Determination of cortex and pith contributions to fruit morphology in apple (Malus×domestica Borkh.) using image analysis

The apple (Malus × domestica) fruit consists of two main fleshy tissues, cortex and pith, that are of diverse origins and contribute differentially to final fruit size. The contribution of these fruit spatial constituents to variation in fruit morphology has not been determined previously. We developed an image-analysis based method that uses two-dimensional images of the fruit sectioned in the proximo-distal plane to determine fruit, cortex, and pith volumes, and key shape descriptors. Fruit volume determined by image-analysis was compared to that determined using water displacement. These two methods were strongly correlated (R2 = 0.95) and displayed good agreement. The cortex volume constituted majority (> 86 %) of the fruit. Variation in cortex volume explained most of the variation in fruit volume (R2 = 0.99), while that of the pith explained a relatively minor portion (R2 = 0.32). Further, variation in pith volume explained only a minor part of cortex volume variation (R2 = 0.24), suggesting that growth in these tissues is regulated by a combination of shared and differential mechanisms. Proportion of the fruit constituted by cortex displayed limited variability across 28 apple cultivars. However, proportion of cortex was substantially lower, and that of pith higher, in a crabapple (Malus sp.) cultivar, Callaway, suggesting that change in tissue proportions contributes to increase in fruit size from that observed in crabapples. Limited variation in the fruit aspect ratio, a key shape descriptor, was noted in the apple cultivars analyzed. A weak relationship was observed between the fruit aspect ratio and that of the core (R2 = 0.29), indicating that differential mechanisms determine shapes of the fruit and its internal tissues. Together, data from this study suggest that growth of cortex and pith tissues is mediated in part by shared, and substantially by differential mechanisms.