The effect of TH-656 on fruit thinning in Satsuma orange I

Abstract

Because of TH-656 [1-(α-naphthaleneacetyl)-3, 5-dimethylpyrazol] showed the higher biological activity towards various kinds of plants than NAA (α-naphthaleneacetic acid), the present authors compared the effect of TH-656 with that of NAA on abscission responses of the petiole explants of primary leaves in Phaseolus vulgaris L. (var. Oohirazayashakugosun). Following the above expriment, they investigated on the effect of the same compound on the thinning of fruit in Citrus Unshiu Mar. (var. Miyagawa) with the expectation of the more remarkable effect as compared to NAA.The results obtained were summarized as follows:1. The inhibiting effect of TH-656 on the abscission of the bean petiole explants increased with the concentrations at more than 10-7M, regardless of the application site of the compound. NAA also inhibited the abscission at all concentrations used in the present experiment, when applied at distal ends of petioles, but in the case of proximal application NAA promoted the abscission at all concentrations lower than 10-4M.2. The effect of TH-656 on the thinning of Satsuma orange fruit was examined with the concentrations of 50, 100, 200 and 400 ppm applied 10 or 30 days after the full bloom, respectively. As the results, the effect of TH-656 was definitely higher than NAA and its increased clearly with the increase of concentration. The result will show that the number of leaves per fruit can be controlled more precisely with TH-656 than NAA through the selection of adequate concentrations. The effects of TH-656 at 50 and 100 ppm were nearly equal to those of NAA at 200 and 300 ppm. When 200 and 400 ppm of TH-656 were applied most of the fruit were dropped off. But the compound applied at those higher concentrations did not cause any injuries to the sprouting leaves, branches and to the fruit harvested. Thus TH-656 is taken to be a very promising compound for the fruit thinning in Satsuma orange.