Fruit of three kiwifruit (Actinidia chinensis) cultivars differ in their degreening response to temperature after harvest

Commercial kiwifruit (Actinidia chinensis) cultivars include both green-fleshed and yellow-fleshed fruit. While both types are green-fleshed during the early stages of fruit development, the yellow-fleshed fruit undergo a marked green to yellow transition associated with degradation of chlorophyll (degreening). To better understand the postharvest temperature sensitivity of the degreening process on off-vine fruit, three kiwifruit cultivars were selected: ‘Zesy003’, ‘Zesh004’ and ‘Hayward’, each having a different degreening behaviour. ‘Zesy003’ is mar- keted as a yellow-fleshed fruit and ‘Hayward’ and ‘Zesh004’as green-fleshed, despite the significant degreening of the last cultivar late in development. The degreening responses of harvested fruit of the three cultivars to a range of temperatures (1 °C–15 °C) were quantified both by flesh-colour measurement and by analysing the expression of the genes for the main enzymes involved in the chlorophyll degradation pathway. Using the newly annotated kiwifruit genome, sequences for genes involved in chlorophyll metabolism were determined, in- cluding STAY GREEN 2 (SGR2), PHEOPHORBIDE HYDROLASE 1 (PPH1), PHEOPHORBIDE A OXYGENASE (PAO1) and RED CHOROPHYLL CATABOLITE REDUCTASE (RCCR) and their expression compared. In spite of the inherently different degreening patterns of the three cultivars, all responded similarly to temperature, showing a faster flesh-colour change at high temperature and a slower or absent colour change at low temperature. In absolute terms, fruit of ‘Zesy003’ degreened faster than ‘Zesh004’ or ‘Hayward’. Expression of two genes in- volved in the chlorophyll degradation pathway, PAO1 and SGR2, was higher in ‘Zesy003’ than in ‘Zesh004’ or ‘Hayward’. SGR2 expression in ‘Zesy003’ was temperature sensitive: it decreased rapidly following harvest, with colder temperatures causing a more rapid down regulation. Likewise PAO1 expression was reduced at lower temperatures. Given the demonstrated activity of SGR2 (and PAO1), this suggests that these genes are important in regulating chlorophyll degradation in yellow-fleshed kiwifruit and partially account for temperature sensi- tivity.