ChileanJAR

It has been propoosed that one of the functions of the enzyme phosphoenolpyruvate carboxylase (PEPCase) in the roots of white lupines consists in providing the carbon required to support the significant quantity of citrate that is excreted by P-starved plants. To demonstrate that citrate excretion by roots is an event more sensitive to P concentration than PEPCase, the activity of the enzyme extracted from roots of white lupines growing in soil as well as its activity and citrate release in plants growing in a nutrient solution were measured. PEPCase activity of plants growing in soil at five P treatments (with P added to obtain shoot P ranging from deficient to adequate) varied from 0.22 to 0.18 m mole NADH mg protein^-1 min^-1 for a shoot P of 0.14 to 0.19%, respectively. In a broad range of P concentrations in nutrient solutions (with P added to obtain shoot P ranging from deficient to near toxicity), the enzyme activity and citrate release were reduced to almost undetectable levels when shoot P was increased to 0.7 and 0.5%, respectively. The results indicate that in vitro PEPCase activity does not significantly change with the range of shoot P from deficient to adequate, and suggest that the mechanism associated with citrate excretion might be impaired at P concentrations lower than those required to inhibit PEPCase activity