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Effect of cold temperature durations of onion sets in store on the incidence of bolting, bulbing and seed yield
Scientia Horticulturae 112 (2007) 16–22 www.elsevier.com/locate/scihorti
Effect of cold temperature durations of onion sets in store on the incidence of bolting, bulbing and seed yield Khalid Mahmud Khokhar a,*, P. Hadley b, S. Pearson b a
Vegetable Crops Research Programme, Horticulture Research Institute, National Agricultural Research Centre, Park Road, Islamabad, Pakistan b Department of Horticulture, School of Plant Sciences, The University of Reading, Reading RG6 6AS, UK Received 30 March 2006; received in revised form 4 September 2006; accepted 4 December 2006
Abstract The experiment was conducted at the experimental area of the School of Plant Sciences, University of Reading during 1996. The planting material comprised of sets (graded to 22.5 mm diameter) of two cultivars, Hygro and Delta. The sets were stored at 5 8C for nine chilling durations, between 10 and 90 days. A control treatment (sets stored at room temperature of 20 8C for days) was also included in the experiment for comparison. Sets of both cultivars treated for 90 days at 5 8C, produced nearly seven times more bolters than those treated for 20 days. Cool temperature treatment for 10 days was too short to induce bolting. Number of florets and percentage of seed bearing florets per umbel increased with lengthening cold durations and this resulted in higher seed yield per umbel. Mean bulb weight per plant was found to increase with shortening the period of low temperature treatment. For bulb crop, storage of sets at 20 8C for 90 days appears to be optimum, as it checked bolting and increased average bulb weight and bulb yields m 2 in both cultivars. # 2007 Elsevier B.V. All rights reserved. Keywords: Chilling durations; Inflorescence emergence; Florets; Umbel; Seed yield; Leaf area; Bulb weight; Bulb yield; Onion; Sets
1. Introduction Bolting, or seedstalk development, of onion (Allium cepa L.) is important for both bulb and seed production. Plants which bolt do not produce marketable bulbs, but seed yields are directly dependent on flower induction and bolting. Onion requires low temperatures or vernalisation for flower induction. Cold temperature storage is necessary in areas of the tropics where little cold weather exists for inflorescence induction. It is also important in regions where spring is followed by very hot summers and flowering is likely to be affected adversely before seed ripening. It is possible that after chilling, spring sowings would produce an early production of flowers by shortening the time to flower initiation, thus saving the seed crop from climatic hazards. Chilling is also necessary because relatively little bolting would otherwise be expected after a warm spring and early summer. In most regions, the mother bulbs of spring-sown cultivars are held in cold storage to fulfill their chilling
requirement and then planted in the spring to produce the seed crop. Several workers have reported that optimum durations to fully vernalise onion cultivars at low temperatures (3–11 8C) are in the range of 7–90 days (Woodbury, 1950; Kruzhilin and Shvedskaya, 1962; Kampen, 1970; Rabinowitch, 1985; Pike, 1986; Bertaud, 1988; Peters, 1990; Rabinowitch, 1990). Flowering in bolting resistant cultivars may be particularly difficult and long treatment at low temperature or large bulb size may be required (Shishido and Saito, 1977). Although a great deal of work has been reported on the incidence of bolting no work appears to have been carried out to study the relation between seed yield in onion sets and their chilling requirements. The present study aims to determine the effect of chilling duration on the incidence of bolting and seed yield using sets in two cultivars of onion (Hygro and Delta). The term chilling is used here to describe the effect of low temperature on the promotion of flowering. 2. Materials and methods
* Corresponding author. E-mail address: [email protected] (K.M. Khokhar). 0304-4238/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2006.12.038
The experiment was conducted at the experimental grounds of the School of Plant Sciences, University of Reading during
K.M. Khokhar et al. / Scientia Horticulturae 112 (2007) 16–22
1996. The planting material comprised of sets (graded to 22.5 mm diameter) of two cultivars (Hygro and Delta). Hygro and Delta are longday cultivars. The sets of these cultivars were obtained from MAAS Bv. Krviningen, Holland, through the courtesy of David O’Conner and Associates, UK. Much of previous work has been done using bulbs rather than sets to produce onion seed. Therefore, the sets of these cultivars were used to study their chilling requirements for floral initiation and seed production. Large sets of these cultivars were used because of the well-established effect of large set size in promoting bolting. The sets were stored at 5 8C for nine chilling durations, between 10 and 90 days. A control treatment (sets stored at room temperature of 20 8C for 90 days) was also included in the experiment for comparison. The land was thoroughly prepared by spreading a 2 cm layer of spent mushroom compost subsoiled to a depth of 50 cm followed by cultivation with a rotavator. No additional fertilizer was incorporated. The sets were planted in the field on April 24, 1996. The experiment was designed as randomized complete block (10 2 factors factorial) with three replications for each treatment. Each treatment consisted of 2 rows 1 m long. The rows were spaced 30 cm apart with 14 plants spaced 7 cm within each row. The crop was irrigated 7 days after planting. Water was applied as required usually with 10 days interval. However, depending on the weather, this interval was increased or decreased. Weeding was done periodically with hand hoe. Observations were recorded on bolting percentage (recorded from plants with emerged inflorescences), time to inflorescence emergence, time to spathe and floret opening, percentage of seed bearing florets per umbel, number of florets per umbel, seed yield per umbel, leaf area, fresh leaf weight and leaf number at the time of inflorescence initiation, average bulb weight, bulb yield m 2 and time to bulb maturity. Time to inflorescence emergence was based on mean data from 28 for each treatment. Observations on time to spathe and floret opening were based on random samples of 10 plants for each replication of each treatment. Percentage of seed bearing florets was based on random samples of five plants for each replication of each treatment. Leaf area was measured with leaf area meter (Delta-T, Cambridge, England). The leaf area thus obtained was multiplied by 2. Time to bulb maturity was recorded when the tops of the plants fell over. Agrometeorological data (temperature) for the post-planting period were also recorded. For statistical analysis of data, the SAS computer package (SAS, 1985) was used and graphs were plotted using Microsoft Excel.
