RUBISCO AND PEP CARBOXYLASE RESPONSES TO CHANGING IRRADIANCE IN A BRAZILIAN CERRADO TREE SPECIES, QUALEA-GRANDIFLORA MART (VOCHYSIACEAE)

The activities of ribulose-1,5-bisphosphate carboxylase-oxygenase, Rub isco (E.C. 4.1.1.39) and phosphoenolpyruvate carboxylase, PEPc (E.C. 4 .1.1.31), and concentrations of protein and chlorophyll were measured in extracts from cotyledons arid first leaves of Qualea grandiflora Ma rt. (Vochysiaceae) seedlings after transfer from high-light (20 days a t 320 mumol m-2 s-1, PAR) to low-light (35 days at 120 mumol m-2 s-1, PAR) conditions. When Tween 20 and glycerol were added to the extracti on medium, Rubisco activities obtained for Qualea grandiflora were com parable to published values for several coniferous species and the bro ad-leaved species, Prunus avium L. Stella, grown in a similar light en vironment. Rubisco activity in cotyledons of Q. grandiflora grown in h igh light for 20 days and then transferred to low light for a further 35 days was similar to the activity in cotyledons of plants grown cont inuously in high light. However, the first leaf above the cotyledons s howed a greater response to the change in irradiance; in high light, R ubisco activity of the first leaf was 1.8 times higher on a fresh weig ht basis and 2.7 times higher on an area basis than that of leaves tra nsferred from high to low light. Fresh weight and chlorophyll concentr ation expressed on a unit leaf area basis were also higher in the high -light treatment. These responses to irradiance are indicative of a sp ecies adapted to growth in an unshaded habitat. The PEPc activity in l eaves was 15% of Rubisco activity, which is typical of species with a C3 photosynthetic pathway. The relatively slow growth rate of Q. grand iflora observed in these experiments could riot be attributed to a low carboxylation capacity per unit leaf area.