Effect of set-size and planting time on the incidence of bolting, bulbing ...

Journal of Horticultural Science & Biotechnology (2008) 83 (4) 481–487

Effect of set-size and planting time on the incidence of bolting, bulbing, and seed yield in two onion cultivars

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 Sets of two onion (Allium cepa L.) cultivars (‘Hygro’ and ‘Delta’), graded into three size classes (12.5, 17.5 and 22.5 mm in diameter) were planted at seven 15-d intervals from 5 December 1995 to 5th March 1996. Flowering and seed production were influenced by set-size. Small sets (12.5 mm in diameter) showed little or no bolting, while the bolting percentage increased with increasing set-size, following a linear relationship. The seed yield per umbel increased with increasing set-size from 12.5 mm to 22.5 mm diameter. Following initiation, inflorescence development (i.e., emergence and floret opening) occurred earlier in plants grown from larger onion sets than from smaller sets. Bolting percentage tended to decrease with late planting of sets (5 March). Maximum seed yield per umbel was observed from the earliest planting (5 December). Delaying planting until 5 March had an enhanced effect on bulb yield. Large sets, being prone to bolting, produced smaller bulb yields than medium or small onion sets. In the earliest planting (5 December), bulbs matured earlier, while the time to bulb maturity was delayed in later plantings.

D

uring previous studies on bolting in onion (Allium cepa L.) sets, little attention was paid to studying the possibility of obtaining a seed crop from sets. During the initiation and development of inflorescences in onion, exposure to low temperatures is essential for floral initiation. Time of flowering may be influenced by the temperature used for cold storage, and by the date of planting (Atkin and Davis, 1954; Aoba, 1960; Jones and Mann, 1963; DeMille and Vest, 1976; Hesse et al., 1979; Currah, 1981; Brewster, 1982a, b). Storage temperatures between 0° – 12°C, and early planting, led to earlier flowering (Jones and Mann, 1963). The time required to initiate inflorescences under low temperatures has been investigated by several workers (Shishido and Saito, 1975; Bertaud, 1988; De Bon and Rhino, 1988) and it has been shown that 20 – 90 d at 7° – 11°C are required to induce flowering across a range of onion cultivars. The effect of bulb size on the frequency of bolting has also been investigated. It has been found that the initiation of inflorescences is favoured by a large set or bulb size (Boswell, 1923; Thompson and Smith, 1938; Heath, 1943; Holdsworth, 1945; Rabinowitch, 1979; Mobli, 1992). It is well-established that onion bulbs must be of a minimum size before they can initiate inflorescences (Heath, 1943; Aura, 1963). Heath et al. (1947) observed that sets of the Spring-planted cultivar ‘Ailsa Craig’, of 4 – 7 g fresh weight (FW), were able to initiate inflorescences, but those with a lower FW could not. It is generally accepted that large bulbs produce more flower stalks and give a higher seed yield per plant than small bulbs (Jones and Emsweller, 1939; Orlowski, 1974; Chiru and Banita, 1980; Levy et al., 1981; Nourai, 1981). Similarly, large sets give higher bulb yields after storage at temperatures which suppress bolting. Cultivars vary considerably in their susceptibility to bolting. Davis and Jones (1944) reported that seedlings of the cultivar ‘White Sweet Spanish’, when transplanted

on 6 September in California, produced 71% bolters, while seedlings of the cultivar ‘Italian Red’ produced no bolters. They also reported that the time of transplanting affected bolting. When transplanted 2 months later, seedlings of the cultivar ‘White Sweet Spanish’ produced only 2.6% bolters. Vasic (1975) reported that Autumn planting of onion sets in Yugoslavia gave lower yields and poorer quality bulbs, and recommended sets 12.5 mm in diameter for Autumn planting. Large-sized sets have been reported to be suitable only for late planting in Spring (Lazic, 1975). In the past, while studies have been made on bolting in onion sets, little attention has been paid to the possibility of obtaining a seed crop from sets. This is desirable because using onion sets rather than bulbs could be economical due to the reduced space requirement for storage compared to bulbs. However, seed should not be obtained from early bolters because this could lead to greater bolting of the cultivar. Factors which influence flowering are important both to seed growers, because the success of a seed crop depends on flowering, and to bulb growers, because yield and quality are reduced by bolting. Manipulation of the planting time for sets or bulbs is important to obtain a good seed crop in onion. The aim of this study was to explore the possibility of producing a seed crop, economically, without cold storage of onion sets, which is expensive. Therefore, a study was undertaken to determine the optimum planting time and most suitable set-size for flowering, bulbing, and seed yield in two onion cultivars (‘Hygro’ and ‘Delta’) in the UK.

