Leaf area is stimulated in Populus by free air CO2 enrichment (POPFACE), through increased cell expansion and production

The effects of free-air CO2 enrichment (FACE) on leaf growth in Populus, wa s studied. For the first time in field conditions, both the production and expansion of leaf cells were shown to be sensitive to atmospheric carbon di oxide. Leaf area expansion rate and final leaf size were stimulated under F ACE for three species (Populus x euramericana (I-214), P. nigra (Jean Pourt et) and P. alba (2AS-11), with the largest effect observed for P. x euramer icana (61%). In this species and in P. nigra, both epidermal cell size and cell number were increased, whereas for P. alba, only cell production was i ncreased in FACE. Two findings suggest that changes in the cell wall may be important in explaining larger leaf cells in FACE: (i) Leaf cell wall exte nsibility of rapidly growing leaves increased in all species in FACE; and ( ii) an increase in xyloglucan endotransglycosylase activity, a cell wall-lo osening enzyme, was increased in FACE and associated with leaf growth rate. The results suggest that the mechanisms by which FACE promotes leaf growth differ, depending on species. Despite this, increases in final leaf size p rovide an important component driving increased biomass accumulation in POP FACE, during this first year of rapid growth, prior to canopy closure. The question as to whether these effects are the result of a direct response to CO2, or are driven indirectly through substrate availability remains unres olved, although evidence from the literature suggests that the latter mecha nism is most likely.