ROLE OF ASYMMETRIC CELL DIVISION IN PTERIDOPHYTE DIFFERENTIATION. II. EFFECT OF CA2+ ON ASYMMETRIC CELL DIVISION,RHIZOID ELONGATION,AND ANTHERIDIUM DIFFERENTIATION IN VITTARIA GEMMAE

Gametophytes of Vittaria graminifolia reproduce vegetatively by means of gemmae. Each gemma consists of a linear array of six cells: four body cells and a knob-shaped terminal cell at each end. When gemmae are shed from the gametophyte onto Knop's mineral medium, the two terminal cells do not divide, but elongate to form primary rhizoids. The body cells undergo asymmetric cell division, and the smaller daughter cells differentiate into either secondary rhizoids or prothalli. When gibberellic acid is included in the medium, antheridia are formed as a result of asymmetric cell division instead of vegetative structures. We studied the effect of Ca²⁺ on asymmetric cell division, rhizoid elongation, and antheridium formation in gemmae cultured on Knop's mineral medium and variations of Knop's medium. Ca²⁺ inhibited the onset of cell division and rhizoid elongation, but was required for differentiation of antheridia. Treatments which lowered the Ca²⁺ content of gemmae (EGTA and dilute HCl extraction, culture on verapamil-containing and Ca²⁺-deficient medium) caused an early onset of cell division and rhizoid elongation. The stimulation of growth was most pronounced when gemmae were deprived of Ca²⁺ during the first 24 hr of culture. The proportion of cell divisions which differentiated into antheridia in response to GA was greatly reduced when the Ca²⁺ status of gemmae was lowered with verapamil and Ca²⁺-EGTA buffers.     

Microscopic analysis and seasonality of gemma production in the freshwater red alga Hildenbrandia angolensis (Hildenbrandiales, Rhodophyta)

The development and release of the unique vegetative propagules of the freshwater encrusting alga Hildenbrandia angolensis Welwitsch ex West et West, gemmae, were studied using several different microscopic and histochemical techniques. In addition, the seasonality of gemma production was monitored bimonthly over a 12‐month period in two spring‐fed streams in Texas, USA. Gemmae differentiate within the thallus and are subsequently released from the surface of the crust. Release of the gemmae most likely occurs by digestion of surrounding cells, as suggested by the presence of starch granules and lipid globules in the region between the released gemma and the thallus. The initial separation of the gemmae from the thallus occurs from the sides of the gemma or the bottom, or possibly simultaneously. Contrary to previous studies, we have observed that gemma production occurs endogenously within the thallus of freshwater Hildenbrandia, rather than on the surface of the crust in raised structures. Histochemical tests and electron microscopic examination indicate that the cells of the gemmae contain a large amount of floridean starch. The starch granules frequently form rings surrounding the nuclei of both gemma and thallus cells; a feature infrequently reported for florideophyte red algae. Our seasonality investigations indicate that large fluctuations in gemma production occur over 1 year, but at least some gemma production continues year‐round in the streams examined.     

GEMMIFEROUS FERN GAMETOPHYTES—VITTARIACEAE

Approximately 10 % of all fern species reproduce vegetatively in the gametophytic stage by means of gemmae. Gametophyte morphologies in these species depart radically from the commonly figured heart-shaped type and expand considerably the opportunities for physiological and morphological studies utilizing fern gametophytes. Original observations on four species of vittarioid ferns are presented and compared with earlier observations on gametophytes of this family. Vittarioid gametophytes grow from a discontinuous marginal meristem which results in a much branched thallus of indeterminant growth. Aerial branches of the gametophytes terminate in gemma production, which proceeds by a regular and predictable sequence of events. The sequence may differ considerably among species but is remarkably constant within species. Archegonia are produced on short ventral branches, and antheridia are produced primarily on germinating gemmae. Ananthacorus angustifolius is the only known member of the Vittariaceae which does not produce gemmae and is considered to represent the primitive condition. In this species antheridia are scattered over the thallus, suggesting that a change in the mode of control of antheridium production may have evolved in the family along with gemma production.     

