Pistillate Stage Research Articles

Divergence in floral scent and morphology, but not thermogenic traits, associated with pollinator shift in two brood-site-mimicking Typhonium (Araceae) species

Flowers which imitate insect oviposition sites probably represent the most widespread form of floral mimicry, exhibit the most diverse floral signals and are visited by two of the most speciose and advanced taxa of insect - beetles and flies. Detailed comparative studies on brood-site mimics pollinated exclusively by each of these insect orders are lacking, limiting our understanding of floral trait adaptation to different pollinator groups in these deceptive systems. Two closely related and apparent brood-site mimics, Typhonium angustilobum and T. wilbertii (Araceae) observed to trap these distinct beetle and fly pollinator groups were used to investigate potential divergence in floral signals and traits most likely to occur under pollinator-mediated selection. Trapped pollinators were identified and their relative abundances enumerated, and thermogenic, visual and chemical signals and morphological traits were examined using thermocouples and quantitative reverse transcription-PCR, reflectance, gas chromatography-mass spectrometry, floral measurements and microscopy. Typhonium angustilobum and T. wilbertii were functionally specialized to trap saprophagous Coleoptera and Diptera, respectively. Both species shared similar colour and thermogenic traits, and contained two highly homologous AOX genes (AOX1a and AOX1b) most expressed in the thermogenic tissue and stage (unlike pUCP). Scent during the pistillate stage differed markedly - T. angustilobum emitted a complex blend of sesquiterpenes, and T. wilbertii, a dung mimic, emitted high relative amounts of skatole, p-cresol and irregular terpenes. The species differed significantly in floral morphology related to trapping mechanisms. Functional specialization and pollinator divergence were not associated with differences in anthesis rhythm and floral thermogenic or visual signals between species, but with significant differences in floral scent and morphological features, suggesting that these floral traits are critical for the attraction and filtering of beetle or fly pollinators in these two brood-site mimics.

A perfume-collecting male oil bee? Evidences of a novel pollination system involving Anthurium acutifolium (Araceae) and Paratetrapedia chocoensis (Apidae, Tapinotaspidini)

It has been known since Stefan Vogel’s observations in 1969 that solitary female oil bees collect fatty floral oils from specialized oil-secreting plants with the aid of hairy patches on either their legs or abdomen, a reward used as food for their larvae and/or to line their brood cells. Similar adaptations are also known from male oil bees, although the purpose of their oil-collecting behavior has not yet been clarified. Here, we describe a novel pollination system involving male Paratetrapedia oil bees and the tropical herb Anthurium acutifolium. We present ultrastructural morphological details of bee and plant structures involved in this interaction and the composition of floral scents likely mediating pollinator attraction. Inflorescences of A. acutifolium were visited almost exclusively by male P. chocoensis oil bees. The bees mopped with a hairy patch of their abdominal sterna 3 across the inflorescence surface. During this activity on both staminate and pistillate stage inflorescences, bees’ abdomens and legs became loaded with pollen and contacted receptive stigmas. In contrast to what has been observed in other angiosperms visited for the collection of fatty floral oils, the inflorescences/flowers of A. acutifolium do not have structures specialized in oil secretion, i.e., elaiophores. These inflorescences, nonetheless, were strongly scented during the time interval they were visited by the bees. Gas chromatography/mass spectrometry (GC/MS) analyses of dynamic headspace floral samples revealed that inflorescences of both anthetic phases emitted scent bouquets consisting mainly of aliphatic esters, indole and uncommmon terpenoids (megastigmanes). Interestingly enough, our data suggest that the unusual floral scent of A. acutifolium is a perfume reward collected by male P. chocoensis oil bees. This pollination system thus bears a remarkable resemblence with the interactions between perfume-collecting male euglossine bees and their preferred flowers, discovered by Stefan Vogel half a century ago.

