Organic Acid Metabolism in Relation to Flooding Tolerance in Roots

Abstract

Higher plants vary in their ability to tolerate flooding and the accompanying anaerobic conditions in the soil. This variation exists not only between species, but also between different ecological races of the same species (Crawford 1966). Frequently, flooding tolerance is associated with the development of aerenchyma. Conway (1940) and Webster (1962) have shown that the roots of many aerenchymatous plants maintain oxygen concentrations adequate for normal growth. However, the functional significance of aerenchyma as a pathway of gaseous diffusion has been questioned by Williams & Barber (1961) and also by Coult (1964). The latter has suggested that aerenchyma may serve either as an oxygen reservoir, or else as a system allowing the maximum amount of root or rhizome structure with the minimum quantity of living tissue, thus achieving an economy in oxygen consumption per unit volume of root or rhizome. The diffusion of oxygen out of the roots of marsh and aquatic plants has been demonstrated by Armstrong (1964), but equally, Heide, Boer-Bolt & Raalte (1963) and Greenwood (1967) studying barley (Hordeum vulgare) and a variety of crop plants, have shown that intolerant species also permit a passage of oxygen from the shoots to the roots. It appears therefore that the facilitation of gaseous diffusion by morphological adaptation is not an entirely satisfactory explanation of flooding tolerance in higher plants, and more attention should perhaps be paid to the possible role of metabolic adaptations to anaerobic environments. That plants differ in their reaction to anoxia has been demonstrated by Leach (1936) and also by Effer & Ranson (1967) who showed that buckwheat (Fagopyrum esculentum) differed from other plants in that it did not develop a Pasteur effect under nitrogen. Physiological differences induced by flooding and related to the distribution of the species in the field have been reported by Crawford (1966, 1967). In these experiments the increased rate of glycolysis including alcohol dehydrogenase (ADH) induction occurred when species intolerant of high water tables were flooded, whereas tolerant species showed no such change. Acetaldehyde is known to be an inducer of ADH activity (Hageman & Flesher 1960; Kolloffel 1968). The response of ADH to acetaldehyde induction differs with the ecology of the plants concerned. Species that are intolerant of flooding show the greatest induction, while tolerant species are relatively little affected (Crawford & McManmon 1968). While these experiments illustrate the effects of anaerobic conditions on species not tolerant of flooding, with their inability to avoid an acceleration of glycolysis, they do little to explain the marked homoeostatic survival properties of flood-tolerant species and their avoidance of ethanol accumulation under conditions of limiting oxygen supply. Mazelis & Vennesland (1957) have suggested that under anaerobic conditions malic acid