REGULATION OF H-TIPS ACCLIMATED TO A LOW-OXYGEN ENVIRONMENT( EXTRUSION AND CYTOPLASMIC PH IN MAIZE ROOT)

We tested the hypothesis that H+ extrusion contributes to cytoplasmic pH regulation and tolerance of anoxia in maize (Zea mays) root tips. W e studied root tips of whole seedlings that were acclimated to a low-o xygen environment by pretreatment in 3% (v/v) O-2. Acclimated root tip s characteristically regulate cytoplasmic pH near neutrality and survi ve prolonged anoxia, whereas nonacclimated tips undergo severe cytopla smic acidosis and die much more quickly. We show that the plasma membr ane H+-ATPase can operate under anoxia and that net H+ extrusion incre ases when cytoplasmic pH falls. However, at an external pH near 6.0, H + extrusion contributes little to cytoplasmic pH regulation. At more a cidic external pH values, net H+ flux into root tips increases dramati cally, leading to a decrease in cytoplasmic pH and reduced tolerance o f anoxia. We present evidence that, under these conditions, H+ pumps a re activated to partly offset acidosis due to H+ influx and, thereby, contribute to cytoplasmic pH regulation and tolerance of anoxia. The r egulation of H+ extrusion under anoxia is discussed with respect to th e acclimation response and mechanisms of intracellular pH regulation i n aerobic plant cells.