Effects of gibberellic acid and dormancy-breaking chemicals on ower development of Rhododendron pulchrum Sweet and R. scabrum Don

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1 Scientia Horticulturae 83 (2000) 331±337 Effects of gibberellic acid and dormancy-breaking chemicals on ower development of Rhododendron pulchrum Sweet and R. scabrum Don Y.-S. Chang *, F.-H. Sung Department of Horticulture, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan, ROC Accepted 19 August 1999 Abstract Flower bud dormancy is a restriction in regulating the azalea owering period. Although many studies have been conducted on ending dormancy by cold temperature and GA treatment, little information is available on the effects of dormancy-breaking chemicals on terminating azalea ower bud dormancy. The purpose of this study was to determine whether dormancy-breaking chemicals affect the termination of ower bud dormancy and precipitate anthesis. Three-year-old pot-grown plants of Rhododendron pulchrum were sprayed with dormancy-breaking chemicals: 500 ppm GA 3, 0.245% cyanamide, 8% mineral oil, 2% potassium nitrate, and 0.5% thiourea on 31 December The results showed that GA 3 and thiourea brought on ower bud anthesis. Fiveyear-old pot-grown plants of R. scabrum were sprayed with similar dormancy-breaking chemicals on 23 December 1997 and exhibited the same results. In addition, potassium nitrate did not hasten ower bud anthesis, but it increased ower diameter. The results suggest that thiourea may be used to regulate the azalea owering period. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Dormancy-breaking chemicals; Flower bud development; Flowering; Gibberellic acid; Rhododendron 1. Introduction In order to survive adverse environmental conditions, deciduous woody plants develop a dormancy mechanism. However, in regions where winter is warm, the * Corresponding author. Tel.: ; fax: address: yschang@ccms.ntu.edu.tw (Y.-S. Chang) /00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S (99)

2 332 Y.-S. Chang, F.-H. Sung / Scientia Horticulturae 83 (2000) 331±337 dormancy mechanism of those plants becomes a main inhibitor of growth in winter. In temperate regions, azalea enter true dormancy in a cold winter (Criley, 1985), but, in Taiwan, where the winter temperature remains suitable for plant growth, ower bud dormancy cannot be broken without a low-enough temperature. This dormancy mechanism is the main problem that limits azalea production in Taiwan. The control of dormancy has been studied widely (Faust et al., 1997). To control the dormancy mechanism, two approaches are possible: (1) preventing plants from entering true dormancy and (2) hastening bud break after plants have already entered true dormancy (Saure, 1985). With regards to azalea pot-plant production, hastening bud break is achieved by low-temperature or gibberellic acid (GA) treatment (Criley, 1985). These methods are dif cult to conduct in an open area and require huge investments. To reduce the cost, easier, less expensive alternatives need to be developed. Interest in arti cial control of budbreak of deciduous fruit tree species is closely connected with commercial attempts to grow these species in warm locations, where the chilling requirements are not ful lled naturally. Erez (1987) pointed out that many chemicals have rest-breaking effects, but only a few are useful for eld treatment. The chemicals which are now used commercially in various places are mineral oil, potassium nitrate (KNO 3 ), thiourea, and cyanamide. All of these chemicals are inexpensive, can effectively break the true dormancy of buds, and improve the production of deciduous fruit trees in warm locations. Although many reports have pointed out the excellent results of dormancybreaking chemicals (Erez et al., 1971; Erez and Lavi, 1985; Fernandez-Escobar and Martin, 1987; Petri, 1987; Shulman et al., 1983), little information is available on the effects of these chemicals on ending ower bud dormancy of oriental plants. The purpose of this study was to determine whether dormancybreaking chemicals (mineral oil, cyanamide, thiourea, and KNO 3 ) affect the termination of azalea ower bud dormancy and precipitate anthesis. The results were compared to those of GA 3 treatment to evaluate the possibility of using dormancy-breaking chemicals in azalea production. 2. Materials and methods Growth conditions. In Experiment 1, Three-year-old rooted cuttings of Rhododendron pulchrum Sweet were bought from a nursery on Yang-Ming Mountain, Taipei, in mid-february The plants, about 60 cm tall, each had 4±5 main shoots and were potted in 15.3 cm plastic pots in a mixture of 5 parts soil: 2 parts peat moss: 2 parts vermiculite: 1 part perlite (by volume). The plants were placed in an open area of the experimental farm at National Taiwan