MATERIALS AND METHODS This study was conducted at the School of Plant Sciences, University of Reading, UK during 1995-1996. The planting material consisted of sets of two onion

482

Onion seed and bulb production through sets

cultivars (‘Hygro’ and ‘Delta’) graded into three sizes [12.5 mm (small), 17.5 mm (medium), and 22.5 mm (large) diameter]. ‘Hygro’ and ‘Delta’ are Spring-sown long-day (LD) cultivars. ‘Delta’ is an open-pollinated cultivar and ‘Hygro’ is an F1 hybrid. Sets of these cultivars were obtained from MAAS Bv. Kruiningen, The Netherlands, by David O’Connor and Associates, Kirton, Boston, UK. Extensive work has been done using bulbs rather than sets to produce onion seed. Therefore, sets of these onion cultivars were used to determine the optimum planting time and suitable set-size for flowering and seed production in the UK. The sets were packed in net bags and stored at 20°C prior to planting. The soil was prepared by spreading a 2 cm-thick layer of spent mushroom compost, which was incorporated to a depth of 50 cm, followed by cultivation with a rotavator. No additional fertiliser was incorporated. Sets were planted in well-prepared beds at seven 15-d intervals from 5 December 1995 to 5 March 1996 using row-to-row and plant-to-plant spacings of 30 cm and 7 cm, respectively. The experiment was designed as a randomised block (3 2 7 factors, which were set-size, cultivar and planting time, respectively) with three replications for each treatment. Each treatment consisted of three 2 m-long rows. The crop was irrigated 7 d after planting. Water was applied as required, usually at 10-d intervals. However, this interval increased or decreased depending on the weather. Weeding was done periodically using a hand hoe. Observations were recorded on bolting percentage (recorded from plants with emerged inflorescences), time to inflorescence emergence, time to floret opening, seed yield per umbel, bulb yield m–2 of bed, and time to bulb maturity. Bolted plants were assessed for recording data on flowering aspects only, not for bulb weight and maturity. Bulb yield was recorded from those plants which did not bolt. Time to inflorescence emergence was based on mean data of bolted plants for each treatment. Observations on time to floret opening were based on random samples of 20 plants for each replication of each treatment. Time to bulb maturity was recorded when the tops of the plants fell over. Bulb weight from bolted bulbs was not recorded and these were left in the field to record data on flowering aspects only. Agrometeorological data (daily minimum and maximum air temperatures) for the post-planting period were obtained from the meteorological office, Reading University, Whiteknights, UK. Plants were sprayed with a mixture of 5 ml l–1 Malathion plus 0.8 g l–1 Benlate on 24 May, 20 June, and 28 July 1996 as a preventive measure against pests and diseases. 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 analysis were plotted by 3-D graphs using ‘Slide Write’ Version 2.0 (1989).

RESULTS Bolting The earliest planting date (5 December) gave the highest level of bolting and lowest bulb yield; whereas all

plantings thereafter showed progressively reduced bolting and increased bulb yield. The bolting percentage in both cultivars (‘Hygro’ and ‘Delta’) increased linearly with increasing set-size and with earlier planting dates (Figure 1A, B), and increased steadily as larger sets were used. In large sets (22.5 mm) of both cultivars, the earliest planting (5 December) gave the highest bolting percentages (100% and 65% for ‘Hygro’ and ‘Delta’, respectively) compared with later plantings (5 March) which produced 60% and 27% bolters for ‘Hygro’ and ‘Delta’, respectively. Small sets (12.5 mm) showed little bolting from earlier plantings, while no induction of flowering was observed from the later plantings. For instance, in small sets (12.5 mm) of both cultivars, the earliest planting (5 December) gave 21% and 15% bolting for ‘Hygro’ and ‘Delta’, respectively, compared with later plantings (19 January) which produced 6% bolters in ‘Hygro’, while no induction of flowering was observed in ‘Delta’ from the 19 January planting and all plantings thereafter. However, in ‘Hygro’, small sets

FIG. 1 Effect of set-size and planting time on percentage of bolting in two cultivars of onion. The plane was fitted by multiple regression analysis. Panel A, [PB = –69.43 (± 6.19) + 7.68 (± 0.34) S –0.50 (± 0.05) T, r2 = 0.97, 15 d.f.]. Panel B, [PB = –49.09 (± 5.65) + 4.97 (±0.32) S –0.42 (± 0.05) T, r2 = 0.95, 12 d.f.]. Where PB, S and T represent percentage of bolting, set-size, and planting time, respectively.

