Effects of bulb temperature on development of Hippeastrum J.C. Doorduin and W. Verkerke Research Station for Floriculture and Glasshouse Vegetables PBG Kruisbroekweg 5 2670 AA Naaldwijk The Netherlands Tel: +31-174-636700 Fax: +31-174-636835 E-mail: j.c.doorduin@pbg.agro.nl Keywords: flower bulbs, Hippeastrum, bulb temperature, bulb growth, bud development flowering, profit Abstract The effects of bulb temperature during the cultivation period on bud development and flowering of Hippeastrum (amaryllis) were investigated. Bulb temperature treatments ranged from 15 to 25 C. Premature flowering occurred at 15 and 17 C. At higher bulb temperature, larger bulbs with more leaves developed, but the percentage of bulb dry matter decreased. The number of potentially flowering buds increased, while the length of the leaf-bud cycle decreased with temperature. Bud desiccation and Fusarium infection occurred at the highest temperature levels. Optimum bulb temperature for flower development was found to be 23 C. INTRODUCTION In The Netherlands amaryllis bulbs (Hippeastrum hybridum) are grown in greenhouses, both in substrate beds and in greenhouse soil. For reasons of energy efficiency, the ambient greenhouse temperature and the temperature of the substrate or greenhouse soil are controlled by separate heating systems. This allows the greenhouse temperature set point to be up to 7 C below the substrate temperature. Under Dutch cultivation conditions, amaryllis plant growth is governed by the light conditions of the season, but bulb development (i.e. flower bud initiation) does not depend of the season. Bulb temperature is probably the most important factor for bulb development rate, and flower buds are initiated at regular intervals (Doorduin, 1990). Dutch growers are advised to use the separate heating systems and keep the temperature of the substrate or greenhouse soil at 20-22 C, while maintaining ambient greenhouse temperature during the cropping period at 14-17 C (Vijverberg, 1981; Van Leeuwen & Buschman, 1991). However, studies with growing cabinets could not fully confirm this advice. Optimal temperatures for amaryllis bulb development were found to be 23/18 C or 24/17 C (day/night, Hayashi & Suzuki, 1970; Ijiro & Ogata, 1997). These levels correspond to an average temperature level of 20.5 C. Such different temperature set points for day and night temperature are not in use in The Netherlands. As far as different set points are used for amaryllis, the difference is mostly smaller than 2 C. In this research we investigated the effect of different substrate temperatures on bulb development at a constant ambient greenhouse set point temperature. The goal was to control and possibly fine-tune the standard advice for Hippeastrum bulb temperature in Dutch growing conditions. MATERIALS AND METHODS Plant stock bulbs of Hippeastrum hybridum cultivar 'Red Lion', circumference 14/16 cm, were planted in fine perlite substrate beds of 70 cm width at a density of 30 bulbs/m 2 net in a wide span greenhouse at December 7th, 1998 (week 50). A nutrient solution especially for amaryllis (ph 5.5 and EC 1.8 ms/cm) was applied according to De Kreij et al. (1997) in a closed cultivation system. At daytime, CO 2 was applied at 600 Proc. 8th Int. Symp. on Flowerbulbs Eds. G. Littlejohn et al. Acta Hort. 570, ISHS 2002 313
ppm. In each bed, substrate temperature could be individually controlled by means of 6 PE tubes 20/25 mm in diameter for cooling and heating at 15 cm depth in the substrate. Substrate temperature was recorded at the level of the bulb disk with PT-100 temperature sensors. Ambient greenhouse temperature set point was 16 C with ventilation at 18 C, and substrate temperature was kept at 22 C until December 28th, 1998 (week 53), when root formation and sprouting commenced. From week 53 onward, a substrate temperature treatment of 15, 17, 19, 20, 21, 23, and 25 C was applied in two repetitions, each consisting of one substrate bed with 128 bulbs. From January 4th, 1999 (week 1) onward, the realised temperatures