YEAR-TO-YEAR CHANGES IN PHENOTYPIC GENDER IN A MONOECIOUS CUCURBIT,APODANTHERA UNDULATA

Year-to-year variation in phenotypic gender in the monoecious cucurbit, Apodanthera undulata Gray was investigated. Small plants produce no flowers. Larger plants produce only staminate flowers (“male” plants), while a somewhat greater threshold size is necessary for pistillate flower production (cosexual plants). Approximately 85% of the plants that bloomed did not change gender group between years. Two measures of phenotypic gender were used: prospective femaleness, a measure standardized to the population floral ratio, and morphological femaleness, an unstandardized measure. Femaleness of cosexes between years was positively correlated; r values were somewhat greater when using morphological femaleness values. Plants that opened only staminate flowers one year were likely to open only staminate flowers the next year. Similarly, cosexes were likely to be cosexes again the following year, with similar femaleness values. Beyond the threshold size for pistillate flower production, plant size was not correlated with femaleness. These patterns suggest that all plants are male until they reach a certain size and that plants in their cosexual phase may have an intrinsic femaleness tendency due to either genotype or microsite effects.     

Modification of sex expression in sponge gourd (Luffa cylindrica L.) roem by mineral nutrient treatments

Effects of three major mineral nutrient elements, nitrogen, phosphorous and potassium, on sex expression in Luffa cylindrica (L.) Roem were investigated in relation to nodal location of the first male and the first female flower on the plant as well as the ratio of the male-female flowers, otherwise known as sex ratio.Potassium-containing nutrient treatments (K, NK & PK) reduced the mean node number of the first staminate inflorescence but did not change the mean node number of the first pistillate flower. These treatments promoted staminate flowering and caused a shift towards maleness while nitrogen- and phosphorous-containing nutrient treatments (N, P, NP & NPK) promoted pistillate flowering and reduced staminate flowering, causing a shift towards femaleness. These results are explained in terms of hormonal balance in the plant. The possible role of mineral nutrients in hormonal balance and thereby in sex expression is discussed.     

Increase in femaleness of three cucurbits by treatment with Ethrel,an ethylene releasing compound

Summary The application of Ethrel (2-chloroethane phosphonic acid), an ethylene-releasing compound, to monoecious cultivars of cucumber and squash and an andromonoecious cultivar of muskmelon, caused a shift towards femaleness in all three species. The increase in femaleness manifested itself in several symptoms: a decrease in the number of staminate (male) flowers, an increase in the number of pistillate (female) or hermaphrodite (perfect) flowers, and a change in flowering pattern, namely, formation of female flowers at lower nodes in cucumber and squash, and formation of hermaphrodite flowers on the main axis in muskmelon where normally only male flowers are formed in this cultivar.     

WITHIN- AND AMONG-TREE VARIATION IN FLOWER AND FRUIT PRODUCTION IN TWO SPECIES OF CARYA (JUGLANDACEAE)

The patterns of variability in the production of staminate inflorescences, pistillate flowers, and fruits for Carya ovata (Mill.) K. Koch and C. tomentosa (Poir.) Nutt. at Hutcheson Memorial Forest (Somerset County, New Jersey) were examined over a four-year period. We sought to determine 1) the patterns of variability in flowering and fruiting (within-trees, among-trees, and between-years), 2) if variable flowering might account for observed nut-bearing patterns, and 3) what were the relative contributions of intrinsic (genetic) and/or extrinsic (environmental) factors in determining flower production and fruit set. In general, our fine-scale analysis of shoots within canopies did not reveal a distinct mast fruiting pattern. While the number of male and female flowers varied significantly (among trees and between years), fruit set was not markedly affected. Variability of flowering and fruiting among shoots within trees was low. Both flowering and fruiting were observed to have a strong genetic and/or microenvironmental basis; however, flowering appeared more plastic and thus more sensitive to yearly environmental vagaries than did fruiting. Favorable environmental conditions at the time of flower differentiation may result in increased flower production; but, this does not necessarily result in increased fruit set. Many pistillate flowers abscise around the time of pollination and fertilization, apparently adjusting fruit set to available shoot resources. Approximately 50% of the pistillate flowers of both species fail to set fruit. Overall, individual trees exhibit their own flowering and fruiting schedules, suggesting the need to account for this level of variability in future studies of mast fruiting.     