K. M. KHOKHAR

FIG. 2 Effect of set-size and planting time on time to inflorescence emergence in two cultivars of onion. The plane was fitted by multiple regression analysis. Panel A, [TIE = 173.43 (± 0.74) –1.41 (± 0.04) S + 0.22 (± 0.005) T, r2 = 0.99, 15 d.f.]. Panel B, [TIE = 184.90 (± 0.61) –1.39 (± 0.034) S + 0.21 (± 0.005) T, r2 = 0.99, 12 d.f.]. Where TIE, S and T represent time to inflorescence emergence, set-size, and planting time, respectively

produced no bolters from relatively late plantings (i.e., 3 February) and all plantings thereafter. Time to inflorescence emergence Time to inflorescence emergence decreased linearly with increasing set-size and also with earlier planting date (Figure 2A, B). In large sets of ‘Hygro’ and ‘Delta’, the inflorescences appeared rapidly from the earliest planting (5 December 1995) taking 142 d and 154 d, respectively; compared with the last planting (5 March 1996) where inflorescences emerged after 161 d and 172 d. In small sets (12.5 mm) of ‘Hygro’ and ‘Delta’, the inflorescences emerged slowly. From 5 December, it took 155 d and 167 d, respectively, for the inflorescences to emerge. Time to floret opening Time to floret opening also decreased linearly with increasing set-size and with earlier planting date (Figure 3A, B). For instance, in large sets (22.5 mm) of ‘Hygro’ and ‘Delta’, the florets opened quickly in the

483

FIG. 3 Effect of set-size and planting time on time to floret opening in two cultivars of onion. The plane was fitted by multiple regression analysis. Panel A, [TFO = 211.41 (± 0.91) –1.40 (± 0.05) S + 0.22 (± 0.007) T, r2 = 0.99, 15 d.f.]. Panel B, [TFO = 220.82 (± 0.84) –1.19 (± 0.05) S + 0.2 (± 0.007) T, r2 = 0.99, 12 d.f.]. Where TFO, S and T represent time to floret opening, set-size, and planting time, respectively.

earliest planting (5 December 1995) taking 180 d and 193 d, respectively; while in the last planting (5 March 1996) the florets opened slowly, taking 200 d and 211 d. Floret opening from small sets of both ‘Hygro’ and ‘Delta’ was relatively slow, occurring after 193 d and 206 d, respectively, from the 5 December planting. Seed yield per umbel Seed yield per umbel increased linearly in both cultivars with increases in set-size and with earlier planting dates (Figure 4A, B). For example, large sets (22.5 mm) of ‘Hygro’ and ‘Delta’ produced nearly fourtimes higher seed yields per umbel from the earliest (5 December) planting (6.3 g and 4.5 g for ‘Hygro’ and ‘Delta’, respectively) compared with the last (5 March) planting (1.8 g and 1.2 g for ‘Hygro’ and ‘Delta’, respectively). Small sets (12.5 mm) produced lower seed yields per umbel than large sets; for example, in the 5 December planting, small sets gave 2.3-times lower

484


FIG. 4 Effect of set-size and planting time on seed yield per umbel in two cultivars of onion. The plane was fitted by multiple regression analysis. Panel A, [SYU = –0.81 (± 0.34) + 0.30 (± 0.02) S –0.04 (± 0.003) T, r2 = 0.97, 15 d.f.]. Panel B, [SYU = –1.71 (± 0.22) + 0.26 (± 0.01) S –0.04 (± 0.002) T, r2 = 0.98, 12 d.f.]. Where SYU, S and T represent seed yield per umbel, set-size, and planting time, respectively.

seed yield for ‘Hygro’ (2.7 g) and three-times lower seed-yield for ‘Delta’ (1.4 g) than those from large sets. Bulb yield In ‘Hygro’, bulb yield m–2 increased curvi-linearly with decreasing set-size, and linearly with delayed plantings (Figure 5A). For example, small sets (12.5 mm) of ‘Hygro’ from the 18 February planting produced 7.4 kg bulbs m–2 compared with those of medium and large sets which produced 7.0 kg and 4.7 kg bulbs m–2, respectively. However, bulb yield m–2 in ‘Hygro’ was maximised (9.8 kg m–2) in medium sets (17.5 mm) from the 5 December planting, which was approx. 26% higher than that of small sets (7.8 kg m–2) and 69% higher than that of large sets (5.8 kg m–2). The bulb yields produced by small and medium sets of ‘Hygro’, from the 5 March planting, were 95% and 308% higher, respectively, than from the 5 December planting. Large sets gave 100% bolting in the 5 December planting, but produced 5.8 kg bulbs m–2 from the 5 March planting.