3 Y.-S. Chang, F.-H. Sung / Scientia Horticulturae 83 (2000) 331± University, were given 50% shading from May to September, and were watered daily during the summer or at 2-to-3-day intervals during autumn and winter. Fertilization consisted of (Peter's) applications at 1 g/l of water at oneweek intervals from March to October and three-week intervals after October to anthesis. Each pot received 250 ml of fertilizer solution. In Experiment 2, Five-year-old rooted cuttings of R. scabrum Don in similar condition to R. pulchrum were bought from a nursery on Yang-Ming Mountain, Taipei, in mid-march The plants were potted in 20.3 cm plastic pots and placed in a plastic greenhouse on the experimental farm at National Taiwan University. Shading, watering, and fertilization were the same as with R. pulchrum, but each pot received 400 ml of fertilizer solution. Chemical application. GA 3 (Sigma, USA), and four dormancy-breaking chemicals, cyanamide (SKW, Trosberg, Germany, 49%), mineral oil (Tolelo Chemicals B.V., Netherlands, 95%), KNO 3 (Sigma, USA, 99.8%), and thiourea (Sigma, USA, 99.9%), were studied. The concentration and application of the chemicals were determined by pretesting. GA 3, cyanamide, mineral oil, KNO 3, and thiourea were applied at dosages of 500 ppm, 0.245%, 8.0%, 2.0%, and 0.5%, respectively. The control plants were sprayed with distilled water. All chemicals except cyanamide were sprayed directly on the tree crown. A surfactant (Tween- 20) at 0.05% was added to each solution. Cyanamide was applied on the wound of a leaf detached from below the ower bud before treatment. Each treatment consisted of one application followed by a second application one week later. In Experiment 1, ve ower buds about cm in length from each plant were chosen for each replicate, for a total of eight replicate plants per treatment. In Experiment 2, ve ower buds about 1.8±2 cm in length from each plant were chosen for each replicate, also totalling eight replicate plants per treatment. The plants from both experiments were arranged in a completely randomized design. Measurement of owering. Bud growth was assessed by determining, at twoweek intervals, the total bud length. The dates of the buds showing color and anthesis and the ower diameter were also recorded. In Experiment 1, the treatments started on 30 December 1995 and ended on 31 March In Experiment 2, the treatments began on 23 December 1997 and ended on 29 April Result Effects of dormancy-breaking chemicals on R. pulchrum ower bud development and owering (Experiment 1). The results of spraying dormancy-breaking chemicals on R. pulchrum ower buds are presented in Tables 1 and 2. Mineral oil and cyanamide had no effects on ower bud development (Table 1) and owering (Table 2). KNO 3 accelerated bud development at the beginning of

4 334 Y.-S. Chang, F.-H. Sung / Scientia Horticulturae 83 (2000) 331±337 Table 1 Effects of chemical treatment on ower bud monthly increment in length in R. pulchrum Sweet (Experiment 1) (from 30 December 1995 to 31 March 1996) a Monthly increment (cm) January February Control 0.32c 1.48b GA 3 (500 ppm) 0.91a 3.79a Cyanamide (0.245%) 0.39bc 1.76b Thiourea (0.5%) 1.01a 2.91ab KNO 3 (2.0%) 0.63b 3.00ab Mineral oil (8.0%) 0.43bc 2.87ab application, but its effects later decreased; it had no in uence on owering. GA 3 was effective on bud growth rate and owering, causing the ower buds to show color 10 days earlier and inducing anthesis 9 days earlier than the control. Among the four dormancy-breaking chemicals, thiourea was the only one effective on ower bud development and owering. It increased bud growth rate and caused the ower buds to show color 11 days earlier and anthesis to occur 10 days earlier than the control. The results suggest that thiourea, like GA 3, may have effects on the growth and owering of R. pulchrum. Effects of dormancy-breaking chemicals on R. scabrum ower bud development and owering (Experiment 2). The results of spraying dormancy-breaking chemicals on R. scabrum ower buds are presented in Tables 3 and 4. The results were consistent with earlier ndings on R. pulchrum. GA 3 and thiourea were the Table 2 Effects of chemical treatment on stages of oral development and ower diameter in R. pulchrum Sweet (Experiment 1) (from 30 December 1995 to 31 March 1996) a Days to buds showing color Days to anthesis Days needed from buds showing color to anthesis Flower diameter (cm) Control 52.3a 60.0a 7.75a 8.01a GA 3 (500 ppm) 42.7b 50.7b 7.63a 7.72a Cyanamide (0.245%) 54.5a 61.9a 7.63a 7.43a Thiourea (0.5%) 41.3b 49.3b 7.57a 7.58a KNO 3 (2.0%) 56.2a 64.0a 7.50a 7.79a Mineral oil (8.0%) 56.8a 64.3a 7.25a 7.78a