Floral characteristics and pollination ecology of Manglietia ventii (Magnoliaceae), a plant species with extremely small populations (PSESP) endemic to South Yunnan of China

Manglietia ventii is a highly endangered plant species endemic to Yunnan province in China, where there are only five known small populations. Despite abundant flowering there is very low fruit and seed set, and very few seedlings in natural populations, indicating problems with reproduction. The causes of low fecundity in M. ventii are not known, largely because of insufficient knowledge of the species pollination ecology and breeding system. We conducted observations and pollination experiments, and analyzed floral scents to understand the pollinator–plant interactions and the role of floral scent in this relationship, as well as the species breeding system. Like the majority of Magnoliaceae, M. ventii has protogynous and nocturnal flowers that emit a strong fragrance over two consecutive evenings. There is a closing period (the pre-staminate stage) during the process of anthesis of a flower, and we characterize the key flowering process as an “open-close-reopen” flowering rhythm with five distinct floral stages observed throughout the floral period of this species: pre-pistillate, pistillate, pre-staminate, staminate, and post-staminate. Flowers are in the pistillate stage during the first night of anthesis and enter the staminate stage the next night. During anthesis, floral scent emission occurs in the pistillate and staminate stages. The effective pollinators were weevils (Sitophilus sp.) and beetles (Anomala sp.), while the role of Rove beetles (Aleochara sp.) and thrips (Thrips sp.) in pollination of M. ventii appears to be minor or absent. The major chemical compounds of the floral scents were Limonene, β-Pinene, α-Pinene, 1,8-Cineole, Methyl-2-methylbutyrate, p-Cymene, Methyl-3-methyl-2-butenoate and 2-Methoxy-2-methyl-3-buten, and the relative proportions of these compounds varied between the pistillate and staminate stages. Production of these chemicals coincided with flower visitation by weevils and beetles. The results of pollination experiments suggest that M. ventii is pollinator-dependent, and low seed set in natural populations is a result of insufficient pollen deposition. Thus, conservation of the species should focus on improving pollination service through the introduction of genetically variable individuals and increase in density of reproducing trees.

Reproductive biology of Magnolia sinica (Magnoliaecea), a threatened species with extremely small populations in Yunnan, China

Magnolia sinica is one of the most threatened trees endemic to Southeast Yunnan. Based on our investigations, only 52 individuals and eight populations are found in the wild. M. sinica has been categorized as Critically Endangered on the IUCN Red List and identified as a “Plant Species with Extremely Small Populations (PSESP)”. Its fruit/seed set is very low and seedlings are rarely found in the wild. It is hypothesized that it may encounter obstacles to reproductive success. This study, therefore, focuses on its reproductive biology, knowledge of which is essential for effective conservation. Flowers of this species are protogynous and nocturnal, and possess a two-day rhythm of sexual presentation. For the first night of anthesis, the flowers are in the pistillate stage during which tepals open at dusk and close approx. 1 h later (except for the open outer ones). They remain closed until the next afternoon, when flowers, now in the staminate stage, re-open and remain so until the tepals drop. Nocturnal beetles enter into the flowers and remain trapped throughout the night as the flower closes, during which time they feed on tepals. Pollen-gathering bees are found to visit the re-opened flowers and the beetles are released during this stage. Two species of Pleocomidae and Curculionidae beetles appear to be effective pollinators.M. sinica is a self-compatible, pollinator-dependent species, and its fruit/seed set can be significantly increased by hand-pollination. No functional seed dispersers have been found in its extant natural habitats. These findings suggest that it may face both pollination and seed disperser insufficiencies in its current fragmented habitats, which may account for its low regeneration. Here we propose conservation strategies based on our findings.

Floral thermogenesis: An adaptive strategy of pollination biology in Magnoliaceae

Floral thermogenesis plays a crucial role in pollination biology, especially in plant–pollinator interactions. We have recently explored how thermogenesis is related to pollinator activity and odour release in Magnolia sprengeri. By analyzing flower temperatures, emission of volatiles, and insect visitation, we found that floral blends released during pistillate and staminate stages were similar and coincided with sap beetle visitation. Thus, odour mimicry of staminate-stage flowers may occur during the pistillate stage and may be an adaptive strategy of Magnolia species to attract pollinators during both stages, ensuring successful pollination. In addition to the biological significance of floral thermogenesis in Magnolia species, we explored the underlying regulatory mechanisms via profiling miRNA expression in M. denudata flowers during thermogenic and non-thermogenic stages. We identified 17 miRNAs that may play regulatory roles in floral thermogenesis. Functional annotation of their target genes indicated that these miRNAs regulate floral thermogenesis by influencing cellular respiration and light reactions. These findings increase our understanding of plant–pollinator interactions and the regulatory mechanisms in thermogenic plants.

