Predicting fruit maturation in kiwifruit (Actinidia deliciosa)

In New Zealand, 6.2% soluble solids concentration (SSC) is the minimum maturity at which kiwifruit destined for export can be harvested. Ambient temperature during the fruit maturation period has been shown to affect the soluble solids accumulation rate. The purpose of the present study was to describe variation in the rate of fruit soluble solids accumulation in different regions and seasons and to develop a means of quantifying the effects of temperature on the rate of fruit maturation. Measurements of fruit SCC were made over a three year period on one orchard in each of six principal kiwifruit-growing regions across New Zealand. The date at which fruit reached 6.2% SSC varied by 34 d across all sites and years. There was an 18 d range in the average date across ail sites, with Riwaka, the coolest site being the earliest, and Kerikeri, the warmest site, being the latest. The variation from year to year across all sites was 12 d. We formulated a non-linear, time-dependent model to predict the rate of change of SSC with time, given the temperature and the time since flowering. This model accounted for nearly 60% of the variation in the date that 6.2% SSC was reached and 70% of the variation in the date that 6.5% SSC was reached, if a common initial SSC was assumed across all sites and seasons. No improvement was obtained if the initial SSC was measured about three months after flowering, rather than assumed. A similar model which used the day of year rather than days since flowering was found to be slightly less accurate. However, all three versions of the model performed better than a linear regression model based on mean temperature for the fruit growth period and the date at which 5.0% SSC was reached.