Effect of temperature and photoperiod on the incidence of bulbing and ...

Journal of Horticultural Science & Biotechnology (2008) 83 (4) 488–496

Effect of temperature and photoperiod on the incidence of bulbing and bolting in seedlings of onion cultivars of diverse origin

By KHALID MAHMUD KHOKHAR Vegetable Crops Research Programme, Horticulture Research Institute, National Agricultural Research Centre, Park Road, Islamabad 44000, Pakistan (e-mail: [email protected]) (Accepted 10 January 2008) SUMMARY The effects of temperature and photoperiod on bulbing and bolting in onion (Allium cepa L.) cultivars: ‘Senshyu Yellow’, ‘Jaune Espagnol’, ‘Hygro’, ‘Sito’, ‘Delta’, ‘Australian Brown’, ‘Rijnsburger Balstora’, and ‘Phulkara’ from Japan, Turkey, The Netherlands, The United Kingdom, Australia, and Pakistan were examined under controlled environments in glasshouse conditions in the UK. Plants of each cultivar, at the three-leaf stage, were transferred to a wide range of photo-thermal regimes consisting of six-set point temperatures (6°, 10°, 14°, 18°, 22° or 26°C) and four photoperiods (8, 11, 14 or 17 h d–1). The bulbing ratio in all cultivars increased curvi-linearly with increasing temperature and lengthening photoperiod. Increasing photoperiod promoted bulbing, while under very short photoperiods (8 h d–1) no cultivars bulbed, even after 60 d of growth. The time to bulb maturity decreased linearly with increasing temperature and lengthening photoperiod. Under low temperatures, the time to floral initiation shortened with lengthening photoperiods (8 – 14 h d–1). Under photo-thermal regimes favouring floral initiation, leaf number increased with both increasing temperature and lengthening photoperiod.

I

t is well-established that long-days (LD) and high temperatures promote onion bulbing (Heath, 1945; Kato, 1964). If day-length is sufficiently short, no bulbing occurs, even at high temperatures (e.g., 34°C; Heath, 1943, 1945; Kato, 1964; Butt, 1968; Steer, 1980; Terabun, 1980), while bulbing is delayed or prevented by low temperatures (Heath, 1943; 1945). In contrast, a period of low temperatures, between 7° – 13°C for 20 – 90 d, leads to flower induction in most onion cultivars (Shishido and Saito, 1975; 1977; Brewster, 1983; Bertaud, 1988; De Bon and Rhino, 1988; Peters, 1990). Extended photoperiods under low temperatures have been shown to promote the initiation of inflorescences (Woodbury, 1950; Shishido and Saito, 1975; Brewster, 1983). However, Holdsworth and Heath (1950) and van Kampen (1970) reported that photoperiod had no effect on the initiation of inflorescences. Brewster (1987b) showed that the rate of progress to flower initiation was more-or-less a linear function of photoperiod between 8 – 20 h d–1 under inducing temperatures. Studies on both bulbs and seedlings have shown that flower initiation starts only after a juvenile phase, by which time the plant has a minimum number of four-to-14 leaves, including leaf initials (Heath and Mathur, 1944; Hartsema, 1947; Heath and Holdsworth, 1948; Ito, 1956; Shishido and Saito, 1976; Rabinowitch, 1985; 1990). Once an inflorescence has been initiated, its rate of development increases with increases in temperature (10° – 30°C) and lengthening of the photoperiod (14 – 16 h d–1; Heath and Mathur, 1944; Heath, 1945; Woodbury, 1950; Brewster, 1982a; 1983; 1987a; Bertaud, 1988).

