EFFECTS OF ANOXIA ON WHEAT SEEDLINGS .2. INFLUENCE OF O2 SUPPLY PRIOR TO ANOXIA ON TOLERANCE TO ANOXIA, ALCOHOLIC FERMENTATION, AND SUGAR LEVELS

Prior exposure of roots of intact wheat seedlings for 15-30 h to hypoxia (0.016-0.06 mol m-3 O2) greatly increased their tolerance to subsequent anoxia, as assessed by the ability of the roots to elongate upon return to air. Such hypoxically pretreated roots had 2-4-fold higher activities of pyruvate decarboxylase (PDC) and 3.5-17-fold higher activities of alcohol dehydrogenase (ADH) in their 0-1 mm apices and 0-5 mm root tips than in apices and tips of roots pretreated in air (0.26-0.31 mol m-3 O2). The ADH/PDC ratio increased 1.3-5-fold during hypoxic pretreatment. Furthermore, the rate of alcoholic fermentation by 0-5 mm tips of the hypoxically pretreated roots was 1.4-4-fold faster than in tips from aerobically pretreated roots. No consistent difference between O2 pretreatment was found for alcoholic fermentation by tissues taken between 10 and 20 mm from the root tip. The observed activities of PDC and rates of alcoholic fermentation indicate that alcoholic fermentation is usually rate-limited by PDC in 0-1 mm apices and 0-5 mm tips of wheat roots. Comparisons with data in the literature indicate that wheat has at most a small Pasteur effect, which may explain why wheat is more intolerant to anoxia than rice. Exogenous glucose delayed the loss of elongation potential in both aerobically and hypoxically pretreated roots. In the absence of glucose, more than 85% of aerobically pretreated roots had lost their elongation potential after 9 h anoxia, compared with 30% in the presence of glucose. After 21 h anoxia nearly all aerobically pretreated roots had lost their elongation potential, compared with 10% and 0% of hypoxically pretreated roots in the absence and presence, of glucose, respectively. The protective effect of glucose was presumably not due to an endogenous sugar deficiency; at the start of anoxia, 0-1 mm apices of aerobically pretreated roots contained sufficient sugar for 23 h of their measured rate of ethanol synthesis yet, 85% of these apices had lost their elongation potential after only 9 h of anoxia. It is suggested that in wheat roots, low rates of synthesis of ethanol and hence of ATP, lead to injury of cells, in turn generating a requirement for exogenous glucose, despite high endogenous sugar concentrations.