Photosynthetic characteristics of mesophyll eells isolated from sunflower (helianthus annuus L.) leaves.

Mesophyll cells were isolated from sunflower leaves by an enzymic procedure. The cell suspensions possessed high photosynthesis rates. The products of cell photosynthesis were similar to the products of leaf disc photosynthesis. The relatively high radioactivity incorporated into malate after (14)CO2 feeding suggests that PEP carboxylase might participate in CO2 fixation. Sunflower leaf extracts possessed a PEP carboxylase activity slightly higher than that of other C3 species. Inhibition of PEP carboxylase by maleate decreased cell photosynthesis by only 15% and the first products of cell photosynthesis were phosphorylated compounds. It is concluded that the high photosynthesis rates displayed by sunflower are not due to a parallel C4 pathway of photosynthesis but are rather dependent, at least in part, on the activity, or the amount, of RuBP carboxylase.