Abstract: Kiwifruit is one of the most recent successful fruit crops on the market. Alongside the well-known green-fleshed
Actinidia chinensis var.
deliciosa ‘Hayward’, other species with different flavour, shape and colour (yellow and red) have been used in breeding programmes to develop new cultivars. Novel coloured kiwifruit are attractive to consumers, but achieving uniform fruit pigmentation, particularly in red-fleshed
Actinidia chinensis var.
chinensis genotypes, is challenging. Colour inconsistency between fruit can affect consumer perception, lowering returns to growers. To investigate the cause of colour inconsistency we focused on a solid red-fleshed genotype characterised by variable outer pericarp red pigmentation, we hypothesised that the carbohydrate supply could be responsible for the variation of the red flesh colour. Early in fruit development (28 days after anthesis, DAA) we adjusted the leaf-to-fruit ratio of the shoots to two different carbohydrate supplies (standard and low). Carbohydrate import or redistribution outside of the shoot was controlled by applying a girdle at the base of the shoot, and this girdle was maintained open until harvest to guarantee long-term control of carbohydrate supply. From 84 DAA we observed a reduction in fruit size by about 30 percent, dry matter was reduced by more than 20 percent and colour development by more than 80 percent when fruit had low carbohydrate supply. Both anthocyanin and carbohydrate metabolites were affected. The concentration of major non-structural carbohydrates (starch, glucose, sucrose, and fructose) were also reduced by more than 50 percent in fruit with low carbohydrate supply from 84 DAA. A minor sugar, galactose, was also dramatically reduced by low carbohydrate supply. By 112 DAA, total anthocyanin concentration in the outer pericarp of fruit with low carbohydrate supply was reduced by more than 80 percent. Despite these significant changes in fruit development and metabolites, transcription for candidate genes considered critical steps in the anthocyanin biosynthetic pathway (i.e.
GT1,
MYB10 and
bHLH5) were unchanged between fruit with the two carbohydrate supplies. Interestingly, in low carbohydrate supply fruit, a vacuolar invertase gene (
INV3) was upregulated at 112 DAA and a beta-amylase gene (
BAM9) was upregulated at 84 and 112 DAA. The upregulation of these two genes could be correlated to the low carbohydrate supply available to the fruit and the need to deplete the starch stored in the fruit to support further development. This work suggested a link between carbohydrate and anthocyanin metabolism, and we now have two further hypothesis to test: i) Galactose is the glycosyl moiety of the main kiwifruit anthocyanin measured in these fruit, and given that galactose content was affected by low carbohydrate supply, repression of colour development could be due to substrate limitation; and/or ii) Carbohydrate depletion could have an effect on an unknown repressor of the anthocyanin pathway. We are currently performing a transcriptomic (RNAseq) experiment to test these hypotheses and results should be available soon.
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