Bulbing of onions (Allium cepa L.): The role of endogenous ethylene*

Abstract Bulbing of onions under naturally increasing daylengths was typically associated with elevated rates of ethylene evolution (10?20 mm³× 10³ g^?1h^?1) during the initial stages of bulbing, followed by a decline to very low rates (1 mm³× 10³ g^?1h^?1) towards bulb maturation. However, detailed comparisons conducted under controlled photoperiodic conditions showed only slight differences in the course of ethylene evolution between inductive and non-inductive lighting regimens. Bulbing was not prevented by hypobaric ventilation or by treatments with the ethoxy analogue of rhizobitoxine, but Ag⁺ interfered with bulbing. Although exogenous ethylene induces bulbing under non-inductive photoperiodic conditions, the role of endogenous ethylene in the natural process requires further clarification.     

Photoperiod and Temperature Affect Sprouting of Onion bulbs (Allium cepa L.)

The effects of photoperiod and temperature on sprouting wereinvestigated using three cultivars of onion which exhibit differentresponses to daylength during bulb growth. Two photoperiods(8 h and 14 h) and two constant temperatures (15 C and 20 C)were used. Two cultivars showed enhanced sprouting and leafemergence in short days, and one sprouted poorly in all conditions.In long days, some plants showed growth of new bulbs withinthe old one, without sprouting. Photomorphogenesis, photoperiodism, Allium cepa L, onion, dormancy, bulb sprouting     

A Comparison of Leek (Allium porrum) and Onion (Allium cepa) Seed Development

Leek and onion seed dry weight increased exponentially for thefirst 31 days after flowering (DAF) but thereafter the increasein dry weight was slower. Before maximum seed dry weight wasreached at 45 DAF in onion and 59 to 66 DAF in leek, seed moisturecontent, seed oxygen uptake and conductivity of the seed steepwater fell from initially high levels. Although some seeds germinated31 DAF in both species, full germination in both was not reacheduntil 66–80 DAF. Tolerance of the seed to artificial dryingimmediately after harvesting occurred 45 DAF in onion and 74DAF in leek. Free nuclear division continued in the endospermuntil 17–22 DAF in onion and until 31–35 DAF inleek but it was not until 45 DAF in onion and 66 DAF in leekthat the embryo and endosperm filled the cavity formed by thepericarp. After formation of cell walls in the endosperm thepattern of change in cell number in both species was similar.The shrunken appearance of the seed coat in leek, which occurredearly in seed development, was associated with the period offree nuclear division in the endosperm and, in addition, thepericarp was thinner than in onion. There was no evidence thatthe shrunken seed coat early in development was associated withself as opposed to open-pollination. Allium porrum, Allium cepa, seed development, endosperm, embryo, cell number, germination, respiration, seed leachates     

Interspecific hybrids between onion (Allium cepa L.) with S-cytoplasm and leek (Allium ampeloprasum L.)

 Interspecific hybrids between Allium cepa and A. ampeloprasum have been generated as a first step for the introduction of S-cytoplasm from onion into leek. Pre-zygotic barriers of crossability were observed after the arrival of pollen tubes at the end of the style when entering the cavity. Nevertheless, micropyle penetration of pollen tubes and the formation of hybrid embryos were also observed. After accomplishing in vitro culture of ovaries and ovules successively, triploid hybrid plants with 24 chromosomes were obtained. Their hybrid nature was confirmed by RAPD analysis, genomic in situ hybridization, and morphological analysis. Southern hybridization with a cytoplasmic probe indicated the transfer of unaltered S-cytoplasm into the hybrid plants. Received: 9 May 1996 / Accepted: 23 August 1996     

