Light dependent activation of CO2 assimilation and the ratio of Rubisco activase to Rubisco during leaf aging of rice (Oryza sativa)

The effects of the ratio of Rubisco activase to Rubisco (activase/Rubisco r atio) on light dependent activation of CO2 assimilation were investigated d uring leaf aging of rice. Changes of photosynthetic CO2 gas exchange rates in relation to step increases of light intensity from two photon flux densi ties of 60 mu mol m(-2) s(-1) (low initial PFD) and 500 mu mol m(-2) s(-1) (high initial PFD) to saturated PFD of 1800 mu mol m(-2) s(-1) were measure d. These photosynthetic activation processes were considered to be limited by the Rubisco activation rate when analyzed by the relaxation method. The relaxation time of low initial PFD gradually declined from 3 to 33 days aft er leaf emergence and showed high and negative correlation to the activase/ Rubisco ratio. The initial rate of Rubisco activation under low initial PFD linearly correlated to the amounts of Rubisco activase, whereas these were almost constant from 3 to 23 days after leaf emergence. But these correlat ions could not be recognized in the case of high initial PFD. Moreover, the relaxation times were more sensitive to intercellular CO2 concentration (C -i) under high initial PFD than under low initial PFD; especially, at C-i b elow 300 mu l l(-1) These results suggest the involvement of the activase/R ubisco ratio in the photosynthetic activation under relatively low initial PFD, and the limitation of photosynthetic activation under relatively high initial PFD by Rubisco carbamylation during leaf aging of rice.