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Fig. 1. Effect of chilling treatments on the percentage of bolting. Bars represent standard deviation.
chilling duration equated to 0.73 and 0.33% increases in bolting in cultivars Hygro and Delta, respectively. 3.2. Time to inflorescence emergence Time to inflorescence emergence in both cultivars decreased significantly (P < 0.001) and approximately linearly with increasing cool temperature durations (Fig. 2). The earliest inflorescences appeared in 90 days cold treatment where inflorescence emergence occurred after 44 and 48 days for cultivars Hygro and Delta, respectively compared with 60 and 65 days for short cold treatment (20 days). A one-day increase in chilling duration equated to 0.21 and 0.25 day reductions in time to inflorescence emergence in cultivars Hygro and Delta, respectively. 3.3. Time to spathe opening Time to spathe opening declined significantly (P < 0.001) and approximately linearly with increasing cool temperature durations (Fig. 3). With 90 days cold treatment, spathe opening occurred 71 and 74 days after planting in cultivars Hygro and Delta, respectively compared with 82 and 86 days at 20 days treatment. A one-day increase in chilling duration equated to 0.16 and 0.19 day reductions in time to spathe opening in cultivars Hygro and Delta, respectively. 3.4. Time to floret opening The time to floret opening decreased significantly (P < 0.001) with increasing cold durations (Fig. 4). Floret
3. Results 3.1. Percentage of bolting Bolting percentage increased significantly (P < 0.001) with increasing cool temperature durations (Fig. 1). Sets treated for 90 days at 5 8C, produced nearly seven times more bolters (56 and 27% for cultivars Hygro and Delta, respectively) than those treated for 20 days (8 and 4% for cultivars Hygro and Delta, respectively). No flowering occurred in the untreated (control) and 10 days low temperature treatment. A one-day increase in
Fig. 2. Effect of chilling treatments on time to inflorescence emergence. Bars represent standard deviation.
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Fig. 3. Effect of chilling treatments on time to spathe opening. Bars represent standard deviation.
Fig. 6. Effect of chilling treatments on number of florets per umbel. Bars represent standard deviation.
increases in seed bearing florets/umbel in cultivars Hygro and Delta, respectively. 3.6. Number of florets per umbel
Fig. 4. Effect of chilling treatments on time to floret opening. Bars represent standard deviation.
opening occurred rapidly (90 and 93 days for cultivars Hygro and Delta, respectively) at low temperature (5 8C) treatment for 90 days, whilst it occurred more slowly after a short period exposure (20 days) at low temperature (98 and 102 days). A one-day increase in chilling duration equated to 0.11 and 0.14 day reductions in time to floret opening in cultivars Hygro and Delta, respectively. 3.5. Percentage of seed bearing florets/umbel Cool temperature treatments significantly (P < 0.001) increased the percentage of seed bearing florets per umbel (Fig. 5). The 90 days cold treatment produced the maximum percentage of seed bearing florets (70 and 60% for cultivars Hygro and Delta, respectively), whilst the lowest (41 and 34%) was recorded after the 20 days temperature treatment. A oneday increase in chilling duration equated to 0.43 and 0.39%
Fig. 5. Effect of chilling treatments on percentage of seed bearing florets per umbel. Bars represent standard deviation.
Florets per umbel increased progressively with increasing cold durations (Fig. 6). The cold treatment of 90 days produced the greatest number of florets per umbel (596 and 578 for cultivars Hygro and Delta, respectively) compared with that of the 20 days treatment which produced 441 and 423 florets per umbel for cultivars Hygro and Delta, respectively. A one-day increase in chilling duration equated to 3.47 and 2.76 increases in number of florets/umbel in cultivars Hygro and Delta, respectively. 3.7. Seed yield per umbel The chilling treatments had a significant effect (P < 0.001) on seed yield per umbel which increased with increasing chilling durations (Fig. 7). Seed yield per umbel following a low temperature treatment (5 8C) for 90 days was approximately four times greater in both cultivars (5.5 and 4.2 g for cultivars Hygro and Delta, respectively) compared with that following a low temperature treatment for 20 days (1.5 and 1.0 g for cultivars Hygro and Delta, respectively). A one-day increase in chilling duration equated to 0.06 and 0.05 g increase in seed weight/ umbel in cultivars Hygro and Delta, respectively. 3.8. Leaf area Cold treatments had a significant effect on leaf area. Leaf area decreased with increasing cold treatments so that in
Fig. 7. Effect of chilling treatments on seed weight/umbel. Bars represent standard deviation.
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Fig. 8. Effect of chilling treatments on leaf area. Bars represent standard deviation. Fig. 10. Effect of chilling treatments on leaf number. Bars represent standard deviation.
cultivars Hygro and Delta, the leaf area recorded after cold treatment for 90 days was 63.7 and 57.9 cm2, respectively, whilst 97.7 and 87.3 cm2 was recorded after 10 days cold treatment (Fig. 8). A one-day increase in chilling duration equated to 4039 and 3745 mm reductions in leaf area in cultivars Hygro and Delta, respectively. 3.9. Fresh leaf weight Fresh leaf weight decreased significantly (P < 0.001) with increasing cold duration (Fig. 9). For example, in cultivars Hygro and Delta, the fresh leaf weight recorded after 90 days cold treatment was 59.4 and 53 g, respectively whilst leaf fresh weight was 81 and 77 g after 10 days cold treatment. However, the maximum fresh leaf weight was recorded in the untreated control (90 and 81 g for cultivars Hygro and Delta, respectively). A one-day increase in chilling duration equated to 0.30 and 0.31 g reduction in fresh leaf weight in cultivars Hygro and Delta, respectively. 3.10. Number of leaves per plant Number of leaves per plant decreased significantly (P < 0.001) with increasing cold durations (Fig. 10). In cultivar Hygro, maximum number of leaves (10.2) were produced after 10 days cold treatment and a minimum leaf number (8.2) was recorded following a 90 days cold treatment. In cultivar Delta, the control treatment produced the maximum leaf number (9.8) followed by 10 days cold treatment (9.6), whilst 90 days treatment produced the lowest leaf number (7.6). A one day increase in chilling duration equated to 0.02 and 0.03 reduction in leaf number in cultivars Hygro and Delta, respectively.
Fig. 9. Effect of chilling treatments on leaf fresh weight. Bars represent standard deviation.
3.11. Average bulb weight The average bulb weight in both cultivars increased significantly (P < 0.001) with shorter cold durations (Fig. 11). In cultivar Hygro, the minimum bulb weight (180 g) was recorded after 90 days cold treatment, whilst bulb weight from 10 days cold treatment was 255 g. In cultivar Delta, the 90-day cold treated sets produced 172 g average bulb weight. The untreated controls produced 1.5 and 1.4 times higher average bulb weight (265 and 246 g for cultivars Hygro and Delta, respectively) than the 90 days cold treatment. A one-day increase in chilling duration equated to 0.95 and 0.84 g reduction in average bulb weight in cultivars Hygro and Delta, respectively. 3.12. Bulb yield m
2
Bulb yield m 2 was significantly decreased with increasing cool temperature durations (Fig. 12). In cultivar Hygro, bulb yield m 2 recorded after the 90 days cool treatment was three times less (1.9 kg m 2) than the 10 day treatment whilst in cultivar Delta it was two times less (2.8 kg m 2). However, the highest yield m 2 (6.2 and 5.7 kg m 2 for cultivars Hygro and Delta, respectively) was recorded in the untreated controls. Oneday increase in chilling duration equated to 0.05 and 0.03 kg m 2 reduction in bulb yield in cultivars Hygro and Delta, respectively. 3.13. Bulb maturity The time to bulb maturity significantly (P < 0.001) decreased with lengthening cold durations (Fig. 13). The cold treatment of 90 days hastened the time of bulb maturity taking
Fig. 11. Effect of chilling treatments on average bulb weight. Bars represent standard deviation.
