PHOTOPERIODIC RESPONSE OF POT CHRYSANTHEMUMS IN TWELVE ALL YEAR-ROUND PRODUCTION CYCLES

Marek Jerzy, Joanna Borkowska

ABSTRACT

Rooted cuttings of 11 pot chrysanthemum cultivars from the Time group were planted every month into pots 14 cm in diameter and treated with short photoperiod from the first day of greenhouse culture.

Key words: Dendranthema grandiflora, Time group, AYR, photoperiodic response..

INTRODUCTION

All chrysanthemum cultivars suitable for controlled cultivation are classified into photoperiodic response groups.

The response time is the number of weeks between the beginning of the short day treatment and the full blooming date. The nominal photoperiodic response is determined at the temperature of 16-18°C and a light intensity above 5000 lx. Early varieties reach full bloom after 6-8 weeks, medium-early after 9-11 weeks, and late after 12-15 weeks. The latter, however, are no longer grown owing to an excessively long cultivation period and the resultant lower profitability of production.

The photoperiodic response of the various chrysanthemum cultivars depends on the light. Strong light has a decisive effect on correct and fast growth of flower buds, while weak light either stops this process completely or retards it considerably [1, 7]. Light intensity affects chrysanthemum growth in quantitative terms, the most crucial being the duration of light of specified strength. Hughes and Cokshull [3] showed that the growth of chrysanthemums could proceed at the same pace both at constant and variable light intensity during the day on condition that the total daily amount of light was the same in both cases.

The chrysanthemum grows faster at higher temperatures, but flower bud formation in short days is optimally fast at moderate temperatures, 17-21°C. Below 17°C and above 21°C night temperatures, bud formation is retarded [6]. A prolonged temperature of 32-35°C may stop chrysanthemum blooming altogether [2, 5, 9].

Maintaining the greenhouse temperature at an optimal level for chrysanthemum growth is very hard under production conditions. The same concerns light. The intensity of sunlight often varies from day to day, and is not constant even throughout a day. Hence one might expect big differences in the generative stage of the plants' development.

The knowledge of a plant's photoperiodic response to the various light and temperature conditions over the year is very important. It can contribute to a better, more accurate planning of the production of pot chrysanthemum cultivars by allowing the estimation of number of the production cycles that can be effected during a year in one and the same greenhouse area.

MATERIALS AND METHODS

The experiment was conducted using 11 small-flowered pot cultivars of the chrysanthemum Dendranthema grandiflora Tzvelev (syn. Chrysanthemum × grandiflorum /Ramat./ Kitam) from the Time group, bred by the English firm Cleangro: 'Brill Time', 'Cool Time', 'Dream Time', 'Energy Time', Esperanto Time', 'Icon Time', 'Jewel Time', 'Quartz Time', 'Solar Time', 'Tattoo Time', and 'Tea Time'. The cultivars differed in colour and shape of the inflorescences, growth vigour, and earliness (tab. 1).

Starting with 2 January 2002, on the second day of each successive month of the year rooted cuttings were planted into pots 14 cm in diameter, 5 cuttings per pot. The number of pots of one cultivar in each production cycle was 20. The medium used was a ready-made Stender peat substrate with a pH of 5.5. The pots with cuttings were placed in a soil bed in the greenhouse at a 30 × 30 cm spacing. After five days stem tips were pinched above the fifth leaf, counting from the base of the stem.

From the moment of potting, the plants were treated with a short day. For the black-out, use was made of a two-layered Obscura A/B+B type of material. In periods of naturally long days, the day was shortened to 10.5 hours. No supplementary illumination was used until the end of February, i.e. in the period of natural insolation deficit. This means that the plants grew at that time under day lengths shorter than 10.5 hours, or, on average, 9 h in November, 8 h in December, 9 h in January, and 10 h in February.

To give a fuller characterisation of the light conditions, Figure 1 shows real insolation in Poznań-Ławica (from IMiGW)over the research period. Figure 2 presents mean monthly temperatures of the greenhouse air.

The plants were drip-irrigated. Fertigation was applied as needed. The plants were fed once a day with 160 ml per pot of medium with EC=20 mS cm^-1. In summer, when the temperatures were high, the feeding occurred twice a day with EC reduced to 1.8 mS cm^-1. In winter the feeding was limited to twice a week.

During the day the plants were additionally nourished with carbon dioxide. With closed ventilators, the CO₂ concentration was kept at 1,100-1,200 µl dm^-3, and with opened ones, at 550-600 µl dm^-3.