FIG. 5 Effect of set-size and planting time on bulb yield in two cultivars of onion. The plane was fitted by multiple regression analysis. Panel A, [BY = 5.11(± 0.44) –0.01 (± 0.001) S2 + 0.061 (± 0.005) T, r2 = 0.92, 17 d.f.]. Panel B, [BY = –16.64 (± 2.95) + 2.38 (± 0.35) S –0.07+ (± 0.01) S2 + 0.06 (± 0.004) T, r2 = 0.94, 17 d.f.]. Where BY, S and T represent bulb yield, set-size, and planting time, respectively. ‘Hygro’ largest sets bolted 100% from 5 December planting.

In ‘Delta’, the number of bulbs m–2 also increased curvilinearly with decreasing set-size and linearly with delayed plantings (Figure 5B). However, for each planting date, medium sets (17.5 mm) produced the highest bulb yield, which was maximum from the 5 March planting (10.3 kg m–2). This was 52% higher than that of small sets (6.8 kg m–2) and 5% higher than that of large sets (9.8 kg m–2). From the 5 March planting, small, medium, and large sets of ‘Delta’ gave 74%, 115% and 180%, higher bulb yields (6.8 kg m–2, 10.3 kg m–2 and 9.8 kg m–2), respectively, compared with those of the 5 December planting (3.9 kg m–2, 4.8 kg m–2 and 3.5 kg m–2, respectively). Time to bulb maturity In both cultivars, the time to bulb maturation decreased linearly with decreasing set-size and with earlier planting date (Figure 6A, B). For instance, in small sets (12.5 mm) of ‘Hygro’ and ‘Delta’, bulbs matured in 209 d and 200 d, respectively, from the

K. M. KHOKHAR

485

who showed that inflorescence initiation began earlier in larger bulbs than in smaller ones. As large amounts of carbohydrates are needed for inflorescence bud formation (Shishido and Saito, 1976), the higher percentage of flowering in larger sets may be due to increased stored carbohydrate reserves in these sets. We found that, in large sets, inflorescences emerged earlier, taking 142 d and 154 d for ‘Hygro’ and ‘Delta’, respectively. In controlled temperature studies, depending on cultivar, the time to inflorescence initiation has been reported to be in the range of 20 – 90 d (Shishido and Saito, 1975; Bertaud, 1988; De Bon and Rhino, 1988). In the present studies, plants were not dissected to observe inflorescence initials, only time to inflorescence emergence was recorded. It appears logical that inflorescences in field plants might have been initiated earlier, and emerged only when favourable conditions prevailed (e.g., high temperature and long days). It was also observed that large sets (22.5 mm) matured later than either medium or small sets.

earliest planting (5 December); while from the last planting (5 March), bulbs matured later, taking 222 d and 212 d. Large sets of ‘Hygro’ and ‘Delta’ were slower to mature, taking 229 d and 222 d, respectively, from the 5 December planting.

DISCUSSION Set-size Flowering and seed production in onion was influenced by set-size. Small sets (12.5 mm in diameter) showed little or no bolting, and the percentage of bolting increased linearly with increasing set-size. The number of seedbearing florets and the seed yield per umbel also increased with increasing set-size (from 12.5 mm to 22.5 mm). Inflorescence development (i.e., emergence and floret opening) following initiation, occurred earlier from plants grown from large sets than in those from smaller sets. This agrees with the findings of Thompson and Smith (1938), Aura (1963), Shishido and Saito (1977), and Mobli (1992)

32

Maximum and minimum air temperatures (oC)

FIG. 6 Effect of set-size and planting time on time to bulb maturity in two cultivars of onion. The plane was fitted by multiple regression analysis. Panel A, [TBM = 200.64 (± 0.71) + 0.65 (± 0.04) S + 0.14 (± 0.005) T, r2 = 0.98, 17 d.f.]. Panel B, [TBM = 186.89 (± 1.13) + 1.0 (± 0.06) S + 0.13 (± 0.008) T, r2 = 0.97, 18 d.f.]. Where TBM, S and T represent time to bulb maturity, set-size, and planting times, respectively. Largest sets of ‘Hygro’ bolted 100% from the 5 December planting.