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Fig. 12. Effect of chilling treatments on bulb yield m 2. Bars represent standard deviation.
Fig. 13. Effect of chilling treatments on time to bulb maturity. Bars represent standard deviation.
104 and 95 days in cultivars Hygro and Delta, respectively compared with that after a cold treatment of 10 days in which bulb matured in 116 and 108 days, respectively. A one-day increase in chilling duration equated to 0.13 day reduction in time to bulb maturity in both cultivars Hygro and Delta. 4. Discussion The results presented in this study have shown that parameters associated with flowering and seed production of onion were significantly influenced by duration of chilling treatments. It is clear from the results that the longer the chilling duration the higher the percentage of bolting. Cool temperature treatment for 10 days was too short to induce bolting. For the cultivars used in this study, it appeared that a period of 90 days of chilling treatment was not optimal as it only induced 56 and 27% bolting in cultivars Hygro and Delta, respectively. This supports the findings of Hwang (1982) that increasing chilling durations increased bolting in onion. The results of the present studies partly support the findings of Kruzhilin and Shvedskaya (1962), Shishido and Saito (1975), Aguiar (1984), Pike (1986), Bertaud (1988), Bon and Rhino (1988) and Currah and Proctor (1990) that depending on cultivars, optimum chilling durations range from 7 to 90 days at 3–11 8C for floral initiation. The results also confirm earlier findings of Thompson and Smith (1938), Heath and Mathur (1944), Heath (1945), Holdsworth and Heath (1950), Aoba (1960), Aura (1963) and Kampen (1970) that inflorescence primordia are initiated during storage
of mother bulbs after sufficient accumulation of low temperatures between 5 and 15 8C. It seems very probable that had sets larger than 22.5 mm been used, they might have fully induced for flowering at 5 8C. The sets used here (22.5 mm) clearly need to be treated for a longer period of time (>90 days) at 5 8C to be fully vernalised. The results have also shown that flower induction and further development of the inflorescence depends on low temperature and also longer storage in the cold. Inflorescence emergence and subsequent development (spathe and floret opening) occurred earlier with increasing durations of low temperature. Woodbury (1950) suggested that conditions like storage of bulbs at low temperature favouring initiation of the inflorescence might also promote its subsequent development. The present study provides detailed information on the duration of chilling on inflorescence initiation and its further development towards flowering which has not done before. Number of florets and percentage of seed bearing florets per umbel increased with lengthening cold durations and this resulted in higher seed yield per umbel. This could be explained by the fact that under longer cold treatments at an inductive temperature (5 8C), the sets received colder stimulus which favoured flowering resulting in the highest number of florets and seed yield per umbel in the 90 days cold treatment. No such study appears to have been carried out in the past to show the effect of chilling duration on number of florets and seed yield in onion cultivars. Leaf area, leaf weight and leaf number decreased with increasing cold treatments. Sets stored at ambient temperature (20 0.5 8C) or for very short period (10 days) at 5 8C produced a greater leaf number with larger areas compared with longer chilling treatments. This is likely to be due to the fact that under these conditions, flowering was not induced and the plants continued to produce leaves rather than florets. There must be a certain minimal number of leaves before an inflorescence can be initiated (Gregory, 1936; Galmarini, 1990). Similarly, there may also be a maximal leaf number above which inflorescence initiation must occur. In the present study the minimum number of leaves (8.2 and 7.6 for cultivars Hygro and Delta, respectively) were recorded at the time of inflorescence initiation, in 90 days chilling treatment. The lower leaf number in chilling treatments suggests earlier inflorescence initiation. Maximum leaves (10.2 and 9.6 for cultivars Hygro and Delta, respectively) were observed in the shortest cool temperature treatment (10 days). It was found that bulbs matured faster with lengthening cold treatments. Mean bulb weight per plant was also found to increase with shortening the period of low temperature treatment. The probable reason for an increase in bulb weight could be late ripening in shorter chilling treatments, which prolonged the growth period compared with longer cold treatments. In the untreated control where the sets were kept at ambient temperature (20 8C), the bulbs matured late. These results support the findings of Heath (1943), Heath et al. (1947), Heath and Holdsworth (1948), Aura (1963), Jones and Mann (1963), Aura (1968), Butt (1968) and Palilov (1969) that under low storage temperatures (0–7 8C) the vegetative cycle is
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reduced and bulbing is accelerated whilst under high temperatures (18–30 8C), both bulbing and ripening are delayed. It was found that bulb yield m 2 in both cultivars decreased with increasing chilling durations. The low bulb yield m 2 in longer chilling treatments is partly due to higher bolting and partly because of the lower average bulb weights. Higher bulb yields m 2 recorded from untreated controls could be partly due to the absence of bolting and partly through greater average bulb weights compared with low temperature treatments. As the results indicated, only 56 and 27% bolting occurred in cultivars Hygro and Delta, respectively after storage of sets at 5 8C for 90 days, which is insufficient to have an economical seed crop. Therefore, sets of both cultivars need to be stored for a longer period (more than 90 days) at 5 8C. Other option could be to use sets larger than 22.5 mm to be fully chilled at relatively shorter period of storage at 5 8C. In this study, the sets were planted rather late (April 24, 1996), the time when very little cold stimulus prevailed during the growing phase to further induce bolting (Fig. 14). Also it seems probable that had the sets planted earlier, these might have been fully induced to flowering. For bulb crop, storage of sets at 20 8C for 90 days appears to be optimum, as it checked bolting in both cultivars. The mean maximum and minimum temperature during crop period from 26 April to August 20, 1996 at Reading ranged from 13.5 to 22.1 8C and from 5 to 12.2 8C, respectively. When bulb onions are grown from sets, the temperature and period of storage of sets prior to planting influence the time of bulb development. Bulbs from sets stored at room temperature of 20 8C for 90 days and planted under the post-planting environmental conditions of Reading, matured approximately 2 weeks later than those stored at 5 8C for 90 days. This delay would be longer, the cooler the climatic conditions after planting. The sets stored at 5 8C for 90 days and planted under similar conditions exhibited highest bolting of 56 and 27% in cultivars Hygro and Delta, respectively. However, bolting percentage would vary from location to location depending upon the post-planting environmental conditions. The bolting percentage would be higher at cooler locations than that of Reading. The variation in bolting has been demonstrated by Kampen (1970) who reported 73 and 84% flowering of bulbs stored at inductive temperature and planting them at two locations (Wageningen and Alkmaar) in Netherlands. However, for bulb crop late planting is desirable because warmer and longer days induce bulbing and check bolting. Published
Fig. 14. Mean temperature during experimental period (from 26 April to 20 August, 1996).
