Bot. Bull. Acad. Sin. (2001) 42: 187-191

Sung and Chang — Dormancy of azalea in Taiwan

Rhododendron mucronatum G. Don grown in subtropical Taiwan does not manifest endodormancy

F.H. Sung and Y.S. Chang*

Department of Horticulture, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan, Republic of China

(Received June 20, 2000; Accepted November 15, 2000)

Abstract. Microscopic observation of bi-weekly sampled terminal buds revealed that most flower buds of R. mucronatum were formed between late-July and mid-August and differentiation of specific floral organs was completed before mid-October. Flower buds continued to enlarge through the winter. When flower buds on two-year-old plants from rooted cuttings were allowed to grow at day / night temperature of 15/13°C, 20/15°C, 25/20°C or 30/25°C, the 30/25°C treatment was observed to inhibit their growth; those subjected to the 15/13°C treatment grew the fastest during the early stages. Neither treatment, however, enabled attainment of anthesis by the end of the experiment. The 20/15°C treatment hastened flower bud growth and reduced the number of days to flowering. Bud growth was slower and flowering percentages were lower at 25/20°C, than at 20/15°C. When plants were transferred to 30/25°C after 2, 4, or 6 weeks in the 15/13°C treatment, the growth rate of flower buds increased greatly. Also, when the 15/13°C treatment was extended, the growth rate of flower buds increased, while the number of days needed for flowering after transferring from the low temperature treatment was reduced. The 15/13°C treatment simulated the winter temperatures in Taiwan. Our finding disclosed that bud growth was fastest, rather than ceasing, under this temperature regimen. Although growth rate decreased near the end of the experiment, it increased immediately and bloomed quickly when plants were transferred to higher temperatures. Thus, our observation indicated that R. mucronatum does not enter endodormancy during winter in subtropical Taiwan lowlands.

Keywords: Dormancy; Ecodormancy; Endodormancy; Flower bud development; Flowering; Temperature.

Introduction

Traditionally, it is believed that azalea flower buds enter dormancy before flowering and that the dormancy is overcome by low temperature. Hence, treatment of 4-6 weeks at 2-9°C is used to break flower bud dormancy for commercial production of potted azaleas (Larson, 1992; Seeley, 1981; Hamrick, 1991). When some azaleas, such as "Redwing" and "Reinhold Ambrosius," are grown at 20°C, they can attain anthesis without low temperature treatments; nevertheless, the uniformity of flowering is lower (Pettersen and Kristofferson, 1969). Brown and Box (1971) and Brown (1973) reported that some azalea cultivars, when grown at 18°C and under a 18-hour daylength, flowered about one-half to one month earlier than when given the traditional cold temperature treatment (5-week at 3°C); the uniformity of flowering was also very high. With temperate zone deciduous fruit species that are grown in tropical regions, Saure (1985) proposed that dormancy may be avoided because the warm temperature, together with the long daylength, is suitable for plant growth. This may be why some deciduous species can

bear fruits twice a year. Accordingly, the early flowering of some azalea cultivars as reported by Brown and Box (1971) and Brown (1973) may be the result of the warm temperature and long daylength having prevented dormancy from occurring in the flower buds. The relationship between temperature and azalea flower bud dormancy needs to be clarified more precisely.

Rododendron mucronatum is widely found in northern Taiwan lowlands. It was introduced in 1925, but there have been very few studies on its flowering habits and flower bud dormancy. Tang (1975) reported on its morphological changes, from vegetative growth to flower bud formation, but did not investigate flower bud dormancy. The purpose of our study has been to observe the flowering habit of R. mucronatum in Taiwan lowlands, and to evaluate the effect of temperature on flower bud development and dormancy. Our findings described here may help to control the flowering behavior of azalea in subtropical regions.

Materials and Methods

Determination of Flower Bud Developmental Stages

The flowering of ten azalea (R. mucronatum) plants, about 50-years-old and in good condition on the National Taiwan University campus, was observed from mid-July,

¹Graduate student and associate professor of the Department of Horticulture, National Taiwan University.

