We tested the hypothesis that ATP levels and energy charge determine the resistance of maize (Zea mays) root tips to anoxia. We focused on root tips of whole maize seedlings that had been acclimated to low O , by exposure to an atmosphere of 3% (v/v) O, in N,. Acclimated anoxic root tips characteristically have higher ATP levels and energy charge and survive longer under anoxia than nonacclimated tips. We poisoned intact, acclimated root tips with either fluoride or mannose, causing decreases in ATP and energy charge to values similar to or, i n most cases, below those found in nonacclimated anoxic tips. with the exception of the highest fluoride concentration used, the poisoned, acclimated tips remained much more tolerant of anoxia than nonacclimated root tips. We conclude that high ATP and energy charge are not components critical for the survival of acclimated root tips during anoxia. The reduced nucleotide status in poisoned, acclimated root tips had little effect on cytoplasmic pH regulation during anoxia. This result indicates that i n anoxic, acclimated root tips either cytoplasmic pH regulation is not dominated by ATP-dependent processes or these processes can continue i n vivo largely independently of any changes i n ATP levels i n the physiological range. The role of glycolytic flux in survival under anoxia is discussed.Evaluation of the relative importance of biochemical parameters as determinants of plant performance under environmental stresses, such as low O,, is complicated by the fact that stresses disturb whole networks of biochemical processes (reviewed by Kennedy et al., 1992;Perata and Alpi, 1993;Ricard et al., 1994). Many observed differences between stressed and unstressed, or tolerant and intolerant, plants may well be manifestations of this general disturbance rather than revelations of fundamental processes responsible for the behavior of a particular plant in different environments.The primary biochemical effect of low O, on plant cells is an inhibition of respiratory ATP synthesis, leading to a switch to a lower overall rate of ATP synthesis via fermentation (Beevers, 1961). Given the central role of ATP in cell function (Stryer, 19881, the "energy crisis" in plant cells evoked by low O, would appear to be the direct cause of the eventual loss of function. The resistance of maize root tips to anoxia can be enhanced by pretreatment of intact seedlings with hypoxia ( 2 4 % O,) (Saglio et al., 1988;Johnson et al., 1989). Associated with this acclimation response is the ability to maintain a high glycolytic rate during long periods of anoxia (Saglio et al., 1988;Hole et al., 1992;Xia and Saglio, 1992) and higher ATP levels and energy charge (Saglio et al., 1988;Johnson et al., 1994). One straightforward interpretation of this phenomenon is that the primary acclimation response leading to improved tolerance of anoxia is the enhanced energy metabolism, which relieves the shortage of ATP and facilitates cell function. Moreover, reported differences between acclimated and nonacclimated root t...