Female-stage Flowers Research Articles

Fruit Set in Avocado: Pollen Limitation, Pollen Load Size, and Selective Fruit Abortion

Avocado is a woody perennial fruit crop originating in Central America and Mexico domesticated and cultivated in the Americas since pre-Columbian times, currently cultivated in subtropical, tropical, and Mediterranean climates worldwide, with increasing importance in international trade. Avocado shows an exuberant flower production that, however, results in a very low fruit set reflected in a massive abscission of flowers and fruitlets. Several factors are involved in this behavior, and, in this work, we have focused on pollination limitation. The results obtained show that pollen deposition takes place at the female and male stages during the avocado flowering season and that the percentage of flowers with pollen on the stigma varies along the flowering season, probably due to changes in temperature that affect not only the floral behavior but also pollinator activity. However, no embryo or endosperm development took place when pollination occurred during the male flowering phase. Thus, the low number of pollen grains landing on the stigmas of female stage flowers observed under natural pollination conditions might not be enough to ensure a good yield. The production of an excess of flowers and subsequent flower drop of most of the flowers provides the opportunity of a selective fruitlet drop depending on the genotype of the embryo since fruits derived from outcrossing showed higher chances of reaching maturity. Moreover, an important competition for resources occurs among developing fruits and new vegetative growth, conferring importance to the time of flower fertilization for effective fruit set.

Effects of storage length and flowering stage of pollen influence its viability, fruit set and fruit quality in ‘Red’ and ‘Lessard Thai’ sugar apple (Annona squamosa) and ‘Gefner’ atemoya (A. cherimola × A. squamosa)

The fruit set of commercial Annonaceae pollinated by natural pollinators is generally insufficient to produce commercially acceptable yields. In addition, several studies have reported that both the pollen source and age affect the potential percent fruit set of sugar apple and atemoya. Annona flowers are hermaphrodite; however, the female part mature before the male, which is known as dichogamy of the protogynous type. The objectives of the present study were to determine the effect of pollen storage duration on pollen viability; the percent fruit set by hand pollination of ‘Red’ sugar apple, ‘Lessard Thai’ green sugar apple and ‘Gefner’ atemoya; and resulting fruit characteristics as a result of pollen source in South Florida. Hand pollination and in vitro pollen germination tests were arranged in a 3×5 factorial design, with three cultivars, five treatments, and five replicates per treatment. The percentage of in vitro pollen germination was the highest for ‘Red’ sugar apple pollen (26.5%) collected from male stage flowers and for pollen collected from female stage flowers collected late afternoon and germinated 18h later (23%). Percent pollen germination was higher for ‘Lessard Thai’ (∼33%) than for ‘Red’ sugar apple and pollen storage for up to 24h was not significantly different than male stage pollen used immediately and pollen collected from female stage flowers and used 18h later. ‘Gefner’ atemoya pollen did not germinate when pollen grains were collected from male stage flowers and when using female stage flowers collected late afternoon and stored for 72h. Fruit set and fruit quality varied significantly as a result of pollen storage treatments. Hand pollination promoted high percentages of fruit set, greater than 80% when female stage flowers were collected late afternoon and hand pollination performed 18h after collection. Fruit harvested as a result of the various pollen storage treatments varied significantly in weight, diameter, and the number of seeds per fruit. The pollen from ‘Lessard Thai’ sugar apple and ‘Gefner’ atemoya may be stored up to 24 and 48h, respectively.

Heterodichogamy and nitidulid beetle pollination in Anaxagorea prinoides, an early divergent Annonaceae