Serial Development of Foliar Gemmae in Tortula (Pottiales, Musci), an Ultrastructural Study

The development and liberation mechanism of foliar gemmae havebeen studied by electron microscopy in two mosses, Tortula latifoliaBruch and Tortula papillosa Wils. The gemmae develop on theadaxial surface of mature leaves from single initial cells onboth the lamina and costa in T. latifolia but only on the costain T. papillosa . Elongation of the initial cell is associatedwith the deposition of a highly extensible new wall whilst theold wall and cuticle in the apical dome rupture. The first divisionis transverse and separates a short basal cell embedded in thefoliar tissue and a distal cell, or gemma primordium, protrudingfrom the leaf surface. Subsequent divisions of the gemma primordiumgive rise to a six-to-eight-celled globose gemma with mucilaginousouter walls. During gemma development the basal cell producesa new wall and elongates again whilst the common wall with thegemma splits apart centripetally along the boundary betweenthe old and new wall in the basal cell; plasmodesmal connectionsare gradually severed and eventually the young gemma remainsconnected to the basal cell only by mucilage. After separationof the first-formed gemma, the basal cell may expand and producea second gemma by the same mechanism. The whole process maybe repeated several times resulting in the formation of a chainof gemmae stuck together by mucilage and which are liberatedonly when the leaves are fully hydrated. Accumulation of abundantlipid deposits in the gemmae after symplasmic isolation reflectsconsiderable photosynthetic autonomy. Abscission; bryophytes; cell wall formation; plasmodesmata; vegetative reproduction     

Dispersal potential of spores and asexual propagules in the epixylic hepatic Anastrophyllum hellerianum

Dispersal ability is of great importance for plants, which commonly occupy spatially and temporally limited substrate patches. Mixed reproductive strategies with abundant diaspore production are favoured in a heterogeneous landscape to ensure successful colonisation at different distances. In bryophytes, long-distance dispersal has been thought to take place primarily by spores, while asexual propagules are important in local dispersal and in the maintenance of colonies. In the present study, we investigated the dispersal potential of two equally sized propagules, sexually formed spores and asexually produced gemmae in the dioecious, epixylic hepatic, Anastrophyllum hellerianum, which inhabits spatially and temporally limited substrate patches. We trapped propagules at different distances (0–10 m) and directions from the source colonies in two experiments: one in a natural habitat within a forest and another involving an artificial set-up in an open habitat. Spore dispersal showed only slight distance dependence both in the open and the forest habitats, presumably as a consequence of wind affecting the dispersal pattern. Gemma dispersal was more strongly distance-dependent in the open habitat than in the forest sites. Considerably more gemmae were deposited during rainy than dry periods, possibly because of the effect of rain drops on gemma release. However, weather conditions had no effect on the dispersal patterns of spores or gemmae. In A. hellerianum, the combination of occasional spore production and practically continuous, massive gemma production facilitates dispersal both on local scale and over long distances. Unlike previously assumed, not only spores but also the asexual propagules may contribute to long-distance dispersal, thus allowing considerable gene flow at the landscape level.     

Development and germination of rhizoidal gemmae of Bryum violaceum

Approximately four to six weeks after transferring gametophores of Bryum violaceum Crundwell & Nyholm to fresh soil, abundant new rhizoids with stalked gemmae were present at the bases of the gametophores. The development of gemmae on rhizoids was followed from single cell initials to multicellular, three-dimensional forms. Mature gemmae, the vegetative diaspores, were pale orange, purple or reddish brown and each had a uniseriate stalk of two to four cells. While remaining on soil, the rhizoidal gemmae showed no in situ germination. However, following the removal of gametophores with rhizoidal gemmae, or rhizoids alone with gemmae, and their placement upon wet filter paper in Petri dishes in light, the gemmae germinated. During germination, the protonemata emerged consistently from mature gemmae at their distal ends, revealing the existence of gemma polarity. In the case of immature gemmae, on the other hand, the protonemata emerged from any surface cell indicating that gemma polarity had not yet been established.     

Comparison of gemma production among three Lophozia species during the growing season

The extent and seasonal pattern of asexual reproduction and ability to germinate in the rare liverwort Lophozia ascendens and the common liverworts L. ventricosa and L. longiflora were studied in the Boubínský prales National Nature Reserve in Šumava Mts. (Bohemian Forest), South Bohemia, Czech Republic. Asexual reproduction was quantified as the number of gemmae produced per individual shoot. Numbers of gemmae per shoot among sampling months differed significantly; increase of gemma production was delayed in L. ascendens in comparison with gemma production of L. ventricosa and L. longiflora. We suggest that gemma production is influenced by environmental factors, mainly air humidity. Germinability of gemmae was low in early spring, highest in August and September and slightly depressed in October. This pattern suggests that rather mild winters in the Czech Republic cause the lower mortality of shoots during winter and the environmental pressure towards the production of dormant gemmae is not a prominent factor affecting the population dynamics of the species under study.     

