Oxalate and ascorbate in Actinidia fruit and leaves

Screening of fruit for oxalate crystals by tissue clarification from 132 genotypes of 15 species of Actinidia vines showed bundles of long raphides in locules that varied in abundance between species. Actinidia macrosperma contained very few raphides in the locules but its seed were packed with raphides. A. eriantha was the other extreme having fruit locules with many raphides. Total oxalate and ascorbate was measured in 134 siblings in an Actinidia chinensis mapping population and this revealed a 4.6 fold variation in oxalate amount (12-55 mg oxalate/100 g fresh weight (FW) and a 4.1 fold variation in ascorbate amount (51-211 mg ascorbate/100 g). We found no correlation between the amounts of oxalate and ascorbate, which supports the hypothesis that if oxalate derives from ascorbate then it is by a regulated process. The results also show that it is possible to select for low fruit oxalate and high ascorbate. Oxalate from three genotypes of A. chinensis and one each of A. deliciosa and A. eriantha was measured in flower buds, flowers, fruits and leaves, at different stages of development. Oxalate was highest in fruit at I week after anthesis (WAA) and declined during fruit development to reach its lowest amount in mature fruit (15-21 mg/100 g FW). Oxalate amounts were up to 46 times higher in leaves than in fruit. A. eriantha mature leaves showed oxalate amounts up to 1588 mg/100 g FW. The high oxalate content of leaves supports the notion that oxalate may serve a defensive role and/or calcium storage function to regulate soluble calcium concentration in the plant.