Heterodichogamy in a natural population of an Annonaceae species from the rainforests of French Guiana is described for the first time. Anaxagorea prinoides had bisexual flowers and two floral morphs within the studied population were protogynous. The population under study comprised 7 mature trees belonging to one morph and 12 to the other. Statistical analyses showed that the two morphs were in a 50:50% ratio, and therefore the temporal sexual pattern of heterodichogamy is given. When anthesis of flowers in the male stage ended in one morph, anthesis started with flowers in the female stage in the complementary morph. Approximately 1 h before the end of anthesis in one morph, flowers of the reciprocal morph started to emit a fruit-like scent. The temporal separation of the female and the male stages of the two different morphs lasted only approximately 1 h. Six of the seven identified compounds in the banana-like floral scent were esters and one was an alcohol. The main compounds examined are known to be components of fruit scents. Nitidulidae beetles of the genus Colopterus were the pollinators of A. prinoides and during flowering were maintained within the population of this species. This was not only due to the fact that the beetles remained sheltered in the pollination chamber of the flowers, but also because upon release from individuals of male-stage flowers at the end of flower anthesis they were attracted by the odoriferous female-stage flowers of other individuals of the same population. Heterodichogamy of A. prinoides appears to be a means by which reproductive success is augmented. Attraction of beetle pollinators by “fruit-imitating” floral scent is not restricted to species of Anaxagorea, but occurs in many representatives of the Annonaceae.

Respiration and temperature patterns in thermogenic flowers of Magnolia ovata under natural conditions in Brazil

The roles of floral thermogenesis in pollination biology include attraction and reward of insects. Magnolia ovata (A.St.-Hil.) Spreng. produces ~56 g, bisexual, protogynous and scented flowers. Two distinct episodes of thermogenesis occur during anthesis: one beginning at about sunset and lasting ~3 h in the female phase and another that occurs synchronously 24 h later and lasting 4 h in the male phase. Female stage flowers produce up to 0.36 W to reach 27.3°C, which is 3.9°C above ambient air. In the male stage, corresponding values are 0.79 W, 29.7°C and 5.4°C, respectively. Most heat is generated in the petals in both phases (74 and 65%). Maximum, mass-specific rate of respiration is 23 nmol s–1 g–1 in the petals and 100 nmol s–1 g–1 in the anthers. The flowers are apparently not thermoregulatory, because respiration rate decreases, rather than increases, with decreasing ambient temperature. Scarab beetles, Cyclocephala literata, enter the floral chamber created by the petals in the female phase, mate, consume floral parts (mainly petals) and then depart in the male phase. Temperatures maintained in the floral chamber are sufficient to provide beetles with significant energy savings during their activities in both phases. Thermogenesis is, therefore, consistent with volatilisation of floral fragrances and energy rewards to beetle visitors.

FLOWERING PATTERN AND PROTANDRY IN BOESENBERGIA LONGIFLORA

Aims Boesenbergia longiflora (Wallich) Kuntze is a perennial protandrous ginger widely distributed in Asian tropics. It usually grows in crevices of calcareous rocks of limestone monsoon rainforests or in sedimentation soil on the roadside, forming patches in the understory. Its floral longev-ity is 2 days. Flowers are in male stage during the first day and in female stage during the second day of flowering. Autogamy is avoided because two sexual functions are not overlapping in a single flower. Our objectives were to investigate if any other floral mechanism reinforced protandry to avoid geitonogamy and to determine the pollinators and their visiting patterns on the flowers in different sexual stages of different populations. Methods Flowering phenology was monitored once a week for two years. Floral morphology was studied and compared for three populations of different habitats. Flowering pattern was studied by investigating number of flowering inflorescences and flowers in male or in female stage in four populations and under cultivation. We also investigated natural seed-sets and plant density for different populations. Pollinators were observed in three populations. Important findings Most of individuals of B. longiflora had only one flowering inflorescence at a time and produced one flower every other day. About 10% of all individuals investigated had male and female stage flowers at the same time, both in four natural populations and under cultivation. The pollination system was generalized with different insects as pollinators, including Apis florae, Bombus sp., Amegilla fimrata, Anthophora plagiata, Amegilla yunnansis, Nmia ellioti and Apis cerana in different populations. Plant and floral sizes did not differ significantly among populations. The flowering pattern of most individuals was one flower every other day, which reinforced protandry and avoided selfing.

