Meteorological and tree-management factors related to soluble solids content of apple fruit and crop load management for producing high soluble solids content fruit in high-density planted ‘Fuji’

Abstract

Sweetness is one of the most important drivers of consumer preference in apples (Malus x domestica Borkh.). Despite the market demand for apples with high sugar content, cultural and agronomic practices applied in the field for producing apples with high soluble solids content (SSC) has not been established. We tried to clarify what factors regulate the SSC of fruit by creating a regression model that predicts SSC with factors related to meteorology and crop load management as variables. The highest correlation with SSC was obtained from six-year meteorological data using a four-week average solar radiation value starting August 28. When the SSC was estimated by a regression model using the average solar radiation (SR), the number of fruits per cm2 trunk cross-sectional area (CL), and the number of fruits per tree (NF), the estimated and measured values were close to each other, indicating that year- and tree-dependent variation in the SSC of fruit can be explained mainly by the SR and the CL or NF, respectively. The four weeks starting August 28 coincided with the time when starch was most accumulated in fruit, and a high correlation was found between the starch content in this period and the SSC at harvest. It could be said that it is necessary to restrict the number of fruits per tree by the period—90 days after full bloom—to produce fruit with high SSC in ‘Fuji’. A target number of fruit per tree for maximizing the cumulative yield of fruit with an SSC of 14 oBrix or more was estimated to be less than that for maximizing the cumulative yield without biennial bearing.