Studies in the respiratory and carbohydrate metabolism of plant tissues - X. The influence of oxygen at high pressures as a stimulant and inhibitor of certain pathways of respiration in carrots

Abstract

In contrast with the behaviour of peas (Turner & Quartley 1956; Pritchard 1959, 1961) but like that of various tissues (Johannsen 1888; Cass 1947; Barker, Quartley & Turner 1960), the rate of CO 2 output of whole carrots was stimulated initially in oxygen at a pressure of 5 atm prior to the strong inhibition characteristic of oxygen poisoning. The inhibition was associated with an accumulation of citrate together with a decrease in the contents of α -ketoglutarate, succinate, malate and oxaloacetate; later pyruvate, alcohol and acetate also increased. As in the earlier work with potatoes, peas and apples (Barker & Mapson 1955; Turner & Quartley 1956; Barker et al . 1960), these changes in the acids were attributed in part to the production of an enzymic ‘block’ first in the tricarboxylic acid cycle between citrate and α -ketoglutarate and later in the oxidation of pyruvate. The earlier observations of the activity of the tricarboxylic acid cycle in carrot slices were thus confirmed (Pritchard 1959; ap Rees & Beevers 1960). The ‘total carbon traffic’, representing the sum of the observed CO 2 output in high oxygen and the calculated amounts of CO 2 that would be derived by oxidation of the accumulations of citrate, pyruvate, alcohol and acetate, increased initially in high oxygen to a rate which was about double that of the CO 2 output in air. Considered in relation to this increased carbon traffic in respiration, there was thus initially in high oxygen a large inhibition of the CO 2 output. Oxygen at high pressures thus produced simultaneous stimula­tory and inhibitory effects on certain phases of the respiratory process in carrots. Initially in high oxygen only a small part of the CO 2 output could be accounted for by the observed concentration changes: e. g. the accumulation of citrate; a part of the ‘unknown’ CO 2 output is provisionally attributed, without direct evidence, to operation of the pentose phosphate pathway, earlier shown to be active in carrot slices (Beevers & Gibbs 1954; ap Rees & Beevers 1960).