用基因枪法介导OSISAP1基因遗传转化洋葱

以洋葱栽培品种‘HG400B’的鳞茎盘胚性愈伤组织为受体,利用基因枪介导法将水稻锌指蛋白基因OSISAP1导入洋葱中。组织化学染色检测到GUS基因在胚性愈伤组织中的瞬间表达活性,PCR、Southern杂交和RT-PCR分析,证实OSISAP1基因已整合到洋葱基因组中并实现高水平表达,转化率约为10%。对获得的转基因植株进行NaC1和NaHCO_3胁迫处理,当总浓度为200 mmol/L、处理1周后,未转基因植株会黄化、枯萎、死亡,而转基因植株却有很强的抗性,能耐受400mmol/L浓度的胁迫,表明OSISAP1基因的导入提高了转基因植株的耐盐碱性。     

Growth, Dry Matter Partition and Radiation Interception in an Overwintered Bulb Onion (Allium cepa L.) Crop

Growth, bulb development, partition of dry weight between leafblades and bulbs, and the interception of solar radiation weremeasured in overwintered crops of five cultivars of bulb onionwith different maturity dates sown on successive dates in threeseasons. The onset of bulbing was later the later maturity ofthe cultivar. Later sowing did not delay the onset of bulbingbut it did delay maturity. There was little mean differencebetween cultivars in the duration of bulb growth defined asthe interval between onset of bulbing and maturity, but therewere considerable differences between cultivars within a season,and between seasons for a given cultivar. Duration of bulb growthranged from 11 to 46 days with a mean of 35 days. Increases in total shoot dry weight during bulb developmentand, in the absence of much bolting, bulb dry-matter yieldswere linearly related to the total radiation intercepted duringbulb growth. These relationships were similar to those reportedfor other crops in Britain. Radiation interception during thephase of bulb growth was low compared with other crops, witha mean value of 49 per cent and a maximum of 65 per cent. Thepercentage of solar radiation intercepted during bulb developmentwas higher from early sowings than from later ones, particularlyin early maturing cultivars. The harvest index was high, withtypically more than 80 per cent of the shoot dry weight in bulbsat maturity. Allium cepa L., onion, blub, growth, partition of dry matter, radiation interception     

Effects of Photoperiod, Nitrogen Nutrition and Temperature on Inflorescence Initiation and Development in Onion (Allium cepa L.)

The effects of photoperiod, nitrogen nutrition and temperatureon inflorescence initiation and development in onion cv. Rijnsburgerand cv. Senshyu Semi-globe Yellow were studied in controlledenvironments. Rates of inflorescence initiation were estimatedusing the data for leaf numbers formed prior to flower formationand the rates of leaf initiation. At 9 °C inflorescenceinitiation was accelerated by long photoperiods particularlyfor cv. Rijnsburger where the average time for initiation was86 days in 8 h and 38 days in 20 h photoperiods. Initiationwas as rapid at 12 °C as at 9 °C but was slower at 6°C. A reduction in the nitrate concentration in the nutrientsolution from 0.012 to 0.0018 M greatly accelerated inflorescenceinitiation particularly in photoperiods and temperatures notconducive to rapid initiation. Cv. Senshyu initiated more slowlythan cv. Rijnsburger and was less sensitive to photoperiod andnitrogen level. The development rate of inflorescences afterinitiation was accelerated by long photoperiods and increasesin temperature from 6 to 12 °C but was retarded by the lowernitrogen level. Allium cepa L., onion, flower initiation, inflorescence development, photoperiod, nitrogen nutrition, temperature, vernalization     

Seed quality effects in bulb onions (Allium cepa L.)

The nature and persistence of seed quality effects produced by a range of seed production treatments was investigated in the autumn-sown onion crop. Genotypically equivalent seed lots raised in different glasshouses within the same year were found to germinate at significantly different rates. The seed lot which emerged earlier produced larger seedlings and this difference in size persisted throughout the growing period. In a comparison between the effects of hand crossing and self-pollination techniques the hand crossing method produced a much lower total seed yield but heavier seeds and larger plants. The induced effects persisted throughout the year but were not significant in the mature bulb characters. The implications of these results upon the control of seed production for plant breeding and genetical experiments is discussed.     

Embryological study on gynogenesis in onion (Allium cepa L.)