Thermogenesis, flowering and the association with variation in floral odour attractants in Magnolia sprengeri (Magnoliaceae).

Magnolia sprengeri Pamp. is an ornamentally and ecologically important tree that blooms at cold temperatures in early spring. In this study, thermogenesis and variation in the chemical compounds of floral odours and insect visitation in relation to flowering cycles were studied to increase our understanding of the role of floral thermogenesis in the pollination biology of M. sprengeri. There were five distinct floral stages across the floral cycle of this species: pre-pistillate, pistillate, pre-staminate, staminate and post-staminate. Floral thermogenesis during anthesis and consisted of two distinct peaks: one at the pistillate stage and the other at the staminate stage. Insects of five families visited M. sprengeri during the floral cycle, and sap beetles (Epuraea sp., Nitidulidae) were determined to be the most effective pollinators, whereas bees (Apis cerana, Apidae) were considered to be occasional pollinators. A strong fragrance was released during thermogenesis, consisting of 18 chemical compounds. Although the relative proportions of these compounds varied at different floral stages across anthesis, linalool, 1-iodo-2-methylundecane and 2,2,6-trimethyl-6-vinyltetrahydro-2H-pyran-3-ol were dominant. Importantly, we found that the floral blends released during the pistillate and staminate stages were very similar, and coincided with flower visitation by sap beetles and the two thermogenic episodes. Based on these results, we propose that odour acts as a signal for a reward (pollen) and that an odour mimicry of staminate-stage flowers occurs during the pistillate stage.

Evolutionary steps in the reproductive biology of Annonaceae

Flowers of Annonaceae are characterized by fleshy petals, many stamens with hard connective shields and numerous carpels with sessile stigmas often covered by sticky secretions. The petals of many representatives during anthesis form a closed pollination chamber. Protogynous dichogamy with strong scent emissions especially during the pistillate stage is a character of nearly all species. Scent emissions can be enhanced by thermogenesis. The prevailing reproductive system in the family seems to be self-compatibility. The basal genus Anaxagorea besides exhibiting several ancestral morphological characters has also many characters which reappear in other genera. Strong fruit-like scents consisting of fruit-esters and alcohols mainly attract small fruit-beetles (genus Colopterus, Nitidulidae) as pollinators, as well as several other beetles (Curculionidae, Chrysomelidae) and fruit-flies (Drosophilidae), which themselves gnaw on the thick petals or their larvae are petal or ovule predators. The flowers and the thick petals are thus a floral brood substrate for the visitors and the thick petals of Anaxagorea have to be interpreted as an antipredator structure. Another function of the closed thick petals is the production of heat by accumulated starch, which enhances scent emission and provides a warm shelter for the attracted beetles. Insight into floral characters and floral ecology of Anaxagorea, the sister group of the rest of the Annonaceae, is particularly important for understanding functional evolution and diversification of the family as a whole. As beetle pollination (cantharophily) is plesiomorphic in Anaxagorea and in Annonaceae, characters associated with beetle pollination appear imprinted in members of the whole family. Pollination by beetles (cantharophily) is the predominant mode of the majority of species worldwide. Examples are given of diurnal representatives (e.g., Guatteria, Duguetia, Annona) which function on the basis of fruit-imitating flowers attracting mainly fruit-inhabiting nitidulid beetles, as well as nocturnal species (e.g., large-flowered Annona and Duguetia species), which additionally to most of the diurnal species exhibit strong flower warming and provide very thick petal tissues for the voracious dynastid scarab beetles (Dynastinae, Scarabaeidae). Further examples will show that a few Annonaceae have adapted in their pollination also to thrips, flies, cockroaches and even bees. Although this non-beetle pollinated species have adapted in flower structure and scent compounds to their respective insects, they still retain some of the specialized cantharophilous characters of their ancestors.