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information describing the effects of cold durations on bolting, inflorescence emergence and its subsequent development towards flowering and seed production is lacking. The present study comprehensively describes the duration of chilling on both seed and bulb yield. Once the chilling requirements of a cultivar is known, it should be possible to plan a sound seed production programme. The present study has revealed that Hygro is a bolting susceptible cultivar because of 56% bolting whilst Delta is bolting resistant with only 27% bolting which was observed in the sets of these cultivars stored at 5 8C for 90 days. Furthermore, longer chilling treatments appear to give earlier inflorescence initiation compared to shorter treatments and this information could be applied in the management of a breeding programme to ensure synchronization of flowering. References Aguiar, P.A.A., 1984. Periodo de vernalizacao dos bulbos de cebola para producao de sementes, no nordeste do Brasil. Pesquisa Agropecuaria Brasileira 19, 197–200. Aoba, T., 1960. The influence of the storage temperature for onion bulbs on their seed production. J. Jpn. Soc. Horticult. Sci. 29, 135–141 (Japanese, En. sum.). Aura, K., 1963. Studies on the vegetatively propagated onions cultivated in Finnland, with special reference to flowering and storage. Ann. Agric. Fenn. 2 (Suppl. 5), 1–74. Aura, K., 1968. Studies on the vegetatively propagated onions cultivated in Finnland, with special reference to flowering and storage. IX. The influence of various storage temperatures on flowering and yield in the North Finnish onion strain. Ann. Agric. Fenn. 7, 183–188. Bertaud, D.S., 1988. Effects of chilling duration photoperiod and temperature on floral initiation and development in sprouted and unsprouted onion bulbs. In: Proceedings of the fourth EUCARPIA Allium Symposium, Institute of Horticultural Research, Wellesbourne, UK, pp. 245–261. Bon, H.De., Rhino, B., 1988. Flowering in the various cultivars of onions (Allium cepa L.) brought to Martinique (French West Indies). In: Proceedings of the fourth EUCARPIA Allium Symposium, I.H.R. Wellesbourne, September 5–9, 1988, pp. 262–266. Butt, A.M., 1968. Vegetative growth, morphogenesis and carbohydrate content of the onion plant as a function of light and temperature under field and controlled conditions. Mededelingen Landbouwhogeschool Wageningen 68, 94–191. Currah, L., Proctor, F.J., 1990. Onions in Tropical Regions. Bulletin 35, Natural Resources Institute, Chatham, UK. Galmarini, C.R., 1990. Characterization of argentine onion requirements. Master of Science Thesis, Instituto Agronomico Mediterraneo de Zaragoza, Spain, p. 71. Gregory, F.G., 1936. The effect of length of day on the flowering of plants. Sci. Horticult. 4, 143–154. Heath, O.V.S., 1943. Studies in the physiology of the onion plant. I. An investigation of factors concerned in the flowering (bolting) of onions grown from sets and its prevention. Part 1. Production and storage of onion sets and field results. Ann. Appl. Biol. 30, 208–220. Heath, O.V.S., 1945. Formative effects of environmental factors as exemplified in the development of the onion plant. Nature 155, 623–626. Heath, O.V.S., Holdsworth, M., 1948. Morphogenic factors as exemplified by the onion plant growth. In: Daneellie, J.F., Brown, R. (Eds.), Symposia of the Society of Experimental Biology, vol. 2. Cambridge University Press, pp. 326–350. Heath, O.V.S., Holdsworth, M., Tincker, M.A.H., Brown, F.C., 1947. Studies in the physiology of the onion plant. III. Further experiments on the effects of storage temperature and other factors on onions grown from sets. Ann. Appl. Biol. 34, 473–502. Heath, O.V.S., Mathur, P.B., 1944. Studies in the physiology of the onion plant II. Inflorescence initiation and development, and other changes in the
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internal morphology of onion sets, as influenced by temperature and day length. Ann. Appl. Biol. 31, 173–187. Holdsworth, M., Heath, O.V.S., 1950. Studies in the physiology of the onion plant. J. Exp. Bot. 1, 353–375. Hwang, T.J., 1982. Effects of low temperature and Gibberellic acid (GA3) on floral physiology of Allium cepa L. M.Sc. thesis, National Chung Hsing University, Taichung, Taiwan. Jones, H.A., Mann, L.K., 1963. Onion and Their Allies. Leonard Hill, London. Van Kampen, Ir.J., 1970. Shortening the breeding cycle in onions. Mededeling No 51. Proefstation Voor de Groonteteelt in de Vollegrond, Nederland (Dutch, En sum). Kruzhilin, A.S., Shvedskaya, Z.M., 1962. Peculiarities of phasic development and morphogenesis in onion (Allium cepa). Fiziologia Rastenii 9, 466–475 [Translated] (cited by Rabinowitch, 1990). Palilov, N.A., 1969. The biological bases of onion storage. Proceedings of the Symposium of vegetable storage, Acta Horticulturae 20, 53–64. Peters, R., 1990. Seed production in onions and some other Allium species. In: Rabinowitch, H.D., Brewster, J.L. (Eds.), Onions and Allied Crops. Vol. I. Botany, Physiology and Genetics. CRC Press, Boca Raton, Florida, pp. 161–176. Pike, L.M., 1986. Onion breeding. In: Basset, M.J. (Ed.), Breeding Vegetable Crops. AVI Publishing Co., Connecticut, pp. 357–394.