疣壶藓尖叶变种的芽胞形成及其生态意义

该研究以疣壶藓尖叶变种(Gymnostomiella vernicosa var.acuminata)为实验材料,在人工培养条件下观察并记录其芽胞产生及脱落过程,以揭示该分类群及相关类群的芽胞形成过程和机制,为疣壶藓尖叶变种的分类提供参考性特征指标,明确苔藓植物芽胞的进化和生态学意义。结果显示,疣壶藓尖叶变种的芽胞形成过程划分为3个阶段:(1)茎表皮细胞的外切向壁局部向外隆起,外突的疣壶藓尖叶变种表皮细胞富含叶绿体,可进行光合作用。(2)芽胞起始细胞横向分裂,形成一个基细胞和一个顶细胞;基细胞经数次横向分裂,下部形成柄状结构,上部的细胞常呈喇叭形的“柄托”;顶细胞经多方向细胞分裂,形成椭球状至圆球状芽胞体,形成初期为富含叶绿体的厚壁细胞,成熟后叶绿体数目逐渐减少,最终变成深棕色至红棕色。(3)由薄壁的柄细胞随机破损使芽胞从母株上脱落。研究表明,在无法预测的多变栖息环境中,相对于有较高的能量、遗传和生态消耗的有性生殖,疣壶藓尖叶变种产生芽胞是一种风险分担策略,可以提高生存几率和有效利用资源,可视为一种扩大和维持种群的有效途径。     

Development and Liberation of Cauline Gemmae in the Moss Aulacomnium androgynum (Hedw.) Schwaegr. (Bryales): An Ultrastructural Study

The leafy shoots of the mossAulacomnium androgynum form clustersof gemmae borne terminally on long pseudopodial axes. The gemmaearise from single initial cells produced by the activity ofa superficial meristem. Mature gemmae comprise an apical anda basal cell with four to seven cells forming two, sometimesthree, tiers in between. The basal cell is connected to thetip of the pseudopodium by a uniseriate filament consistingof an abscission (tmema) cell and a stalk cell. The first divisionof the initial cell produces a proximal cell and a distal cell.The proximal cell elongates without further division formingthe stalk of the gemma; the distal cell gives rise to a lowerand an upper cell by transverse division. The upper cell dividesrepeatedly by oblique septa forming the apical and middle cellsof the gemma; the basal cell and tmema cell arise from a transversedivision of the lower cell. The first two divisions in gemmadevelopment are highly asymmetrical and exogenous, i.e. preceededby cell expansion. A broad interphase cortical band of microtubulesis associated with intercalary cellular growth during this stage.Subsequent gemma development follows an endogenous pattern withcellular expansion following the completion of proliferativedivisions and involving a conventional system of cortical microtubules.While elongating to about four times its original length withoutdeposition of a distinct new wall the tmema cell undergoes cytoplasmicdegeneration and eventually breaks, causing gemma liberation.The stalk cell elongates about eight-fold and its contents alsodegenerate after gemma liberation. Plasmodesmata in the basaland stalk cells are obliterated by the deposition of additionalwall materials. The highly electron-opaque outer walls of themature gemmae and tips of the stalk cells are water-repellant.The gemmae are dispersed either in water films or by air currents. Abscission; asexual reproduction; bryophytes; morphogenesis; microtubules; ultrastructure     

