Variation in pedicel-fruit retention force in sweet cherry (Prunus avium L.)

Zhao, Y., A. Dahl, B. Athanson, E. Smith, M. Whiting and N. Oraguzie

Sweet cherry is one of the most labor-intensive fruit crops due to hand harvesting. Successful mechanical or mechanically-assisted sweet cherry harvest requires a low retention force between the fruit and pedicel. To enable the adoption of future harvest technologies, it is necessary to identify cultivars and their progeny that combine low pedicel-fruit retention force (PFRF) with excellent fruit quality.

Experiments were conducted during 2009, 2010 and 2011, at the Washington State University Irrigated Agriculture Research and Extension Center, Prosser, USA. A total of 19 cultivars and 12 F1 seedlings in 2009, 29 cultivars and 19 F1 seedlings in 2010 and 29 cultivars only in 2011 were assessed for PFRF and their fruit quality attributes. Identical trees from 18 cultivars were used in both 2010 and 2011. Individuals were evaluated at commercial maturity by experienced horticulturists and assessed within 24 hours of harvest in the lab. PFRF and stem length were recorded in the field in 2009 on fruit harvested with pedicel on, while in 2010 and 2011, PFRF was recorded with the fruit on the tree, to capture the force required to detach the fruit from the tree for comparison with fruit-pedicel retention force which was our main interest.  Fifty fruit were randomly sampled from each cultivar in 2009 and 30 in 2010 and 2011, while 10 were tracked in 2009 and 15 in 2010 and 2011 for measurement of fruit weight, size, exocarp color, firmness, and soluble solids content.  The remaining samples were divided into 3 groups of 5 fruit for measurement of titratable acidity along with other quality attributes mentioned above, to provide enough replication for statistical analysis.

A total of 1249, 676 and 615 PFRF values were recorded in 2009, 2010 and 2011, respectively. The data was adjusted to account for the force required to separate the pedicel from the tree (recorded in 58% and 71% of the fruit samples in 2010 and 2011, respectively), before analysis of variance for PFRF. Both F1 seedlings and the cultivars assessed showed a wide range of variation for PFRF in all three years, with values ranging from 0.14 to 2.27 kg/f. The frequency distribution showed continuous variation (Fig. 1), suggesting that PFRF is a quantitative trait. The F1 seedlings had lower PFRF (0.56 kgf in 2009 and 0.88 kgf in 2010) compared to cultivars (0.81 kgf in 2009 and 1.03 kg/f in 2010)(P<0.0001). Correlations between PFRF and fruit quality attributes were generally low (Fig.2), suggesting that PFRF has minimal influence on fruit quality. In 2010 and 2011, significant differences for PFRF were observed among varieties and between years, as well as for the interaction between variety and year (p<0.0001). This highlights the importance of both genetics and environment on PFRF. Interestingly, ‘4.10.5-31’, a progeny of ‘Lapins’ and ‘Chelan’, had the lowest mean PFRF in 2009 (0.27 kg/f) and 2010 (0.55 kg/f) suggesting transgressive segregation since both parents are high PFRF cultivars. These results suggest that PFRF can be improved by breeding although the significant genotype x year interaction highlights the importance of accurate phenotyping to maximize the genetics.

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