Rootstock and crop load effects on 'Honeycrisp' fruit quality and photosynthetic performance

Rootstock and crop load are two critical factors in apple orchard management that can affect scion photosynthetic performance and fruit quality. However, the interactions between both factors on the physiology of ‘Honeycrisp’ apple under Washington State semiarid climate conditions have not been studied extensively yet. The objectives of this research were (I) to assess the effects of selected rootstocks and crop load levels on ‘Honeycrisp’ fruit maturity and quality and (II) to determine the photosynthetic performance and proportions of carbohydrates in leaves and fruit of ‘Honeycrisp’ under different rootstock and crop load combinations. In 2019 and 2020, ‘Honeycrisp’ trees grafted on five rootstocks (‘M.9-T337’, ‘B.9’, ‘G.11’, ‘G.41’ and ‘G.935’) were hand-thinned to 3, 5 and 7 fruit/cm2 trunk cross-sectional area (TCSA). Bitter pit incidence was evaluated at harvest, and fruit quality parameters were assessed after one month of storage (1 °C and regular atmosphere, RA). In 2020, ‘Honeycrisp’ trees grafted on ‘M.9-T337’, ‘G.41’ and ‘G.935’, were hand-thinned to low and high crop load levels. Through the season, leaf gas exchange and chlorophyll fluorescence measurements were conducted, followed by sampling for carbohydrate composition determination. Fruits from all trees were harvested at maturity and processed for carbohydrate analysis two months after storage (1 °C, RA). Fruit from ‘G.11’ thinned to 3 fruit/cm2 TCSA presented the highest bitter pit incidence across all treatments. Among rootstocks, ‘B.9’ enhanced fruit coloration and induced earlier maturation, lower firmness and titratable acidity. In general, fruit size increased with decreasing cropping levels. Maturity was delayed, and red color development was reduced when trees were thinned to 7 fruit/cm2 TCSA. Rootstock ‘G.935’ and high crop load enhanced carbon assimilation of ‘Honeycrisp’, as indicated by increased leaf net photosynthesis. Leaves from high cropping trees also had a lower concentration of soluble carbohydrates. Neither rootstock nor crop load had a major impact on fruit carbohydrates composition. Overall, the results of this study provided insights about the impact of different rootstock and crop load levels on ‘Honeycrisp’, which could help optimize management strategies for this cultivar under the unique growing conditions of Washington State