Bicarbonate Fixation and Malate Compartmentation in Relation to Salt-induced Stoichiometric Synthesis of Organic Acid

Abstract

The relationship of malate synthesis to K(+) absorption from solutions of K(2)SO(4) and KHCO(3) was compared in nonvacuolate barley (Hordeum vulgare) root tips and whole excised roots. The comparison has permitted separation of the process which evokes organic acid synthesis from that which leads to stoichiometry between net acid equivalents formed and excess K(+) absorbed from K(2)SO(4), on the one hand, and total K(+) absorbed from KHCO(3), on the other. Both in tips and in roots K(+) uptake from 20 mN salt solution exceeds malate synthesis in the first hour. In vacuolate roots the expected stoichiometry is achieved with time. When root tips are transferred to dilute CaSO(4), malate is rapidly metabolized, and K(+) is lost to the solution. By contrast, in excised whole roots the malate level remains unchanged, the salt-induced organic acid presumably being retained in the vacuole. In excised roots malonate leads to a marked drop in malate levels in untreated roots as well as in roots which have experienced salt-induced net malate synthesis. In consequence, it is contended that malonate makes available normally sequestered vacuolar malate.The general hypothesis is offered that the bicarbonate level of the cytoplasm controls organic acid synthesis by phosphoenolpyruvate carboxylase, and that the cytoplasmic bicarbonate level is raised either by exchange of cytoplasmic H(+) for external cation, or by bicarbonate absorption directly. Stoichiometry, in turn, is achieved by the accumulation in the vacuole of the double salt of malate.