Oxygen deficiency and root metabolism: Injury and acclimation under hypoxia and anoxia

Oxygen deficiency in the rooting zone occurs with poor drainage after rain or irrigation, causing depressed growth and yield of dryland species, in contrast with native wetland vegetation that tolerates such conditions. This review examines how roots are injured by O-2 deficiency and how metabolism changes during acclimation to low concentrations of O-2. In the root apical meristem, cell survival is important for the future development; metabolic changes under anoxia help maintain cell survival by generating ATP anaerobically and minimizing the cytoplasmic acidosis associated with cell death. Behind the apex, where cells are fully expanded, ethylene-dependent death and lysis occurs under hypoxia to form continuous, gas-filled channels (aerenchyma) conveying O-2 from the leaves. This selective sacrifice of cells may resemble programmed cell death and is distinct from cell death caused by anoxia. Evidence concerning alternative possible mechanisms of anoxia tolerance and avoidance is presented.