Varietal Differences in Oxalic Acid Concentration and Enzyme Activity Related to Oxalic Acid Biosynthesis in Carambola (Averrhoa carambola L.)

Abstract

Oxalic acid concentrations in the fruits and leaves of a tart cultivar and a sweet cultivar of carambola (Averrhoa carambola L.) ware determined. (1) The concentrations of soluble and insoluble oxalic acids in the leaves of the tart cultivar ware higher than those of the sweet cultivar. The highest concentration of total oxalic acid, 7.6 g/100 g fr. wt. was detected in the mature leaves of the tart cultivar. (2) The oxalic acid concentration gradually decreased with fruit maturation. As compared with leaves, varietal difference in soluble or insoluble oxalic acid of fruits was apparent. A larger proportion of insoluble to total oxalic acid was obtained in the sweet cultivar, although the total oxalic acid concentrations in both cultivars were similar. Therefore, the soluble oxalic acid concentration would be consistent with the relative sourness of fruits. (3) Enzyme activity of glyoxlate oxidase or ascorbate oxidase, which relates to the biosynthetic pathways of oxalic acid production changed with the growth stages of leaves and fruits of carambola. The highest activity of glyoxlate oxidase was obtained in the mature yellow green leaves and a five-fold difference was recognized between tart and the sweet cultivars. Activity of glyoxlate oxidase was predominantly in any growth stages; therefore, it may locate on the main route for oxalic acid biosynthesis in carambola.