APICAL DOMINANCE,POLARITY, AND ADVENTITIOUS GROWTH IN MARCHANTIA POLYMORPHA

Benzylaminopurine, indoleacetic acid, and the auxin inhibitors transcinnamic acid, triiodobenzoic acid, and dinitrophenol were employed to elucidate the role of apical dominance and hormones during regeneration of thalli and gemmae of Marchantia polymorpha L. The cytokinin suppressed normal gemma germination and led to the development of nodular, callus-like growths. When removed from the influence of benzylaminopurine, the site and magnitude of normal thallus outgrowths varied with the length of time that the tissue had remained on the cytokinin-containing medium. This aberrant germination was not influenced by the incorporation of indoleacetic acid into the medium. Exogenous auxin neither accelerated nor inhibited the regeneration of normal thallus growth on excised vegetative discs. Transcinnamic acid and dinitrophenol inhibited regeneration. Auxin reversed this suppression. Triiodobenzoic acid did not significantly affect regeneration. Autoradiographs demonstrated a pronounced accumulation of labelled auxin in the midribs and the acropetal regions of excised thallus discs. This evidence suggests that there is an endogenous, basipetal auxin gradient in Marchantia; that the maintenance of this gradient is vital to normal growth and regeneration of the thallus; and that high endogenous concentrations of cytokinin destroy this polarity by increasing the auxin-synthesizing capacity of the tissue.     

LIGHT-DEPENDENT INHIBITION OF CELL DIVISION AND RHIZOID ELONGATION IN GEMMAE OF THE FERN,VITTARIA GRAMINIFOLIA

Gametophytes of the shoe-string fern Vittaria graminifolia produce linear, six-celled propagules called gemmae. The terminal cells of each gemma elongate into primary rhizoids in culture, and the inner body cells divide asymmetrically to produce prothallial or rhizoid initials. The initiation of both asymmetric cell division and rhizoid elongation is delayed by light intensities greater than 2 w/m². The maximal rates of cell division and rhizoid elongation are unaltered. A 24-hr pulse of high light intensity delays cell division and rhizoid elongation to the same extent, whenever applied during the first 3 d of culture. The model we propose for cell division hypothesizes the existence of a preparatory phase of finite duration prior to mitosis that is sensitive to light intensity. If a cell is irradiated by light intensities greater than 2 w/m² while in the preparatory phase, its entrance into mitosis is delayed. A similar model is proposed for the initiation of rhizoid elongation. Despite the fact that both cell division and rhizoid elongation are dependent on photosynthesis, direct measurements of CO₂-uptake rates show that the inhibitory effects of high light intensities are not due to an inhibition of photosynthesis.     

PROBLEMS IN THE IDENTITY AND ORIGIN OF THE APPALACHIAN VITTARIA GAMETOPHYTE,A SPOROPHYTELESS FERN OF THE EASTERN UNITED STATES

Populations of a fern gametophyte presumed to be of the genus Vittaria occur commonly in the uplands of the southeastern United States. The gametophytes occur on non-calcareous rock outcrops of various composition in areas which provide continuous moisture and protection from temperature extremes. Gametophytes in these habitats are robust and long lived, frequently forming the dominant vegetation in areas covering several square feet. Reproduction is exclusively vegetative by production of gemmae. Although sex organs are present in most populations, viable sporophytes are never produced. The gametophytes have previously been considered most likely to be V. lineata, which occurs in Florida, but morphological and physiological comparisons do not support that conclusion. Significant differences between Appalachian and Florida gametophytes occur in growth form, growth rate, cold hardiness, sporophyte production, and patterns of gemma production. The distribution of the Appalachian gametophytes correlates with old, unglaciated land masses suggesting antiquity rather than recent introduction. Present evidence favors the interpretation that the Appalachian Vittaria gametophytes either belong to a tropical American species from which they have long been separated, or that they represent a distinct species of which the sporophyte no longer exists.     

Central projections of the sensory hairs on the gemma of the ant Diacamma: substrate for behavioural modulation?

In the ant genus Diacamma, all workers eclose from their cocoons with little clublike thoracic appendages, called gemmae. Whether these gemmae are mutilated determines individual behaviour, and ultimately reproductive role, in two of the three species examined. The gemmae are covered with sensory hairs, which probably serve a mechanoreceptive function. The sensory afferents arising from these hairs were stained and traced into the central nervous system (CNS). They feature widely distributed collaterals invading all three thoracic ganglia as well as the suboesophageal and the second abdominal ganglia. The multisegmental arborization pattern of the gemma afferents is very similar to that of wing-hair afferents of other ants (queens and males) or other insects in general. This implies that gemmae and wings are homologous structures. We discuss the morphology of the gemma afferents with respect to their possible involvement in the behavioural changes associated with mutilation. The neuronal processing may be modulated by (1) the decrease of sensory input onto interneurons (suggested by the afferents' extensive arborizations); or (2) by the effect of neuromodulatory substances (suggested by the finding that terminals occur within the cell body rind of the ganglion).     