*Corresponding author. Tel: 866-2-23630231-3340; Fax: 866-2-23635849; E-mail: yschang@ccms.ntu.edu.tw

Botanical Bulletin of Academia Sinica, Vol. 42, 2001

1995 to mid-March, 1996. One bud was randomly sampled from each plant at two-week intervals, and the developmental stage was observed microscopically using fixed and fresh sections. The developmental stages were identified according to the nine stages of Kohl and Scaroni (1956), Tang (1975), and Bodson (1983). These stages are shown in Table 1. In addition, the lengths of buds, corollas, stamens and styles, the dates of emergence of buds with color, and the dates of anthesis were recorded.

Temperature Treatments

Two-year-old plants from rooted cuttings of R. mucronatum were bought from a nursery in Yang-Ming Mountain, Taipei, in mid-February 1995. Each plant was about 60-cm tall and had 4-5 main shoots. They were potted in 15.3-cm plastic pots, containing a mixture of 5:2:2:1, V: V: V: V, soil: peat moss: vermiculite: perlite. The plants were placed in an open area of the Experimental Farm of National Taiwan University, Taipei, Taiwan. The plants were given 50% shade from May to September, and watered daily during the summer and at 2- to 3-day intervals during autumn and winter. A 20-20-20 fertilizer mixture (Peter's) was dissolved at a rate of 1 g / liter of water and applied weekly from March to October and at three-week intervals from October to anthesis, each pot receiving 250 ml of solution. Ten flower buds of about 1.0 cm length were sampled from each plant, and a total of three plants were employed per treatment. Plants were moved into growth rooms at day/night temperatures of 30/25°C, 25/20°C, 20/15°C, and 15/13°C. Because a long day may prevent dormancy occurring, the temperature experiment was conducted under short-day conditions to avoid the influence of daylength. The temperature experiment was conducted under natural daylengths because days in Taiwan were short during the experimental period. The natural daylength ranged from 11.5 to 12.6 hours during the experimental period, which spanned from 27 October 1995 to 2 February 1996. Bud growth, measured at two-week intervals, was based on total bud length. The dates of flower buds showing color and undergoing anthesis were also recorded.

Temperature Shift Treatments

Six flower buds, each about 1.2-1.3 cm long, were sampled from each plant. The plant source and cultiva

Figure 1. Flower developmental stage and flower bud length of R. mucronatum grown on campus of National Taiwan University.

tion methods were the same as for those used in temperature treatment investigation. Twelve plants were moved into a 15/13°C growth room on 16 November 1995. The natural daylength ranged from 11.5 to 12.6 hours during the experiment period. After 2, 4, and 6 weeks, three plants were transferred to a 30/25°C growth room. All treatments ended on 31 January 1996. Total bud lengths were measured at two-week intervals, and dates when all buds showed color and underwent anthesis, as well as flower diameter, were recorded. The percentage of total flower buds that bloomed before the experiment ended was also calculated as flowering percentage.

Results

In the Taiwan lowlands, flowers of R. mucronatum that bloomed during the main flowering season were formed between mid-July and mid-August (Figure 1). Differentiation of flower organs was completed by late October, after which the buds grew slowly, but steadily. Flower buds developed color in late February, and underwent anthesis in mid-March. During the period of our observations, there were about four months during which the flower buds remained in Stage 6, but the buds did not cease growing because of the lower winter temperatures. Although the length of corollas, styles, and stamens did not increase significantly in January 1996 (Figure 2), an increase in standard errors of means indicated that a large variance existed in the development of buds.

