Abstract
The capacity of apple trees to produce fruit of a desired diameter, i.e., fruit-bearing capacity (FBC), was investigated by considering the inter-tree variability of leaf area (LA). The LA of 996 trees in a commercial apple orchard was measured by using a terrestrial two-dimensional (2D) light detection and ranging (LiDAR) laser scanner for two consecutive years. The FBC of the trees was simulated in a carbon balance model by utilizing the LiDAR-scanned total LA of the trees, seasonal records of fruit and leaf gas exchanges, fruit growth rates, and weather data. The FBC was compared to the actual fruit size measured in a sorting line on each individual tree. The variance of FBC was similar in both years, whereas each individual tree showed different FBC in both seasons as indicated in the spatially resolved data of FBC. Considering a target mean fruit diameter of 65 mm, FBC ranged from 84 to 168 fruit per tree in 2018 and from 55 to 179 fruit per tree in 2019 depending on the total LA of the trees. The simulated FBC to produce the mean harvest fruit diameter of 65 mm and the actual number of the harvested fruit >65 mm per tree were in good agreement. Fruit quality, indicated by fruit's size and soluble solids content (SSC), showed enhanced percentages of the desired fruit quality according to the seasonally total absorbed photosynthetic energy (TAPE) of the tree per fruit. To achieve a target fruit diameter and reduce the variance in SSC at harvest, the FBC should be considered in crop load management practices. However, achieving this purpose requires annual spatial monitoring of the individual FBC of trees.
Highlights
The number of apples per tree is negatively correlated to the mean fruit fresh mass (FM), coloration (Palmer et al, 1997), soluble solids content (SSC) (Link, 2000; Serra et al, 2016), and flower set in the following season (Handschack and Schmidt, 1991)
Sigmoid-growth functions were applied to interpolate the measured values of FM and Cfruit and model the increase of FM and Cfruit during fruit development (Supplementary Equations 1, 2)
The simulated growth curves showed a horizontal shift explaining the difference of fruit FM and C content multiplicative distributed over the season (Figure 1)
Summary
The number of apples per tree is negatively correlated to the mean fruit fresh mass (FM), coloration (Palmer et al, 1997), soluble solids content (SSC) (Link, 2000; Serra et al, 2016), and flower set in the following season (Handschack and Schmidt, 1991). A high percentage of flowers and later fruitlets will be naturally shed in flower or fruit abscission, often too many fruit remain on the tree. The distribution of fruit throughout the canopy may not be uniform, Fruit Bearing Capacity which is one reason for the variability of fruit quality within the tree. Crop load management is required to adjust the number of fruit per tree. Various strategies to obtain one to two fruit per flower cluster widely distributed in the canopy exist, targeting a high percentage of high-quality fruit and, high crop value in the current and sufficient flower bud initiation for the subsequent growing season (Costa et al, 2018). For developing efficient crop load management, the information on the optimal number of fruit per tree is crucial
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