RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE AND PHOSPHOENOLPYRUVATE CARBOXYLASE ACTIVITIES OF PHOTOAUTOTROPHIC CALLUS OF PLATYCERIUM-CORONARIUM (KOENIG EX OF MUELL.) DESV. UNDER CO2 ENRICHMENT

The in vitro activities of ribulose-1,5-bisphosphate carboxylase/oxyge nase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) were measure d in cell-free extracts of Platycerium coronarium callus cultured for up to 42 days under photoautotrophic conditions with CO2 enrichment. W ith an increase in CO2 in the culture environment to 10% (v/v) at low light, the apparent photoautotrophic fixation of CO2 by Rubisco declin ed, whereas the non-photoautotrophic CO2 fixation by PEPC activity was enhanced. Hence, photosynthesis appears to play a lesser role in prov iding carbon skeletons and energy with prolonged culture in a CO2-enri ched environment. Instead, the anaplerotic supply of C-skeletons by PE PC may be important under such a situation. Short-term (HCO3-)-C-14 fi xation experiments indicated that photoautotrophic callus cultured for 3 weeks with 10% CO2 enrichment assimilated less (CO2)-C-14 than the control (0.03% CO2). Analyses of C-14-metabolites indicated that about 50% of the total soluble (CO2)-C-14 fixed was in the organic acid fra ction and 35% in the amino acid fraction. Despite the changes in the i n vitro Rubisco/PEPC activity-ratio, no significant change in the C-14 distribution pattern was apparent in response to increasing sucrose o r CO2 concentrations. The suppression of Rubisco activity and total ch lorophyll content in high sucrose or elevated CO2 concentrations sugge sts an inhibition of the capacity for photoautotrophic callus growth u nder these conditions.