Intraspecific Variation of Floral Scent Chemistry in Magnolia kobus DC. (Magnoliaceae)

was examined at 32 sites in Japan (109 female-stage flowers from 52 plants) by GC-MS. Major chemical compounds (a total of 36 chemicals) emitted from the flowers were: linalool (and its oxides), limonene, cis- and trans-β-Ocimene, benzaldehyde, benzyl alcohol, benzyl cyanide, and 2-aminobenzaldehyde. Linalool and its oxides were the most abundant components of floral scents in 21 individuals. The rate at which chemical volatiles were emitted ranged from 0.002 to 0.929 μg/flower/hour (average 0.211). High quantitative and qualitative variation in floral scent chemistry among individuals was found throughout the range of M. kobus, especially in central Honshu. The high variability in floral scent chemistry may be due to the importance of visual cues in the reproductive biology of M. kobus which flowers in early spring, resulting in decreased selection for specific floral scent profiles. Alternatively, different scent compounds or chemical profiles may be equally effective in attracting pollinators.

Cockroach pollination and breeding system of Uvaria elmeri (Annonaceae) in a Lowland mixed‐dipterocarp forest in Sarawak

Tropical forest plants are known to be pollinated by a diverse array of animals. Here we report on the pollination of a woody climber species, Uvaria elmeri (Annonaceae), by cockroaches in a lowland mixed-dipterocarp forest in Sarawak, Malaysia. To the best of our knowledge, this is the first report of pollination by cockroaches. The cauliflorous flowers in the understory are protogynous and bloomed for 50 h. An odor similar to decayed wood or a mushroom was secreted by flowers and was stronger during the male stage. Pollinators were cockroaches (Blattellidae) and drosophilid flies (Drosophilidae). Cockroaches, the main pollinators, visited flowers during both female and male stages at night, feeding on stigmatic exudate and pollen. Drosophilids, the secondary pollinators. mainly visited female-stage flowers during daytime, fed on stigmatic exudate. and laid eggs on stigmas. Neither autogamy nor self-compatibility was observed. Fruit production appeared to be pollen-limited. The fruit set, which was 2% of flowers in natural condition, was significantly lower than the 30% fruit set obtained by artificial cross-pollination. We discuss the traits of cockroaches as pollinators and the breeding system of U. elmeri.

Pollination disruption by European honeybees in the Australian bird-pollinated shrubGrevillea barklyana (Proteaceae)

European honeybees (Apis mellifera) were less efficient pollinators ofGrevillea barklyana than nectar-feeding birds. Nectar-collecting honeybees did not contact reproductive parts of flowers. Pollen-collecting honeybees preferentially visited malestage flowers but rarely visited female-stage flowers. Fruit set on caged inflorescences that allowed access to honeybees but excluded birds was reduced by more than 50% compared to inflorescences that were visited by both types of visitors. Further, fruit set on caged inflorescences was less than on bagged inflorescences that excluded both birds and honeybees, indicating that pollen removal by bees decreased opportunities for delayed autonomous selfing in the absence of birds. Although fruit set was not pollen-limited at the study site, pollen removal by honeybees would decrease fruit set in small populations where birds are scarce. In addition, pollen removal by honeybees would reduce opportunities for outcrossing and reproductive success through male function. Although honeybees have been in Australia for insufficient time to have exerted selection on floral traits, evolutionary shifts in response to these animals are likely to occur in the future.

?Ixoroid? secondary pollen presentation and pollination by small moths in the Malagasy treeletIxora platythyrsa (Rubiaceae)

The pollination biology ofIxora platythyrsa (Rubiaceae) was studied in NW. Madagascar. The plant displayed cream-yellow, nocturnally fragrant, nectariferous, tubular and strongly protandrous flowers. These had an “ixoroid” secondary pollen presentation mechanism: prior to anthesis, anthers exhausted their pollen onto unripe stylar heads. From this position pollen of male-stage flowers later adhered to primarily the probosces of small visiting nocturnal noctuid and geometrid moths. — Pollen was subsequently raked off moths' probosces by receptive, copiously papillose stigmas of female-stage flowers. Principal pollination adaptation was probably to the noctuid moth subfam.Sarrothripinae.

Consequences of flower heliotropism for reproduction in an alpine buttercup (Ranunculus adoneus).

