Modelling of colour development in the fruit of Actinidia chinensis 'Hort16A'

Colour development in fruit of Actinidia chinensis var. chinensis 'Hort16A', the new yellow-fleshed cultivar produced commercially in New Zealand, was monitored during three seasons in four kiwifruit-producing districts of New Zealand. Fruit were destructively harvested to measure flesh colour between flowering and harvest. Flesh colour of the outer pericarp remained green (115° hue angle) until c. 140 days after mid bloom (DAMB). Then flesh colour changed following a sigmoid like pattern to a yellow hue (97-100° hue angle) by 220 DAMB. The change in hue angle is shown to be well represented by the complementary log-log function. The scale parameter describing this transition was independent of site and season, whereas timing of the transition between the upper and lower asymptote was dependant on both site and season because of a small dependence on the average of the maximum temperature over the period of 100-150 DAMB. Each degree C increase in maximum temperature during the 100-150 DAMB delays colour development by c. 3 days. Distribution of individual hue angles about the mean value was best described by a constrained beta distribution defined by two parameters, which in turn uniquely specify the average hue angle and variance of the population at that time. For hue angles near the mid range, which occurs before harvest maturity, this beta distribution is well approximated by a Normal distribution with the same mean and variance, but at harvest maturity a beta distribution is a better description. When the average hue angle is 103°, the difference between the 97.5 percentile on the Normal approximation and the beta distribution is c. 0.6. This difference rises rapidly as the hue angle drops further.