Haploid induction in onion can, to date, be induced only via gynogenesis by culturing unfertilized flowers, ovaries or ovules. The process of haploid embryo induction has been macroscopically well studied, but only limited data exist from microscopic examination of ovule development status at the inoculation stage and of the origin of gynogenic embryos. Microscopic studies were carried out using individual donor plants with relatively high embryo induction frequencies (45.9 embryos formed per 100 flowers, on average, for 2 years). Ovaries from flower bud culture were fixed at 1 week intervals up to the 7th week of culture. These were compared with pollinated ovaries at 1 or 2 weeks after pollination. In total, 1428 unfertilized embryo sacs were examined. The results indicate that, at the time of inoculation, ovules within ovaries 2.0–3.0 mm in diameter contained two- or four-nucleate embryo sacs in the smallest ovaries to mature embryo sacs in the largest ovaries. It seems likely that the embryos are actually induced from ovaries cultured at the immature stage. After 1 or 2 weeks in culture, the egg apparatus primarily consisted of distinctly enlarged synergids and the egg cell, which was often detached from the micropylar pole. But free nuclear endosperm was also formed. From the 2nd to 7th week in culture, formation of haploid embryos (from globular to the almost mature cylindrical stage) was detected in 5.7% of the ovules. Their origin, for several reasons, was most likely the egg cell. In addition, ovules containing endosperm only (3.6%) and ovules containing the egg apparatus (0.5%) or both endosperm and embryo (0.4%) were detected. This observation is probably unique and has not yet been reported in other species studied. Received: February 2001 / Revision accepted: 20 April 2001     

Production and characterization of somatic hybrid plants between leek (Allium ampeloprasum L.) and onion (Allium cepa L.)

 Results are reported on the production and characterization of somatic hybrids between Allium ampeloprasum and A. cepa. Both symmetric and asymmetric protoplast fusions were carried out using a polyethylene-based mass fusion protocol. Asymmetric fusions were performed using gamma ray-treated donor protoplasts of A. cepa and iodoacetamide-treated A. ampeloprasum protoplasts. However, the use of gamma irradiation to eliminate or inactivate the donor DNA of A. cepa proved to be detrimental to the development of fusion calli, and thus it was not possible to obtain hybrids from asymmetric fusions. The symmetric fusions yielded a high number of hybrid calli and regenerated plants. The analysis of the nuclear DNA composition using interspecific variation of rDNA revealed that most of the regenerated plants were hybrids. Flow cytometric analysis of nuclear DNA showed that these hybrid plants contained a lower DNA content than the sum of the DNA amounts of the parental species, suggesting that they were aneuploid. A shortage of chromosomes in the hybrids was confirmed by genomic in situ hybridization. Chromosome counts in metaphase cells of six hybrids revealed that these plants lacked 2–7 leek chromosomes. One hybrid showed also the loss of onion chromosomes. The hybrids had an intermediate phenotype in leaf morphology. The application of these somatic hybrids in breeding is discussed. Received: 7 April 1997 / Accepted: 10 September 1997     

Bulb Development in Onion (Allium cepa L.) I. Effects of Plant Density and Sowing Date in Field Conditions

Early and later maturing cultivars of both spring- and autumn-sownonions were sown on two dates at 25, 100 and400 plants m^–2.High levels of nutrients and irrigation were applied. Both high plant density and early sowing advanced the date atwhich bulb scales, rather than leaf blades, were initiated atthe shoot apex and so advanced the date of maturity by up to46 d. The later maturing spring-sown cultivar showed greaterresponses to density and sowing date than the earlier maturingspring-sown cultivar which in turn was more responsive thanthe autumn-sown cultivars. For each cultivar, maturity date increased linearly with decreasesin the percentage radiation intercepted by the leaf canopy. Onion, Allium cepa L., bulbing, competition, spacing     

Water Transport in Onion (Allium cepa L.) Roots (Changes of Axial and Radial Hydraulic Conductivities during Root Development)