were very close to the aimed values (r 2 = 0.99). Based on a 24-hours average, the standard deviation ranged from 0.2 at 15 C till 0.6 at 25 C. The average greenhouse temperature was 18.9 C and at the sunniest days the temperature rose to 27 C. Bulb and bud development was studied by sampling three plants per repetition at 5-7 week intervals (Doorduin, 1990); buds from stage V (Blaauw, 1931) were recorded. Finally, all remaining bulbs were harvested at November 29th, 1999 (week 48). The bulbs were dried, sorted, and prepared for 16 weeks at 13 C, planted in pots at April 6th, 2000 (week 14), and grown in a greenhouse at set point heating of 22 C and ventilation at 24 C to evaluate the effects of the temperature treatment on flowering. Data from September 6th, 1999 (week 36) correspond with the standard commercial growing period for dry selling bulbs; therefore these data were used to evaluate the effects on bulb growth. For a precise estimation of the effects on bud development and percentage flowering, data from the latest harvest, November 29th, 1999 (week 48), were used. Data was analysed with ANOVA and if appropriate, exponential models were fit. RESULTS AND DISCUSSION The number of leaves and the bulb circumference increased with bulb temperature, while both the bulb dry matter percentage and the percentage of dry matter allocated to the bulb decreased. At 25 C, also the number of outer scales decreased (Table 1). At the lowest temperatures, both bud initiation was depressed and premature flowering occurred (Table 2, Figure 1). The premature flowering could be expected, because these temperatures are also applied to induce flower bud elongation of harvested mature bulbs (Vijverberg, 1981). Bulb development was clearly stimulated at higher temperatures. Over the temperature range applied, both the total number of buds and potentially flowering buds increased (Figure 1), while the length of the bud-leaf cycle (bud plastochron) was reduced with 47% (Figure 2). Unlike the results of Hayashi & Suzuki (1970), the cycle of one flower bud per four leaves remained constant at all temperature treatments, which confirms the observations of Blaauw (1931). The calculated amount of leaf/bud cycles per year thus increased from 3 to 6, which is more than found previously (Rees, 1972; 1985). The number of stems and calyxes per bulb, and the number of calyxes per stem increased (Table 3). A good correlation (97%) was found between the number of potentially flowering buds and the number of stems. This confirms that the amount of flowering stems can be predicted from bud observations (Doorduin, 1990). At the highest temperature levels bud desiccation occurs (Figure 1, Table 2). This confirms the results of Ijiro & Ogata (1997). Also, the bulbs became infected with Fusarium. From these results it can be concluded that bulb size and flowering decrease at lower temperatures, and that for a better yield, bulb temperature can be increased to at least 23 C without adverse effects on bulb quality. Such a higher temperature optimum contrasts with the earlier reports where optimal bulb temperature was found to be 20.5 C (Hayashi & Suzuki, 1970; Ijiro & Ogata, 1997). These different results could partly be explained from the differences in experimental set up. We controlled the actual bulb temperature in a greenhouse experiment, while the authors mentioned controlled ambient temperature in a growing cabinet, without separate temperature control systems for substrate and ambient temperature. Furthermore we used smaller temperature intervals that allowed for a better estimation of the temperature effect, and differences in light 314
