'Compartmentation' of acids in plant tissues.

The cells of higher plants, in contrast with those of other organisms, contain relatively large amounts of organic acids. Krebs & Lowenstein (1960) calculated that the concentrations of individual organic acids in Escherichia coli are about 0-01 mM, in liver or kidney cells 0 1 mm, and in yeast cells 1 mm. By using the same type of calculation it can be estimated that many plant tissues contain citrate and malate at concentrations of 10 mm, and some contain aconitate or isocitrate at this concentration. Succinate and fumarate are often present at a concentration of 1 mm, but other acids occur at lower concentrations. Another distinguishing feature of plant cells is the large vacuole which frequently accounts for more than 90% of the volume. Stiller (1959) has given theoretical reasons for supposing that a correspondingly large proportion of the total acid is present in the vacuole. She has calculated that, if malic acid in Bryophyllum leaves were confined to the cytoplasm, its concentration would be 7-8M, but, if it were in the vacuole also, its average concentration would be 0-25M, a more credible value. Her experimental results also indicate 'compartmentation' of malate, since the malate formed from 14CO2 remained asymmetrically labelled, but asymmetrically labelled malate provided to the tissue from outside became randomized. BennetClark & Bexon (1943) have experimental evidence that individual acids may be constrained inside the vacuole so that an apparent diffusion gradient is maintained between the vacuole and the cytoplasm. The vacuole need not be considered as the only storage reservoir of metabolites, however, since a degree of 'compartmentation' within the cytoplasm itself has been invoked as a rational explanation of a variety of results in which labelling patterns and specific activities of metabolites have been measured after providing 14C_ labelled intermediates to various cells (Porter & May, 1955; Steward, Bidwell & Yemm. 1958; Lajtha, Berl & Waelsch, 1959; Berl, Lajtha & Waelsch, 1961; Cowie & Walton, 1956; Cowie & Bolton, 1957; Cowie & McClure, 1959).