Planting times The bolting percentage tended to decrease with later planting of sets (5 March). This concurs with a report by Lawande and Kale (1986) that, in late plantings, the plants are still small when the flower-inducing temperature prevails, and the tendency to bolt is therefore retarded. Maximum bolting was observed in the earliest planting (5 December) which is probably an effect of low temperatures during Winter and early Spring (Figure 7). The large sets showed higher percentages of bolting, while the small sets showed very little or no bolting. It can be argued that plants grown from small sets were too small to be fully induced by the low temperatures prevailing after the earlier plantings. Maximum seed yield per umbel was observed from the earliest planting (5 December). It can be argued that, following earlier planting, low temperatures prevailed which favoured bolting and increased the number of florets per umbel, resulting in higher seed yields. Delaying planting (to 5 March) had an enhanced effect on bulb yield. This agrees with the findings of Mondal et al. (1986) who showed that bulb growth in the Spring-sown main crop in the UK occurred during August and September under declining photoperiods and decreasing average temperatures. Similar results

24 16 8 0 0

50

100

150

200

250

300

-8 Days after 5 December

5

9

9

1

FIG. 7 Daily maximum and minimum air temperatures during the experimental period (from 5 December 1995 to 17 August 1996).

486


have been reported by Vasic (1975), who observed lower yields from Autumn planting of sets in Yugoslavia. Large sets, being prone to bolting, gave lower bulb yields than medium or small sets. The effect of late planting of sets described here confirms reports by Holdsworth (1945) and Heath and Holdsworth (1948) that late planting reduced the incidence of bolting, presumably because the onset of bulbing suppressed flower emergence, and bulbing occurred rapidly in the long-days and high temperature conditions of Summer. As indicated above, the low-to-mild temperatures which prevailed during the growing phase of onion plants in earlier plantings resulted in increased bolting from large sets. As a result, large sets gave lower bulb yields than either medium or small sets. Bulb yields in both cultivars were highest from medium sets (17.5 mm) from the 5 March planting. These results are in general agreement with those of Vasic (1975) and Lazic (1975), who recommended small- and large-sized sets for earlier (Autumn) and late (Spring) plantings, respectively, to obtain higher bulb yields. This also supports the finding by Heath et al. (1947) that the effect of set-size on bulb yield is complicated by the effect of bolting which depresses yield, otherwise large sets gave consistently higher yields than small sets. It was also found that, from the earlier planting (5 December), the bulbs matured earlier and the

time to bulb maturity was longer with late plantings. From the present study it can be inferred that, after the 5 December planting, conditions were favourable for the initiation of inflorescences and for obtaining higher seed yields. Under these conditions, large sets (22.5 mm) of ‘Hygro’ and ‘Delta’ gave 100% and 65% bolting, respectively. Flowering will occur in the first week of June, and finally ripe seed will be ready for harvest in the middle of August. However, for ‘Delta’, planting sets before 5 December could induce a higher percentage of bolters, resulting in higher seed yields because of being subjected to a relatively prolonged spell of cold. However, for a bulb crop, a 5 March planting, using medium-sized sets (17.5 mm diameter), seemed to be optimum, because the long-days and high temperatures suppressed bolting in mediumsized sets during the growing phase. There appears to be little need to vernalise the onion seed crop artificially in the UK, as low temperature Winter conditions favour the induction of onion sets and the Summer is long enough for the seed crop to ripen. Onion seed is not usually produced in the UK because the damp Summer climate leads to diseases of the seed stalks (e.g., downy mildew, Botrytis). However, seed can be produced on a small commercial scale if fungicides are sprayed to prevent diseases.

REFERENCES ATKIN, J. D. and DAVIS, G. N. (1954). Altering onion flowering dates to facilitate hybrid seed production. California Agricultural Experiment Station Bulletin, No. 746. Davis, CA, USA. 16 pp. AOBA, T. (1960). The influence of the storage temperature for onion bulbs on their seed production. Journal of the Japanese Society for Horticultural Science, 29, 135–141. (In Japanese with English summary). AURA, K. (1963). Studies on vegetatively propagated onions cultivated in Finland, with special reference to flowering and storage. Annals of Agriculturae Fenniae, 2 (Suppl. 5), 1–74. BERTAUD, D. S. (1988). Effects of chilling duration, photoperiod and temperature on floral initiation and development in sprouted and unsprouted onion bulbs. Proceedings of the 4th EUCARPIA Allium Symposium, Institute of Horticultural Research, Wellesbourne, UK. 245–261. BOSWELL, V. R. (1923). Influence of time of maturity of onions on the behaviour during storage and the effect of storage temperature on subsequent vegetative and reproductive development. Proceedings of the American Society for Horticultural Science, 20, 234–239. BREWSTER, J. L. (1982a). Flowering and seed production in overwintered cultivars of bulb onions. I. Effects of different raising environments, temperatures and day lengths. 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 day lengths and the rate of inflorescence development. Journal of Horticultural Science, 57, 103–108. CHIRU, C. R. and BANITA, I. (1980). Experience of the Buzau Vegetable Research and Production Station, Romania in onion seed production. Productia Vegetala, Horticultura, 29, 9–12. (In Romanian).

