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Down-Regulation of Photosynthesis in Fast-Growing Poplar Trees
Davey, P.A., Olcer, H., Zakhleniuk, O., Bernacchi, C.J., Calfapietra, C., Long, S.P. and Raines, C.A. 2006. Can fast-growing plantation trees escape biochemical down-regulation of photosynthesis when grown throughout their complete production cycle in the open air under elevated carbon dioxide? Plant, Cell and Environment 29: 1235-1244.

Trees grown for long periods of time in elevated CO2 environments often, but not always, exhibit some degree of photosynthetic acclimation or down regulation, which is typically characterized by modestly reduced rates of photosynthesis (compared to what might be expected on the basis of short-term exposure to CO2-enriched air) that result from a long-term decrease in the activity and/or amount of rubisco, the primary plant carboxylating enzyme.

What was done
At the EuroFACE facility near Viterbo in Central Italy, where three plantation poplar genotypes - white poplar (Populus alba), robusta poplar (Populus x euramericana) and black poplar (Populus nigra) - were grown over a complete multi-year production cycle under conditions of adequate soil water and nutrients at either 370 or 550 ppm CO2, the authors measured total daily photosynthetic carbon assimilation together with a number of related plant physiological parameters and processes.

What was learned
Davey et al. report that "diurnal photosynthesis in poplar trees grown in elevated CO2 over four growing seasons showed a sustained increase in photosynthesis of between 35 and 60% prior to coppicing," and that "this increase in daily photosynthesis is maintained during the re-growth following coppicing in P. x euramericana, which produced the most biomass at coppice." These observations and their other data indicate, in their words, that "no long-term photosynthetic acclimation to CO2 occurred in these plants," and that "poplar trees are able to 'escape' from long-term, acclamatory down-regulation of photosynthesis through a high capacity for starch synthesis and carbon export." Also of great importance, they note that "Wittig et al. (2005) show that the canopy photosynthetic carbon gain in these species is proportional to wood increment, implying that the increased photosynthesis will result in more carbon in wood."

What it means
The seven scientists say their findings "show that the acclamatory loss of the initial increase in photosynthetic rate under elevated CO2 is not inevitable," and that "poplar species, selected for rapid growth, may be well suited to a future elevated CO2 environment and particularly suited to afforestation projects aimed to increase carbon uptake into wood in the near term."

Wittig, V.E., Bernacchi, C.J., Zhu, X.G., Calfapietra, C., Ceulemans, R., Deangelis, P., Gielen, B., Miglietta, F., Morgan, P.B. and Long, S.P. 2005. Gross primary production is stimulated for three Populus species grown under free-air CO2 enrichment from planting through canopy closure. Global Change Biology 11: 644-656.

Reviewed 18 October 2006