Differences between tomato (lycopersicon esuculentum mill.) and Cucumber (cucumis sativus l.) in Ethanol, lactate and malate metabolisms and cell sap ph of roots under hypoxia

Abstract

Plant growth in nutrient solution with a low dissolved oxygen concentration (DO) of 1 ppm is greatly inhibited in tomato (Lycopersicon esculentum Mill.), but almost normal in cucumber (Cucumis sativus L.) (Guo · Tachibana, 1997). To elucidate the mechanisms for the high tolerance of cucumber to root-zone hypoxia, the metabolisms of ethanol, lactate and malate, and the cell sap pH in intact and excised roots under hypoxia were compared between tomato 'Hausu-Momotaro' and cucumber 'Sharp I'. In intact roots of tomato, concentrations of ethanol and lactate as well as the activity of alcohol dehydrogenase (ADH) and lactate dehydrogenase (LDH) were markedly increased on exposure to 1 ppm DO for 3 and 6 days compared to 4 ppm DO. Similar results were obtained with excised tomato roots incubated at 0, 1, and 4 ppm DOs for 24 and 48 hr. Concentrations of malate decreased at low DOs in both intact and excised tomato roots. Contrarily, in cucumber the increase in ethanol concentrations and ADH activity at low DOs was slight in intact roots and did not occur in excised roots. Lactate concentrations and LDH activity did not increase in both types of hypoxic roots, except that excised roots incubated at 0 ppm DO exhibited a slight increase in lactate concentrations but to a much lesser extent than tomato roots. On the other hand, malate concentrations were greatly increased in both intact and excised roots at 1 ppm DO; excised roots, incubated at 0 ppm DO, showed no increase in realate concentrations. The cell sap pH of intact tomato roots grown 3 days at 1 ppm DO fell from 5.70 to 5.51 and that at 0 ppm DO to 5.22. However, in intact cucumber roots it did not change significantly at 1 ppm DO and arose markedly from 5.98 to 6.33 after 3 days at 0 ppm DO. The results strongly suggest that the metabolic properties of roots to stimulate synthesis of malate in response to low O2 in the root tissue and consequently prevent cytoplasmic acidosis because of lactate accumulation are responsible for the high tolerance of cucumber to root-zone hypoxia.