The hydraulic architecture of developing onion (Allium cepa L. cv Calypso) roots grown hydroponically was determined by measuring axial and radial hydraulic conductivities (equal to inverse of specific hydraulic resistances). In the roots, Casparian bands and suberin lamellae develop in the endodermis and exodermis (equal to hypodermis). Using the root pressure probe, changes of hydraulic conductivities along the developing roots were analyzed with high resolution. Axial hydraulic conductivity (Lx) was also calculated from stained cross-sections according to Poiseuille's law. Near the base and the tip of the roots, measured and calculated Lx values were similar. However, at distances between 200 and 300 mm from the apex, measured values of Lx were smaller by more than 1 order of magnitude than those calculated, probably because of remaining cross walls between xylem vessel members. During development of root xylem, Lx increased by 3 orders of magnitude. In the apical 30 mm (tip region), axial resistance limited water transport, whereas in basal parts radial resistances (low radial hydraulic conductivity, Lpr) controlled the uptake. Because of the high axial hydraulic resistance in the tip region, this zone appeared to be "hydraulically isolated" from the rest of the root. Changes of the Lpr of the roots were determined by measuring the hydraulic conductance of roots of different length and referring these data to unit surface area. At distances between 30 and 150 mm from the root tip, Lpr was fairly constant (1.4 x 10-7 m s-1 MPa-1). In more basal root zones, Lpr was considerably smaller and varied between roots. The low contribution of basal zones to the overall water uptake indicated an influence of the exodermal Casparian bands and/or suberin lamellae in the endodermis or exodermis, which develop at distances larger than 50 to 60 mm from the root tip.     

Genetic analyses of bolting in bulb onion (Allium cepa L.)

Key message

We present the first evidence for a QTL conditioning an adaptive trait in bulb onion, and the first linkage and population genetics analyses of candidate genes involved in photoperiod and vernalization physiology.

Abstract

Economic production of bulb onion (Allium cepa L.) requires adaptation to photoperiod and temperature such that a bulb is formed in the first year and a flowering umbel in the second. ‘Bolting’, or premature flowering before bulb maturation, is an undesirable trait strongly selected against by breeders during adaptation of germplasm. To identify genome regions associated with adaptive traits we conducted linkage mapping and population genetic analyses of candidate genes, and QTL analysis of bolting using a low-density linkage map. We performed tagged amplicon sequencing of ten candidate genes, including the FT-like gene family, in eight diverse populations to identify polymorphisms and seek evidence of differentiation. Low nucleotide diversity and negative estimates of Tajima’s D were observed for most genes, consistent with purifying selection. Significant population differentiation was observed only in AcFT2 and AcSOC1. Selective genotyping in a large ‘Nasik Red × CUDH2150’ F₂ family revealed genome regions on chromosomes 1, 3 and 6 associated (LOD > 3) with bolting. Validation genotyping of two F₂ families grown in two environments confirmed that a QTL on chromosome 1, which we designate AcBlt1, consistently conditions bolting susceptibility in this cross. The chromosome 3 region, which coincides with a functionally characterised acid invertase, was not associated with bolting in other environments, but showed significant association with bulb sucrose content in this and other mapping pedigrees. These putative QTL and candidate genes were placed on the onion map, enabling future comparative studies of adaptive traits.     

Purification and Characterization of Two Distinct NAD(P)H Dehydrogenases from Onion (Allium cepa L.) Root Plasma Membrane