conditions and cultivar could also play a role. Practical Consequences Our results confirm the current advice to growers, a bulb temperature of 20-22 C, as a safe option. Reduction of the energy input by maintaining lower temperatures leads to a decrease in both bulb size and in flowering and a considerable profit decrease. But we also found that the upper limit of bulb temperature can be safely increased to 23 C. In this temperature range heating costs increase with ƒ 1.25/m 2 per degree. Calculations based on fitted data in the range of 21-23 C show that bulb circumference increases with 0.5 cm per degree and the number of calyxes per bulb increases with 0.35 per degree. For growers of dry selling bulbs with 32 bulbs/m 2 and an estimated value of ƒ 0.20 per cm circumference increase, a profit increase of ƒ 2.00-4.60/m 2 per degree can be expected. For growers of cut flowers, with 24 bulbs/m 2 and a price per calyx of ƒ 0.50 (Van Meggelen, 2000), a profit increase of ƒ 2.52-5.88/m 2 per degree can be expected. Literature cited Blaauw, A.H. 1931. Orgaanvorming en periodiciteit van Hippeastrum hybridum. Meded. Laboratorium voor Plantenphysiologisch Onderzoek Wageningen 32:1-90. Doorduin, J.C. 1990. Growth and development of Hippeastrum grown in glasshouses. Acta Hort. 266:123-131. Hayashi, I. and M. Suzuki. 1970. Studies on the growth and flowering of Hippeastrum hybridum. Bull. Kanagawa Hort. Exp. St. 18:171-188. Ijiro, Y. and R. Ogata. 1997. Effect of ambient temperature on the growth and development of Amaryllis (Hippeastrum hybridum hort.) bulbs. J. Japan Soc. Hort. 66:575-579. Kreij, C. de, Voogt, W., van den Bos, A.L., Baas, R. 1997. Voedingsoplossingen voor de teelt van Hippeastrum in gesloten teeltsystemen. PBG Brochure VG 10, 21 pp. Rees, A.R. 1972. The growth of bulbs. Academic Press, London. Rees, A.R. 1985. Hippeastrum. In: A.H. Halevy, ed., Handbook of Flowering, pp. 294-296, CRC Press, Boca Raton. Van Leeuwen, A.J.M. and Buschman, J.C.M. 1991. The Hippeastrum (amaryllis) as a cut flower. Herbertia 47:93-102. Van Meggelen, I. 2000. Hippeastrum. Vakblad voor de Bloemisterij 55 (22a): p. 42. Vijverberg, A.J. 1981. Growing Amaryllis. Grower Book, London. 315
Tables Table 1. The effect of bulb temperature T ( C) on bulb growth. Number of leaves, bulb circumference C (cm), bulb dry matter DM (g), bulb percentage dry matter %DM, percentage of total dry matter allocated to the bulbs %DMA and the number of scales external of the first bud. T leaves C DM %DM %DMA scales ( C) (cm) (g) 15 9.5 32.8 93.8 20.7 76.7 6.5 17 12.7 34.7 106.2 19.8 74.2 6.2 19 12.5 37.3 118.4 19.6 72.5 6.0 20 12.7 37.5 124.7 19.1 73.2 5.5 21 13.5 38.0 117.0 17.6 67.1 6.0 23 16.9 39.3 112.7 16.4 60.4 5.5 25 18.5 39.0 106.2 15.2 53.7 3.9 p *** * NS *** ** * LSD 5% 1.4 2.6 1.5 6.9 1.2 *** = p < 0.001; ** = p < 0.01; * = p < 0.05; NS = not significant Table 2. The effect of bulb temperature T ( C) on bulb development. Total number of buds, number of potentially flowering buds (>20 mm), number of desiccated buds, number of premature flowering buds, and the length of the leaf-bud cycle LBC (weeks). T number of buds LBC ( C) total potentially flowering desiccated premature flowering (weeks) 15 4.8 1.3 0.0 1.5 15.4 17 5.5 1.5 0.0 2.0 12.7 19 6.7 3.2 0.5 0.3 9.7 20 6.3 3.7 0.2 0.0 10.4 21 6.8 3.5 0.7 0.2 9.3 23 7.5 3.8 1.0 0.0 8.3 25 7.7 3.3 1.5 0.0 8.1 p ** ** ** ** *** LSD 5% 0.8 0.8 0.6 0.3 1.9 *** = p < 0.001; ** = p < 0.01 316
Table 3. The effect of bulb temperature T ( C) on flowering. Number of stems per bulb, number of calyxes per bulb, and number of calyxes per stem. T ( C) stems per bulb calyxes per bulb calyxes per stem 15 1.6 3.6 2.3 17 1.9 4.7 2.5 19 3.1 10.1 3.2 20 3.6 12.4 3.4 21 3.7 13.5 3.6 23 3.6 14.2 3.9 25 3.6 13.1 3.7 p *** *** *** LSD 5% 0.4 1.4 0.3 *** = p < 0.001 317
Figurese 8 7 6 number of buds 5 4 3 2 1 0 14 16 18 20 22 24 26 bulb temperature ( C) Fig. 1. The effect of temperature on the total number of buds (closed circles, r 2 = 0.90), the number of potentially flowering buds (open circles), number of desiccated buds (open triangles, r 2 = 0.81), and number of premature flowering buds (closed triangles). For potentially flowering buds and premature flowering buds no exponential models were fitted. 18 16 LBC (weeks) 14 12 10 8 6 14 16 18 20 22 24 26 bulb temperature ( C) Fig. 2. The effect of bulb temperature on the length of the leaf/bud cycle LBC (bud plastochron, r 2 = 0.92). 318