Qualitative and Quantitative Analysis of Endogenous Jasmonic Acid in Bulbing and Non-Bulbing Onion Plants

Bulb development in onion plants (Allium cepa L.) is consideredto be regulated by bulbing and anti-bulbing hormones. Sincebulbing involves the disruption of microtubules, both jasmonicacid (JA) and methyl jasmonate (JAMe) are candidates for thebulbing hormone because of their microtubule-disrupting activitiesand wide distribution in higher plants. To survey JA and JAMein onion plants, we developed a radioimmunoassay (RIA) for JAMethat is sensitive enough to detect femtomole amounts of JAMe.Using this RIA, we detected JA in leaf blades, leaf sheathsand roots of onion plants, but no JAMe was detected in any tissue.The endogenous levels of JA in leaf blades, leaf sheaths androots of 4-week-old bulbing and non-bulbing onion plants weredetermined by gas chromatography/selected ion monitoring with[²H₂]JA as an internal standard. The amount of JA per plantin leaf sheaths of bulbing onion plants was about three timeshigher than that of non-bulbing onion plants, although the differencein levels of JA in leaf blades between bulbing and non-bulbingonion plants was quite small, and the level of JA in roots ofbulbing onion plants was lower than that of non-bulbing onionplants. However, the relationship between endogenous JA andthe development of onion bulbs remains to be clarified. (Received June 3, 1992; Accepted October 1, 1992)     

The Initiation and Growth of Narcissus Bulbs

Plants of two Narcissus cultivars were dissected periodicallyover a year to study initiation, growth, and dry-weight changesof flowers and daughter bulb units in relation to the positionof the daughter bulbs in the branching system. A Narcissus bulbis a branching system comprising terminal and lateral bulb units;the former bear both terminal and lateral bulb units, but lateralscontain only terminal ones. This basic pattern may be modifiedby the failure of lateral bulb units or the development of supernumeraryones. Differences in bulb unit size, in scale and leaf number,and in flowering, related to position in the branching system,are probably due to the initiation of lateral bulb units somemonths after terminal ones. This affects the growth and behaviourof the laterals and daughter bulb units borne by these lateralsin the next generation. Bulb units live about four years; after the loss of their leafblades and flowers, the scales and leaf bases act as storageorgans whose weight increases or decreases according to thecarbohydrate status of the plant. Growth of bulb units showsan externally controlled alternation of rapid and slow growth.This is likely to be an effect of day length on bulbing.     

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     

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     

Studies in the Physiology of the Onion Plant: VI. A SENSITIVE MORPHOLOGICAL TEST FOR BULBING AND ITS USE IN DETECTING BULB DEVELOPMENT IN STERILE CULTURE

Bulb development in the onion plant, which occurs in responseto the stimulus of long days, has two main morphological features:swelling of the base of the pseudostem and the formation ofscales from leaf initials produced at the stem apex. The swellingmay be expressed as the ‘bulbing ratio’, i.e. theratio of the maximum diameter at the base to the minimum atthe neck, but this is a somewhat insensitive measure wider adverselight conditions and no ratio can be specified at which bulbingis first apparent. In the absence of bulbing, leaf initialshave at all stages of development a blade longer than the leafsheath, giving a ‘leaf ratio’ of blade/sheath atall times greater than unity. In scale formation, when bulbdevelopment begins, the sheath of a small initial starts togrow sooner than in foliage leaf formation and blade growthis suppressed; the leaf ratio therefore falls rapidly belowunity and this is diagnostic of bulbing. Characteristic graphsof leaf ratio plotted against leaf number from the apex demonstratethe stage of bulbing reached and a fall in leaf ratio from nearthe apex in such a graph can indicate the beginning of bulbingbefore any actual scales (with ratios less than unity) havebeen formed. In normally grown plants, differentiation intofoliage leaves or scales appears to take place when the initialsreach about 1 mm in length. When the incidence of bulbing within a treatment is very variable,difficulties arise in averaging the leaf ratio curves for thedifferent plants. Mean number of scales per plant, or mean minimumleaf ratio, may then be used as measures of bulbing. Heavy shading in winter was found to delay but not prevent thebulbing of those plants which survived. ‘Buds’, consisting of a portion of stem with theapex surrounded by leaf initials, but no scales, were dissectedout from onion sets and cultured aseptically in tubes. Limitedgrowth was obtained and rooting occurred in some cases. In thepresence of sucrose, swelling of leaf sheaths and reductionof the leaf ratios took place; these responses increased withsucrose concentration. Glucose similarly stimulated bulbingbut high osmotic pressure caused by mannitol or mineral nutrientshad no effect. Rooting appeared to reduce the degree of bulbing,possibly by competing for carbohydrate supply. The bulb developmentin these sterile cultures was most unusual in occurring in shortphotoperiods and the possibility of a residue of ‘bulbinghormone’ being stored in sets is discussed.     

