SOIL AERATION AND PLANT-ROOT METABOLISM

Plant roots commonly experience temporary periods of oxygen deprivation when soil becomes flooded or waterlogged, and warm temperatures encourage rapid consumption of O2 by soil microorganisms and roots. This paper describes some of the practical situations in the field where excess soil water, particularly from irrigation, causes depletion of roil oxygen. It also reviews briefly some of the earliest physiological responses of the roots to such conditions. Below the critical oxygen pressure, an increasing fraction of the root volume becomes anaerobic. In such anoxic cells, survival is possible for at least several hours by the generation of ATP during glycolysis and fermentation. However, cytoplasmic acidosis through accumulation of lactic acid, as well as leakage of H+ from the vacuole to the cytoplasm, contribute to cell death. Additionally, re-exposure to O2 may generate reactive O2 species (superoxide radicals) than can damage cellular membranes. Prior acclimation of roots to a partial shortage of O2 (causing tissue hypoxia) allows them to remain viable during later, more extreme conditions (tissue anoxia) that would otherwise be lethal. Experiments using the root tips of maize show that hypoxia induces changes in gene expression for enzymes of fermentation that are associated with a more rapid fermentation, a higher energy status, and a continued ability to synthesize proteins during anoxia. An alternative resistance mechanism involves avoidance of extreme O2-deficiency. In maize, hypoxia alters the structure of newly emerging roots, resulting from an ethylene-dependent lysis of cortical cells to form aerenchyma. Aerenchyma facilitates the internal transfer of O2 to the roots by diffusion from the leaves. Changes in activities of various key enzymes involved in aerenchyma formation that are induced by hypoxia are described briefly.