In 2004, Clark et al. released the first commercial primocane-fruiting (PF) blackberry cultivars, Prime-Jan® (cv. APF-8) and Prime-Jim® (cv. APF-12) (Clark et al., 2005). ‘Prime-Ark® 45’ (cv. APF-45) followed in 2009 (University of Arkansas, 2010). ‘Prime-Jim’® and ‘Prime-Jan’® have had some production limitations. Both cultivars have exhibited poor summer heat tolerance in field trials (Clark et al., 2005; Drake and Clark, 2003) and controlled-environment studies (Stanton, 2005), resulting in low berry weight, crumbly berries, and poor flavor. Self-incompatibility was also thought to be a factor in fruit set (J.R. Clark, personal communication).
Stanton et al. (2007) concluded that high temperatures (35.0/23.9 °C day/night) induced male sterility and affected fruit set in both ‘Prime-Jim’® and ‘Prime-Jan’®. ‘Prime-Jim’® appeared to have less tolerance for high temperatures than ‘Prime-Jan’®. Although negative effects associated with high temperatures have been confirmed through controlled studies, low fertility has not been eliminated as a contributor to low fruit set and poor fruit quality.
Fertility problems (concluded from decreased berry set, drupelet set, and/or seed number) as well as reduced pollen quality have been reported for many species of Rubus (Daubeny, 1969, 1971; Hellman and Clark, 1984; Keep, 1968; Nybom 1985, 1986; Redalen, 1976). Selection A-593, a genotype in the lineage of ‘Prime-Jim’® and ‘Prime-Jan’®, showed reduced fruit set in self-pollinated flowers (Perry and Moore, 1985). Selfed A-593 had ≈40% fruit set; when crossed with ‘Cheyenne’, fruit set increased to 60%. Fruit weight as well as mean seed number was also higher for A-593 crossed with ‘Cheyenne’ compared with self-pollinated A-593. Although cross-pollination was superior to self-pollination, fruit set and weight were still lower than the other genotypes in the study, implying that low fertility was a problem for A-593 (Perry, 1981; Perry and Moore, 1985). Because A-593 was a foundational parent in the Arkansas PF blackberry breeding effort and is found in the pedigree of all released and potential releases from the program, this finding is of note. If this low fertility characteristic was transmitted to PF genotypes, this could become an issue if these cultivars or subsequent PF genotypes are grown in solid blocks where other sources of pollen are not available. There is a need to evaluate fertility in ‘Prime-Jim’® and ‘Prime-Jan’® as well as in subsequent generations that are believed to have better fruit set and heat tolerance based on field observations between 2005 and 2007. Some commercially important thornless floricane-fruiting (FF) cultivars also need to be evaluated because the fertility of each has not been thoroughly assessed.
The objectives of this study were to: 1) evaluate the effects of self- and cross-pollination on floricane and primocane fertilization and fruit set among a set of PF and FF genotypes in a field setting; and 2) evaluate different components of flower fertility in a controlled environment.
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