Towards understanding the role of temperature in apple fruit growth responses in three geographical regions within New Zealand

The influence of temperature on growth of 'Royal Gala' apples on MM.106 roo tstock was studied using trees from one site in each of three regions of Ne w Zealand (Canterbury, Hawkes Bay and Nelson) over three consecutive season s. Temperatures were recorded as hourly means over each season. Growing deg ree day accumulation with a base of 10 degrees C (GDD(10)) and 5 degrees C (GDD(5)) were calculated, commencing at pollination. Crop loads were establ ished at bloom to a very low level to minimize competition between fruit an d provide a clear response of fruit growth to environmental conditions. Som e trees were shaded with 70% shadecloth for 7 d beginning either 60 d after pollination (DAP) or 90 DAP. In the third season, other trees were thinned to a medium crop load to compare their fruit growth with that from trees b earing low crop loads. There was a highly significant correlation between G DD(10) at 30 DAP and time from pollination to harvest (r(2) = 0.96). On the low crop load trees, fruit weight at 50 d was closely correlated with GDD( 10) accumulated over the 50 DAP (r(2) = 0.71). Furthermore fruit weight at 50 DAP was also significantly correlated with fruit weight at harvest (r(2) = 0.68) These results support an hypothesis that a potential maximum fruit size is set by about 50 DAP, determined by total fruit cell number, result ing from a temperature-responsive cell division growth phase. Under conditi ons of no Limitations after the cell division phase, all fruit cells would expand to their optimum size to provide the maximum fruit weight achievable for that cell number. Factors which limited carbohydrate availability, suc h as higher crop loads and shading of trees, reduced final fruit size. In t his study, shading for 7 d at 90 DAP did not reduce final fruit size and sh ading at 60 DAP reduced final fruit size only in some cases. Differences in fruit weight at harvest induced by low and high crop loads were expressed only at the sites that achieved large fruit weight in that season and exper ienced greatest early season GDD(10) accumulation (Hawkes Bay, Nelson). In a cooler region (or season), represented by Canterbury in this example, a f ruit weight response to crop load was much less apparent. Ideas towards dev eloping a practical methodology for predicting fruit weight at harvest are discussed.