CURRAH, L. (1981). Onion flowering and seed production. Scientia Horticulturae, 32, 26–46. 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). Proceedings of the 4th EUCARPIA Allium Symposium, Institute of Horticultural Research, Wellesbourne, UK. 262–266. DEMILLE, B. and VEST, G. (1976). The effect of temperature and light during bulb storage on traits related to onion seed production. Journal of the American Society for Horticultural Science, 101, 52–53. 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. 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. Annals of Applied Biology, 30, 208–220. HEATH, O. V. S. and HOLDSWORTH, M. (1948). Morphogenic factors as examplified by the onion plant. In: Growth. (Daneellie, J.F. and Brown, R., Eds.). Symposia of the Society for Experimental Biology, 2, 326–350. HEATH, O. V. S., HOLDSWORTH, M., TINKER, M. A. H. and BROWN, F. C. (1947). Studies on the physiology of the onion plant. III. Further experiments on the effects of storage temperature and other factors on onions grown from sets. Annals of Applied Biology, 34, 473–502. HOLDSWORTH, M. (1945). A comparative study of onion varieties in relation to bolting and yield when grown from sets. Annals of Applied Biology, 32, 22–34.

K. M. KHOKHAR HESSE, P. S., VEST, G. and HONMA, S. (1979). The effect of 4 storage treatments on seed yield components of 3 onion inbreds. Scientia Horticulturae, 11, 207–215. JONES, H. A. and EMSWELLER, S. L. (1939). Effect of storage, bulb size, spacing and time of planting on production of onion seed. California Agricultural Experiment Station Bulletin, No. 628. Davis, CA, USA. 14 pp. JONES, H. A. and MANN, L. K. (1963). Onions and Their Allies. Leonard Hill, London, UK. 286 pp. LAWANDE, K. E. and KALE, P. N. (1986). Effect of monthly planting round the year on yield, bolting, self–topping and twin bulb formation in onions. Journal of Maharashtra Agricultural University, 11, 167–170. LAZIC, B. (1975). The effect of the size of the sets on the intensity of growth, maturity and qualitative properties of onion. Acta Horticulturae, 52, 259–262. LEVY, D., BEN–HERUT, Z., ALBASEL, N., KAISI, F. and MANASRA, I. (1981). Growing onion seeds in an arid region drought tolerance and the effect of bulb weight, spacing and fertilization. Scientia Horticulturae, 14, 1–7. MOBLI, M. (1992). Quantitative Effects of Bulb Size, Pre- and Post-Planting Environment on Flowering and Seed Production in Onion, Allium cepa L. Ph.D. Thesis, University of Reading, UK. 296 pp. MONDAL, M. F., BREWSTER, J. L., MORRIS, G. E. L. and BUTLER, H. A. (1986). Bulb development in onion. I. Effects of plant density and sowing date in field conditions. Annals of Botany, 58, 187–195. NOURAI, A. H. (1981). Effect of bulb size and in–row spacing on onion seed yield. Annual Report, Hudeiba Research Station, Sudan. 35 pp.

487

ORLOWSKI, M. (1974). The influence of onion bulb sizes and their planting density on the quantity and quality of the seed crop with consideration of economical effects. Proceedings of the 19th International Horticultural Congress, Merbein, Victoria, Australia. Volume 16. 633 pp. RABINOWITCH, H. D. (1979). Doubling of onion bulbs as affected by size and planting date of sets. Annals of Applied Biology, 93, 63–66. SAS (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). 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. SLIDE WRITE PLUS. (1989). Advanced Graphics Software Inc., Sunnyvale, CA, USA. (http://www.slidewrite.com/TrialGuide.asp). VASIC, A. (1975). Influence of the sowing time and the size of sets on the yield and marketable quality of onion. Acta Horticulturae, 52, 253–258.