Accumulation and scavenging of reactive oxygen species and nitric oxide correlate with stigma maturation and pollen–stigma interaction in sunflower

During the course of stigma development in sunflower (bud, staminate and pistillate stages), correlation is evident among the accumulation of reactive oxygen species (ROS), nitric oxide (NO), and the activities of ROS scavenging enzymes [superoxide dismutase (SOD) and peroxidase (POD)]. Confocal image analysis shows a gradual increase in ROS and NO accumulation in the stigmatic papillae, which may provide immunity to the developing stigma and function as signalling molecules. A novel, NO-specific probe (MNIP-Cu) has been employed for the detection and quantification of intracellular NO. Mn-SOD (mitochondrial) and Cu/Zn-SOD (cytoplasmic) exhibit differential expression during the staminate stage of stigma development. An increase in total SOD activity at the staminate stage is followed by a peak of POD activity during pistillate stage, thereby indicating the sequential action of the two enzymes in scavenging ROS. An increase in the number of POD isoforms is observed with the passage of stigma development (from three at bud stage to seven at pistillate stage), and two POD isoforms are unique to pistillate stage, thereby highlighting their role in ROS scavenging mechanism. ROS and NO accumulation exhibit reverse trends during pollen–stigma interaction.

Pollination and floral scent differentiation in species of the Philodendron bipinnatifidum complex (Araceae)

The Philodendron bipinnatifidum complex of Philodendron subgenus Meconostigma may comprise four species, which because of only slight and not very distinct morphological differences are not all unanimously recognized as good species. To find out whether these species are reproductively isolated, we studied the flowering rhythm, thermogenesis and pollination biology of three species of this complex, namely of P. bipinnatifidum, P. aff. bipinnatifidum (provisionally named “P. form selloum”) and P. mello-barretoanum in Brazil. Of the first two mentioned taxa, floral scent was collected and scent compounds were identified by GC–MS. The results showed that the coastal forest species P. bipinnatifidum has a two-, or three-night flowering rhythm, with the pistillate stage in the first night and the staminate stage lasting the second and sometimes also the third night. Strong thermogenesis with extended heating periods of several hours during the first part of the usual two subsequent nights and the maximum temperatures of up to 40 °C absolute heating of the spadices occurred in the pistillate and staminate stages. Concomitant with the heating periods, relatively low amounts of principally (Z)-2-pentenyl acetate and (Z)-jasmone were emitted by both the pistillate and staminate stage inflorescences. The dynastid scarab beetle Cyclocephala variolosa was the only pollinator attracted. The upland forest P. form selloum always had a two-night flowering rhythm with the pistillate stage in the first and the staminate stage in the subsequent night. This world-record holder of thermogenesis can heat up to the remarkable 45 °C during a relatively short period in the evening of the pistillate stage. During the thermogenic period, enormous amounts of principally 4-methoxystyrene and 3,4-dimethoxystyrene were produced and which could attract a large number of female and male individuals of the dynastid scarab beetle Erioscelis emarginata. In the staminate stage of P. form selloum, temperature elevation is significantly lower and the scent compounds are different from the pistillate stage. The cerrado biome species P. mello-barretoanum has a flowering rhythm similar to P. form selloum, reaching a maximum heating of about 40 °C during the pistillate stage. The sole pollinator attracted was Cyclocephala atricapilla. The differences observed and analyzed among the taxa, including the flowering rhythm, thermogenic activities, scent compounds emitted, pollinating dynastid scarab beetles attracted, as well as slight morphological differences and apparent geographical exclusiveness noted in these three taxa are strong indicators that P. bipinnatifidum, P. form selloum and P. mello-barretoanum are different enough to be considered good species. The morphological affinities of these species might be a hint that speciation has been a recent event and/or also that reproductive isolation based on different, non-overlapping distribution areas, different scent compounds and different pollinators was effective enough to need further morphological differentiation.

