LONG-TERM KINETICS OF THE LIGHT-DEPENDENT REGULATION OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE ACTIVITY IN PLANTS WITH AND WITHOUT2-CARBOXYARABINITOL 1-PHOSPHATE

The light-dependent modulation of ribulose-1,5-bisphosphate carboxylas e/oxygenase (Rubisco) activity was studied in two species: Phaseolus v ulgaris L., which has high levels of the inhibitor of Rubisco activity , carboxyarabinitol 1-phosphate (CA1P), in the dark, and Chenopodium a lbum L., which has little CA1P. In both species, the ratio of initial to fully-activated Rubisco activity declined by 40-50% within 60 min o f a reduction in light from high a photosynthetic photon flux density (PPFD; > 700 mumol . m-2 . s-1) to a low PPFD (65 +/- 15 mumol . m-2 . s-1) or to darkness, indicating that decarbamylation of Rubisco is su bstantially involved in the initial regulatory response of Rubisco to a reduction in PPFD, even in species with potentially extensive CA1P i nhibition. Total Rubisco activity was unaffected by PPFD in C. album, and prolonged exposure (2-6 h) to low light or darkness was accompanie d by a slow decline in the activity ratio of this species. This indica tes that the carbamylation state of Rubisco from C. album gradually de clines for hours after the large initial drop in the first 60 min foll owing light reduction. In P. vulgaris, the total activity of Rubisco d eclined by 10-30% within 1 h after a reduction in PPFD to below 100 mu mol . m-2 . s-1, indicating CA1P-binding contributes significantly to the reduction of Rubisco capacity during this period, but to a lesser extent than decarbamylation. With continued exposure of P. vulgaris le aves to very low PPFDs (< 30 mumol . m-2 . s-1), the total activity of Rubisco declined steadily so that after 6-6.5 h of exposure to very l ow light or darkness, it was only 10-20% of the high-light value. Thes e results indicate that while decarbamylation is more prominent in the initial regulatory response of Rubisco to a reduction in PPFD in P. v ulgaris, binding of CA1P increases over time and after a few hours dom inates the regulation of Rubisco activity in darkness and at very low PPFDs.