Highly purified plasma membrane fractions were obtained from onion (Allium cepa L.) roots and used as a source for purification of redox proteins. Plasma membranes solubilized with Triton X-100 contained two distinct polypeptides showing NAD(P)H-dependent dehydrogenase activities. Dehydrogenase I was purified by gel filtration in Sephacryl S-300 HR, ion-exchange chromatography in DEAE-Sepharose CL-6B, and dye-ligand affinity chromatography in Blue-Sepharose CL-6B after biospecific elution with NADH. Dehydrogenase I consisted of a single polypeptide of about 27 kD and an isoelectric point of about 6. Dehydrogenase II was purified from the DEAE-unbound fraction by chromatography in Blue-Sepharose CL-6B and affinity elution with NADH. Dehydrogenase II consisted of a single polypeptide of about 31 kD and an isoelectric point of about 8. Purified dehydrogenase I oxidized both NADPH and NADH, although higher rates of electron transfer were obtained with NADPH. Maximal activity was achieved with NADPH as donor and juglone or coenzyme Q as acceptor. Dehydrogenase II was specific for NADH and exhibited maximal activity with ferricyanide. Optimal pH for both dehydrogenases was about 6. Dehydrogenase I was moderately inhibited by dicumarol, thenoyltrifluoroacetone, and the thiol reagent N-ethyl-maleimide. A strong inhibition of dehydrogenase II was obtained with dicumarol, thenoyltrifluoroacetone, and the thiol reagent p-hydroxymercuribenzoate.     

In Vitro Flowering Induction in Date Palm (Phoenix dactylifera L.)

This article reports on a histological and morphological study on the induction of in vitro flowering in vegetatively propagated plantlets from different date palm cultivars. The study aimed to further explore the control of in vitro flower induction in relation to the photoperiodic requirements in date palm and to come up with a novel system that may allow for early sex determination through plant cycle reduction. In fact, the in vitro reversion of a shoot meristem from a vegetative to a reproductive state was achieved within 1–5 months depending on the variety considered. This reversion was accompanied by several morphological transformations that affected the apical part of the leafy bud corresponding mainly to a size increase of the prefloral meristem zone followed by the appearance of an inflorescence. The flowers that were produced in vitro were histologically and morphologically similar to those formed in vivo. The histological examination of the in vitro flowering induction process showed that the conversion into inflorescences involved the entire apical vegetative meristem of the plantlet used as a starting material and brought about a change in its anatomical structure without affecting its phyllotaxis and the leaf shape. Through alternating between hormone-free and hormone-containing media under different light/dark conditions, the highest flower induction rates were obtained with a basal Murashige and Skoog medium. A change in the architectural model of date palm was induced because unlike the natural lateral flowering, in vitro flowering was terminal. Such in vitro flower induction allowed a significant reduction in plant cycle and can, therefore, be considered a promising candidate to save time for future improvement and selection programs in date palm.     

Bulb Development in Onion (Allium cepa L.) II. The Influence of Red: Far-red Spectral Ratio and of Photon Flux Density

Bulb development was followed in four onion cultivars growingin controlled environments under 17 h days in factorial combinationsof photon-flux densities (PFD) of 111 and 333 µmol m^–2s^–1 with red: far-red spectral ratios (R : FR) of 2.88and 1.41. A PFD of 111with R : FR of 0.86 was used in a secondexperiment. The lower the R : FR or the higher the PFD the higherthe mean bulbing ratio and the ratio of bulb plus sheath toleaf blade dry weight and the earlier the bulb swelling. LowerR : FR accelerated scale initiation as did the higher PFD underR : FR 2.88 but not under R : FR 1.41. In high PFD bulb swellingoccurred before scale initiation because many sheaths of bladedleaves thickened, but in low PFD and low R : FR bulb swellingand scale initiation were concurrent. In the low PFD a low R: FR increased soluble carbohydrate concentrations in sheathsand scales compared to high R : FR. In high PFD, soluble carbohydrateconcentrations did not vary with R : FR and were higher thanunder low PFD. Onion, Allium cepa L., bulb, photon flux density, red: far-red ratio, light spectral quality, soluble carbohydrate     

Induction of direct somatic organogenesis in onion (Allium cepa L.) using a two-step flower or ovary culture