Quinones and respiratory activity in Spoendonema epizoum

Conidia, gemmae, submerged mycelium and surface cultures of different ages of Sporendonema epizoum were analyzed for isoprenoid quinones. Respiratory activities of the fungus, metabolizing endogenous substrates and glucose, were determined. Q-9 was the only quinone detected; its concentration, lowest in conidia, increases during germination and then decreases in submerged culture. Q-9 concentrations in surface cultures are much higher than those in submerged cultures and are not related to culture age. Changes in Q-9 concentration and respiratory activity are roughly parallel; a peak is reached during gemma formation.     

Reproductive modes in the epixylic hepatic Anastrophyllum hellerianum

The frequency and dynamics of sexual and asexual reproduction were investigated in a dioecious epixylic hepatic, Anastrophyllum hellerianum, which has declined in recent decades in Finland as a consequence of forestry practices. In our investigation asexual reproduction by gemmae was the dominant mode of reproduction and specialised gemmiparous shoots were present in all colonies studied. The proportions of dead shoots were considerably higher among sex-expressing than among non-sexexpressing shoots. Our results suggest that lower reproductive investment is required for asexual than for sexual reproduction. For instance, no trade-off is detected between asexual reproduction and survival of the gemmiparous shoots in A. hellerianum. Sexual reproduction occurred only in 12% of the colonies and it was promoted by the following factors: medium shoot density, high proportion of sex-expressing shoots, an even sex ratio and very short distances between individuals representing opposite sexes. The ratio of dead males to dead females was significantly female-biased, which suggests higher mortality among female shoots. At the level of individual shoots, more spores than gemmae were produced. However, as a consequence of the low frequency of sporophyte-bearing shoots, gemma production highly exceeded spore production at the colony level. Furthermore, cultivation tests of the propagules showed that gemmae germinate faster than spores.     

The control of in vitro direct main stem formation of Lilium longiflorum derived from receptacle culture, and rapid propagation by using in vitro stem nodes

The control of in vitro direct main stem formation by culturing receptacles, and a protocol for the micropropagation of Lilium longiflorum using in vitro main stem nodes derived from receptacle culture were developed. Receptacles from flowers cultured on MS medium containing 1.0 mg l^–1 gibberellic acid (GA₃) and 0.5 mg l^–1 6-benzyladenine (BA) resulted in direct main stem formation after 3 months culture. These stems were isolated and cut into nodal stem segments, which were then cultured on MS medium supplemented with 0.2 mg l^–1 BA. Shoots formed on each node after one month culture. These shoots were subcultured on MS medium containing 0.5 mg l^–1 BA for their mass propagation. An average of 30 vigorous and uniform shoots were formed per single shoot after each subculture. A cyclic and continuous system of propagation by multiplication of shoots was developed. Shoots were rooted on 1/2 MS medium containing 0.2 mg l^–1-naphthaleneacetic acid (NAA). One hundred plantlets that were acclimatized in the greenhouse had a 100% survival. A comparison was made with the traditional culture of explants derived from bulb-scales and with that from main stems.     

喀斯特山区粗裂地钱风兜亚种冬季芽胞杯的形态多样性特征及芽胞传播观察

芽胞杯是地钱属特有的无性繁殖器官,关于其冬季形态特征及繁殖传播的行为研究较少。现以贵州喀斯特山区常见的粗裂地钱风兜亚种（Marchantia paleacea subsp.diptera）为代表,在最冷的冬季1月份,对其芽胞杯、杯内芽胞产量及传播方式进行野外定点观察和采样分析。结果显示：（1）冬季芽胞杯形态多样。根据其颜色和杯内芽胞特点将其划分为4个生长时期：未成熟期（透明）、成熟期（绿色）、衰退期（紫色）和衰亡期（紫黑色）,反映了冬季芽胞杯生长发育的不同阶段。（2）各生长时期的芽胞杯数量不同,表现出有序的凋亡特征。在统计的708个芽胞杯中,4个时期芽胞杯数量分别为62、209、254和183个,且不同时期的芽胞杯内芽胞的平均产量明显不同,不同时期单杯芽胞的平均产量分别为42、131、87和0 个;（3）冬季芽胞杯及芽胞在配子体上的密度较高,每平方米分别达到10 139和754 889个;（4）除春夏季常见的被雨滴敲打传播外,通过重力作用传播是冬季芽胞的一种重要传播方式。冬季粗裂地钱风兜亚种配子体上的芽胞杯处在不同的生长时期,形成的芽胞仍十分丰富,这对该物种适应喀斯特山区最冷月严苛环境条件具有积极的意义。     