Rabinowitch, H.D., 1985. Onions and other edible Alliums. In: Halevy, A.H. (Ed.), CRC Handbook of Flowering, Vol. I. CRC Press, Boca Raton, Florida, pp. 398–405. Rabinowitch, H.D., 1990. Physiology of flowering. In: Rabinowitch, H.D., Brewster, J.L. (Eds.), Onions and Allied Crops. Vol. I. Botany, Physiology and Genetics. CRC Press, Boca Raton, Florida, pp. 133–134. S.A.S., 1985. Statistical Analysis System User’s Guide: Statistics. S.A.S. Inc., Cary, North Carolina. Shishido, Y., Saito, T., 1975. Studies on the flower bud formation in onion plants. I. Effects of temperature, photoperiod and light intensity on the low temperature induction of flower buds. J. Jpn. Soc. Horticult. Sci. 44, 122– 130 (Japanese, En. Sum). Shishido, Y., Saito, T., 1977. Studies on the flower bud formation in onion plants. III. Effects of physiological conditions on the low temperature induction of flower buds in bulbs. J. Jpn. Soc. Horticult. Sci. 46, 310– 316 (Japanese, En. Sum). Thompson, H.C., Smith, O., 1938. Seedstalk and bulb development in the onion (Allium cepa L.). Cornell University Agricultural Experiment Station Bulletin, No. 708. Woodbury, G.W., 1950. A study of factors influencing floral initiation and seedstalk development in the onion, Allium cepa Linn. Idaho Agricultural Experiment Station Research Bulletin No. 18.
Effect of cold temperature durations of onion sets in store on the incidence of bolting, bulbing and seed yield Khalid Mahmud Khokhar a,*, P. Hadley b, S. Pearson b a
Vegetable Crops Research Programme, Horticulture Research Institute, National Agricultural Research Centre, Park Road, Islamabad, Pakistan b Department of Horticulture, School of Plant Sciences, The University of Reading, Reading RG6 6AS, UK Received 30 March 2006; received in revised form 4 September 2006; accepted 4 December 2006
Abstract The experiment was conducted at the experimental area of the School of Plant Sciences, University of Reading during 1996. The planting material comprised of sets (graded to 22.5 mm diameter) of two cultivars, Hygro and Delta. The sets were stored at 5 8C for nine chilling durations, between 10 and 90 days. A control treatment (sets stored at room temperature of 20 8C for days) was also included in the experiment for comparison. Sets of both cultivars treated for 90 days at 5 8C, produced nearly seven times more bolters than those treated for 20 days. Cool temperature treatment for 10 days was too short to induce bolting. Number of florets and percentage of seed bearing florets per umbel increased with lengthening cold durations and this resulted in higher seed yield per umbel. Mean bulb weight per plant was found to increase with shortening the period of low temperature treatment. For bulb crop, storage of sets at 20 8C for 90 days appears to be optimum, as it checked bolting and increased average bulb weight and bulb yields m 2 in both cultivars. # 2007 Elsevier B.V. All rights reserved. Keywords: Chilling durations; Inflorescence emergence; Florets; Umbel; Seed yield; Leaf area; Bulb weight; Bulb yield; Onion; Sets
1. Introduction Bolting, or seedstalk development, of onion (Allium cepa L.) is important for both bulb and seed production. Plants which bolt do not produce marketable bulbs, but seed yields are directly dependent on flower induction and bolting. Onion requires low temperatures or vernalisation for flower induction. Cold temperature storage is necessary in areas of the tropics where little cold weather exists for inflorescence induction. It is also important in regions where spring is followed by very hot summers and flowering is likely to be affected adversely before seed ripening. It is possible that after chilling, spring sowings would produce an early production of flowers by shortening the time to flower initiation, thus saving the seed crop from climatic hazards. Chilling is also necessary because relatively little bolting would otherwise be expected after a warm spring and early summer. In most regions, the mother bulbs of spring-sown cultivars are held in cold storage to fulfill their chilling
requirement and then planted in the spring to produce the seed crop. Several workers have reported that optimum durations to fully vernalise onion cultivars at low temperatures (3–11 8C) are in the range of 7–90 days (Woodbury, 1950; Kruzhilin and Shvedskaya, 1962; Kampen, 1970; Rabinowitch, 1985; Pike, 1986; Bertaud, 1988; Peters, 1990; Rabinowitch, 1990). Flowering in bolting resistant cultivars may be particularly difficult and long treatment at low temperature or large bulb size may be required (Shishido and Saito, 1977). Although a great deal of work has been reported on the incidence of bolting no work appears to have been carried out to study the relation between seed yield in onion sets and their chilling requirements. The present study aims to determine the effect of chilling duration on the incidence of bolting and seed yield using sets in two cultivars of onion (Hygro and Delta). The term chilling is used here to describe the effect of low temperature on the promotion of flowering. 2. Materials and methods
* Corresponding author. E-mail address: [email protected] (K.M. Khokhar). 0304-4238/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2006.12.038
The experiment was conducted at the experimental grounds of the School of Plant Sciences, University of Reading during
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1996. The planting material comprised of sets (graded to 22.5 mm diameter) of two cultivars (Hygro and Delta). Hygro and Delta are longday cultivars. The sets of these cultivars were obtained from MAAS Bv. Krviningen, Holland, through the courtesy of David O’Conner and Associates, UK. Much of previous work has been done using bulbs rather than sets to produce onion seed. Therefore, the sets of these cultivars were used to study their chilling requirements for floral initiation and seed production. Large sets of these cultivars were used because of the well-established effect of large set size in promoting bolting. The sets were stored at 5 8C for nine chilling durations, between 10 and 90 days. A control treatment (sets stored at room temperature of 20 8C for 90 days) was also included in the experiment for comparison. The land was thoroughly prepared by spreading a 2 cm layer of spent mushroom compost subsoiled to a depth of 50 cm followed by cultivation with a rotavator. No additional fertilizer was incorporated. The sets were planted in the field on April 24, 1996. The experiment was designed as randomized complete block (10 2 factors factorial) with three replications for each treatment. Each treatment consisted of 2 rows 1 m long. The rows were spaced 30 cm apart with 14 plants spaced 7 cm within each row. The crop was irrigated 7 days after planting. Water was applied as required usually with 10 days interval. However, depending on the weather, this interval was increased or decreased. Weeding was done periodically with hand hoe. Observations were recorded on bolting percentage (recorded from plants with emerged inflorescences), time to inflorescence emergence, time to spathe and floret opening, percentage of seed bearing florets per umbel, number of florets per umbel, seed yield per umbel, leaf area, fresh leaf weight and leaf number at the time of inflorescence initiation, average bulb weight, bulb yield m 2 and time to bulb maturity. Time to inflorescence emergence was based on mean data from 28 for each treatment. Observations on time to spathe and floret opening were based on random samples of 10 plants for each replication of each treatment. Percentage of seed bearing florets was based on random samples of five plants for each replication of each treatment. Leaf area was measured with leaf area meter (Delta-T, Cambridge, England). The leaf area thus obtained was multiplied by 2. Time to bulb maturity was recorded when the tops of the plants fell over. Agrometeorological data (temperature) for the post-planting period were also recorded. For statistical analysis of data, the SAS computer package (SAS, 1985) was used and graphs were plotted using Microsoft Excel.
