RESPONSES OF RUBISCO ACTIVATION AND DEACTIVATION RATES TO VARIATIONS IN GROWTH-LIGHT CONDITIONS

Basil (Ocimum basilicum) and impatiens (Impatiens wallerana) were grow n in sun, shade, or fluctuating light (15 min sun, 15 min shade) to ex amine the effects of growth-light conditions on the rates of light-ind uced Rubisco activation and deactivation. Rubisco activation and deact ivation rates were determined from gas-exchange measurements of photos ynthesis following a step increase in PFD. Rubisco deactivation rates were also determined from biochemical analyses of leaf extracts. There were no significant differences in Rubisco activation rate among the growth conditions or between the two species. However, there were sign ificant differences in Rubisco deactivation rate among the growth cond itions in basil and between the two species. In basil, Rubisco deactiv ated more slowly following a decrease in PFD in sun-and fluctuating-li ght grown plants than in shade grown plants. Slower rates of Rubisco d eactivation during periods at low PFD resulted in higher activation st ates at the onset of increased PFD. Thus, the contribution of Rubisco activation to the induction process was less for basil plants grown un der sun and fluctuating light than for those grown under shade. Impati ens deactivated Rubisco more rapidly than in basil, but there was no s ubstantial effect of the three growth-light conditions on Rubisco deac tivation rates in impatiens.