The role of ethylene in photoperiodic control of bulbing in Allium cepa

Bulb formation in Allium cepa L. cv. Dorata di Parma, was promoted by ethylene (Ethrel 120 mg l⁻¹) treatments. Application of the antiethylene complex silverthiosulphate (0.01 m M ) inhibited ethylene-induced bulb formation, but did not affect photoperiod-induced bulb formation. When the inhibitor of ethylene biosynthesis, aminoethoxyvinylglycine, was applied at 0.04 to 4 mg l⁻¹ to onion plants under inductive photoperiodic conditions the bulb promotion was unaffected. Therefore, Ethrel-induced bulb formation does not appear to be related to endogenous control of bulbing.     

Bulb Development in Onion (Allium cepa L.) III. Effects of the Size of Adjacent Plants, Shading by Neutral and Leaf Filters, Irrigation and Nitrogen Regime and the Relationship between the Red: Far-red Spectral Ratio in the Canopy and Leaf Area Index

Field experiments were done to investigate why onion crops witha high leaf area index (LAI) initiate bulb scales and maturesooner than those with a low LAI. When small plants were growneither mixed with large plants or in pure stands, those in themixed stands initiated bulb scales earlier than those in purestands. The timing of bulb development therefore depended onthe size of neighbouring plants and the LAI of the stand andwas not simply determined by the size or age of individual plants.Shading plots with neutral filters which caused no spectralchanges slightly accelerated bulb development compared to unshadedplots but shading by a canopy of climbing pea and bean leaveshad a larger effect. Lack of irrigation advanced bulb maturitybut in the highly irrigated treatments plots of high plant densitystill initiated bulb scales and matured before those of lowdensity. Quantitative relationships between the change in R : FR andeither LAI or total radiation interception were determined foronion leaf canopies. It is suggested that the decrease in thered to far-red spectral ratio (R : FR) as LAI increases maybe the cause of the earlier bulb scale initiation and maturitythat occurs as LAI increases. Onion, Allium cepa (L.), spacing, competition, leaf area index, red: far red, irrigation, nitrogen, shading, bulbing     

Studies in the Physiology of the Onion Plant: IV. THE INFLUENCE OF DAY-LENGTH AND TEMPERATURE ON THE FLOWERING OF THE ONION PLANT

At temperatures above about 17° C. inflorescence initiationin growing onion plants, as in stored sets, is suppressed whetherthe plants are kept in long or short days. Independently ofcurrent day-length and of previous day-length treatment, ifthe plants are sufficiently large initiation begins very shortlyafter the temperature falls below c. 15° C. Emerged infiorescencesappear some ten or so weeks later. Small plants are unable toinitiate inflores cences under any of the conditions tested,and actual size (perhaps leaf area) rather than leaf or nodenumber seems to be the important factor. Inflorescence emergenceis suppressed at high temperatures in short days or long days;in long days bulb formation also suppresses emergence at lowertemperatures. In long days at temperatures sufficiently lowfor bulbing to be delayed, however, emergence is accelerated.Plants which have produced bulbs in long days in the summershow a delay of inflorescence emergence in the following winter.     

The dynamics of the flavour precursors, the S-alk(en)yl-L-cysteine sulphoxides, during leaf blade and scale development in the onion (Allium cepa)