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Fig. 1. Effect of chilling treatments on the percentage of bolting. Bars represent standard deviation.
chilling duration equated to 0.73 and 0.33% increases in bolting in cultivars Hygro and Delta, respectively. 3.2. Time to inflorescence emergence Time to inflorescence emergence in both cultivars decreased significantly (P < 0.001) and approximately linearly with increasing cool temperature durations (Fig. 2). The earliest inflorescences appeared in 90 days cold treatment where inflorescence emergence occurred after 44 and 48 days for cultivars Hygro and Delta, respectively compared with 60 and 65 days for short cold treatment (20 days). A one-day increase in chilling duration equated to 0.21 and 0.25 day reductions in time to inflorescence emergence in cultivars Hygro and Delta, respectively. 3.3. Time to spathe opening Time to spathe opening declined significantly (P < 0.001) and approximately linearly with increasing cool temperature durations (Fig. 3). With 90 days cold treatment, spathe opening occurred 71 and 74 days after planting in cultivars Hygro and Delta, respectively compared with 82 and 86 days at 20 days treatment. A one-day increase in chilling duration equated to 0.16 and 0.19 day reductions in time to spathe opening in cultivars Hygro and Delta, respectively. 3.4. Time to floret opening The time to floret opening decreased significantly (P < 0.001) with increasing cold durations (Fig. 4). Floret
3. Results 3.1. Percentage of bolting Bolting percentage increased significantly (P < 0.001) with increasing cool temperature durations (Fig. 1). Sets treated for 90 days at 5 8C, produced nearly seven times more bolters (56 and 27% for cultivars Hygro and Delta, respectively) than those treated for 20 days (8 and 4% for cultivars Hygro and Delta, respectively). No flowering occurred in the untreated (control) and 10 days low temperature treatment. A one-day increase in
Fig. 2. Effect of chilling treatments on time to inflorescence emergence. Bars represent standard deviation.
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Fig. 3. Effect of chilling treatments on time to spathe opening. Bars represent standard deviation.
Fig. 6. Effect of chilling treatments on number of florets per umbel. Bars represent standard deviation.
increases in seed bearing florets/umbel in cultivars Hygro and Delta, respectively. 3.6. Number of florets per umbel
Fig. 4. Effect of chilling treatments on time to floret opening. Bars represent standard deviation.
opening occurred rapidly (90 and 93 days for cultivars Hygro and Delta, respectively) at low temperature (5 8C) treatment for 90 days, whilst it occurred more slowly after a short period exposure (20 days) at low temperature (98 and 102 days). A one-day increase in chilling duration equated to 0.11 and 0.14 day reductions in time to floret opening in cultivars Hygro and Delta, respectively. 3.5. Percentage of seed bearing florets/umbel Cool temperature treatments significantly (P < 0.001) increased the percentage of seed bearing florets per umbel (Fig. 5). The 90 days cold treatment produced the maximum percentage of seed bearing florets (70 and 60% for cultivars Hygro and Delta, respectively), whilst the lowest (41 and 34%) was recorded after the 20 days temperature treatment. A oneday increase in chilling duration equated to 0.43 and 0.39%
Fig. 5. Effect of chilling treatments on percentage of seed bearing florets per umbel. Bars represent standard deviation.
Florets per umbel increased progressively with increasing cold durations (Fig. 6). The cold treatment of 90 days produced the greatest number of florets per umbel (596 and 578 for cultivars Hygro and Delta, respectively) compared with that of the 20 days treatment which produced 441 and 423 florets per umbel for cultivars Hygro and Delta, respectively. A one-day increase in chilling duration equated to 3.47 and 2.76 increases in number of florets/umbel in cultivars Hygro and Delta, respectively. 3.7. Seed yield per umbel The chilling treatments had a significant effect (P < 0.001) on seed yield per umbel which increased with increasing chilling durations (Fig. 7). Seed yield per umbel following a low temperature treatment (5 8C) for 90 days was approximately four times greater in both cultivars (5.5 and 4.2 g for cultivars Hygro and Delta, respectively) compared with that following a low temperature treatment for 20 days (1.5 and 1.0 g for cultivars Hygro and Delta, respectively). A one-day increase in chilling duration equated to 0.06 and 0.05 g increase in seed weight/ umbel in cultivars Hygro and Delta, respectively. 3.8. Leaf area Cold treatments had a significant effect on leaf area. Leaf area decreased with increasing cold treatments so that in
Fig. 7. Effect of chilling treatments on seed weight/umbel. Bars represent standard deviation.
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Fig. 8. Effect of chilling treatments on leaf area. Bars represent standard deviation. Fig. 10. Effect of chilling treatments on leaf number. Bars represent standard deviation.
cultivars Hygro and Delta, the leaf area recorded after cold treatment for 90 days was 63.7 and 57.9 cm2, respectively, whilst 97.7 and 87.3 cm2 was recorded after 10 days cold treatment (Fig. 8). A one-day increase in chilling duration equated to 4039 and 3745 mm reductions in leaf area in cultivars Hygro and Delta, respectively. 3.9. Fresh leaf weight Fresh leaf weight decreased significantly (P < 0.001) with increasing cold duration (Fig. 9). For example, in cultivars Hygro and Delta, the fresh leaf weight recorded after 90 days cold treatment was 59.4 and 53 g, respectively whilst leaf fresh weight was 81 and 77 g after 10 days cold treatment. However, the maximum fresh leaf weight was recorded in the untreated control (90 and 81 g for cultivars Hygro and Delta, respectively). A one-day increase in chilling duration equated to 0.30 and 0.31 g reduction in fresh leaf weight in cultivars Hygro and Delta, respectively. 3.10. Number of leaves per plant Number of leaves per plant decreased significantly (P < 0.001) with increasing cold durations (Fig. 10). In cultivar Hygro, maximum number of leaves (10.2) were produced after 10 days cold treatment and a minimum leaf number (8.2) was recorded following a 90 days cold treatment. In cultivar Delta, the control treatment produced the maximum leaf number (9.8) followed by 10 days cold treatment (9.6), whilst 90 days treatment produced the lowest leaf number (7.6). A one day increase in chilling duration equated to 0.02 and 0.03 reduction in leaf number in cultivars Hygro and Delta, respectively.
Fig. 9. Effect of chilling treatments on leaf fresh weight. Bars represent standard deviation.
