Percent Drupelet Set Varies among Blackberry Genotypes

Twenty-one genotypes of blackberry (Rubus L. subgenus Rubus Watson) were evaluated for percent drupelet set in 1993 and eleven genotypes were evaluated in 1994. Commercial cultivars were chosen to represent types (trailing, erect, and semi-erect) of blackberries grown in the United States. Secondary fruit were picked when green but developing a red blush during the early- and late-ripening season. Drupelets and pistils per fruit were counted to calculate percent set. In 1994, ripe fruit ranging in size were harvested for 'Boysen', 'Marion', 'Thornless Evergreen', and 'Chester Thornless' and drupelet per fruit were counted. Drupelet set among genotypes ranged from 40% to 86% in 1993 and 39% to 78% in 1994. Most genotypes had a higher percent set on early flowers compared to later ones. Drupelet set and number were not correlated with fruit weight among genotypes. In 'Boysen', 'Marion', 'Thornless Evergreen', and 'Chester Thornless', drupelet number was correlated with fruit weight, although the relationship in 'Chester Thornless' was not as strong as the others. It is unclear what factors limit drupelet set in the genotypes studied. The weight of a blackberry fruit is a function of the number of drupelets, the weight of each drupelet, and the receptacle weight. Fruit weight could be increased by increasing one or more of these components. In red raspberry, fruit with many drupelets are firmer than fruit with a similar weight having fewer, larger drupelets (Robbins and Sjulin, 1989). Increased seed weight was also an important factor associated with increased fruit cohesiveness (Robbins and Sjulin, 1989). However, with >95% of the black- berry production in Oregon used for processing, cultivars with large seeds (or receptacles) are undesirable. Larger fruit size, with corresponding higher yields, fruit with small receptacles, and increased fruit firmness are desirable traits for the Oregon process- ing industry. The first step in breeding for increased fruit weight is to evaluate the extent of variation for the main components related to fruit weight and, if possible, determine which traits are most important. Drupelet number is a function of the number of ovaries/pistils per flower and the percent drupelet set. Drupelet weight and number, as well as seed weight, vary among blackberry genotypes (Darrow and Sherwood, 1931; Moore et al., 1974). While Moore et al. (1974) studied the relationship between seed (drupelet) number and berry weight in erect blackberries, no one has studied percent drupelet set or compared different types of blackberries (erect, semi-erect, trailing) for this trait. The blackberry fruit is an aggregation of drupelets that develop independently while adhering to a common receptacle. Although ovule abortion frequently takes place, there is no abscission of the individual blackberry ovaries as occurs in many other types of fruit. Thus, the drupelets and unset ovules can be counted to determine percent drupelet set. This project was undertaken to study the variation among blackberry genotypes (representing different types of blackber- ries) for number of set and unset drupelets and percent drupelet set and their relationship to fruit weight.