Compartmentation of malate in relation to ion absorption in beet.

Malate in beet discs treated in different salt solutions was labeled by a 30 min pulse of (14)CO(2), and subsequent changes in specific activity were followed for several hr. In treatments which resulted in net acid synthesis in response to excess cation absorption, malate specific activity fell slowly after removal of (14)CO(2). In solutions where no net acid synthesis occurred, and from which cation and anion were absorbed equally, malate specific activity fell rapidly when (14)CO(2) was removed. The foregoing suggests that the net synthesis of organic acids in response to excess cation absorption leads to the removal of organic anions from cytoplasmic metabolic pools as counter-ions in salt transport to the vacuole.The latter hypothesis was further examined by direct measurement of the distribution of labeled malate between cytoplasm and vacuole using the wash-exchange method of compartmental analysis, previously described for inorganic ions. The method satisfied the criterion of exchange specificity necessary for this purpose. Much higher retention of label in the cytoplasm was observed in KCl solutions (no net synthesis) than in K(2)SO(4) solutions (net synthesis) after 3 hr (14)CO(2) fixation and subsequent wash-exchange. The observed distribution is consistent with the rapid removal of organic anions to the vacuole during net acid synthesis. The significance of organic acid transport in relation to metabolism is discussed.