GROWTH AND PHYSIOLOGY OF ONE-YEAR-OLD POPLAR (POPULUS) UNDER ELEVATEDATMOSPHERIC CO2 LEVELS

The effects of elevated atmospheric CO2 concentrations on the ecophysi ological responses (gas exchange, chlorophyll a fluorescence, Rubisco activity, leaf area development) as well as on the growth and biomass production of two poplar clones (i.e. Populus trichocarpa x P. deltoid es clone Beaupre and P. x euramericana clone Robusta) were examined un der open top chamber conditions. The elevated CO2 treatment (ambient 350 mu mol mol-1) stimulated above-ground biomass of clones Robusta a nd Beaupre after the first growing season by 55 and 38 %, respectively . This increased biomass production under elevated CO2 was associated with a significant increase in plant height, the latter being the resu lt of enhanced internode elongation rather than an increased productio n of leaves or internodes. Both an increased leaf area index (LAI) and a stimulated net photosynthesis per unit leaf contributed to a signif icantly higher stem biomass per unit leaf area, and thus to the increa sed above-ground biomass production under the elevated CO2 concentrati ons in both clones. The larger LAI was caused by a larger individual l eaf size and leaf growth rate; the number of leaves was not altered by the elevated CO2 treatment. The higher net leaf photosynthesis was th e result of an increase in the photochemical (maximal chlorophyll fluo rescence Fm and photochemical efficiency Fv/Fm) as well as in the bioc hemical (increased Rubisco activity) process capacities. No significan t differences were found in dark respiration rate, neither between clo nes nor between treatments, but specific leaf area significantly decre ased under elevated CO2 conditions. (C) 1995 Annals of Botany Company