Early flower bud development and plant growth regulators to improve return bloom of pears

Three major ontogenetic processes are distinguished in pome trees: flower induction, initiation, and differentiation and after that anthesis. Flower initiation is an irreversible process marked by histological changes and intensive mitotic activity. The formation of an apical dome is the first visible signal of floral initiation. The use of plant growth regulators (PGRs) to increase flower formation in apples has been extensively used, however, the results are not always satisfactory, probably because of varietal differences and excessive crop load during flower induction and differentiation. There is limited information on the early floral development and the use of PGRs on return bloom of pears, thus, the objective of this research was to determine the time of visible transition from vegetative to floral meristem and investigate the effect of various PGRs on return bloom of pears. Studies were carried out in New York State, USA and Southern Brazil. Spur buds on one-year-old wood of various Asian and European pear varieties were sampled weekly throughout the spring/summer (June/September) of 2017 in Geneva, NY, USA. Samples were dissected under a dissecting microscope and classified as floral (domed meristem) or vegetative (flat meristem). Pear varieties differed in their time of flower initiation. For some varieties, doming or flower initiation began as early as 65 days after full bloom (DAFB) (‘Shinseiki’) or as late as 95 DAFB for ‘Comice’. Ethephon alone or combinations of Ethephon and NAA sprayed between 40 and 100 DAFB (ethephon 100-300 mg L^‑1 or naphthaleneacetic acid 5-10 mg L^‑1) significantly increased return bloom of ‘Rocha’ and ‘Packhams Triumph’ pear when compared to the untreated control trees in southern Brazil. Rates, timing and interaction with crop load still need to be fine-tuned for optimal usage. Our studies clearly show that it is critical to understand and identify the timing of flower induction/initiation of each variety before implementing any strategy to increase return bloom, including the use of PGRs.