A novel method for direct organogenesis in onion (Allium cepa L.) resulting in the formation of multiple shoot structures induced on mature flower buds or ovaries in a two-step culture procedure is described. Flowers were cultured on an induction medium containing 2 mg/l 2,4-dichlorophenoxyacetic acid (2,4-D) and 2 mg/l 6-benzylaminopurine (BAP). After 6 days (superior to 3 or 12 days), flowers or extracted ovaries were transferred to a differentiation medium containing 2 mg/l thidiazuron (TDZ). Medium solidification with gellan gum was superior to agar or agar/gellan gum mixture. A similar regeneration frequency was achieved at high (100 g/l) and lower (50 g/l) sucrose content. Regeneration was obtained from all 12 cultivars or inbred lines examined, although the efficiency and the occurrence of hyperhydricity varied depending on genotype and procedure used. Studies of plant growth regulators revealed that in the induction medium, the auxin 2,4-D was superior to 5 mg/l naphthaleneacetic acid or picloram, which partially or completely inhibited regeneration. Omitting cytokinin in the induction medium or substitution of BAP with 2 mg/l 2iP lowered regeneration, while substitution with 1 mg/l TDZ was equally effective. In the differentiation medium, lower concentrations of TDZ (1 and 0.5 mg/l) or substitution of TDZ with 5 mg/l BAP were equally or less effective. Received: 14 October 1998 / Revision received: 19 November 1998 / Accepted: 30 November 1998     

Variations in gynogenic response among long-day onion (Allium cepa L.) accessions

Long-day onion genotypes originating from Europe, North America and Japan were tested for gynogenic induction ability. Haploid induction using an efficient single-step flower culture induction procedure was at least as productive as the previously studied double-step flower/ovary culture, while undesired callusing of flower bases did not substantially lower the induction frequency. Differences in embryo yields among different accessions and among different donor plants within accessions were very pronounced. Three extremely responsive accessions were found, with average embryo yields of 18.6–22.6%, while individual donor plants produced up to 51.7% embryos. In terms of geographic origin, genetic material bred in America was on average almost five and nine times more responsive than European and Japanese material, respectively. The stability of high induction frequency was confirmed by culturing flowers from the same individual donor plants in two subsequent growth years. Of the regenerants, 90.5% were haploid and 88.2% of analysed diploid regenerants were homozygous. The results indicate that the genotype of donor plants has a crucial influence on haploid induction ability and that the less labour intensive single-step flower induction procedure is an efficient method of obtaining a high-frequency homozygous-embryo induction rate. Received: 20 November 1997 / Revision received: 13 March 1998 / Accepted: 30 March 1998     

Agrobacterium tumefaciens-mediated transformation and transgenic-plant regeneration of onion (Allium cepa L.)

 An Agrobacterium tumefaciens-mediated transformation method has been developed for onions (Allium cepa L.) using immature embryos as the explant source. Transgenic plants were recovered from the open-pollinated onion cultivar Canterbury Longkeeper at a maximum transformation frequency from immature embryos of 2.7%. The method takes between 3–5 months from explant to primary regenerant entering the glasshouse. Multiple-shoot formation from primary transgenic material made possible the clonal multiplication of transformants. The binary vector used carried the nptII antibiotic resistance gene and the m-gfp5-ER reporter gene. Transgenic cultures were initially screened for their ability to fluoresce and to grow in the presence of geneticin (5–25 mg/l). The transgenic nature of individual plants was confirmed by Southern blot analysis. Received: 12 October 1998 / Revision received: 17 May 1999 Accepted: 14 June 1999     

Fidelity of Hymenoptera and Diptera pollinators in onion (Allium cepa L.) pollination

Onion (Allium cepa L.) is protandrous in nature and requires cross‐pollination to avoid inbreeding. The pollination potential of native bees (Hymenoptera) and true flies (Diptera) was assessed in the perspective of finding the best pollinators for onion cross‐pollination and seed multiplication. The community of pollinators was composed of four bee species and twelve true fly species. Episyrphus balteatus, Eupeodes sp., Musca domestica and Eristalinus aeneus were the most abundant pollinators. The maximum pollinator activity was observed from 12 to 24 days after opening of the flowers. The pollination effectiveness of tested bees (Apis dorsata and Apis florea) was greater than true flies (E. balteatus, Eupeodes sp., M. domestica, E. aeneus and Callihoridae sp.) in terms of Spears values.