Three thinning chemicals, J-455, NAA and ethephon were applied to 'saijo' and 'Fuyu' adult trees by spraying at the concentration of 50 or 25 ppm 10 days before full bloom in order to compare their effects on the abscission and ethylene evolution of flower buds and blossoms (young fruits) following treatments.
The peak in the drop of flower buds and blossoms (young fruits) induced by J-455 sprays appeared 4 days in 'Fuyu' or 6 days in 'Saijo' after the applications regardless of the concentrations. The time courses of their drop induced by NAA and ethephon sprays exhibited almost the same patterns with those in the case of J-455 treatments, even though ethephon was less effective in flower bud thinning than the other two chemicals.
NAA spray at 50 ppm was most effective in thinning of flower bud among all treatments in each cultivar showing 61.8% in 'Saijo' or 54.2% in 'Fuyu' as the rate of flower bud drop. J-455 spray at 50 ppm resulted in the rate of 43.6% or 41.1% in each cultivar. When J-455 was applied at 25 ppm, the rates of flower bud drop became less than one third of those obtained at 50 ppm in both cultivars. So far as the blossom drop was concerned, J-455 applications caused higher rates than the other treatments.
The applications of NAA and J-455 in both cultivars, especially at 50 ppm in 'Saijo' gave rise to severe leaf droop lasting for several weeks but no such symptoms were observed in them when ethephon was applied even at 50 ppm.
NAA-induced ethylene evolution of flower buds in 'Fuyu' reached its peak 6-24 hrs after the treatments and then it depreciated sharply. The ethylene evolution stimulated by J-455 maintained a high level as long period as 24-72 hrs after spraying. The ethephon-treated flower buds released lower amount of ethylene than the NAA- and J-455-treated ones and exhibited no obvious peak in the time course of its evolution.