However, high temperatures (e.g., 30°C for cv. ‘Rijnsburger’) during early development of the scape can result in flower abortion because of competition for assimilates from bulbing, which may be induced if the day-length is appropriate for the cultivar (van Kampen, 1970). When breeding onions, the interval between one generation and the next needs to be minimised. Normally, a 2 year seed-to-seed cycle occurs, with 1 year needed to produce the bulbs which then produce seed in the following year (van Kampen, 1970; Pike, 1986; Bertaud, 1988; Dowker, 1990). We need to understand how to achieve an early induction of flowering, for rapid seed production in breeding programmes, and how to prevent bolting in seed-sown, over-wintered crops. There have been limited studies on the effects of temperature and photoperiod on flowering of onions during their growing phase, using only a narrow range of environmental conditions (Shishido and Saito, 1975). However, some work has been reported on the effect of temperature on onion plants (Davis and Jones, 1944; Shishido and Saito, 1976; 1977). The present studies were carried out to examine the quantitative effects of a wide range of photo-thermal conditions on the initiation of inflorescences and bulb development in a diverse range of onion genotypes, with the aim of defining the optimum environment for rapid seed production and for bulbing. Information on an extended range of post-planting environments would also be useful for planning a seed production programme. A seed-to-seed production method would help to shorten the onion seed production cycle.

K. M. KHOKHAR MATERIALS AND METHODS Seeds of eight onion (Allium cepa L.) cultivars: ‘Senshyu Yellow’, ‘Jaune Espagnol’, ‘Hygro’, ‘Sito’, ‘Delta’, ‘Australian Brown’, ‘Rijnsburger Balstora’ and ‘Phulkara’, originating in Japan, Turkey, the Netherlands, the United Kingdom, Australia, and Pakistan (Table I) were sown on 15 April 1996 in P 80 plug trays (TEKU Plug Trays; BHGS Ltd., Evesham, UK) where each cell had a diameter of 3 cm, using a peat-based seed-sowing modular compost (SHL; William Sinclair Horticulture and Leisure Limited, Lincoln, UK), mixed thoroughly with 20% (v/v) perlite. After filling the trays, the compost was firmed lightly then levelled-off. Sangral 111 (William Sinclair Horticulture and Leisure Limited) was applied as a supplement, as liquid feed at each watering with an EC of 1.5 dS m–1 (13.0 mM N; 2.50 mM P; 3.80 mM K), and a pH of 5.8. The trays were immediately placed in a glasshouse where the temperature averaged 22.3°C. One month after sowing, when seedlings had reached the three-leaf stage, plants were re-potted into 7.5 cmdiameter pots using a potting medium consisting of 80% (v/v) Fisons Levington M2 peat-based compost, and 20% (v/v) medium-size Silvaperl perlite (William Sinclair Horticulture Limited). A factorial combination of eight cultivars, four photoperiods and six temperature treatments gave a total of 192 treatment combinations, in each of which were nine plants. To study the combined effects of temperature and photoperiod on the bulbing and flowering responses of eight onion cultivars, plants were placed on moveable trolleys and exposed (23 May 1996) to 24 different photo-thermal regimes created by a factorial combination of six temperatures and four photoperiods. Sixteen additional plants of each cultivar were also placed in each treatment combination for dissection, in order to observe inflorescence initials. Two plants of each cultivar were selected at random from the extra plants and dissected 12 weeks after shifting the plants to the photo-thermal regimes, and thereafter at 14 d intervals until the end of the experiment. Controlled glasshouse compartments (3.7 m 7 m) provided minimum temperatures of 6°, 10°, 14°, 18°, 22°, or 26°C. Each compartment was equipped with four controlled photoperiod chambers, sealed from exterior light sources. Day-lengths were extended inside each chamber by low irradiance lighting (PAR 11 µmol m–2 s–1) provided by one 40 W tungsten plus one 15 W compact fluorescent light bulb, to provide 8, 11, 14 or 17 h d–1. All trolleys were removed from the photoperiod chambers at 08.00 h so that plants were exposed to 8 h of natural daylight in the glasshouse before being returned to the photoperiod chambers at 16.00 h each day. In all treatments, the lamps were switched on automatically at 16.00 h to provide day-lengths of 8, 11, 14 or 17 h d–1.

