生理活性物質によるウンシュウミカンの着花・結実の制御とその過程における内生植物ホルモンの動態

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It is very important to control alternate bearing in satsuma mandarin (Citrus unshiu Marc.) and to maintain regular yields year after year. The fluctuation of fruit production may be able to control by appropriate pruning, fertilization, and fruit thinning, but less labor-intensive and immediate effective method is needed. The control of flowering or fruit setting with plant growth regulators has been investigated for supporting to stabilize the production. However, the technique of chemical control for improving flower and fruit set of satsuma mandarin has never founded. Gibberellin A_3 (GA_3) is known to reduce flowering of satsuma mandarin, when it sprayed in autumn or winter. If the excessive endogenous GAs inhibit floral initiation, the anti-GA substances may induce the flower set through the reduction of GA level. The physiological fruit drop also affects the production. Ethylene is the main hormone related to senescence and abscission, and it promotes the physiological fruit drop occurred 10 days to 2 months after anthesis in satsuma mandarin. The physiological fruit drop may be prevented by inhibition of endogenous ethylene release. This study has been undertaken with a practical and physiological view to clarifying the properties of new gibberellin synthesis inhibitors to promote the flower initiation and etylene synthesis inhibitor to inhibit the physiological fruit drop in satsuma mandarin. 1. The effects of new-groups of inhibitors of GA biosinthesis; compounds with a nitrogen-containing heterocycle (paclobutrazol, uniconazole-P) and cyclohexatrione (prohexadione-Ca), on flower setting of satsuma mandarin were evaluated. Paclobutrazol, uniconazole-P, and prohexadione-Ca were applied by spraying with 1,000 ppm solutions in winter. There was a significant increase in the number of flowers by paclobutrazol treatments made on 10 and 30 Jan., uniconazole-P at 30 Jan., and prohexadione-Ca at 20 Dec., 10, and 30 Jan. On the contrary, GA_3 application reduced the number of flowers and inhibited bud sprout break. Foliar applications of the above 3 GA inhibitors at 100, 300, 1,000 and 3,000 ppm were made on 30 Jan. Paclobutrazol at 100 to 1,000 ppm, uniconazole-P at 100 ppm and prohexadione-Ca at 300 and 1,000 ppm were effective for the flower induction. Treatments with 100 ppm paclobutrazol and 3000 ppm prohexadione-Ca induced the dropping of flowers and fruitlets. To determine the optimum time(s) of application, trees were sprayed with paclobutrazol, uniconazole-P, and prohexadione-Ca on 10 Jan., 30 Jan., and 20 Feb., repeatedly. With any agent, thrice treatments were most effective in promoting flowering, but once and twice applications were sufficient for a significant increase in flower bud formation with any chemicals. 2. Seasonal changes of contents of endogenous GAs in vegetative organs and effect of harvesting date on endogenous GA synthesis were surveyed. Simultaneously, the influence of paclobutrazol on the contents of GAs in leaves was evaluated. GA_1 content in leaf of vegetative shoot increased from 26 Jul. to 26 Sep., decreased to 28 Jan., and increased again to 24 Mar. On the contrary, GA_<20> and GA_<19> -like activities were the lowest on 26 Sep.; their maximum activities were recorded on 29 Nov. and 28 Jan., respectively. The trends of GAs in the axillary buds were closely paralleled with those in the leaves. A foliar spray with 1,000 ppm paclobutrazol on 1 Feb. increased the numbers of flowers, and decreased GA_<20> and GA_<19> -like activities in leaf, but it did not affect GA_1 activity. The early harvest in October increased the number of flowers the following season compared to the late harvest in December. GA_<20> activity in the leaves of the vegetative shoots on the early harvested tree was lower than those of the late harvested tree at the end of November and January. Thus, paclobutrazol treatment reduced the level of GA_1 precursor, but not GA_1, which is in agreement with the metabolism of endogenous GAs in the early harvested tree. The inhibition of GA synthesis by paclobutrazol is one of the physiological evidence which indicate that GA plays a role in flowering in satsuma mandarin. 3. Inhibition of physiological fruit drop in satsuma mandarin with AVG (2-aminoethoxy vinyl glycine); ethylene synthesis inhibitor, was evaluated. AVG sprayed to fruits of 7- and 33-year-old satsuma tree reduced physiological fruit drop effectively. AVG improved leaf-fruit ratio of poor fruiting young tree. The most suitable content of AVG was 100 to 300 ppm. The AVG application at end of petal fall phase to end of style fall reduced fruit drop more than at full bloom. The repetition of AVG treatment added to the effect. 4. Influence of AVG on ethylene production by the fruit and the contents of other hormones in the fruit were evaluated. Ethylene production of fruit treated AVG at end of petal fall phase was reduced significantly during 2 weeks after application. This inhibition disappeared by 4 weeks after application. ACC content in fruit treated AVG at end of petal fall phase was lower than non-treated fruit during 2 weeks after application. These results indicate that efficacy of AVG continued for 2 to 3 weeks after treatment. IAA content in fruit treated AVG increased after treatment and peaked 3 weeks after application. ABA content of non-treated fruit increased at style drop phase, while that of treated fruit kept low content. But this difference disappeared by 3 weeks after application.
大阪府立大学の論文
1997-03-31

生理活性物質によるウンシュウミカンの着花・結実の制御とその過程における内生植物ホルモンの動態

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