IMPROVED CYTOPLASMIC PH REGULATION, INCREASED LACTATE EFFLUX, AND REDUCED CYTOPLASMIC LACTATE LEVELS ARE BIOCHEMICAL TRAITS EXPRESSED IN ROOT-TIPS OF WHOLE MAIZE SEEDLINGS ACCLIMATED TO A LOW-OXYGEN ENVIRONMENT

We tested the hypothesis (J.-H. Xia and P.H. Saglio [1992] Plant Physi ol 100: 40-46) that the enhanced ability of maize (Zea mays) root tips to survive anoxia, elicited by a 4-h exposure to 3% O-2 (''acclimatio n''), is due to less cytoplasmic acidosis early in anoxia. Cytoplasmic pH and fermentation reactions were monitored in excised and intact (a ttached) maize root tips by simultaneous in vivo C-13- and P-31-NMR sp ectroscopy. We demonstrate that both excised and intact acclimated roo t tips have significantly higher cytoplasmic pH values under anoxia. T his reduction in cytoplasmic acidosis is greater in intact root tips. Remarkably, cytoplasmic pH does not change when root tips are transfer red from 3% O-2 to anoxia. The earlier observation of considerable lac tate efflux and lowered intracellular lactate in excised, acclimated r oot tips (ibid.) was extended to intact seedlings. The predominant fer mentation end product retained in the cells of acclimated root tips is alanine. We discuss the relationship between cytoplasmic pH and level s of intracellular lactate and alanine in sugar-replete roots, and the role of cytoplasmic pH in determining survival under anoxia.