The effect of three plant bioregulators on pollen germination, pollen tube growth and fruit set in almond prunus dulcis (Mill.) D.A. Webb. Cvs. Non pareil and carmel

Background: In commercial almond Prunus dulcis (Mill.) D.A.Webb. orchards in Chile, the percentage of fruit set is low and commonly reaches 5–30%. As almond trees bloom during a cool period and also suffer from self-incompatibility, any factor that can improve pollination, pollen germination and pollen tube growth or extend the effective pollination period, such as the application of plant bioregulators (PBRs), should be beneficial for fruit production. Results: Three plant bioregulators (PBRs): brassinolide (BL), gibberellic acid (GA3) and kinetin (KN) were evaluated for pollen germination and pollen tube growth in vitro, as well as for fruit set in almond cultivars Non Pareil and Carmel, in central Chile, during the 2013 and 2014 seasons. For pollen germination in vitro, the BL concentration of 10 mg L^-1 had the highest value in Non Pareil (97.7%), after 4 h germination in 2014 growing season (the control was 90.9%). KN at a concentration of 50 μL L^-1 induced the longest pollen tube growth of 1243.4 μm in Carmel after 8 h germination in 2013 (the control was 917.7 μm). In Non Pareil, the highest percentage of fruit set (31.0%) was achieved in 2014 by spraying during bloom at pink bud stage with KN at 50 μL L^-1 (the control was 16.7%). Conclusions: A significant favorable effect of the tested PBRs was observed in pollen germination and pollen tube growth in vitro, as well as on fruit set in Non Pareil and Carmel almonds.