POLLINATION ECOLOGY OF A SEAGRASS,THALASSIA TESTUDINUM (HYDROCHARITACEAE), IN ST. CROIX

The reproductive ecology of a dioecious Caribbean seagrass, Thalassia testudinum was studied in St. Croix, U.S. Virgin Islands. Flowering in Thalassia coincided with spring tides with anthesis and anther dehiscence in staminate plants occurring at night. As predicted by search theory, pollen is dispersed in negatively buoyant rafts of pollen grains (that superficially resemble strings of frog spawn) which are bound by a slime of apparent thecal origin. Dispersal of pollen is submarine and approximately two-dimensional along the plane of the substratum. The stigmas on pistillate plants are linear, stiff, and densely papillate; they protrude from the mouth of the subtending leaf. Floral ratios of staminate to pistillate flowers were 60 to 1 with an average distance of 4.0 cm from pistillate flower to the closest staminate flowers. Populations included a high proportion of short shoots with numerous (> 100) leaf scars indicating ages for these axes of up to 3–4 years. Nonflowering shoots can be sexed and frequency of flowering can be assessed from scars of old inflorescences. Preliminary results suggest that there is no regular pattern of flowering, since the interval between flowering events is variable.     

Resource partitioning to male and female flowers of Spinacia oleracea L. in relation to whole-plant monocarpic senescence

Male plants of spinach (Spinacea oleracea L.) senesce following flowering. It has been suggested that nutrient drain by male flowers is insufficient to trigger senescence. The partitioning of radiolabelled photosynthate between vegetative and reproductive tissue was compared in male (staminate) versus female (pistillate) plants. After the start of flowering staminate plants senesce 3 weeks earlier than pistillate plants. Soon after the start of flowering, staminate plants allocated several times as much photosynthate to flowering structures as did pistillate plants. The buds of staminate flowers with developing pollen had the greatest draw of photosynthate. When the staminate plants begin to show senescence 68% of fixed C was allocated to the staminate reproductive structures. In the pistillate plants, export to the developing fruits and young flowers remained near 10% until mid-reproductive development, when it increased to 40%, declining to 27% as the plants started to senesce. These differences were also present on a sink-mass corrected basis. Flowers on staminate spinach plants develop faster than pistillate flowers and have a greater draw of photosynthate than do pistillate flowers and fruits, although for a shorter period. Pistillate plants also produce more leaf area within the inflorescence to sustain the developing fruits. The (14)C in the staminate flowers declined due to respiration, especially during pollen maturation; no such loss occurred in pistillate reproductive structures. The partitioning to the reproductive structures correlates with the greater production of floral versus vegetative tissue in staminate plants and their more rapid senescence. As at senescence the leaves still had adequate carbohydrate, the resources are clearly phloem-transported compounds other than carbohydrates. The extent of the resource redistribution to reproductive structures and away from the development of new vegetative sinks, starting very early in the reproductive phase, is sufficient to account for the triggering of senescence in the rest of the plant.     

Flower structure and potential bisexuality in Freycinetia reineckei (Pandanaceae), a species of the Samoa Islands

