Effects of elevated [CO2] and/or ozone on limitations to CO2 assimilation in soybean (Glycine max)

Soybean (Glycine max) was grown in open-top fieldchambers at ambient (360 molmol^-1) or doubled [CO2] either in charcoal-filteredair (20 nmol mol^-1 [O3]) or in non-filtered airsupplemented to 1,5 x ambient [O3] (70 nmol mol^-1)to determine the major limitations to assimilation under conditions ofelevated [CO2] and/or [O3]. Through plant ontogeny, assimilation versusintercellular CO2 concentration (A/Ci) responses weremeasured to assess the limitations to assimilation imposed by the capacityfor Rubisco carboxylation, RuBP regeneration, and stomatal diffusion.In thevegetative 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 thepotential rate of assimilation by Rubisco capacity(Ap) was increased. Also in elevated [CO2]:A was 51% of Ap; the relativestomatal limitation (%Stomata) was 5%; and therelative RuBP regeneration limitation (%RuBP) was 44%.In ambient [CO2], O3 gradually decreased A per unitleaf area, but had little effect on Ap and therelative limitations to assimilation where A remained51% 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 wasreduced by [O3] at ambient [CO2]; A increased to 72%of Ap in elevated [CO2] and/or [O3]-fumigated air; the%Stomata increased; and the %RuBPdecreased, to become non significant in elevated [CO2] from the beginningof seed growth on, and in O3-fumigated air at ambient [CO2] at the seedmaturation stage. The decrease in %RuBP occurredconcomitantly with an increase in Amax andChl. Significant [CO2] x [O3] interactions support thelack of an O3 effect on assimilation and its limitations at elevated [CO2]during seed maturation. These data suggest that elevated [CO2] alleviatedsome of the effects of O3 on photosynthesis.Keywords:CO2 by O3 interactions, elevated [CO2], O3 fumigation, Rubiscocarboxylation efficiency, RuBP regeneration.