5 Y.-S. Chang, F.-H. Sung / Scientia Horticulturae 83 (2000) 331± Table 3 Effects of chemical treatment on ower bud monthly increment in length in R. scabrum Don (Experiment 2) (from 23 December 1997 to 14 April 1998) a Monthly increment (cm) January February March Control 0.26b 0.16bc 0.86c GA 3 (500 ppm) 0.48a 2.48a 3.31a Cyanamide (0.245%) 0.35ab 0.19bc 1.79bc Thiourea (0.5%) 0.46a 0.37b 1.84bc KNO 3 (2.0%) 0.34ab 0.15bc 1.26c Mineral oil (8.0%) 0.24b 0.11c 2.63ab most effective chemicals on ower bud development and owering. Cyanamide and mineral oil had no effects. Although KNO 3 could not increase bud growth rate nor hasten anthesis, it, like GA 3 and thiourea, could signi cantly increase ower diameter. Another nding was that the later the bud showed color, the shorter the time period was needed from buds showing color to anthesis. It appears that thiourea may have the potential to regulate the owering period of azaleas and that KNO 3 may increase ower diameter. 4. Discussion In an earlier report, Joiner et al. (1982) indicated that 100 mg/l thiourea was not effective on dormant `Redwing' and `Alaska' azalea ower buds. In our results, Table 4 Effects of chemical treatment on stages of oral development and on ower diameter in R. scabrum Don (Experiment 2) (from 23 December 1997 to 29 April 1998) a Days to buds showing color Days to anthesis Days needed from buds showing color to anthesis Flower diameter (cm) Control 104.3a 108.7a 4.47b 7.034b GA 3 (500 ppm) 93.2bc 98.7bc 5.46ab 7.526a Cyanamide (0.245%) 98.6abc 103.3abc 4.71ab 7.236ab Thiourea (0.5%) 90.2c 96.1c 5.94a 7.604a KNO 3 (2.0%) 100.4ab 105.0ab 4.63ab 7.591a Mineral oil (8.0%) 95.5abc 101.2abc 5.67ab 7.238ab

6 336 Y.-S. Chang, F.-H. Sung / Scientia Horticulturae 83 (2000) 331±337 however, 0.5% thiourea (5000 mg/l) accelerated R. pulchrum and R. scabrum ower bud development and precipitated anthesis (Tables 1±4). Although the cultivars we used in our study were different from the type they used, the concentration we applied, which was 50 times greater than what they used, was probably the main reason why the results differed indicating perhaps that the concentration they used was insuf cient. One explanation for the favorable effect exhibited in our study is the dormancybreaking ability of thiourea. Thiourea stimulates ethylene production (Esashi et al., 1975). Vieira and Barros (1994) assumed that thiourea-released dormancy was due to ethylene production. Nell et al. (1983) reported that after `Redwing' azalea plants received cold or GA 3 treatment, ethylene production was detected in their ower buds. Moreover, R. nudi ora was brought into bloom by 24- or 48- hour treatment with ethylene chlorhydrin and ethylene dichloride vapor (Denny and Stanton, 1928). These reports support the inference that the ability of thiourea to hasten azalea owering may be due to its stimulation of ethylene production, resulting in the termination of true dormancy. Another explanation for the earlier induction of ower bud development and anthesis by thiourea is cytokinin (CK) activity. Halmann (1990) pointed out that urea-containing compounds may display cytokinin activity. Joiner et al. (1982) reported that the addition of N 6 benzyl adenine (BA) or kinetin to GA 3 treatment decreased the time to anthesis in `Alaska' and `Redwing' azaleas more than did GA 3 treatment alone. Therefore, thiourea may improve bud development and owering by acting as a cytokinin. Cyanamide and mineral oil exhibited no effects on ower bud development and anthesis in our results. Erez (1987) pointed out that these two chemicals mainly break true dormancy. In subtropical Taiwan, many azalea cultivars may not enter true dormancy because the average winter temperature, about 15±178 C, remains suitable for plant growth. According to Sung (1996), ower buds of R. pulchrum and R. scabrum produced in the Taiwan lowlands continued to grow instead of proceeding to endodormancy in the winter. Since the ower buds did not enter endodormancy, this may explain why chemicals such as cyanamide and mineral oil did not affect ower bud development. Although the anthesis of R. scabrum could not be hastened, the ower diameter was increased by KNO 3 treatment. Furthermore, KNO 3 could hasten R. pulchrum anthesis when applied on ower buds that were about to show color (unpublished data). Dubey and Pessarakli (1994) stated that nitrogen assimilation and distribution in whole plants decreased in low temperatures. Erez et al. (1971) pointed out that a low level of nitrate reductase during the prebloom period resulting from a shortage of NO 3 might become a limiting factor in ower bud development. Thus, the role of KNO 3 on azalea ower bud development and anthesis may be that of a nutrition provider. To summarize, thiourea may increase azalea ower bud development and hasten owering, and KNO 3 may improve ower quality. These two dormancy-