In Freycinetia reineckei the staminate flower (on the staminate spikes) comprises 3 or 4 (sometimes 2) stamens and a pistillode with 2 (sometimes 4) carpellodes, and the pistillate flower (on the pistillate spikes) is formed of a pistil with 2 (sometimes 4) carpels and of 3 or 4 (sometimes 2) staminodes. This perfect floral homology, also observed in all the other species that were studied with both pistillate and staminate material, strongly suggests that the flower of Freycinetia is basically and potentially bisexual, and may explain the occasional sexual lability and bisexuality of that flower (occurrence of both pistillate and staminate inflorescences, and/or of bisexual inflorescences with bisexual flowers and/or unisexual flowers, on the same individuals) in some species, and also the frequent occurrence of bisexual spikes in this species. These may be partitioned into pistillate, staminate, mixed and sterile zones. In the pistillate zones the flowers have the same aspect and structure as the pistillate flowers. In the staminate zones the flowers generally comprise 3 or 4 (sometimes 2) stamens and a ‘semi-pistil’ some have both stamens and staminodes. The semi-pistils are intermediate between pistils and pistillodes in length, aspect and structure, but always have placentas and ovules. In the mixed zones the flowers are generally formed of a pistil and 3 or 4 (sometimes 2) stamens, and are therefore true hermaphrodite flowers; some have both stamens and staminodes. In the sterile zones the flowers comprise a semi-pistil and 3 or 4 (sometimes 2) staminodes. The staminodes are anatomically very similar to the stamens, especially in the staminate, mixed, and sterile zones, in which they exhibit a wide range of variation in length, aspect and structure. The perfect floral homology as generic character on one hand, and the occasional bisexuality both with and without bisexual flowers and other aspects of sex expression (e.g. occurrence of both pistillate and staminate shoots on the same individuals) in some species on the other hand, seem to indicate that Freycinetia is a basically monoecious, sex changing genus.     

Protandry,synchronized flowering and sequential phenotypic unisexuality in neotropical Pentagonia macrophylla (Rubiaceae)

Summary The neotropical shrub Pentagonia macrophylla Benth. (Rubiaceae) has protandrous two-day flowers. Synchronous development among flowers on a single individual results in sequential phenotypic unisexuality: the entire plant alternates gender from day to day. Pistillate flowers produce more nectar than staminate flowers at comparable hours, but this difference does not result in different paterns of visitation to male and female flowers by hummingbird pollinators. Rare periods of bisexuality occur due to asynchronous floral development within or between inflorescences and are always followed by return to a synchronized pattern of alternation of gender. Reestablishment of synchrony usually involves a timeframe shift in the pattern of flowering (i.e., a plant produces staminate flowers on days when it previously would have been pistillate). It is suggested that timeframe shifts occur in response to inadequate pollination and serve to either desynchronize the plant from neighboring conspecifics or to temporarily allow self-pollination.     

Flowering and fruiting phenology of Normanbya normanbyi (W. Hill) L. H. Bailey (Arecaceae), a palm endemic to the lowland tropical rainforest of north‐eastern Australia

Abstract Normanbya normanbyi (W. Hill) L. H. Bailey (Arecaceae) is a monoecious, arborescent palm with a very small distribution area within the Daintree rainforest in north‐eastern Australia. Our 2‐year study was focused on the reproductive phenology at the individual and population level. At the population level flowering peaked in the dry season, whereas fruiting was confined to the wet season. Each palm can bear up to three inflorescences/infructescences at the same time. Flowering of each inflorescence is separated from each other by a couple of weeks. A single inflorescence consists of about 1900 staminate and 800 pistillate flowers. The flowering of N. normanbyi is protandrous with a staminate phase lasting 40 days and a pistillate phase of approximately 2 weeks. Between both phases is a non‐flowering phase of about 9 days. Fruit ripening takes 21 weeks, with an average of about 280 ripe fruit per tree. Comparison of three study plots revealed a moderate synchrony of flowering and fruiting initiation in this species of palm. The male phase of flowering shows a higher degree of synchrony than the female phase at the population level. Seasonal regularity of flowering and fruiting peaks appears to be predictable. The general flowering and fruiting phenology of N. normanbyi follows a subannual pattern with a strong tendency towards a continual pattern.     

Reproductive Biology of the Monoecious Understory Palm Prestoea schultzeana in Amazonian Ecuador1

Prestoea schultzeana is a monoecious, protandrous palm in the forest understory of Amazonian Ecuador. We studied its leaf production, population density, sexual expression, phenology, pollination, and the specificity of the floral visitors. On average, 1.4 leaves and 0.9 inflorescences are produced per individual per year. The number of staminate flowers per inflorescence is relatively constant compared with the number of pistillate flowers which varies greatly. Flowering occurs in staminate and pistillate phases of approximately 19 and 0–7 days duration, respectively. Flowers open in the morning, and staminate flowers abscise in the afternoon of the same day whereas pistillate flowers last for two days. Flowers are whitish-yellow with a sweet odor and produce nectar. They were visited by Coleoptera (Chrysomelidae, Curculionidae, Nitidulidae, Ptiliidae, Staphylinidae), Hemiptera, Diptera (Drosophilidae, Syrphidae, Ceratopogonidae), Lepidoptera (Nymphalidae), and Hymenoptera (Formicidae, Halictidae). All examined individuals of the syrphid fly Copestylum sp. visiting pistillate flowers carried 100–500 grains of P. schultzeana pollen. Pollen occurred on all body parts, but especially on the legs, and this makes Copestylum sp. the most important pollinator. Most floral visitors were also frequent on the flowers of co-occurring plant species; notably the palm Hyospathe elegans shared most visitor species with P. schultzeana.     

