GROWTH AND PHOTOSYNTHESIS UNDER HIGH AND LOW IRRADIANCE OF ARABIDOPSIS-THALIANA ANTISENSE MUTANTS WITH REDUCED RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE ACTIVASE CONTENT/

Photosynthesis and growth to maturity of antisense ribulose-1,5-bispho sphate carboxylase/oxygenase (Rubisco) activase Arabidopsis thaliana w ith reduced concentrations of activase relative to wild-type (Wt) plan ts were measured under low (200 mu mol m(-2) s(-1)) and high (600 mu m ol m(-2) s(-1)) photosynthetic photon flux density growing conditions. Both growth and photosynthesis were significantly reduced in an Arabi dopsis clone (R100) with 30 to 40% Wt activase, an effect that was mor e pronounced in high light. The aboveground biomass of the antisense c lone R100 reached 80% of Wt under low light and 65% of Wt under high l ight. Decreased growth in the antisense plants was attributed to reduc ed relative rates of growth and leaf area expansion early in developme nt; all plants attained similar values of relative rates of growth and leaf elongation by 21 d after planting. Reductions in photosynthesis were attributed to decreased Rubisco activation in the antisense plant s. Rubisco constituted about 40% of total soluble protein in both Wt a nd clone R100 under both light regimes. Activase content was 5% and 1. 4% of total soluble protein in Wt and clone R100, respectively, and al so was unaffected by growth irradiance. The stoichiometry of Rubisco t o activase was estimated at 20 Rubisco active sites per activase tetra mer in Wt Arabidopsis and 60 to 80 in the transgenic clone R100. We co nclude that Wt Arabidopsis does not contain Rubisco activase in great excess of the amount required for optimal growth.