489

Inside the chambers, plants were ventilated continuously at an average air speed of 0.2 m s–1 to maintain temperature. Actual mean diurnal temperatures within each compartment were calculated from the temperatures recorded on a data-logger (Datataker, DT 500; Data Electronics, Letchworth Garden City, UK), linked to aspirated PT 100 temperature sensors (15 s scans, logged each hour). The actual mean temperature for each respective developmental stage (i.e., bulbing, neckfall, floral initiation) for each photo-thermal combination was calculated. Plants were sprayed with 0.8 g l–1 BenlateTM (DuPont, Wilmington, DE, USA) 6 weeks after planting as a preventive measure against fungal diseases. This was repeated every 14 d until the end of the experiment. The plants were watered three-times a week, or more frequently when required. Observations on bulbing ratios, times to floral initiation, times to bulb maturation, and leaf numbers were recorded. Bulbing ratios were recorded at 30 d and at 60 d. The bulbing ratio was calculated by dividing the maximum plant leaf base diameter by the minimum neck diameter. Plant leaf base and neck diameters were measured with vernier callipers. A bulbing ratio equal to or greater than 2.0 was taken as indicative of definite bulbing (Terabun, 1965; Kedar et al., 1975; Wright and Sobeih, 1986; Brewster et al., 1987). Random samples of two plants from the extra plants in each treatment were dissected to observe inflorescence initials using a binocular microscope. The samples were fixed in 70% (v/v) methanol and kept in a refrigerator prior to processing for scanning electronic microscopy (SEM). The time to bulb maturation was recorded when neckfall started. The number of leaves was recorded at the time of inflorescence initiation. Analysis of variance was performed using SAS software (SAS, 1985). Regression analysis was performed with Microsoft EXCEL following the procedure of Gomez and Gomez (1984). Fitted planes from multiple regression analyses were plotted by 3-D graphs using ‘SlideWrite’ Version 2.0 (1989). Inflorescence initiation in ‘Phulkara’ took place only at a photoperiod of 8 h d–1. Therefore, multiple regression analysis for the time taken to inflorescence initiation was not possible, and 3-D graphs could not be plotted for this cultivar. However, the results are described and discussed.

RESULTS Bulbing ratio Bulbing ratios increased curvi-linearly with increasing temperature and lengthening photoperiod for all cultivars examined (Figure 1A–P). For example, after 60 d, the bulbing ratio under a 14 h d–1 photoperiod

TABLE I Details of onion cultivars used in this study Name of cultivar

Source

Country of origin

Day-length status

Season

‘Hygro’ ‘Delta’ ‘Senshyu Yellow’ ‘Jaune Espagnol’ ‘Sito’ ‘Australian Brown’ ‘Rijnsburger Balstora’ ‘Phulkara’

Moles Seeds, UK Moles Seeds, UK A.L. Tozer Ltd, UK Genetic Resources Unit, HRI, Wellsbourne, UK Nickerson Seeds Ltd, UK Genetic Resources Unit, HRI, Wellsbourne, UK A.L. Tozer Ltd, UK NARC, Pakistan

Netherlands Netherlands Japan Turkey UK Australia Netherlands Pakistan

Long Intermediate Long Intermediate Long Intermediate Long Short

Spring Spring Autumn Spring Spring Spring Spring Autumn

490

Bulbing and bolting in onion cultivars

FIG. 1 Effect of temperature, photoperiod and time on the bulbing ratio in eight onion cultivars of diverse origin at 30 d and 60 d (Panels A – P). 2 (r = 0.72 – 0.91).