This study was undertaken to determine the patterns of accumulation and loss of the flavour precursors. (+) S-1-propyl-L-cysteine sulphoxide, (+) trans-S-1-propenyl-L-cysteine sulphoxide, and (+) S-1-methyl-L-cysteine sulphoxide, during the development and senescence of the leaf blades and scales of a brown onion ( Allium cepa L. cv. Pukekohe Longkeeper).
The levels of the flavour precursors were related to the ontogeny of the individual leaf blade and scale, and the ontogeny of the entire plant. Leaf blades which developed on a young or bulbing onion contained all 3 flavour precursors (total of about 50–70 mg leaf⁻¹); but as each attached scale developed, the leaf blades lost their flavour precursors. All 3 flavour precursors increased in the developing scales and decreased in the senescing scales. Leaf blades which developed on an older, ripening onion contained, and then lost, only (+) S-1-propyl-L-cysteine sulphoxide, whilst the scales accumulated only (+) S-1-propyl-L-cysteine sulphoxide; (+) S-1-methyl-L-cysteine sulphoxide and (+) trans-S-1-propenyl-L-cysteine sulphoxide were minimal. In the main scales of the onion, which did not senesce during ripening, there was a transition between these two patterns. These scales accumulated all 3 flavour precursors with (+) S-1-propyl-L-cysteine sulphoxide remaining constant at about 30 mg/scale; however there was a 10 fold loss of the other 2 flavour precursors (from 20 to about 2 mg/scale). The base plate (true stem) contained mainly (+) S-1-propyl-L-cysteine sulphoxide, which increased 5 fold in amount during bulbing. The other 2 flavour precursors were present at much lower levels.
A recycling of flavour precursors is suggested, with the leaf blades supplying flavour precursors to scales, and in turn older senescing scales recycling their flavour precursors to developing younger scales.     

Leaf and root degeneration in early maturing onions: A physiological disorder

Three onion cvs., four sowing dates, two plant densities and two photoperiod regimes were employed to elucidate the factor(s) responsible for leaf and root degeneration which occur frequently in crops of early maturing onions in the Arava valley, Israel, causing heavy economic losses. It was found that the symptoms developed in early maturing genotypes which showed the abnormal sequence of early bulbing in the autumn, followed by the reversal of bulbing later in the season. At this time of season root regrowth is restrained due to low soil temperatures and therefore mineral and water supply to the sprouting foliage is likely to be limited. These interpretations appear to be correct for the problem has been avoided in several seasons by the prevention of early bulbing by sowing later, by keeping population density at standard rates, and by the prevention of mineral deficiency by application of extra nitrogen.     

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     

Qualitative and Quantitative Analysis of Endogenous Gibberellins in Onion Plants and Their Effects on Bulb Development

Evidence has been reported that bulb development in onion plants (Allium cepa L.) is controlled by endogenous bulbing and anti-bulbing hormones, and that gibberellin (GA) is a candidate for anti-bulbing hormone (ABH). In this study, we identified a series of C-13-H GAs (GA₁₂, GA₁₅, GA₂₄, GA₉, GA₄, GA₃₄, and 3-epi-GA₄) and a series of C-13-OH GAs (GA₄₄, GA₂₀, GA₁ and GA₈) from the leaf sheaths including the lower part of leaf blades of onion plants (cv. Senshu-Chuko). These results suggested that two independent GA biosynthetic pathways, the early-non-hydroxylation pathway to GA₄ (active GA) and early-13-hydroxylation pathway to GA₁ (active GA), exist in onion plants. It was also suggested that GA₄ and GA₁ have almost the same ability to inhibit bulb development in onion plants induced by treatment with an inhibitor of GA biosynthesis, uniconazole-P. The endogenous levels of GA₁ and GA₄, and their direct precursors, GA₂₀ and GA₉, in leaf blades, leaf sheaths, and roots of 4-week-old bulbing and non-bulbing onion plants were measured by gas chromatography/selected ion monitoring with the corresponding [²H]labeled GAs as internal standards. In most cases, the GA levels in long-day (LD)-grown bulbing onion plants were higher than those of short-day (SD)-grown non-bulbing onion plants, but the GA₁ level in leaf blades of SD-grown onion plants was rather higher than that of LD-grown onion plants. Relationship between the endogenous GAs and bulb development in onion plants is discussed.     

Pectic Enzyme Production by Fusarium oxysporum f. sp, ccpac: Induction by Cell Walls from Different Parts of Onion Bulbs at Different Growth Stages

Fusarium oxysporum f. sp. cepae produced significantly different amounts of pectic enzymes when grown on cell walls from morphologically different parts of onion bulbs. Cell walls from stem plate tissue of both tolerant and susceptible onion genotypes allowed a rapid and high production of both exo-polygalacturonase and endo-pectin-frans-eliminase. Bulb scale cell walls from susceptible genotypes induced synthesis of these enzymes at much lower rates and levels, whereas bulb scale cell walls from tolerant genotypes gave poor induction of pectic enzyme synthesis. Leaf sheath cell walls from both susceptible and tolerant genotypes were poor inducers of enzyme synthesis. Enzyme induction by cell walls from leaf sheaths and bulb scales of tolerant genotypes increased dramatically during ageing. Differences in pectic enzyme accumulation on cell walls were not related to fungal growth. These patterns of enzyme induction could help to explain susceptibility or tolerance of bulb scale and leaf sheath tissue of the different genotypes.     