The flowers of the alpine snow buttercup Ranunculus adoneus track the sun's movement from early morning until mid-afternoon. Individual blooms last up to a week: younger female stage flowers show greater solar tracking fidelity than older hermaphrodite or dehisced flowers. Flowers aligned parallel to the sun's rays reach mean internal temperatures several degrees Celsius above ambient air temperature. As a flower's angle of deviation from the sun increases beyond 45 degrees, internal flower temperature is significantly reduced. Fly pollinators are seen disproportionately often on flowers aligned with the sun; this is due, in part, to their greater residence time on tracking flowers. Fly visitation is important to fecundity. When flies were excluded from flowers, some selfed seeds were matured, but total seed production was much less than in either handoutcrossed or open-pollinated flowers. Flowers that were tethered at random angles to prevent solar tracking set fewer, smaller seeds than unmanipulated control flowers. Reductions in seed weight were statistically consistent at different times in the season, but tethering influenced seed number per flower most strongly in the early season. Effects of tethering on maternal reproduction could be due to either pollinator diserimination, post-pollination developmental processes, or both. In a second series of experiments using fluorescent dye particles as pollen analogs, tethered donor flowers dispersed dye to as many recipients as paired control flowers, suggesting that solar tracking fidelity may have little effect upon this component of male reproduction.

Effect of low temperatures during flowering on floral cycle and pollen tube growth in nine avocado cultivars

The floral response of the type A avocado cultivars ‘Reed’, ‘Wurtz’, ‘Rincon’ and ‘Jalna’, and the type B cultivars ‘Bacon’, ‘Ryan’, ‘Edranol’, ‘Sharwil’ and ‘Hazzard’ was tested under growth-cabinet conditions of 17° C day, 12° C night, with a 12-h photoperiod and photon flux density of 400 μE m −2 s −1 (400–700 nm). Most of the flowers of all type A cultivars and the type B cultivar ‘Bacon’ had both a female and a male stage. None of the flowers of the other type B cultivars had a female stage but opened in the male stage only. Hand pollination resulted in some ovule penetration in the cultivars with female stage flowers. In those with only male-stage flowers, the pollen tubes grew no further than the style. Maintenance of fertility under low-temperature conditions during flowering appeared to be partly, but not entirely, linked to the type A flowering cycle.

Pollination Biology of Asarum caudatum (Aristolochiaceae) in Northern California

The flowers of Asarum caudatum Lindl. are protogynous and receptive for cross-pollination for approximately one week. After this period, the anthers dehisce near the stigmas and self-pollination occurs. Based largely on studies of garden plants in Europe, Vogel proposed that this species is allogamous but my observations of eight populations in northern California indicate that autogamy predominates because (a) the frequency of flower visitation is extremely low, (b) pollen was found on stigmas of only 8% of female-stage flowers, (c) emasculated flowers set few fruits and seeds compared to controls (fruit set: 20 vs. 99%; seed set: 5 vs. 67%), and (d) fruit and seed sets of bagged flowers were equivalent to those of controls. Fly eggs and larvae were present in some flowers in each population but I found no evidence to support Vogel's hypothesis that mating and ovipositing fun- gus gnats pollinate this species. There has been much speculation about the pollination biology of Wild Ginger (Asarum L.) but few documented field observations of pollination (Peattie 1940; Stebbins 1974). The dark maroon flowers of this familiar understory herb of forests in Asia, North America, and Europe resemble flowers that are pollinated by carrion insects, but they lack the charac- teristic foul odor associated with the sapromyophilous syndrome (Meeuse 1961; Faegri and van der Pijl 1979; Proctor and Yeo 1972). Visitors such as carrion flies, gnats, and beetles have been suggested as possible pol- linators (Muller 1883; Blanchen 1900; Knuth 1909; Morley 1970; Vogel 1973, 1978; Niehaus 1976; Moldenke 1976). Direct self-pollination has been reported for A. europaeum L. (Daumann 1972; Werth 1951; Kugler 1934) and A. canadense (Wildman 1950). Two species of the segregate genus Hexastylis Raf. are also autogamous but require intrafloral move- ment of pollen by insects (Otte 1977). In contrast, Vogel (1973, 1978) proposed that A. caudatum Lindl. is predominantly cross-pollinated. He contends that flowers of this species mimic mushrooms and are polli- nated by fungus gnats that mate and oviposit in them. The majority of his evidence was collected from garden specimens in Europe; in this paper, I report on the pollination biology of A. caudatum in native hab- itats of northern California.