K. M. KHOKHAR

491

492


increased from 2.07 and 1.50 at 13.7°C to 2.95 and 2.51 at 29.2°C for ‘Senshyu Yellow’ and ‘Jaune Espagnol’, respectively. Similarly, after 60 d, the bulbing ratio at 29.2°C increased from 1.42 and 1.33 with an 8 h d–1 photoperiod, to 4.20 and 3.42 with a 17 h d–1 photoperiod for ‘Senshyu Yellow’ and ‘Jaune Espagnol’, respectively. For ‘Senshyu Yellow’, ‘Jaune Espagnol’, ‘Australian Brown’ and ‘Phulkara’ there was a significant interaction (P < 0.001) between temperature and photoperiod, such that the bulbing ratio was maximised under a photoperiod of 17 h d–1 at a mean temperature of 25°C. In ‘Hygro’ and ‘Delta’, there was also a significant interaction (P < 0.001) between photoperiod and time, such that the effect of photoperiod was more pronounced after 60 d than after 30 d. In ‘Sito’, there was a significant interaction (P < 0.001) between temperature, photoperiod and time, such that the effect of temperature and photoperiod on the bulbing ratio was more pronounced after 60 d than after 30 d. There was an interaction between temperature and time in ‘Rijnsburger Balstora’, such that the effect of temperature on the bulbing ratio was more pronounced after 60 d than after 30 d. ‘Senshyu Yellow’, ‘Jaune Espagnol’, ‘Hygro’, ‘Sito’, ‘Delta’, ‘Australian Brown’ and ‘Rijnsburger Balstora’ (intermediate to LD types), bulbed under 14 h d–1 and 17 h d–1 photoperiods. The short-day (SD) cultivar, ‘Phulkara’ (Pakistan), bulbed under photoperiods of 11 h d–1, 14 h d–1 and 17 h d–1. However, no bulbing was observed under an 8 h d–1 photoperiod in any cultivar, at any mean temperature. In all cultivars except ‘Sito’, bulbing occurred earlier (i.e., 30 d) after exposure to the different inductive photo-thermal regimes. For example in ‘Senshyu Yellow’ and ‘Phulkara’, bulbing occurred after 30 d at 11°C under a 17 h d–1 photoperiod, and from 14° – 29°C under 14 h d–1 and 17 h d–1 photoperiods, while ‘Phulkara’ also bulbed under an 11 h d–1 photoperiod from 25° – 29°C. Bulbing in ‘Jaune Espagnol’, ‘Hygro’ and ‘Rijnsburger Balstora’ occurred after 30 d at temperatures from 25° – 29°C under a 17 h d–1 photoperiod, but ‘Hygro’ also bulbed from 19° – 23°C at a 17 h d–1 photoperiod. ‘Delta’ bulbed from 23° – 29°C under 14 h d–1 and 17 h d–1 photoperiods, and also at 19°C under a 17 h d–1 photoperiod. In ‘Australian Brown’, bulbing also occurred after 30 d from 19° – 29°C under 14 h d–1 and 17 h d–1 photoperiods, and from 11° – 14°C under a 17 h d–1 photoperiod. However ‘Sito’ was late, and bulbing was observed only after 60 d at 25° – 29°C under 14 h d–1 and 17 h d–1, and at 20° – 23°C under 17 h d–1 photoperiods. Bulbing occurred faster under a longer photoperiod (17 h d–1) and at higher temperatures in all cultivars. In ‘Jaune Espagnol’, bulbing occurred after 30 d at 17 h d–1 at higher temperatures from 25º – 29ºC, but after 60 d under 14 h d–1, and 17 h d–1 from 13º – 29ºC. Time to floral initiation Under low temperatures, all genotypes tended to initiate flowers rather than to bulb. Time to floral initiation in ‘Senshyu Yellow’, ‘Hygro’ and ‘Delta’ decreased linearly with decreasing temperature and increasing photoperiod (Figure 2A, C, E). In ‘Jaune Espagnol’, ‘Sito’, ‘Australian Brown’ and ‘Rijnsburger Balstora’, time to floral initiation also decreased linearly with decreasing temperature, but decreased curvi-