3.11. Average bulb weight The average bulb weight in both cultivars increased significantly (P < 0.001) with shorter cold durations (Fig. 11). In cultivar Hygro, the minimum bulb weight (180 g) was recorded after 90 days cold treatment, whilst bulb weight from 10 days cold treatment was 255 g. In cultivar Delta, the 90-day cold treated sets produced 172 g average bulb weight. The untreated controls produced 1.5 and 1.4 times higher average bulb weight (265 and 246 g for cultivars Hygro and Delta, respectively) than the 90 days cold treatment. A one-day increase in chilling duration equated to 0.95 and 0.84 g reduction in average bulb weight in cultivars Hygro and Delta, respectively. 3.12. Bulb yield m
2
Bulb yield m 2 was significantly decreased with increasing cool temperature durations (Fig. 12). In cultivar Hygro, bulb yield m 2 recorded after the 90 days cool treatment was three times less (1.9 kg m 2) than the 10 day treatment whilst in cultivar Delta it was two times less (2.8 kg m 2). However, the highest yield m 2 (6.2 and 5.7 kg m 2 for cultivars Hygro and Delta, respectively) was recorded in the untreated controls. Oneday increase in chilling duration equated to 0.05 and 0.03 kg m 2 reduction in bulb yield in cultivars Hygro and Delta, respectively. 3.13. Bulb maturity The time to bulb maturity significantly (P < 0.001) decreased with lengthening cold durations (Fig. 13). The cold treatment of 90 days hastened the time of bulb maturity taking
Fig. 11. Effect of chilling treatments on average bulb weight. Bars represent standard deviation.
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Fig. 12. Effect of chilling treatments on bulb yield m 2. Bars represent standard deviation.
Fig. 13. Effect of chilling treatments on time to bulb maturity. Bars represent standard deviation.
104 and 95 days in cultivars Hygro and Delta, respectively compared with that after a cold treatment of 10 days in which bulb matured in 116 and 108 days, respectively. A one-day increase in chilling duration equated to 0.13 day reduction in time to bulb maturity in both cultivars Hygro and Delta. 4. Discussion The results presented in this study have shown that parameters associated with flowering and seed production of onion were significantly influenced by duration of chilling treatments. It is clear from the results that the longer the chilling duration the higher the percentage of bolting. Cool temperature treatment for 10 days was too short to induce bolting. For the cultivars used in this study, it appeared that a period of 90 days of chilling treatment was not optimal as it only induced 56 and 27% bolting in cultivars Hygro and Delta, respectively. This supports the findings of Hwang (1982) that increasing chilling durations increased bolting in onion. The results of the present studies partly support the findings of Kruzhilin and Shvedskaya (1962), Shishido and Saito (1975), Aguiar (1984), Pike (1986), Bertaud (1988), Bon and Rhino (1988) and Currah and Proctor (1990) that depending on cultivars, optimum chilling durations range from 7 to 90 days at 3–11 8C for floral initiation. The results also confirm earlier findings of Thompson and Smith (1938), Heath and Mathur (1944), Heath (1945), Holdsworth and Heath (1950), Aoba (1960), Aura (1963) and Kampen (1970) that inflorescence primordia are initiated during storage
of mother bulbs after sufficient accumulation of low temperatures between 5 and 15 8C. It seems very probable that had sets larger than 22.5 mm been used, they might have fully induced for flowering at 5 8C. The sets used here (22.5 mm) clearly need to be treated for a longer period of time (>90 days) at 5 8C to be fully vernalised. The results have also shown that flower induction and further development of the inflorescence depends on low temperature and also longer storage in the cold. Inflorescence emergence and subsequent development (spathe and floret opening) occurred earlier with increasing durations of low temperature. Woodbury (1950) suggested that conditions like storage of bulbs at low temperature favouring initiation of the inflorescence might also promote its subsequent development. The present study provides detailed information on the duration of chilling on inflorescence initiation and its further development towards flowering which has not done before. Number of florets and percentage of seed bearing florets per umbel increased with lengthening cold durations and this resulted in higher seed yield per umbel. This could be explained by the fact that under longer cold treatments at an inductive temperature (5 8C), the sets received colder stimulus which favoured flowering resulting in the highest number of florets and seed yield per umbel in the 90 days cold treatment. No such study appears to have been carried out in the past to show the effect of chilling duration on number of florets and seed yield in onion cultivars. Leaf area, leaf weight and leaf number decreased with increasing cold treatments. Sets stored at ambient temperature (20 0.5 8C) or for very short period (10 days) at 5 8C produced a greater leaf number with larger areas compared with longer chilling treatments. This is likely to be due to the fact that under these conditions, flowering was not induced and the plants continued to produce leaves rather than florets. There must be a certain minimal number of leaves before an inflorescence can be initiated (Gregory, 1936; Galmarini, 1990). Similarly, there may also be a maximal leaf number above which inflorescence initiation must occur. In the present study the minimum number of leaves (8.2 and 7.6 for cultivars Hygro and Delta, respectively) were recorded at the time of inflorescence initiation, in 90 days chilling treatment. The lower leaf number in chilling treatments suggests earlier inflorescence initiation. Maximum leaves (10.2 and 9.6 for cultivars Hygro and Delta, respectively) were observed in the shortest cool temperature treatment (10 days). It was found that bulbs matured faster with lengthening cold treatments. Mean bulb weight per plant was also found to increase with shortening the period of low temperature treatment. The probable reason for an increase in bulb weight could be late ripening in shorter chilling treatments, which prolonged the growth period compared with longer cold treatments. In the untreated control where the sets were kept at ambient temperature (20 8C), the bulbs matured late. These results support the findings of Heath (1943), Heath et al. (1947), Heath and Holdsworth (1948), Aura (1963), Jones and Mann (1963), Aura (1968), Butt (1968) and Palilov (1969) that under low storage temperatures (0–7 8C) the vegetative cycle is
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reduced and bulbing is accelerated whilst under high temperatures (18–30 8C), both bulbing and ripening are delayed. It was found that bulb yield m 2 in both cultivars decreased with increasing chilling durations. The low bulb yield m 2 in longer chilling treatments is partly due to higher bolting and partly because of the lower average bulb weights. Higher bulb yields m 2 recorded from untreated controls could be partly due to the absence of bolting and partly through greater average bulb weights compared with low temperature treatments. As the results indicated, only 56 and 27% bolting occurred in cultivars Hygro and Delta, respectively after storage of sets at 5 8C for 90 days, which is insufficient to have an economical seed crop. Therefore, sets of both cultivars need to be stored for a longer period (more than 90 days) at 5 8C. Other option could be to use sets larger than 22.5 mm to be fully chilled at relatively shorter period of storage at 5 8C. In this study, the sets were planted rather late (April 24, 1996), the time when very little cold stimulus prevailed during the growing phase to further induce bolting (Fig. 14). Also it seems probable that had the sets planted earlier, these might have been fully induced to flowering. For bulb crop, storage of sets at 20 8C for 90 days appears to be optimum, as it checked bolting in both cultivars. The mean maximum and minimum temperature during crop period from 26 April to August 20, 1996 at Reading ranged from 13.5 to 22.1 8C and from 5 to 12.2 8C, respectively. When bulb onions are grown from sets, the temperature and period of storage of sets prior to planting influence the time of bulb development. Bulbs from sets stored at room temperature of 20 8C for 90 days and planted under the post-planting environmental conditions of Reading, matured approximately 2 weeks later than those stored at 5 8C for 90 days. This delay would be longer, the cooler the climatic conditions after planting. The sets stored at 5 8C for 90 days and planted under similar conditions exhibited highest bolting of 56 and 27% in cultivars Hygro and Delta, respectively. However, bolting percentage would vary from location to location depending upon the post-planting environmental conditions. The bolting percentage would be higher at cooler locations than that of Reading. The variation in bolting has been demonstrated by Kampen (1970) who reported 73 and 84% flowering of bulbs stored at inductive temperature and planting them at two locations (Wageningen and Alkmaar) in Netherlands. However, for bulb crop late planting is desirable because warmer and longer days induce bulbing and check bolting. Published
Fig. 14. Mean temperature during experimental period (from 26 April to 20 August, 1996).