Pollination ecology of Magnolia ovata may explain the overall large flower size of the genus

Flowering and fruiting biology of Magnolia ovata was studied in Atlantic forests in the interior of São Paulo State, Brazil. The large, bisexual flowers are protogynous, nocturnal, thermogenic and emit a strong scent in two consecutive evenings. In the first night of anthesis, the flowers are in the pistillate stage and thermogenesis starts at about sunset and lasts about 3h. In the second night, the flowers enter the staminate stage and produce heat for 4h. Heat is generated by the petals, gynoecium and anthers. Temperatures measured inside the petals reach 26.7°C and 31.9°C in the pistillate and staminate stages, 6.0 and 10.6°C above ambient air, respectively. In the pistillate stage, the perianth opens after sunset and closes tightly a few hours later, and remains closed until the next evening. The initial opening and closing, however, is not synchronous for all flowers during the night. In the following evening, flowers in the staminate stage again open and remain so until the petals drop. Scent compounds, analyzed by GC–MS, contain C5-branched chain compounds, aliphatics, benzenoids and monoterpenoids. Emission of the most prominent compound, C5-branched methyl 2-methyl butyrate, commences before flower opening and continues throughout anthesis, but is accentuated in the thermogenic pistillate and staminate stages. Female and male individuals of only one beetle species, the dynastid scarab Cyclocephala literata, are attracted to the scented flowers in both pistillate and staminate stages. Once inside the flowers they feed on the petals and mate. Tests with synthetic methyl 2-methyl butyrate indicate that this compound is a strong attractant for the beetles. Because this scent compound is strongly emitted in both pistillate and staminate stages, the beetles fly indiscriminately between flowers of both stages. This behavior enhances pollen mixing and effective cross-pollination of the self-compatible species. The evolutionary history of Magnolia appears to be influenced by an ancestral condition of dynastid scarab beetle pollination. Large magnolia flowers are best explained as an archaic structure resulting from the initial association of tropical American species of section Talauma with large and voracious dynastid beetles.

Pollination of ‘criollo’ avocados (Persea americana) and the behaviour of associated bees in subtropical Mexico

SUMMARYThe flowering characteristics, contribution of self versus cross pollination and bee visitation were studied in ‘criollo’ avocado cultivars in Yucatán, Mexico. The pattern of pistillate and staminate stages (I and II, respectively), stigma receptiveness and the production of fruit in bagged and open panicles was followed in two commercial orchards. The number of bee visits across the day and the number of pollen grains on the body of workers of honey bees and two native stingless bee species were compared. Environmental variables were also recorded to determine their influence on bee visit rates. Few stigmas were morphologically receptive during the second day of opening of individual flowers (stage II). There was no overlapping between stages I and II. Open panicles produced significantly more fruit (60% on average) than bagged ones. Bee visits to flowers of both type A and B cultivars were significantly more frequent during the pistillate stage than during the staminate stage. Apis mellifera had an overall higher frequency of visits compared with the other two species. The number of pollen grains on the bees was not statistically different between A. mellifera (1644 ± 90.1 [s.d.]) and Trigona nigra (1356 ± 71.3) but it was significantly less in Nannotrigona perilampoides (781 ± 16.1). Environmental humidity had an influence on the visits of A. mellifera (r = −0.88) and N. perilampoides (r = 0.92) but not on T. nigra. It is concluded that self pollination is of limited influence in fruit production in avocado cultivars in subtropical Mexico and that both native stingless bees and honey bees potentially are efficient pollinators of this crop.

GROWTH STAGE, FLOWERING PATTERN, YIELD, AND HARVEST DATE PREDICTION OF FOUR TYPES OF CUCUMBER TESTED AT 10 PLANTING DATES

GROWTH STAGE, FLOWERING PATTERN, YIELD, AND HARVEST DATE PREDICTION OF FOUR TYPES OF CUCUMBER TESTED AT 10 PLANTING DATES

The Cost of Flowers in Nigella degenii Inferred from Flower and Perianth Removal Experiments

