CONTROL OF PHOTOSYNTHESIS IN AMARANTHUS-EDULIS MUTANTS WITH REDUCED AMOUNTS OF PEP CARBOXYLASE

Mutants of the NAD-ME plant have been created using sodium azide. Thes e contain reduced activities of PEP carboxylase ranging from 5 to 100% of wild-type. Those with greater than 50% of the wild-type PEPC conte nt showed a lower maximum rate of photosynthesis and reduced carboxyla tion efficiency compared to the wild-type plants. The PEPC from these heterozygotes was less sensitive to malate inhibition than the PEPC of the wild-type plants and also had an increased phosphorylation state. Mutants containing 45 and 49% of wild-type PEPC exhibited a greater A (max) than was observed for the 55, 66 and 70% mutants. We postulate t hat there is a compensatory mechanism that activates PEPC when the PEP C protein is less than 55% of the wild-type. Control coefficients were measured for PEP carboxylase from wild-type and heterozygous (55%) pl ants. Results suggest that activation of PEPC by phosphorylation and m etabolites may be more an 'on-off' switch than a means of fine adjustm ent of PEPC activity in response to varying factors such as PEPC conte nt, CO2 or temperature changes.