linearly with increasing photoperiod (Figure 2B, D, F, G). Thus, time to floral initiation under an 8 h d–1 photoperiod decreased from 185 d and 175 d at 13.1°C, to 183 d and 170 d at 10.9°C for ‘Senshyu Yellow’ and ‘Jaune Espagnol’, respectively. Time to floral initiation at 13.1°C decreased from 185 d and 175 d with an 8 h d–1 photoperiod, to 181 d and 168 d with an 11 h d–1 photoperiod for ‘Senshyu Yellow’ and ‘Jaune Espagnol’, respectively. Adding an interaction term did not significantly improve these relationships. ‘Senshyu Yellow’, ‘Jaune Espagnol’, ‘Hygro’, ‘Sito’, ‘Delta’, ‘Australian Brown’ and ‘Rijnsburger Balstora’ (intermediate to LD types), initiated flowers under 8 h d–1 and 11 h d–1 at temperatures from 11° – 13°C. In ‘Jaune Espagnol’, ‘Hygro’, ‘Sito’, ‘Australian Brown’ and ‘Rijnsburger Balstora’, inflorescences were also initiated under 14 h d–1 at mean temperatures from 11° – 13°C. ‘Rijnsburger Balstora’ initiated inflorescences even at relatively warm temperatures (19°C) under photoperiods of 11 h d–1 and 14 h d–1. However, in the short-day ‘Phulkara’, inflorescence initiation occurred only at a photoperiod of 8 h d–1 at temperatures from 11° – 13°C. Time to bulb maturity The time to bulb maturity decreased linearly with increasing temperature and also with lengthening photoperiod in all cultivars (Figure 3A–H). For example, for ‘Senshyu Yellow’ and ‘Australian Brown’, time to bulb maturity at a 14 h d–1 photoperiod decreased from 98 d and 90 d at 22.7°C, and to 67 d and 61 d at 29.1°C, respectively. Similarly, time to bulb maturity at 29.1°C decreased from 67 d and 60 d at a 14 h d–1 photoperiod, and to 51 d and 55 d at a 17 h d–1 photoperiod for ‘Senshyu Yellow’ and ‘Rijnsburger Balstora’, respectively. However, there was an interaction between photoperiod and temperature such that increasing photoperiods decreased the time to bulb maturity under all temperatures, but the effect of photoperiod was more pronounced at lower temperatures. The short-day ‘Phulkara’ was earliest to mature, taking 49 d to mature under a 17 h d–1 photoperiod at a temperature of 29°C, whereas other cultivars (intermediate to LD type) matured in 51 – 55 d under the same photo-thermal regime.

DISCUSSION The results of these experiments show that the dynamics of bulbing and bolting in onion plants are influenced significantly by photoperiod and temperature. Increasing photoperiods promoted bulbing, but, under short photoperiods (8 h d–1), plants did not bulb even after a long period of growth. This confirms previous findings (Heath, 1943; Heath and Mathur, 1944; Austin, 1972; Kedar et al., 1975; Steer, 1980) that increasing photoperiod promotes bulbing, and short photoperiods delay or, if sufficiently short, completely inhibit bulbing. Increasing temperatures promoted bulbing and hastened maturity. This strengthens previous findings (Heath, 1945; Kato, 1964) that LD and higher temperatures cause earlier bulbing and maturity. It was shown earlier that bulbing did not occur at lower temperatures of 10° – 15.5°C (Thompson and Smith,

K. M. KHOKHAR

493

FIG. 2 Effect of temperature and photoperiod on the time to floral initiation in seven onion cultivars of diverse origin (Panels A – G). r2 = 0.96 – 0.99. = no response, = response.

494


FIG. 3 Effect of temperature and photoperiod on the time to neckfall in eight onion cultivars of diverse origin (Panel A – H). r2 = 0.90 – 0.98, = no response, = response.

