Russeting and Relative Growth Rate Are Positively Related in ‘Conference’ and ‘Condo’ Pear

Russeting is an important surface disorder in fruit and mechanical growth stresses, among other factors, are considered causal in russet induction. To test this hypothesis, fruit development and russeting were monitored on a whole fruit level and also in the calyx, cheek, and neck region of developing 'Conference' and 'Condo' pear fruit (Pyrus communis L.). To quantify growth, the pear fruit was geometrically modeled as approximating to half of a prolate spheroid for the calyx region and two truncated cones for the cheek and neck regions, respectively. Mass and surface area of 'Conference' and 'Condo' fruit increased in a single sigmoidal pattern with time. Fruit volume, determined by buoyancy, using a hydrostatic balance, and the Archimedes' principle was closely related to that predicted by the model from fruit geometry. Growth rates of surface area in 'Conference' and 'Condo' peaked at approximate to 90 and 100 days after full bloom (DAFB), respectively, and were highest in the calyx followed by the cheek and neck regions. Relative growth rates, calculated by dividing growth rates by the absolute surface area present at that time, were at maximum during early development and thereafter continuously declined. In general, relative growth rates were highest for the cheek region, intermediate in the calyx, and lowest for the neck. 'Conference' fruit were always more russeted than 'Condo' with russeting generally decreasing from calyx to cheek and neck. Furthermore, russeting increased rapidly in 'Conference' during early development until approximate to 70 DAFB, particularly in the calyx and cheek regions and, to a lesser extent, in the neck region. There was little change in russeting after approximate to 70 DAFB. Plotting rates of russeting vs. relative growth rates in surface area indicate a positive and common relationship across regions where russeting increased when relative growth rates exceeded 0.03/day. Thus, differential growth rates between regions within 'Conference' or 'Condo', but not across the two cultivars, accounted for topical differences in russeting.