'Chambourcin' grapevine (Vitis vinifera × Vitis L.) berries display sustained post-veraison accumulation of oxalic acid

A substantial accumulation of organic acids (OAs) occurs during pre-veraison grape berry development, which results in low pH conditions and contributes to the organoleptic properties of the fruit.The main OAs in most grape berries are tartaric acid (TA) and malic acid (MA). MA is accumulated via a primary metabolic shunt, whereas TA is accumulated via ascorbic acid (AA; vitamin C) metabolism. Oxalic acid (OX) is also derived from AA in competition with TA, and results from a cleavage between the second and third carbon atoms. Oxalic acid is subsequently biomineralised as calcium oxalate crystals in idioblast cells. We have examined the biosynthesis of TA, MA, and OX during grape berry development in four grapevine cultivars. We show that in the Vitis vinifera L. × Vitis L. complex inter-specific hybrid, 'Chambourcin', there is a high capacity to accumulate OX during post-veraison fruit development. OX content increased from approx. 1 mg g^-1 to 3 mg g^-1 berry fresh weight between 14 and 16 weeks post-anthesis, resulting in OX levels that were similar to the MA levels in mature fruit. Whether OX accumulation occurs due to increased biomineralisation or biosynthesis,our results show that 'Chambourcin' grape berries represent the only documented case of post-veraison accumulation of OX.