Formation of onion bulblets in vitro and viability during medium-term storage

 The influences of light conditions, sucrose and ethylene on in vitro formation and storability of onion (Allium cepa L.) bulblets were studied in various accessions. Light, sucrose and ethylene influenced bulb formation. Storability was primarily enhanced by a high sucrose concentration (100 g/l) in the culture medium. The bulbing process was characterised by changes in bulbing ratio, leaf length, number of leaves and leaf development time. The viability of bulbs after 1 year of in vitro storage at low temperatures was determined by their growth reaction in subsequent subcultures, growth after transfer into the greenhouse and tetrazolium staining. Sufficient sprouting of bulblets previously stored at –1  °C demonstrated the possibility of storing them in a low-temperature, slow-growth culture. Received: 8 June 2000 / Revision received: 5 October 2000 / Accepted: 5 October 2000     

In vitro bulb development in shallot (Allium cepa L. Aggregatum Group): effects of anti-gibberellins,sucrose and light

Bulbing was studied in shallot plants cultured in vitro. Bulbing occurred under a 16 h photoperiod with fluorescent + incandescent light and 30-50 g 1(-1) sucrose in the culture medium. Exogenous gibberellin (10 microM GA3) inhibited leaf and root growth and bulbing. When added to the medium at a concentration of 10 microM, three inhibitors of gibberellin biosynthesis (ancymidol, flurprimidol and paclobutrazol) promoted bulb formation and the percentage of bulbing. When ancymidol was used in combination with GA3, it did not reverse the effect of GA3 applied alone. Under treatments with 30-70 g l(-1) sucrose, bulbing ratios greater than those found in control plants were achieved by addition of ancymidol, and bulb fresh weight was increased in the same way. Ancymidol caused a 66% decrease in sucrose content in leaf bases but greatly increased the glucose, fructose and fructan contents. The increase in fructan content by ancymidol could result from the three-fold rise in total [14C]sucrose uptake per plant from the culture medium associated with a marked increase in leaf base labelling at the expense of root labelling. The possible role of ancymidol is discussed and evidence supports a major regulatory role for gibberellins in bulbing.     

Bulbing in Long-day Onion (Allium cepa L.) Cultured In Vitro: Comparison Between Sugar Feeding and Light Induction

Bulbs were obtained on onion plants cultured in vitro. No bulbinghappened under long days with fluorescent light and 30–40g l^–1 sucrose. Bulb formation was mainly dependent eitheron sucrose concentration in the culture medium, or on lightspectral composition. Raising the sucrose concentration from40 to 120 g l^–1 increased plant basal swelling and stoppedfurther vegetative development. These plants were not dormant.When fluorescent light was enriched in incandescence duringa long day period, bulbs were obtained in two months and underwenta consecutive dormancy. Bulb, dormancy, light spectral quality, photoperiod, R: FR ratio, sugar, tissue culture     

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     

Role of phytochrome in photoperiodic regulation of bulbing and growth in the long day plant Allium cepa

Plant elongation in Allium cepa L. cv. Dorata di Parma was stimulated by end-of-day far-red radiation, while the same treatment was ineffective with respect to bulbing response. It was concluded that the P_fr-dependent reactions which control bulbing are completed during the long daily light period (18 h). Day breaks of 3 h fluorescent white light, in the middle of the inductive photoperiod were inhibitory to bulbing. Repeated brief far-red irradiations could substitute for continuous far-red irradiations lasting 3 h in the middle of the photoperiod. Red light alone or applied immediately after each far-red irradiation inhibits bulb formation.     

Changes in Microtubules in Onion Leaf Sheath Cells during Bulb Development

Cortical microtubules are oriented transversely to the cellaxis in leaf sheath cells of onion plants (Allium cepa L. cv.Osaka-Okute) that have not started bulb formation. As the bulbdevelops and the leaf sheath cells swell, the microtubules becomedisoriented and scattered and finally disappear. The microtubuleinhibitors colchicine and cremart [O-ethyl O-(3-methyl-6-nitrophenyl)N-sec-butylphosphorothioamidate]cause swelling of leaf sheath cells and make the basal partof the plant bulbous. The cortical microtubules may have animportant role in regulating bulb development in onion plants. (Received August 21, 1982; Accepted December 6, 1982)