The plants were retarded using the preparation B-Nine 85 SP at a concentration of 0.3%; for the first time when lateral shoots, after pinching the tip of the main stem, attained a length of 10-15 mm, and for the second time, 7-10 days later. These practices were applied in 10 production cycles starting from January to October. In the November cycle the growth and development of the plants was poor and delayed, so their retardation was unnecessary. In the December cycle the plants grew a bit better, hence they were treated with B-Nine once, when flower buds were already visible on lateral shoots 5-7 cm long.

The experiment was carried out in a greenhouse of the Horticultural Farm of Maria and Marek Szaj, at Dachowa near Poznań, from 2 January 2002 to 9 March 2003. The photoperiodic response of the plants was determined, i.e., the time from the beginning of the short day treatment to full bloom, when half of all the inflorescences were fully opened.

RESULTS

The photoperiodic responses of the various chrysanthemum cultivars as expressed by the number of days from the beginning of short photoperiod to full bloom, equivalent in this experiment to the duration of the entire greenhouse culture, differed with the season of the year, but they were always longer than the nominal response established by the breeder (tab. 2).

Plants grown in the cycles starting in February to September and ending with full bloom in April to October flowered after an average of two months. An exceptionally longer photoperiodic response was recorded in the cycle embracing the height of summer, i.e. from June to August, when the temperature during the day was very high, exceeding in August the permissible 26°C.

The shortest photoperiodic responses, close to the nominal one, were recorded in cycles starting in February and August.

In the last two cycles falling in the autumn-winter insolation deficit in Poland, flowering was seriously retarded. In most of the cultivars grown from November, the photoperiodic response was roughly twice as long as the nominal one. They came into bloom after an average of 3.5 months of cultivation as against 2.5-3 months in the other varieties.

DISCUSSION

The duration of real insolation (fig. 1), which is an indicator of light intensity, differed widely in the all-year-round production cycle of 11 pot chrysanthemum cultivars from the Time group, ranging from about 250 hours per month between May and August to just under 50 hours per month in the period of autumn-winter insolation deficit, i.e. in November, December and January. Equally variable was the greenhouse temperature: from 14°C in November to 26°C in August by day, and from 13°C in November to 22°C in August by night (fig. 2).

The night temperature lower than the 16-18°C optimal for chrysanthemums was maintained from October 2002 to February 2003. This was also the period of the lowest insolation in the year. As a result, photoperiodic response lengthened considerably. In most varieties grown from November, the response was even twice as long as the nominal one.

In the summer, especially in the cycle falling in the beginning of the season, one could observe a marked detrimental effect of excessive temperatures exceeding in July and August 25-26°C by day and 21-22°C by night. It consisted in a noticeable retardation of the macroscopic development of flower buds and the lengthening of photoperiodic response in almost all chrysanthemum cultivars.

The level of insolation and the greenhouse temperature also controlled the photoperiodic response of other pot chrysanthemum cultivars bred by Yoder Brothers Inc. [4] and miniature spray chrysanthemum cultivars grown for cut flowers [8]. As follows from these studies, in production conditions the photoperiodic response of various chrysanthemum cultivars is almost always longer than the nominal one. It is necessary to take this fact into account when planning an all-year-round production cycle, especially in scheduling room for chrysanthemum cultivation in the height of summer and the autumn/winter period when photoperiodic response lengthens considerably.

REFERENCES

1. Cockshull K. E., Hughes A. P., 1972. Flower formation in Chrysanthemum morifolium: the influence of light level. J. Hort. Sci. 47, 113-127.

2. Cockshull K. E., Kofranek A. M., 1994. Hight night temperatures delay flowering, produce abnormal flowers and retard stem growth of cut-flower chrysanthemums. Sci. Hortic. 56, 217-234.

3. Hughes A. P., Cockshull K. E., 1971. A comparison of the effect of diurnal variation in light intensity with constans light intensity on growth of Chrysanthemum morifolium ‘Bright Golden Anne’. Ann. Bot. 35, 927-932.

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6. Lint P. J. A. L. de, Heij G., 1987. Effect of day and night temperature on growth and flowering of chrysanthemum. Acta Hortic. 197, 53-61.

7. Machin B., 1997. Pot chrysanthemum production. Grower Guide 5, 3-74.

8. Przymęska J., Jerzy M., 2004. Response of spray chrysanthemum to a long-day vegetative growth period in all year-round culture. Folia Univ. Agric.Stetin., Agricultura 236(94), 159-168.

9. Whealy C. A., Neil T. A., Barret J. E., Larson R. A., 1987. Hight temperature effects on growth and floral development of chrysanthemum. J. Am. Soc. Hortic. Sci. 112, 464-468.

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