Elimination of ornithogalum mosaic virus in the Ornithogalum cv. Rogel through meristem tip culture and chemotherapy

Virus-indexed Ornithogalum ev. Rojel plants were produced by eliminating Ornithogalum mosaic virus (OMV) through meristem-tip culture. The best plantlet regeneration was obtained from meristems derived from adventitious buds which developed on leaf explants taken from mother plants at the flowering stage. Acyclovir had no effect as an anti-viral compound on plantlet regeneration or virus elimination. Adenine arabinoside retarded plantlet development at concentrations of 10 mg l^-1 and higher, while 5.0 mg l^-1 suppressed the virus concentration beneath a detectable level in young plants. All the mature plants, however, tested positively for the presence of OMV.Abbreviations BA 6-benzyladenine - NAA -naphthaleneacetic acid - OMV Ornithogalum mosaic virus     

中国新记录种——摩拉维采真藓(新拟)

该文首次报道了摩拉维采真藓(新拟)在中国的分布。研究表明:(1)摩拉维采真藓的主要识别特征为:叶常聚集在茎顶成莲座状,叶倒卵状披针形至匙形,具长毛尖,具分化边缘,中肋消失于叶尖下,叶细胞长菱形至六边形,叶腋处着生大量单列细胞构成的分枝或不分枝的丝状芽胞。(2)通过对摩拉维采真藓的命名和系统位置的讨论,确认该种是真藓属细叶真藓组的有效种。(3)摩拉维采真藓与近缘种细叶真藓和幽美真藓有诸多相似特征:莲座状的茎顶、有分化边和菱形至六边形中上部细胞的倒卵形叶,但该种以具有大量叶腋生丝状芽胞和叶中肋不及顶等特征区别于细叶真藓的无腋生芽胞、叶中肋突出叶尖成长芒状,以具有叶湿时平展、干时卷曲和叶腋有芽胞等特征区别于幽美真藓的叶湿时内凹、干时紧贴于茎和无腋生芽胞;拟三列真藓、圆叶真藓和灰黄真藓的部分种群都曾报道有与摩拉维采真藓相似的腋生丝状芽胞,但摩拉维采真藓的假根集生于植株基部、叶有狭分化边、叶缘平直、中肋消失于叶尖下而区别于拟三列真藓的茎中下部密被假根、叶有宽分化边、叶缘背卷、中肋及顶或短出,摩拉维采真藓有分化边和长毛尖的倒卵状披针形区别于圆叶真藓叶有无分化边和圆钝叶尖的卵圆形叶,区别于灰黄真藓有中肋及顶和短尖的卵状披针形叶;柔叶真藓有与摩拉维采真藓相似、中肋不及顶的叶,但无芽胞而易与新记录种区分。(4)该种在北温带有较广泛分布,形成欧洲-北亚-中亚-西亚和北美两个主要分布区;作者在四川和新疆等地的发现可以推测摩拉维采真藓在中国可能有更广泛的分布。     

Interspecific hybrid between Allium cepa and Allium sativum

Summary Interspecific hybrids between Allium cepa and Allium sativum were obtained using the fertile clone A. sativum as the male parent. The nascent embryos which formed shortly in interspecific hybridization between A. cepa and A. sativum were rescued by ovule culture at an early stage. The zygotes or proembryos developed in Murashige and Skoog medium containing 5.7×10^-8 M indole-3-butyric acid (IBA). Once developed, the embryos were taken out of the ovule and cultured on embryo culture medium where they regenerated into whole plants. The hybridity of the plants obtained was examined by morphological observation, chromosome analysis, and ribosomal RNA gene analysis. The analyses proved that the plants were mature sexual hybrids between A. cepa and A. sativum. Each hybrid plant had keeled but fistulose leaves and formed a bulb resembling that of A. cepa. The hybrids produced not only S-propenyl-l-cysteine sulfoxide, which is the major flavor precursor in A. cepa, but also S-allyl-l-cysteine sulfoxide (alliin), which is characteristic of A. sativum.