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 mol mol^-1) or doubled CO2] either in charcoal-filtered air (20 nmol mol^-1 O3]) or in non-filtered air supplemented to 1,5 x ambient O3] (70 nmol mol^-1) to determine the major limitations to assimilation under conditions of elevated CO2] and/or O3]. Through plant 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 treatment effects of elevated CO2] and/or O3] were observed on Rubisco carboxylation efficiency (CE), light and CO2-saturated assimilation capacity (Amax), and chlorophyll content (Chl). However, for plants grown in elevated CO2], the assimilation rate at growth CO2] (A) was 60% higher than at ambient CO2] up to the seed maturation stage, and the potential rate of assimilation by Rubisco capacity (Ap) was increased. Also in elevated CO2]: A was 51% of Ap; the relative stomatal limitation (%Stomata) was 5%; and the relative RuBP regeneration limitation (%RuBP) was 44%. In ambient CO2], O3 gradually decreased A per unit leaf area, but had little effect on Ap and the relative limitations to assimilation where A remained 51% of Ap, %Stomata was 27%, and %RuBP was 22%.During reproduction, CE declined for plants grown in elevated CO2] and/or O3]; Ap was unaffected by elevated CO2], but was reduced by O3] at ambient CO2]; A increased to 72% of Ap in elevated CO2] and/or O3]-fumigated air; the %Stomata increased; and the %RuBP decreased, to become non significant in elevated CO2] from the beginning of seed growth on, and in O3-fumigated air at ambient CO2] at the seed maturation stage. The decrease in %RuBP occurred concomitantly with an increase in Amax and Chl. Significant CO2] x O3] interactions support the lack of an O3 effect on assimilation and its limitations at elevated CO2] during seed maturation. These data suggest that elevated CO2] alleviated some of the effects of O3 on photosynthesis.Keywords: CO2 by O3 interactions, elevated CO2], O3 fumigation, Rubisco carboxylation efficiency, RuBP regeneration.