Evaluation of the importance of lactate for the activation of ethanolic fermentation in lettuce roots in anoxia

According to the Davies–Roberts hypothesis, plants primarily respond to oxygen limitation by a burst of lactate production and the resulting pH drop in the cytoplasm activates ethanolic fermentation. To evaluate this system in lettuce (Lactuca sativa L.), seedlings were subjected to anoxia and in vitro activities of alcohol dehydrogenase (ADH, EC 1.1.1.1), pyruvate decarboxylase (PDC, EC 4.1.1.1) and lactate dehydrogenase (LDH, EC 1.1.1.27) and concentrations of ethanol, acetaldehyde and lactate were determined in roots of the seedlings. The in vitro activities of ADH and PDC in the roots increase in anoxia, whereas no significant increase was measured in LDH activity. At 6 h, the ADH and PDC activities in the roots kept in anoxia were 2.8- and 2.9-fold greater than those in air, respectively. Ethanol and acetaldehyde in the roots accumulated rapidly in anoxia and increased 8- and 4-fold compared with those in air by 6 h, respectively. However, lactate concentration did not increase and an initial burst of lactate production was not found. Thus, ethanol and acetaldehyde production occurred without an increase in lactate synthesis. Treatments with antimycin A and salicylhydroxamic acid, which are respiratory inhibitors, to the lettuce seedlings in the presence of oxygen increased the concentrations of ethanol and acetaldehyde but not of lactate. These results suggest that ethanolic fermentation may be activated without preceding activation of lactate fermentation and may be not regulated by oxygen concentration directly.