Predicting harvest date for kiwifruit: Variation of soluble solids concentration with mean temperature

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Abstract
The increase in soluble solids concentration (SSC) of kiwifruit ( Actinidia deliciosa cv. Hay ward) maturing on the vine was monitored in 35 orchards in eight regions of New Zealand during 1981 and in 37 orchards during 1982. During these 2 years the date of commercial maturity (SSC of 6.2%) ranged from 16 April at Nelson to 25 May at Kerikeri, a spread of 39 days. It tended to be earliest at the southern‐most and hence coolest sites. A simple model that has been used by others to predict the date of commercial maturity was tested. This model relates the time taken for fruit to increase in SSC from 5.0 to 6.5% to the mean air temperature during that period. Our data confirmed that the rate of increase in SSC increased as the mean temperature decreased from 15 to 11°C. However, the predictive model was not consistent between or within seasons. There is some evidence that this lack of consistency is caused, at least in part, by factors other than temperature influencing the rate of SSC increase. The rate of increase in SSC was inversely correlated with the date that fruit in the orchard reached a SSC of 5.0%. This suggests that the state of vine development influences the rate of maturation. It is also possible that the description of temperature was inappropriate, either in the use of mean temperatures, or because air temperatures recorded at standard meteorological sites did not adequately represent the temperature of the fruit.

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