I examined the costs of producing and maintaining floral structures in Nigella degenii (Ranunculaceae). To test for such costs, I subjected plants of two populations to flower or perianth removal and then evaluated allocation to subsequently produced flowers, fruits, and seeds. Based on data from a flower removal experiment, the amount of resources allocated to flowers during the staminate and pistillate stages was about half as great as the amount of resources devoted to fruit maturation. Plants on which the perianths were removed at the bud stage produced more flowers or had a higher percent fruit set and seed viability than did plants from which the perianths were removed immediately after the growth of the sepals and petals. Removal of fully developed perianths at the onset of flowering increased flower number, fruit set, or seed viability relative to plants on which the perianths remained until senescence. Some of these effects were population specific, but plants in low and high watering treatments responded similarly to perianth removal. Given these observations, I propose that the costs of producing and maintaining floral structures represent a significant drain on the water and/or energy budget of a N. degenii plant and that some trade‐offs may be specific to the population in which they have been detected.

Floral fragrance compounds of some anthurium (ARACEAE) species and hybrids

Floral fragrance compounds of five species Anthurium armeniense, A. fragrantissimum, A. lindenianum, A. ochranthum and A.roseospadix, and three hybrids of A. armeniense were studied using headspace adsorption and solvent extraction for sample preparation followed by capillary GC and GC-MS analysis. Twenty eight compounds were identified and their levels (%) assessed at the scent-emitting pistillate stage of the spadix. Variation of compounds in the headspace in response to the time of day were determined in a hybrid, A. antioquiense × ‘Tatsuta Pink Obake’ (UH1299), which emitted strong fragrance all day. Chemical profiles from the headspace and from spadices soaked in CH2Cl2 were compared in three species. Combination of these two data sets would provide more complete chemotaxonomic profiles in Anthurium.

The Pollination Biology of Tuckahoe, Peltandra virginica (Araceae)

This study describes the specialized brood-site-based pollination system of Peltandra virginica Kunth (Araceae) with the chloropid fly Elachiptera formosa Loew and provides experimental evidence that gender-related changes in floral odor composition synchronize pollinator behavior with the blooming sequence. P. virginica is protogynous and does not self-pollinate because of a strong temporal separation in sexual function, and it is dependent upon insects for pollination because the spathe completely surrounds the spadix during the pistillate stage. Field observations conducted in central New Jersey and southeastern Pennsylvania showed that E. formosa is closely associated with P. virginica inflorescences. Within the floral chamber, E. formosa adults feed on pollen, mate, and find oviposition sites while the larvae complete their development. Although drosophilid and syrphid flies were collected and reared from inflorescences of both sexual stages, only E. formosa emerged from pistillate-stage inflorescences and adult E. formosa rapidly transferred fluorescent dye particles between inflorescences of both sexual stages in laboratory enclosures. These findings indicate that this fly species is the primary pollinator at our study locations. Field censuses demonstrated that although E. formosa visited P. virginica inflorescences of both sexual stages with equal frequency, the female flies preferentially oviposited within pistillate-stage inflorescences. Analysis of floral volatiles with gas chromatography-mass spectrometry showed that only the spathes emit the floral fragrance and that the composition of the floral volatiles changed during the transition from pistillate to staminate stage. A particularly noticeable change occurred in the emission ratio of the two primary floral odor components, 4,5,7-trimethyl-6,8-dioxabicyclo[3.2.1.]octane and an unidentified chemical analog with molecular weight of 142. The relationship between floral volatile composition and fly oviposition behavior was evaluated using sham inflorescences placed in the P. virginica study population. The sham inflorescences were constructed by covering spadices dissected from mature inflorescence buds with spathes excised from different-age inflorescences. After a 2-hr-long exposure period, sham inflorescences fitted with pistillate-stage spathes contained more E. formosa eggs than the sham inflorescences with staminate-stage spathes. However, the highest number of ovipositions occurred in sham inflorescences fitted with spathes excised from mature inflorescence buds. The fragrance emitted by these spathes was composed almost entirely of the two principal odor components. The decline in ovipositions observed in both real and sham inflorescences corresponded to an increase in the emission ratio of 4,5,7-trimethyl-6,8-dioxabicyclo[3.2.1.]octane to the molecular weight 142 analog. This suggests that gravid flies searching for oviposition sites used the ratio of the two primary floral volatile components as an inflorescence gender or age recognition cue.