K. M. KHOKHAR 1938); however, the results of our experiments show that, in bolting-resistant cultivars, bulbing can take place at lower temperatures (11° – 13.8°C) under a longer photoperiod (17 h d–1). However, the present studies support the findings of Heath (1943; 1945), who reported that lower temperatures could greatly delay or prevent bulbing. With an increase in temperature, bulb development under long photoperiods was accelerated; while, under short photoperiods (8 – 11 h d–1), no bulbing occurred even at warm temperatures. This is in agreement with previous reports of other workers (Heath, 1943; 1945; Kato, 1964; Butt, 1968; Steer, 1980; Terabun, 1980) who showed that, with an increase in temperature, the rate of bulb development increases; but, if day-lengths are sufficiently short, no bulbing will occur, even at warm temperatures. Under low temperatures, the time to floral initiation was hastened with decreasing temperatures and lengthening photoperiods (8 – 14 h d–1). This confirms previous findings (Shishido and Saito, 1975; Brewster, 1983) that extended photoperiods under low inducing temperatures promoted inflorescence initiation. The present studies confirm the findings of Holdsworth and Heath (1950), van Kampen (1970), Brewster (1982b) and Bertaud (1988) that inflorescence development after initiation is positively correlated with photoperiod, but are in conflict with some reports (Holdsworth and Heath, 1950; van Kampen, 1970) indicating no effect of photoperiod on inflorescence initiation. Our studies have shown that inflorescences in most onion cultivars (‘Senshyu Yellow’ ‘Delta’ ‘Jaune Espagnol’, ‘Hygro’, ‘Sito’, ‘Australian Brown’) can initiate under short-to-intermediate day-lengths (8 h d–1, 11 h d–1 or 14 h d–1), at temperatures from 11° – 13°C. Initiation of inflorescences in ‘Rijnsburger Balstora’ can take place under a short photoperiod (8 h d–1) at temperatures from 11° – 13°C, and under short-tointermediate day-lengths (11 h d–1 and 14 h d–1) from 11° – 19°C. ‘Phulkara’ can initiate inflorescences only under short-days (8 h d–1) at temperatures from 11° – 13°C. Inflorescences in most cultivars initiated at 11° – 13°C after 154 – 184 d, except in ‘Rijnsburger Balstora’, where initiation took place at 11° – 18.9°C after 162 – 185 d. This confirms the findings of Shishido and Saito (1975, 1977), Brewster (1983), Bertaud (1988), De Bon and Rhino (1988) and Peters (1990) that low temperatures between 7° – 13°C for 20 d – 90 d were optimum for flower induction in most onion cultivars and that bolting-resistant varieties required a longer cold stimulus compared to normal Spring-sown cultivars. Most cultivars in the present study were bolting-resistant and required a longer cold stimulus (154 – 185 d) for flower induction. ‘Rijnsburger Balstora’ also initiated

495

inflorescences at relatively higher temperatures (18.9°C) than the other cultivars studied here. Cultivars adapted to warmer conditions have a lesser requirement for cold, and possibly a higher optimum vernalisation temperature. This could be attributed to genotype, and confirms reports (Sinnadurai, 1970) that certain cultivars (for example ‘Bawku’) flowered at ambient night temperature of 15° – 21°C. These studies have clearly shown that photoperiod and temperature are extremely important environmental variables for flowering and bulbing in onion, and that different cultivars show variations in their responses. Considerable variability exists in the global environment. It is therefore possible to identify suitable photo-thermal regimes for flowering or bulbing for a particular cultivar in a particular region. It is also possible to combine the fitted relationships for bulbing and flowering for each cultivar to have the full photothermal response for flowering and bulbing. Flowering in ‘Senshyu Yellow’ and ‘Delta’ will occur under shorter photoperiods (8 h d–1 and 11 h d–1) at temperatures from 11° – 13°C, while bulbing will occur under intermediate-to-LD (14 h d–1 and 17 h d–1) at temperatures from 11° – 29°C. ‘Jaune Espagnol’, ‘Hygro’ and ‘Sito’ will initiate flowers under short-tointermediate day-lengths (8 h d–1, 11 h d–1 and 14 h d–1) at mean temperatures from 11° – 13°C. However, these will bulb under intermediate day-length (14 h d–1) at temperatures from 20° – 29°C. Under a long photoperiod (17 h d–1), bulbing will occur at temperatures from 11° – 29°C. ‘Australian Brown’ will flower under short photoperiods (8 h d–1 and 11 h d–1) at temperatures from 11° – 13°C, and under intermediate photoperiods (14 h d–1) at 11°C. However, the latter will bulb under intermediate day-lengths (14 h d–1) at temperatures from 14° – 29°C. Under a long photoperiod (17 h d–1), bulbing will occur from 11° – 29°C. Flowering in ‘Rijnsburger Balstora’ will occur under short photoperiod (8 h d–1) at temperatures from 11° – 13 °C and under short-to-intermediate daylengths (11 h d–1 and 14 h d–1) from 11° – 19°C. However, it will bulb under intermediate day-lengths (14 h d–1) at temperatures from 23° – 29°C, and under a long photoperiod (17 h d–1) from 11° – 29°C. ‘Phulkara’ will flower under short-day (8 h d–1) at temperatures from 11° – 13°C. However, it will bulb under a wide range of photoperiods (11 h d–1, 14 h d–1 and 17 h d–1) at temperatures from 11° – 29°C. The bulbing ratio, bulb maturity, and floral initiation of onion plants are largely dependent on temperature and photoperiod. Thus, plant development can be modified towards bulbing, or towards flowering under suitable photo-thermal regimes.

