Plant Physiol, January 2003, Vol. 131, pp. 177-185

Spatial and Temporal Effects of Free-Air CO₂ Enrichment (POPFACE) on Leaf Growth, Cell Expansion, and Cell Production in a Closed Canopy of Poplar¹

School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton S016 7PX, United Kingdom (G.T., P.J.T., F.Z.Z., V.J.A.); CNR, Institute of Biometeorology, P.le delle Cascine, 18 50144 Firenze, Italy (F.M.); and Department of Forest Environment and Resources, University of Tuscia, Viterbo, Italy (E.K.)

Leaf expansion in the fast-growing tree, Populus × euramericana was stimulated by elevated [CO₂] in a closed-canopy forest plantation, exposed using a free air CO₂ enrichment technique enabling long-term experimentation in field conditions. The effects of elevated [CO₂] over time were characterized and related to the leaf plastochron index (LPI), and showed that leaf expansion was stimulated at very early (LPI, 0-3) and late (LPI, 6-8) stages in development. Early and late effects of elevated [CO₂] were largely the result of increased cell expansion and increased cell production, respectively. Spatial effects of elevated [CO₂] were also marked and increased final leaf size resulted from an effect on leaf area, but not leaf length, demonstrating changed leaf shape in response to [CO₂]. Leaves exhibited a basipetal gradient of leaf development, investigated by defining seven interveinal areas, with growth ceasing first at the leaf tip. Interestingly, and in contrast to other reports, no spatial differences in epidermal cell size were apparent across the lamina, whereas a clear basipetal gradient in cell production rate was found. These data suggest that the rate and timing of cell production was more important in determining leaf shape, given the constant cell size across the leaf lamina. The effect of elevated [CO₂] imposed on this developmental gradient suggested that leaf cell production continued longer in elevated [CO₂] and that basal increases in cell production rate were also more important than altered cell expansion for increased final leaf size and altered leaf shape in elevated [CO₂].