Pollen movement to flowering canopies of pistillate individuals of three rain forest tree species in tropical Australia

This study measured the quantities of effective pollen vectors and their pollen loads arriving at the canopies of dioecious tropical rain forest trees in north-east Queensland. Population flowering synchrony, effective pollinator populations and pollen loads transferred between staminate and pistillate trees were compared among three insect-pollinated tree species. All three were visited by a wide range of insects, 75% of which (mostly 3–6 mm long) carried conspecific pollen. Fewer than 8% of individual insects were found to be carrying single-species pollen exclusively and none could be described as specialist pollen foragers. The introduced honeybee carried greater quantities of pollen than any native species but was not necessarily a reliable pollinator. The brief flowering periods in Neolitsea dealbata (3–4 weeks) and Litsea leefeana (4–5 weeks) populations were synchronized among individuals. Flowering in the Diospyros pentamera population extended over 15 weeks and most individuals were in flower for most of this period. Staminate trees began flowering earlier, produced more flowers and attracted relatively more insects than did pistillate trees, suggesting a density-dependent response of pollinators to flowering performance. Pollen was trapped in greater quantities on insects at staminate trees than at pistillate trees. Insect numbers increased at peak flowering periods and Diptera were the most abundant flower visitors. Anthophilous Coleoptera were more numerous at staminate than at pistillate trees in all three tree species populations. Larger quantities of pollen were mobilized during peak flowering times although the greatest quantities were transferred to pistillate canopies towards the end of the population flowering periods. Diptera carried pollen more often to pistillate N. dealbata and L. leefeana trees than did other groups whereas Coleoptera carried pollen more often to pistillate D. pentamera trees. The two contrasting flowering performances in the three tree species are discussed with reference to mechanisms that facilitate pollen transfer between staminate and pistillate trees.     

Pistillate and staminate flower development in dioecious Pistacia vera (Anacardiaceae)

The development of staminate and pistillate flowers in the dioecious tree species Pistacia vera L. (Anacardiaceae) was studied by scanning electron microscopy with the objective of determining organogenetic patterns and phenology of floral differentiation. Flower primordia are initiated similarly in trees of both sexes. Stamen and carpel primordia are initiated in both male and female flowers, and the phenology of organ initiation is essentially identical for flowers of both sexes. Vestigial stamen primordia arise at the flanks of pistillate flower apices at the same time functional stamens are initiated in the staminate flowers. Similarly, a vestigial carpel is initiated in staminate flowers at the same time the primary, functional carpel is initiated in pistillate flower primordia. Differences between the two sexes become apparent early in development as, in both cases, development of organs of the opposite sex becomes arrested at the primordial stage. Male flowers produce between four and six mature functional stamens and female flowers produce a gynoecium with one functional and two sterile carpels.     

Induced potential for male sex expression inRicinus communis L. by ethyl hydrogen-1-propylphosphonate

Summary Spray application of ethyl hydrogen-1-propylphosphonate (NIA 10637) to juvenile plants ofRicinus communis L. at concentrations ranging from 4,000 to 16,000 ppm caused a change in the normal pattern of sex expression resulting in the transformation of the female into male flowers. Intergrades of sex expression were observed in plants treated with 4,000 and 8,000 ppm of the chemical and the plants were wholly male in those that had received 16,000 ppm, but showing delay in flowering. Pollen sterility was observed in the treated plants.Abbreviations CEPA (2-Chloroethyl)phosphonic acid - TIBA 2,3,5-triiodobenzoic acid - CCC (2-chloroethyl)trimethylammonium chloride - GA₃ gibberellic acid - Kn kinetin     

