Differences in apple fruit shape are independent of fruit size

Abstract

Fruit quality is crucial in breeding new apple varieties. Before tasting, consumers assess freshness and flavor based on the physical appearance of fruit. Understanding how fruit quality traits such as shape and size vary across diverse apples [Malus domestica (Suckow) Borkh.] provides a foundation for future breeding efforts. We analyzed images of 5724 apples representing 743 different trees and 534 unique accessions from Canada's Apple Biodiversity Collection to quantify variation in fruit shape and size. To achieve this, we used a pseudo-landmarking approach paired with traditional linear measurements including length, width, area, solidity, circularity, and aspect ratio. We also incorporated previously collected fruit weight measurements from the same trees. Using a comprehensive measure of shape, we determined that the primary source of variation in apple fruit shape, or morphometric principal component 1 (PC1) which explained 22.7% of the variation, was most highly correlated with the width to length (aspect) ratio of the fruit (ρ = −0.964, p < 1 × 10−15). In contrast, PC1 was not significantly correlated with differences in fruit size as measured using area and harvest weight. Our findings indicate that two critical aspects of morphological variation in apple—fruit shape and size—are independent, suggesting it is possible to select for a diverse range of fruit shapes while maintaining a consistent and marketable size.