The growth rate of flower buds increased when the temperature was lowered (Figure 3). In the early stages, buds of the 15/13°C treatment grew significantly faster than those of the other temperature treatments. Unlike flower buds of the 20/15°C treatment, which grew dramatically at the end of experiment and attained anthesis, those of 15/13°C declined in growth after 2 months. The growth rate of buds in the 25/20°C and 30/25°C treatments did not differ greatly, although a few flowers opened at the end of

Table 1. The developmental stages of azalea terminal buds.

Stages Description

0 Vegetative growth

1 Apex broadens

2 Sepal formation

3 Petal formation

4 Stamen formation

5 Carpel formation

6 Elongation of style

7 Bud shows color

8 Full bloom

Sung and Chang — Dormancy of azalea in Taiwan

25/20°C treatment. The growth rate was the lowest among buds of the 30/25°C treatment; furthermore, about 30% of the buds aborted (data not shown). Except for the 15/13°C treatment, no flower buds of R. mucronatum ceased growing during the experiment.

Flower buds grew slowly at 15/13°C and remained so when left continuously in the temperature regime (Table 2). The slower growth rate in continuous 15/13°C was attributable to decreased growth toward the end of the experiment. The flower bud growth rate increased twofold with a change in temperature regimes. Buds that received only two weeks of 15/13°C temperature treatment displayed color and bloomed the earliest, but the flowering percentage was only 58.82%. Plants that received 4 and 6 weeks of 15/13°C did not differ in dates of color display or in anthesis and flowering percentage. Nevertheless, as is evident in Table 3, the longer the low temperature treatment, the faster did the buds show color

and attain anthesis. However, because of the shorter development time, the diameter of flowers was smaller.

Discussion

In the temperate zone, winter temperatures range from 0°C to 10°C, unsuitable for azalea plant growth. Their flower buds become dormant. However, in the subtropical Taiwan lowlands, the winter temperature is about 15°C with a minimum temperature always above 10°C (Chen and Wu, 1978). The continuous increase in the length of the flower buds as observed in this study indicated that the flower buds did not cease growing. The lengths of individual flower organs also increased in early winter, but ceased in late winter. Phytotron experiment confirmed that when buds were subjected to a 15/13°C regimen, simulating winter temperatures of north Taiwan lowlands, they grew faster than at higher temperature treatments during

Figure 2. Lengths of floral organs of R. mucronatum grown on campus of National Taiwan University.

Figure 3. Effect of temperature on flower bud growth of R. mucronatum.

Table 2. Effect of temperature shift on flower bud development of R. mucronatum.

Average bud growth rate Days to Days to Flowering

Temperature treatment (mm/day) buds show flowering percentage

Before shift After shift^a color (%)

15/13°C 8 weeks 0.085b^b _ _ _ _

15/13°C 6 weeks®30/25°C 2 weeks 0.110aB^c 0.333aA 68.4a 77.6a 97.43a

15/13°C 4 weeks®30/25°C 4 weeks 0.111aB 0.199aA 64.4a 74.5a 86.33a

15/13°C 2 weeks®30/25°C 6 weeks 0.114aB 0.228aA 53.8b 66.2b 58.82b

^a Means the average growth rate before flower buds showed color.

^b Means within the same column followed by different lowercases were significantly different at the 0.05 level of Duncan's Multiple Range Test.

^c Means within the same row followed by different uppercases were significantly different at the 0.05 level of Duncan's Multiple Range Test.

Botanical Bulletin of Academia Sinica, Vol. 42, 2001

Table 3. Effect of temperature shift on flowering of R. mucronatum.

