosynthetic utilization of sunflecks: a temporally patchy resource on a time scale of seconds to minutes. In: Caldwell, M.M. and Pearcy, R.W. (Eds.). Exploitation of Environmental Heterogeneity by Plants. Academic Press, San Diego, California, USA, pp. 175-208.

Tang, Y.U., Washitani, I., Tsuchiya, T. and Iwaki, H. 1988. Fluctuation of photosynthetic photon flux density within a Miscanthus sinensis canopy. Ecological Research 3: 253-266.

Tinoco-Ojanguren, C. and Pearcy, R.W. 1993. Stomatal dynamics and its importance to carbon gain in 2 rain-forest piper species. 2. Stomatal versus biochemical limitations during photosynthetic induction. Oecologia 94: 395-402.

Valladares, F., Allen, M.T. and Pearcy, R.W. 1997. Photosynthetic responses to dynamic light under field conditions in six tropical rainforest shrubs occurring along a light gradient. Oecologia 111: 505-514.