Involved in ascorbic acid (vitamin C) biosynthesis. L-galactonate + NADP(+) = aldehydo-D-galacturonate + H(+) + NADPH Cofactor biosynthesis; L-ascorbate biosynthesis. Expressed specifically in the receptacle tissue of the fruit. Up-regulated during ripening, with a highest expression in fully mature red fruit. Belongs to the aldo/keto reductase family.