The relationship between phenology of pistillate flower organogenesis and mode of heterodichogamy in Juglans regia L. (Juglandaceae)

 We investigated the degree of organogenesis completed at the end of the growing season in pistillate flowers of heterodichogamous Juglans regia, English or Persian walnut. Terminal buds from paired cultivars, one each protandrous and protogynous, chosen to represent early, midseason and late leafing walnuts, were examined by scanning electron microscopy. Results indicate that pistillate floral primordia in protandrous individuals had not progressed beyond involucre initiation during the season prior to bloom. In protogynous individuals, floral differentiation had progressed to the initiation of perianth primordia. These observations are compared with an earlier report on staminate flower differentiation in the same cultivars where a comparable, but opposite, relationship exists. We conclude that the degree of differentiation in both staminate and pistillate flowers that must be completed between the time growth resumes in the spring and anthesis is a developmental determinant of the mode of heterodichogamy in walnut. Received: 15 June 1996 / Revision accepted: 25 October 1996     

SEX EXPRESSION,BREEDING SYSTEM,AND POLLEN BIOLOGY OF RICINOCARPOS PINIFOLIUS: A CASE OF ANDRODIOECY?

A population of 54 Ricinocarpos pinifolius (Euphorbiaceae) plants contained male plants, which produced only staminate flowers, and hermaphrodites, which produced staminate and pistillate flowers. The fraction of pistillate flowers ranged continuously from 0 to 0.68. Insect pollination was effective and fruit set virtually complete except for losses to herbivores. Self pollen, outcross pollen from male plants, and outcross pollen from hermaphrodites were all equivalent in viability, germination, tube growth, ovule penetration, and fruit setting ability. Inbreeding depression was manifested as late abortion of some selfed seeds. Geitonogamous selfing is largely prevented by temporal separation of male and female functions within plants. This temporal separation, combined with population-wide synchrony of flowering, may create unusual conditions allowing male plants at low frequency to match hermaphrodites in reproductive success.     

SEX DIFFERENTIAL FLORAL LONGEVITY,NECTAR SECRETION,AND POLLINATOR FORAGING IN A PROTANDROUS SPECIES

Sex differential nectar production, floral longevity and pollinator foraging were examined in Lobelia cardinalis, a self-compatible, protandrous species that is hummingbird pollinated. The staminate phase of the flowers lasts significantly longer and produces significantly more nectar (total sugar) per day than the pistillate phase of the flowers. Additional pollen is presented throughout the staminate phase. Because inflorescences of L. cardinalis mature acropetally, the nectar reward on any given day is greatest at the top of the inflorescence (where staminate phase flowers are located). Hummingbirds appear to be sensitive to this pattern of nectar presentation as they most commonly began foraging in the middle of an inflorescence and proceeded upward. This foraging pattern tends to promote outcrossing and suggests that staminate phase flowers are visited more often than pistillate phase flowers. We conclude that L. cardinalis emphasizes the male function at anthesis. Others have hypothesized that the features of this species are a logical consequence of intrasexual selection, but further research is needed before we place great confidence in a sexual selection interpretation of our data.     

THE INTRAFLORAL PHENOLOGY OF STREPTANTHUS TORTUOSUS (BRASSICACEAE)

The sequence of floral events during anthesis was examined in Streptanthus tortuosus to determine the relationship between the male and female floral phases. The flowers are strongly protandrous. In the staminate phase, the anthers mature sequentially over a 3–4-day period. Because pollinators quickly remove pollen from the anthers, sequential anther maturation prolongs the male phase relative to what it would be if anthers did not mature sequentially. Pollen applied to the stigma during the staminate phase does not adhere readily and does not germinate. The length of the pistillate phase depends on pollinator activity, as pollination accelerates the abscission of floral parts. Unpollinated flowers remain pistillate for 3–4 days, during which time stigmatic receptivity declines gradually. In the field, 72% to 80% of flowers are staminate at any time, indicating that the staminate phase is three times longer than the pistillate phase when pollinators have access to the flowers. The consequences of the relative length of the floral phases and the schedule of stigmatic receptivity are discussed in terms of outcrossing mechanism, floral longevity, and sexual selection models.     

