The Effects of Anoxia and Carbohydrates on the Growth and Viability of Rice, Pea and Pumpkin Roots

Abstract

Intact or excised roots, immersed in anaerobic aqueous agar media with or without additions of sugar, were made anoxic by exposing the shoots (or cut-ends) to oxygen-free nitrogen. Polarographic monitoring of the internal oxygen status showed that roots rapidly became anoxic; also extension growth immediately declined and was soon halted. Growth was re-started only by re-aeration and, in sugar deficient media, apices survived only if the period of anoxia had not exceeded 4 h (rice), 6 h (pea) and 12 h (pumpkin). Utilizable carbohydrate supplied exogenously could reduce the rate of decline in growth rate (rice and pumpkin) but could not of itself indefinitely sustain or induce growth; it enhanced viability, however, and rice became the most tolerant of anoxia (c. 44 h). Oxygen was essential both to sustain and initiate root extension but growth could cease and apical death ensue from an insufficiency of carbohydrate even in the presence of oxygen. It is concluded that the normal response of roots to anoxia is a hypersensitive one arising from sugar deficiency. The discussion relates the results to recent reports concerning carbohydrate and energy levels in anoxic and aerated roots, to work on ultrastructural change under anoxia, and to the subject of flood tolerance. The results are not thought to accord with a metabolic theory of flood-tolerance based upon differences in the accumulation and phytotoxicities of ethanol and other by-products of anaerobic respiration. They are considered to be more in keeping with the view that flood-tolerance in roots is chiefly a property of internal aeration and the potential for producing well-ventilated roots in response to soil wetness.