BIOSYNTHESIS AND ROOT EXUDATION OF CITRIC AND MALIC-ACIDS IN PHOSPHATE-STARVED RAPE PLANTS

Rape (Brassica napus L.) plants are efficient users of rock phosphates. This has been ascribed to the excretion of malic and citric acids from the roots during phosphorus deficiency, resulting in a decrease of the pH in the rhizosphere and a consequent solubilization of the rock phosphate. In this paper the biosynthesis and excretion of organic acids were investigated, partly by comparing physiological reactions to P deficiency of rape with those of an inefficient user of rock phosphate from the same family, hedge mustard (Sisymbrium officinale Scop.). In the leaves of both species, both citrate levels and phosphoenolypyruvate carboxylase were nearly doubled in P-deficient plants, and the citrate/sugar ratio in the phloem exudate into EDTA solution was doubled as well. After exposure of the shoots of rape plants to (CO2)-C-14, in the light, the specific activities of citrate in the root and in the nutrient solution were ten times higher than those of malate, and also much higher than the respective activities of citrate in P-sufficient controls. It is suggested that citrate produced in the leaf is exported to the roots, where it is accumulated in the excretion region. Malic acid which is prevailing in the exudate is probably newly synthesized within the excreting root segment. When no phosphate is supplied, this excretion zone is located 1-2 cm behind the root tips. However, when rock phosphate is applied locally this zone shifts along the root to that part which is in direct contact with the rock phosphate particles. No accumulation and excretion of organic acids could be observed with the roots of P-deficient Sisymbrium.