Refining the Estimation of Kiwifruit Size from Linear Fruit Dimensions

The ability to predict kiwifruit yield and size profile allows fine tuning of on-orchard vine management processes to maximise fruit growth rate. Fruit growth curves for kiwifruit are usually determined from repeated measurements of fruit length (L), and the minimum (D1) and maximum (D2) diameters, leading to models for estimating fruit weight. A reduction in estimated fruit weight has been reported for fruit late in development. In this paper, the relationship between fruit dimensions and fruit size (weight and/or volume) has been investigated for Actinidia chinensis var. deliciosa 'Hayward' and A. chinensis var. chinensis 'Zesy002' (marketed as Zespri (R) SunGold Kiwifruit; commonly called Gold3). It was hypothesised that a change in the relative dimensions of the fruit might occur as the fruit softened. It was found that the three linear dimensions of the fruit used in the estimation of fruit weight did decrease slightly as the fruit changed to rapid softening, with a larger change in 'Zesy002' than 'Hayward'. A consistent gradual increase in fruit density occurred for both cultivars over the period fruit were monitored, even after the cessation of dry matter accumulation. It is suggested that the apparent late season 'shrinkage' of kiwifruit when monitoring fruit growth on the vine is largely an artefact of the application of a single model based on linear fruit dimensions at all stages of development. This approach disregards other changes in the fruit shape and softening associated changes that affect fruit density. Inclusion of a fruit age element into the equations estimating fruit size improves the accuracy, but without defining specifically the aspect of biology driving the changing relationship.