Fertility Improvements In Primocane-Fruiting Blackberry (Rubus L. spp.) Genotypes

Variation in berry set and fruit size of the recently introduced primocane-fruiting (PF) cultivars has been observed, particularly in diverse climates. Therefore, there is a need to assess overall fertility and heat tolerance of this new type of blackberry. Five PF genotypes were tested for fertility in a field setting using primocane flowers with three pollination treatments.  Treatments included open-pollinated undisturbed, emasculated selfed, and emasculated and cross pollinated. Berry set was not significantly different between cross- and self-pollination treatments or genotypes.  The open-pollinated undisturbed flowers had the highest berry set and emasculated selfed flowers had the lowest. Berry set among pollination treatments ranged from 44 to 84%.  Cross pollination resulted in larger berries for ‘Prime Jim’^® and APF-45.   Rating for drupelet set was higher for ‘Prime Jim’^® and APF-59 for cross pollination compared to self pollination.  ‘Prime Jim’^® had significantly lower berry set and drupelet set rating than the five other PF genotypes. Screening for heat tolerance in a field setting can take multiple seasons or sites.  Having a screening protocol to be used in a laboratory setting would be beneficial to breeding programs because of the amount of time that could be saved.  In an effort to create a protocol, pollen from six PF genotypes was placed on germination media and incubated at three temperatures (23°C, 30°C, and 35°C) for 48 hours.  Media was then examined for germinated pollen grains.  There was no significant interaction between genotypes and incubation temperature.  Significantly more pollen germinated for APF-31, APF-45, and APF-59 than ‘Prime-Jim’^®.  Pollen germination decreased as temperature increased for all genotypes.  This suggests that higher pollen viability, and likely not improved heat tolerance, enhanced pollen germination.  While this study did not provide a successful heat tolerance screening protocol, it did reinforce findings from the field study.  Our results indicate that self fruitfulness appears to be adequate in all PF genotypes evaluated and that the newer-generation genotypes show improvement in fertility over ‘Prime-Jim’^®.