EFFECTS OF ELEVATED [CO2] AND OR OZONE ON LIMITATIONS TO CO2 ASSIMILATION IN SOYBEAN (GLYCINE-MAX)/

Soybean (Glycine max) was grown in open-top field chambers at ambient (360 mu mol mol(-1)) or doubled [CO2] either in charcoal-filtered air (20 nmol mol(-1) [O-3]) or in non-filtered air supplemented to 1.5 x a mbient [O-3] (70 nmol mol(-1)) to determine the major limitations to a ssimilation under conditions of elevated [CO2] and/or [O-3]. Through p lant ontogeny, assimilation versus intercellular CO2 concentration (A/ Ci) responses were measured to assess the limitations to assimilation imposed by the capacity for Rubisco carboxylation, RuBP regeneration, and stomatal diffusion. In the vegetative stages, no significant treat ment effects of elevated [CO2] and/or [O-3] were observed on Rubisco c arboxylation efficiency (CE), light and CO2-saturated assimilation cap acity (A(max)), and chlorophyll content (Chl). However, for plants gro wn in elevated [CO2], the assimilation rate at growth [CO2] (A) was 60 % higher than at ambient [CO2] up to the seed maturation stage, and th e potential rate of assimilation by Rubisco capacity (A(p)) was increa sed. Also in elevated [CO2]: A was 51% of A(p); the relative stomatal limitation (%Stomata) was 5%; and the relative RuBP regeneration limit ation (%RuBP) was 44%. In ambient [CO2], O-3 gradually decreased A per unit leaf area, but had little effect on A, and the relative limitati ons to assimilation where A remained 51% of A(p), %Stomata was 27%, an d %RuBP was 22%. During reproduction, CE declined for plants grown in elevated [CO2] and/or [O-3]; A(p) was unaffected by elevated [CO2], bu t was reduced by [O-3] at ambient [CO2]; A increased to 72% of A(p) in elevated [CO2] and/or [O-3]-fumigated air; the %Stomata increased; an d the %RuBP decreased, to become non significant in elevated [CO2] fro m the beginning of seed growth on, and in O-3-fumigated air at ambient [CO2] at the seed maturation stage. The decrease in %RuBP occurred co ncomitantly with an increase in A(max) and Chl. Significant [CO2] x [O -3] interactions support the lack of an O-3 effect on assimilation and its limitations at elevated [CO2] during seed maturation. These data suggest that elevated [CO2] alleviated some of the effects of O-3 on p hotosynthesis.