Breeding behaviour of Kunzea pomifera (Myrtaceae): self-incompatibility, intraspecific and interspecific cross-compatibility

To examine breeding system characteristics of the endemic Australian prostrate shrub Kunzea pomifera, artificial hybridisations were undertaken using thirteen different genotypes of K. pomifera, to elucidate: (1) self-incompatibility, (2) intraspecific cross-compatibility in the species and (3) interspecific cross-compatibility with each of K. ambigua and K. ericoides. K. pomifera exhibited very low self-compatibility, with the barrier to self-fertilisation being prevention of pollen-tube growth in the style or ovary. Following intraspecific pollination amongst a number of different genotypes of K. pomifera, 38.4% of pollinated flowers developed fruit; arrest of compatible pollen-tubes in the style, preventing fertilisation, contributes to the low fruit set in this species. Interspecific compatibility was examined between K. pomifera (pistillate parent) and K. ambigua (staminate parent) where seed set per pollinated flower (4.47) was not significantly different from intraspecific crosses (4.66). In crosses between K. pomifera (pistillate parent) and K. ericoides as staminate plant, 0.037% of pollinated flowers produced fruit, with 0.0075 seeds per pollinated flower. Reproductive barriers between these two species were evident in the style of K. pomifera, where the growing tips of the K. ericoides pollen-tubes swelled and ceased to grow.     

Ant pollination of the palaeoendemic dioeciousBorderea pyrenaica (Dioscoreaceae)

The importance of ants for pollination in the dioeciousBorderea pyrenaica (Dioscoreaceae), a Tertiary palaeoendemic plant of the Pyrenees (NE Iberian Peninsula) was studied. The frequency of different visitors (ants, lady beetles, andDiptera) to staminate and pistillate flowers was quantified, and their effectiveness as pollinators was examined by means of fruit and seed set in selective experimental exclusions. Although ants were less abundant on flowers than other visitors, they were the most effective pollinators. Some qualitative factors of this mutualistic ant-plant interaction may account for their effectiveness: the small size of the ants, their high visitation rate to pistillate flowers, and the lack of reduction in viability of the pollen transported on the integument. In addition, the sedentary nature of ants assures their presence during the flowering period. The most abundant floral visitors ofB. pyrenaica were therefore not the most effective pollinators.     

Preformation and distribution of staminate and pistillate flowers in growth units of Nothofagus alpina and N. obliqua (Nothofagaceae)

Background and Aims

The distribution and differentiation times of flowers in monoecious wind-pollinated plants are fundamental for the understanding of their mating patterns and evolution. Two closely related South American Nothofagus species were compared with regard to the differentiation times and positions of staminate and pistillate flowers along their parent growth units (GUs) by quantitative means.

Methods

Two samples of GUs that had extended in the 2004–2005 growing season were taken in 2005 and 2006 from trees in the Lanín National Park, Patagonia, Argentina. For the first sample, axillary buds of the parent GUs were dissected and the leaf, bud and flower primordia of these buds were identified. The second sample included all branches derived from the parent GUs in the 2005–2006 growing season.

Key Results

Both species developed flowering GUs with staminate and/or pistillate flowers; GUs with both flower types were the most common. The position of staminate flowers along GUs was similar between species and close to the proximal end of the GUs. Pistillate flowers were developed more distally along the GUs in N. alpina than in N. obliqua. In N. alpina, the nodes bearing staminate and pistillate flowers were separated by one to several nodes with axillary buds, something not observed in N. obliqua. Markovian models supported this between-species difference. Flowering GUs, including all of their leaves and flowers were entirely preformed in the winter buds.

Conclusions

Staminate and pistillate flowers of N. alpina and N. obliqua are differentiated at precise locations on GUs in the growing season preceding that of their antheses. The differences between N. alpina and N. obliqua (and other South American Nothofagus species) regarding flower distribution might relate to the time of anthesis of each flower type and, in turn, to the probabilities of self-pollination at the GU level.Key words: Branch, bud, growth unit, Markovian models, Nothofagus alpina, N. obliqua, Patagonian forests, pistillate flower, preformation, staminate flower