Severa1 enzyme activities were measured in extracts from acclimated and nonacclimated maize (Zea mays) root tips at pH 6.5 and 7.5, corresponding to cytoplasmic pH in anaerobiosis or aerobiosis, respectively, to determine what causes the decline of the glycolytic flux observed in anoxia in nonacclimated tips. We found that phosphorylation of hexoses by kinases was a major limiting step of glycolysis in anoxia. When fructose was substituted for glucose, glycolysis was slightly enhanced and survival improved, but neither matched that of acclimated tips. Decrease of kinase activities was not the result of proteolytic degradation but was more likely the result of inhibition by interna1 factors (low pH and low ATP). There was no evidence of induction during the hypoxic pretreatment of isoenzymes better adapted to the anoxic cellular environment. A preacclimation in hypoxia allows excised maize (Zea nzays) root tips supplemented with externa1 Glc to survive severa1 days in strict anoxia instead of less than 10 h in the absence of pretreatment Xia and Saglio, 1992). However, in the absence of exogenous Glc, they do not survive significantly longer than nonacclimated tips. We have shown that, below a critica1 threshold of glycolytic flux, survival was compromised (Xia et al., 1995). An hypoxic pretreatment induces an increase of maximal in vitro catalytic activities of many enzymes involved in sugar metabolism, a higher ATP level correlated with a higher energy charge (reviewed by Ricard et al., 1994), and a better regulation of cytoplasmic pH linked to an efficient efflux of lactic acid (Xia and Roberts, 1994;Xia et al., 1995;Xia and Saglio, 1992). However, the role of these modifications in the subsequent survival of tissues in anoxia is not obvious. For example, we have shown that in HPT root tips the level of ATP is not critica1 for survival or for cytosolic pH regulation in anoxia (Xia et al., 1995), and the increase in glycolytic enzyme activities is not correlated with a higher glycolytic rate during the 90 min after the transfer of tissues from air to strictly anoxic conditions. During this period, the glycolytic rate (measured as the sum of ethanol plus lactate) remains strictly identical in HPT and NHPT * Corresponding author; e-mail saglio@bordeaux.inra.fr; fax 56-84-32-45. root tips. It is only after 60 to 90 min that the glycolytic flux declines and then almost stops in NHPT root tips even in the presence of added Glc, whereas it remains sustained in HPT tips (Xia and Saglio, 1992). Such a decline of ethanol production during the hours or days after the transfer to anoxia appears to be common to many plants (Smith and ap Rees, 1979;Raymond et al., 1985; Kimmerer and Mac Donald, 1987). Exceptions are rice seeds and seedlings, which are resistant to anoxia and in which the rate of ethanol production increases during at least the first 2 d in the absence of O, (Raymond et al., 1985).How glycolytic flux is modified in HPT and NHPT root tips and what is responsible for its decline in NHPT tips early in anoxia (...