Temperature treatment Days from temperature Days from temperature Flower shift to buds show color shift to anthesis diameter

15/13°C 6 weeks®30/25°C 2 weeks 18.4b^a 27.6c 7.14b

15/13°C 4 weeks®30/25°C 4 weeks 29.4a 39.7b 7.06b

15/13°C 2 weeks®30/25°C 6 weeks 33.8a 46.2a 7.71a

^a Means within the same column followed by different lowercases were significantly different at the 0.05 level of Duncan's Multiple Range Test.

the early stages. Lang (1987) suggested that dormancy should be defined as "the temporary suspension of visible growth of any plant structure containing a meristem." Applying that definition, it can be concluded that R. mucronatum did not manifest dormancy in the northern lowlands of Taiwan. The bud growth rate, however, decreased after about two months at 15/13°C. When buds from the 15/13°C treatment were transferred after 2-6 weeks to a higher (30/25°C) temperature, growth resumed and flowering quickly followed. These results also indicated that R. mucronatum does not enter "endodormancy," or dormancy that is controlled by physiological factors within the dormant structure (Lang, 1987). The growth rate only decreased among flower buds that were kept at 15/13°C continuously. It is possible that buds treated continuously in 15/13°C gradually entered "ecodormancy," or dormancy that is controlled by environmental stresses evoking nonspecific responses (Lang, 1987). The temperature was unfavorable to the final developmental stage of the flower buds, possibly explaining why the lengths of corollas, styles, and stamens did not increase continuously in January 1996. Pemberton and Wilkins (1985) also pointed out that the "Prize" azalea may not exhibit endodomancy. Therefore, it seems that the flower buds of some azalea species do not necessarily enter endodormancy before flowering, but they can enter endodormancy because of unsuitable growth temperatures in winter.

The four temperature treatments in our experiment caused the buds to grow faster as the temperature decreased, indicating that high temperatures inhibited flower bud development of R. mucronatum. This finding is similar to the report by Larson and Biamonte (1972), that "Red American Beauty" azaleas formed flower buds rapidly at 30°C, but subsequent development was delayed if the plants were retained at this temperature. Bud growth resumed only after the plants were moved to a cooler temperature (22-18°C). In our investigation, flower buds grown at 15/13°C grew faster in the early stages than at other temperatures, rather than entering dormancy. It is possible that azalea flower buds require low temperatures during flower development, but the low temperatures may be involved in physiological processes other than in overcoming dormancy. However, the length of the buds grown in 15/13°C did not continue to increase, but possibly ceased, and anthesis was unattained by the end of our experiments, indicating a need of higher temperature for completing the final developmental stage preceding anthesis. Base on results of our temperature shift

treatments, we propose that four weeks of the low temperature treatment should be sufficient to satisfy the low temperature requirement of R. mucronatum.

To summarize, when R. mucronatum plants were placed under a 15/13°C treatment, similar to the average winter temperature in the northern Taiwan lowlands, the growth rate of their flower buds increased, instead of ceasing. Even though growth rate of buds decreased, it could be immediately increased and hastened to flower by transferring the plants to a higher temperature. Hence R. mucronatum grown in lowlands of subtropical Taiwan show no endodormancy, but may show ecodormancy in later developmental stages in winter. We conclude that R. mucronatum flower buds need about a month of low temperature to induce blooming. The low temperature requirement is related to some physiological reactions other than overcoming endodormancy.

Literature Cited

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Brown, W.L. 1973. Flowering of azalea with controlled photoperiods and gibberellic acid. J. Amer. Soc. Hort. Sci. 98: 300-303.

Brown, W.L. and C.O. Box. 1971. Effects of succinic acid-2,2-dimethylhydrazide and photoperiod temperature manipulation on flowering and vegetative bypassing in azalea cvs. Red Wing and Alaska. J. Amer. Soc. Hort. Sci. 96: 823-825.

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Larson, R.A. and R.L. Biamonte. 1972. Response of azaleas to precisely controlled temperatures. J. Amre. Soc. Hort. Sci. 97: 491-493.

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Sung and Chang — Dormancy of azalea in Taiwan

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and gibberellic acid in floral development of evergreen azalea rhododendron. J. Amer. Soc. Hort. Sci. 110: 730-737.

Pettersen, H. and T. Kristofferson. 1969. The effect of daylength and temperature on flowering in the azalea cultivars `Redwing' and `Reinhold ambrosius'. Acta Hortic. 14: 27-38.

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