496

Bulbing and bolting in onion cultivars REFERENCES

AUSTIN, R. B. (1972). Bulb formation in onions as affected by photoperiod and spectral quality of light. Journal of Horticultural Science, 47, 493–504. 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 4th EUCARPIA Allium Symposium, Institute of Horticultural Research, Wellesbourne, UK. 245–261. BREWSTER, J. L. (1982a). Flowering and seed production in overwintered cultivars of bulb onions. I. Effects of different raising environments, temperatures and daylengths. Journal of Horticultural Science, 57, 93–101. BREWSTER, J. L. (1982b). Flowering and seed production in overwintered cultivars of bulb onions. II. Quantitative relationships between mean temperatures and daylengths and the rate of inflorescence development. Journal of Horticultural Science, 57, 103–108. BREWSTER, J. L. (1983). Effects of photoperiod, nitrogen nutrition and temperature on inflorescence initiation and development in onion (Allium cepa L.). Annals of Botany, 51, 429–440. BREWSTER, J. L. (1987a). The effect of temperature on the rate of sprout growth and development within stored onion bulbs. Annals of Applied Biology, 111, 463–467. BREWSTER, J. L. (1987b). Vernalization in the onion - a quantitative approach. In: Manipulation of Flowering. (Atherton, J.G., Ed.). Butterworths, London. UK. 171–183. BREWSTER, J. L., LAWES, W. and WHITLOCK, A. J. (1987). The phenology of onion bulb development at different sites and its relevance to incomplete bulbing (‘thick-necking’). Journal of Horticultural Science, 62, 371–378. 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. DAVIS, G. N. and JONES, H. A. (1944). Experiments with the transplant onion crop in California. California Agricultural Experiment Station Bulletin No. 682, Davis, CA, USA. 20 pp. DE BON, H. and RHINO, B. (1988). Flowering in the various cultivars of onions (Allium cepa L.) brought to Martinique (French West Indies). In: Proceedings of the 4th EUCARPIA Allium Symposium, Institute of Horticultural Research, Wellesbourne, UK. 262–266. DOWKER, B. D. (1990). Onion breeding. In: Onions and Allied Crops. Volume I. (Rabinowitch, H.D. and Brewster, J.L., Eds.). CRC Press Inc., Boca Raton, FL, USA. 215–232. GOMEZ, K. A. and GOMEZ, A. A. (1984). Statistical Procedures for Agricultural Research. 2nd Edition. John Wiley and Sons, Ltd., New York, USA. 680 pp. HARTSEMA, A. M. (1947). The periodical development of the onion (Allium cepa L.), var. Giant Zittau. Mededelingen Landbouwhogeschool Wageningen, 48, 265–300. (In Dutch with English summary). HEATH, O. V. S. (1943). Studies on 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 2. Effects of day length and temperature on onions grown from sets and general discussion. Annals of Applied Biology, 30, 308–322. 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. and MATHUR, P. B. (1944). Studies on the physiology of the onion plant. II. Inflorescence initiation and development, and other changes in the internal morphology of onion sets, as influenced by temperature and day length. Annals of Applied Biology, 31, 173–187. HEATH, O. V. S. and HOLDSWORTH, M. (1948). Morphogenic factors as exemplified by the onion plant. In: Growth. (Daneellie, J.F. and Brown, R., Eds.). Symposia of the Society for Experimental Biology, 2, 326–350. HOLDSWORTH, M. and HEATH, O. V. S. (1950). Studies on the physiology of the onion plant. Journal of Experimental Botany, 1, 353–375.