Expression of Andromonoecy and Pollination of Tuberous Lomatiums (Umbelliferae)

Tuberous lomatiums (Lomatium, Umbelliferae) constitute a well-de- fined infrageneric group that is characterized by adaptation to a short, early spring growing season followed by a long summer drought. All tuberous lomatiums are andromonoecious and all exhibit 'a pattern of increasing ratios of hermaphroditic to staminate flowers in successively maturing inflorescences. Floral visitation by a variety of Diptera, Hymenoptera, and Coleoptera effects outcrossing and may also mediate geitonogamous selfing. Xenogamy is promoted by protogyny of hermaphroditic flowers, the arrangement of staminate and perfect flowers, and the sequence of floral maturation. Geitonogamy is favored by the morphology of the inflorescences, overlap of the staminate and pistillate stages of different flowers on the same indi- vidual, and the behavior of pollinators. Andromonoecy and high pollen production favor both xenogamy and geitonogamy. The floral features of tuberous lomatiums ensure relatively high levels of seed production even though abundance and activity of pollinators may be limited by harsh environmental conditions. The characteristic and comparatively uniform inflorescences of the Umbelliferae undoubtedly contributed to early recognition of this nat- ural family. Apparent uniformity may account, in part, for what until recently has been a paucity of investigations into the biological impor- tance of floral features of umbellifers. Studies by Bell (1971), Bell and Lindsey (1978), Brehm (1979), Cruden (1976), Cruden and Hermann- Parker (1977), Lindsey (1982), Lloyd (1973, 1980a, b), Schlessman (1978, 1980), and Webb (1979, 1980) have revealed considerable variation with

Inheritance of Sex Expression from Crosses of Dioecious Cucumber (Cucumis sativus L.)1

Abstract The hybrids and segregating populations obtained by crossing 4 gynoecious with 4 androecious lines were analyzed to determine the inheritance of sex expression in dioecious cucumber (Cucumis sativus L.). Sex expression of all F1 hybrids was characterized by plants with a continuous pistillate stage of flowering on the main stem. This included both gynoecious and predominantly female plants. No reciprocal cross differences were observed in the F1 and backcross generations. Backcrosses to the gynoecious parents produced plants with a continuous female stage. Backcrosses to the androecious parent produced plants with continuous pistillate, monoecious (without a continuous pistillate stage), and androecious expression in a 2:1:1 ratio, respectively. The F2 generation segregated 12:3:1 continuous pistillate, monoecious, and androecious phenotypes, respectively. Two major loci, a and acr, with epistasis are proposed to control sex expression. The a locus permits male (aa) as opposed to female (A –) flower expression. The acr locus conditions the intensity of femaleness.

Study of the inheritance of sex expression in the cucumber. The interaction of major genes with modifying genetic and non-genetic factors

Two major genes and a complex of polygenes affecting sex expression inCucumis sativus L. as well as their interaction with some nongenetic factors were investigated in the present study. The genetic factorst was found to affect sex by inducing a shift of the predetermined flowering pattern of this plant, in the direction of its base. As this pattern is composed of a staminate stage followed by a mixed (staminate-pistillate) stage and a pistillate stage, a double dose ofst will induce a change from the normal monoecious sex expression (to absolute gynoecism. The second major gene studied,m, known previously to control sex in the individual flower (m/m—andromonoecious,M—monoecious), also interacts with factors affecting the flowering pattern by inducing male tendency. Using an ordinary commercial monoecious stock additional modifying factors for sex expression were demonstrated, by eight generations of selection for high (in male direction) and for low (in female direction) node number to the first pistillate flower. A comparison of the means and frequency distributions of node number of the two selected lines, their F1 and F2, indicated polygenic control of this character. It was found that two non-genetic factors, day length and gibberellic acid (GA), may mimic the genetic factors for sex expression. Furthermore, evidence was presented, indicating that this sex controlling ability of GA and the modifying genes may be based on physiological conditions common to those two genetic and nongenetic factors. The overall mechanism of sex control in the cucumber and in other plants was discussed and a hypothetic model for the evolution of dioecism in flowering plants was proposed.