Control of C-4 photosynthesis: effects of reduced activities of phosphoenolpyruvate carboxylase on CO2 assimilation in Amaranthus edulis L.

Heterozygous mutants of Amaranthus edulis deficient in PEP carboxylase (PEP C) have been used to study the control of photosynthetic carbon assimilatio n. A reduction in PEPC activity led to a decrease in the initial slope of t he relationship between the CO2 assimilation rate and the intercellular CO2 concentration and to a decrease in photosynthesis at high light intensitie s, consistent with a decrease in the capacity of the C-4 cycle in high ligh t, PEPC exerted appreciable control on photosynthetic flux in the wild-type , with a relatively high flux control coefficient of 0.35 in saturating lig ht and ambient CO2. The flux control coefficient was decreased in low light or increased in low CO2 or in plants containing lower PEPC activity, Howev er, the rate of CO2 assimilation decreased down to about 55% PEPC, followed by an up-turn in the light-saturated photosynthetic rate as PEPC was furth er reduced, suggesting the existence of a mechanism that compensates for th e loss of PEPC activity. The amounts of photosynthetic metabolites, includi ng glycine and serine, also showed a biphasic response to decreasing PEPC, There was a linear relationship between the activity of PEPC and the activa tion state of the enzyme. A possible mechanism of compensation involving ph otorespiratory intermediates is discussed.