ITO, K. (1956). Studies on the bolting of onion. I. Relation between the flower bud formation and the bulb division. Journal of the Japanese Society for Horticultural Science, 25, 187–193. (In Japanese with English summary). KATO, T. (1964). Physiological studies on bulb formation and dormancy in onion plant. III. Effects of external factors on the bulb formation and development. Journal of the Japanese Society for Horticultural Science, 33, 53–61. (In Japanese with English summary). KEDAR, N., LEVY, D. and GOLDSCHMIDT, E. E. (1975). Photoperiodic regulation of bulbing and maturation of ‘Bet Alpha’ onions (Allium cepa L.) under decreasing daylength conditions. Journal of Horticultural Science, 50, 373–380. PETERS, R. (1990). Seed production in onions and some other Allium species. In: Onions and Allied Crops. Volume I. (Rabinowitch, H. D. and Brewster, J. L., Eds.). CRC Press Inc., Boca Raton, FL, USA. 161–176. PIKE, L. M. (1986). Onion breeding. In: Breeding Vegetable Crops. (Bassett, M.J., Ed.). AVI Publishing Co., Westport, CT, USA. 357–394. RABINOWITCH, H. D. (1985). Onions and other edible Alliums. In: CRC Handbook of Flowering. Volume I. (Halevy, A.H., Ed.). CRC Press Inc., Boca Raton, FL, USA. 398–405. RABINOWITCH, H. D. (1990). Physiology of flowering. In: Onions and Allied Crops. Volume I. (Rabinowitch, H.D. and Brewster, J.L., Eds.). CRC Press Inc., Boca Raton, FL, USA. 133–134. S.A.S. (1985). Statistical Analysis System User’s Guide: Statistics. SAS. Inc., Cary, NC, USA. (http://www.asu.edu/sas/sasdoc/sashtml/). SHISHIDO, Y. and 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. Journal of the Japanese Society for Horticultural Science, 44, 122–130. (In Japanese with English summary). SHISHIDO, Y. and SAITO, T. (1976). Studies on the flower bud formation in onion plants. II. Effects of physiological conditions on the low temperature induction of flower buds on green plants. Journal of the Japanese Society for Horticultural Science, 45, 160–167. (In Japanese with English summary). SHISHIDO, Y. and 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. Journal of the Japanese Society for Horticultural Science, 46, 310–316. (In Japanese with English summary). SINNADURAI, S. (1970). A note on the bulbing and flowering habit of the Bawku onion. Tropical Agriculture (Trinidad), 47, 77–79. SLIDE WRITE PLUS (1989). Advanced Graphics Software Inc., Sunnyvale, CA, USA. (http://www.slidewrite.com/TrialGuide.asp). STEER, B. T. (1980). The bulbing response to daylength and temperature of some Australian cultivars of onion (Allium cepa L.). Australian Journal of Agricultural Research, 31, 511–518. TERABUN, M. (1965). Studies on the bulb formation in onion plants. I. Effects of light quality on the bulb formation and the growth. Journal of the Japanese Society for Horticultural Science, 34, 196–204. (In Japanese with English summary). TERABUN, M. (1980). Effect of cyclic lighting on photoperiodic responses in onion plant. Journal of the Horticultural Association of Japan, 49, 375–379. THOMPSON, H. C. and SMITH, O. (1938). Seedstalk and bulb development in the onion (Allium cepa L.). Cornell University Agricultural Experiment Station Bulletin, No. 708. 21 pp. VAN KAMPEN, J. (1970). Shortening the breeding cycle in onions. Mededelingen Proefstation Voor de Groenteteelt in de Vollegrond in Nederland. 51, 72. (In Dutch with English summary). WRIGHT, C. J. and SOBEIH, W. Y. (1986). The photoperiodic regulation of bulbing in onions (Allium cepa L.). I. Effects of irradiance. Journal of Horticultural Science, 61, 331–335. 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. 27 pp.