Response to Inbreeding of Early Seedling Growth and Fruit Traits in a Prunus domestica L. Breeding Population

This is a continuation of an inbreeding study done in 2002 that tested for the presence of inbreeding depression in a Prunus domestica (European plum) breeding population at the University of California at Davis. That study found inbreeding depression affecting seed germination and growth traits in the first season of growth. This study tested the same population for inbreeding depression in relation to field survival (2003-2008), young tree growth traits (height (2004) and cross sectional trunk area (2004-2008)), and fruit traits (fruit set (2004-2008), fruit size and soluble solids content at harvest (2007)). The test population was created with controlled crosses using 20 elite parents. The final population was made up of 11 self-pollinated families (inbreeding coefficient F=0.5), 10 full-sibling families (F=0.25), 11 non-related crosses (F=0) and a control set of random-matings (average F=0.23) that had accumulated inbreeding over 2 to 5 generations with intervening cycles of selection. In spring of 2003, part of each treatment group was planted at either 0.79 m spacing or 1.22 m spacing within a randomized block design at the UC Davis pomology orchards. Survival of seedlings from self-pollinated parents was significantly lower (89%) than non-related progenies (97%). Tree spacing significantly affected tree growth and time of first fruit set, with the 0.79 m spaced trees being smaller but setting fruit a year earlier than trees at the 1.22 m spacing. Trees resulting from greater inbreeding had smaller mean trunk cross sectional areas when grown at the 1.22 m spacing.