Fruit maturation and the soluble solids harvest index for ‘Hayward’ kiwifruit

•Increased rate of soluble solids (SS) accumulation can start before nett starch loss.•The period between the end of starch accumulation and nett starch loss is variable.•Developmentally programmed nett starch loss may be pre-empted by low temperature.•‘Hayward’ is late maturing – low temperature & increased SS accumulation may coincide.•Chilling tolerance for storage is associated with dark seeds and cessation of growth.

A harvest index of 6.2% soluble solids content (SSC) has long been used for New Zealand-grown ‘Hayward’ kiwifruit destined for storage and export, based on the soluble solids accumulation rate at this point having increased because of starch breakdown in response to low night temperatures. Fruit maturation in ‘Hayward’ and other kiwifruit cultivars has been investigated through monitoring changes in SSC, soluble carbohydrates and starch content as fruit developed on the vine. The initial increase in the rate of soluble solids accumulation was not necessarily the result of starch breakdown, occurring before a nett decrease in starch and possibly the direct result of imported soluble carbohydrates. It is suggested that SSC should be considered in the context of starch metabolism, for which there is a progression from starch accumulation to starch degradation, with a period of variable duration of no nett change in starch between these. The increased rate of soluble solids accumulation of fruit at 6.2% SSC may occur through either import of soluble carbohydrate or starch breakdown. Hence fruit at the same SSC may differ physiologically, thereby affecting fruit postharvest performance. The loss of temperature sensitivity of ‘Hayward’ kiwifruit occurred over a period of weeks and was initially associated with the change to dark seed coats, and was complete in association with the cessation of growth.