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information describing the effects of cold durations on bolting, inflorescence emergence and its subsequent development towards flowering and seed production is lacking. The present study comprehensively describes the duration of chilling on both seed and bulb yield. Once the chilling requirements of a cultivar is known, it should be possible to plan a sound seed production programme. The present study has revealed that Hygro is a bolting susceptible cultivar because of 56% bolting whilst Delta is bolting resistant with only 27% bolting which was observed in the sets of these cultivars stored at 5 8C for 90 days. Furthermore, longer chilling treatments appear to give earlier inflorescence initiation compared to shorter treatments and this information could be applied in the management of a breeding programme to ensure synchronization of flowering. References Aguiar, P.A.A., 1984. Periodo de vernalizacao dos bulbos de cebola para producao de sementes, no nordeste do Brasil. Pesquisa Agropecuaria Brasileira 19, 197–200. Aoba, T., 1960. The influence of the storage temperature for onion bulbs on their seed production. J. Jpn. Soc. Horticult. Sci. 29, 135–141 (Japanese, En. sum.). Aura, K., 1963. Studies on the vegetatively propagated onions cultivated in Finnland, with special reference to flowering and storage. Ann. Agric. Fenn. 2 (Suppl. 5), 1–74. Aura, K., 1968. Studies on the vegetatively propagated onions cultivated in Finnland, with special reference to flowering and storage. IX. The influence of various storage temperatures on flowering and yield in the North Finnish onion strain. Ann. Agric. Fenn. 7, 183–188. Bertaud, D.S., 1988. Effects of chilling duration photoperiod and temperature on floral initiation and development in sprouted and unsprouted onion bulbs. In: Proceedings of the fourth EUCARPIA Allium Symposium, Institute of Horticultural Research, Wellesbourne, UK, pp. 245–261. Bon, H.De., Rhino, B., 1988. Flowering in the various cultivars of onions (Allium cepa L.) brought to Martinique (French West Indies). In: Proceedings of the fourth EUCARPIA Allium Symposium, I.H.R. Wellesbourne, September 5–9, 1988, pp. 262–266. Butt, A.M., 1968. Vegetative growth, morphogenesis and carbohydrate content of the onion plant as a function of light and temperature under field and controlled conditions. Mededelingen Landbouwhogeschool Wageningen 68, 94–191. Currah, L., Proctor, F.J., 1990. Onions in Tropical Regions. Bulletin 35, Natural Resources Institute, Chatham, UK. Galmarini, C.R., 1990. Characterization of argentine onion requirements. Master of Science Thesis, Instituto Agronomico Mediterraneo de Zaragoza, Spain, p. 71. Gregory, F.G., 1936. The effect of length of day on the flowering of plants. Sci. Horticult. 4, 143–154. Heath, O.V.S., 1943. Studies in the physiology of the onion plant. I. An investigation of factors concerned in the flowering (bolting) of onions grown from sets and its prevention. Part 1. Production and storage of onion sets and field results. Ann. Appl. Biol. 30, 208–220. Heath, O.V.S., 1945. Formative effects of environmental factors as exemplified in the development of the onion plant. Nature 155, 623–626. Heath, O.V.S., Holdsworth, M., 1948. Morphogenic factors as exemplified by the onion plant growth. In: Daneellie, J.F., Brown, R. (Eds.), Symposia of the Society of Experimental Biology, vol. 2. Cambridge University Press, pp. 326–350. Heath, O.V.S., Holdsworth, M., Tincker, M.A.H., Brown, F.C., 1947. Studies in the physiology of the onion plant. III. Further experiments on the effects of storage temperature and other factors on onions grown from sets. Ann. Appl. Biol. 34, 473–502. Heath, O.V.S., Mathur, P.B., 1944. Studies in the physiology of the onion plant II. Inflorescence initiation and development, and other changes in the
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Rabinowitch, H.D., 1985. Onions and other edible Alliums. In: Halevy, A.H. (Ed.), CRC Handbook of Flowering, Vol. I. CRC Press, Boca Raton, Florida, pp. 398–405. Rabinowitch, H.D., 1990. Physiology of flowering. In: Rabinowitch, H.D., Brewster, J.L. (Eds.), Onions and Allied Crops. Vol. I. Botany, Physiology and Genetics. CRC Press, Boca Raton, Florida, pp. 133–134. S.A.S., 1985. Statistical Analysis System User’s Guide: Statistics. S.A.S. Inc., Cary, North Carolina. Shishido, Y., Saito, T., 1975. Studies on the flower bud formation in onion plants. I. Effects of temperature, photoperiod and light intensity on the low temperature induction of flower buds. J. Jpn. Soc. Horticult. Sci. 44, 122– 130 (Japanese, En. Sum). Shishido, Y., Saito, T., 1977. Studies on the flower bud formation in onion plants. III. Effects of physiological conditions on the low temperature induction of flower buds in bulbs. J. Jpn. Soc. Horticult. Sci. 46, 310– 316 (Japanese, En. Sum). Thompson, H.C., Smith, O., 1938. Seedstalk and bulb development in the onion (Allium cepa L.). Cornell University Agricultural Experiment Station Bulletin, No. 708. Woodbury, G.W., 1950. A study of factors influencing floral initiation and seedstalk development in the onion, Allium cepa Linn. Idaho Agricultural Experiment Station Research Bulletin No. 18.