Root metabolism in the black mangrove ( Avicennia germinans (L.) L): Response to hypoxia

Abstract

Oxygen concentrations in the roots of Avicennia germinans (L.) L decreased from 16 to less than 2% within 1 hr after exposure to hypoxic conditions and were still low when the experiment was terminated after 96 hr. The intact roots responded metabolically by increasing the capacity for alcoholic fermentation (as indicated by significant increases in alcohol dehydrogenase activity). Ethanol, the potentially toxic product of alcoholic fermentation, was low in control and hypoxic roots, but may have diffused from the roots into the surrounding hydroponic solution during the course of the experiment. Although no change in PEP carboxylase activity occurred, increased activity of malate dehydrogenase in combination with low malic enzyme activity may have caused the relatively greater accumulation of malate in the hypoxic roots. The adaptive significance of malate accumulation in A. germinans could not be determined from the data, however. Alanine concentrations also increased in the hypoxic roots. Lactate, however, did not accumulate even though lactate dehydrogenase activity increased significantly. Although root adenine nucleotide concentrations and the adenylate energy charge (AEC) ratio in the hypoxic roots were significantly lower than those of the aerobic controls, the AEC of the hypoxic roots was still relatively high (0.69) after 4 days of the treatment, possibly due to the anaerobic generation of ATP via alcoholic fermentation. Metabolic adaptations in A. germinans seedlings may be as important as internal oxygen diffusion in allowing this species to tolerate flooded soil conditions.