7 Y.-S. Chang, F.-H. Sung / Scientia Horticulturae 83 (2000) 331± breaking chemicals may have bene ts on commercial azalea production. Further research should be performed to ascertain the effects of thiourea and KNO 3 and to study the mechanisms of these chemicals on ower bud development and owering. References Criley, R.A., Rhododendrons and azaleas. In: Halevy, A.H. (Ed.), CRC Handbook of Flowering, vol. IV. CRC press, Boca Raton, pp. 180±197. Denny, F.E., Stanton, E.N., Chemical treatments for shorting the rest period of pot-grown woody plants. Am. J. Bot. 15, 327±336. Dubey, R.S., Pessarakli, M., Physiological mechanisms of nitrogen absorption and assimilation in plant under stressful conditions. In: Pessarakli, M. (Ed.), Handbook of Plant and Crop Physiology. Marcel Dekker, New York, pp. 605±625. Erez, A., Chemical control of budbreak. HortScience 22, 1240±1243. Erez, A., Lavee, S., Samish, R.M., Improved methods for breaking rest in the peach and other deciduous fruit species. J. Am. Soc. Hort. Sci. 96, 519±522. Erez, A., Lavi, B., Breaking bud rest of several deciduous fruit tree species in the Kenyan highlands. Acta Hort. 58, 239±247. Esashi, Y., Hata, H., Katoh, H., Germination of cocklebur seeds: interactions between gibberellic acid, benzyladenine, thiourea, KNO 3 and gaseous factors. Aust. J. Plant Physiol. 2, 569±579. Faust, M., Erez, A., Rowland, L.J., Wang, S.Y., Norman, H.A., Bud dormancy in perennial fruit trees: physiological basis for dormancy induction, maintenance, and release. HortScience 32, 623±629. Fernandez-Escobar, R., Martin, R., Chemical treatments for breaking rest in peach in relation to accumulated chilling. J. Hort. Sci. 62, 457±461. Halmann, M., Synthetic plant growth regulators. Adv. Agron. 43, 47±105. Joiner, J.N., Washington, O., Johnson, C.R., Nell, T.A., Effect of exogenous growth regulators on owering and cytokinin levels in azaleas. Sci. Hort. 18, 143±151. Nell, T.A., Bodnaruk Jr., W.H., Joiner, J.N., Sheehan, T.J., Ethylene evolution and owering of cold- and GA-treated `Redwing' azalea. HortScience 18, 454±455. Petri, J.J., Breaking dormancy of apple trees with chemicals. Acta Hort. 199, 109±117. Saure, M.C., Dormancy release in deciduous fruit trees. Hort. Rev. 7, 239±289. Shulman, Y., Nir, G., Fanberstein, L., Lavee, S., The effect of cyanamide on the release from dormancy of grapevine buds. Sci. Hort. 19, 97±104. Sung, F.-H., Studies on owering habits and ower development of Hirado azaleas. Master thesis. Department of Horticulture, National Taiwan University, pp. 29±39 (in Chinese). Vieira, H.D., Barros, R.S., Responses of seed of Stylosanthes humilis to germination regulators. Physiol. Plant. 92, 17±20.