Biologia reprodutiva e germinação de sementes em Adenostemma brasilianum (Pers.) Cass. (Asteraceae) / Reproductive biology and seed germination of Adenostemma brasilianum. (Pers.) Cass. (Asteraceae)

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

Specific studies on the reproductive biology and seed germination of Adenostemma brasilianum are inexistent. Aiming to fill this gap, the objective of this study was to investigate its life history (reproductive phenology and fruit dispersion process), reproductive biology, including tests on the efficiency of two types of materials in isolating the heads from their floral visitors, and visitation frequency. Best seed germination conditions, longevity and physical and physiological quality of the seeds were also verified. The research was carried out from March 2005 to December 2006 in Mata do Paraíso (MP), a seasonal semi-deciduous forest with 195 ha, and Horto Botanico (HB) at the Department of Plant Biology of the Universidade Federal de Viçosa (UFV), both sites located in Viçosa (2045S and 4251W), Minas Gerais. For life history and phenology studies, 50 seedlings were monitored until death, with the following observations being made: presence or absence of floral buds, open flowers diasporas in dispersion, herbivores, and dead individuals; 140 heads were marked in floral buds which were observed daily from the first open flowers until dispersion, characterizing various aspects of flower and head biology, including ASP (pollen secondary presentation mechanism). For the breeding system, the following five tests wee performed: AE-spontaneous self-pollination; AG- agamospermy; AM- manual self-pollination; PC- cross-pollination and PA- open pollination. Except for PA, the heads were isolated with failete bags. The averages were compared by the t test at 0.005% of probability. The tests to verify the efficiency of the two materials (muslin and failete) were: AE, AG and AM and in each one of them the heads were isolated with muslin and failete and the averages compared by the Qui-square test. Assessment of the floral visitors was performed in both areas and visitation frequency was recorded only in MP, between 07:00 am and 05:00 pm., for 30 minutes every hour during four non-consecutive days, totaling a sampling effort of 20 hours. The visitor contacting the reproductive organs of the flowers was considered a pollinator. For the germination tests, four repetitions with 50 seeds each originated from MP were performed, with the seeds being submitted to different temperatures (20C, 25C and 30C) and light schemes (light supplementation- SUP, environmental light- AMB, continuous darkness- ESC) and daily seed count (27 consecutive days). Longevity was tested by storing seeds at 20C for two, four, six, twelve, and eighteen months, later submitting them to the same analyses described for the newly collected seeds. To characterize their physical quality, the natural population seeds were classified as perfect (with carbonized pericarp and embryo) and imperfect (without hyaline and embryo). Dormancy tests and imbibitions site were developed with tetrazolium and aniline blue, respectively Since A. brasilianum is annual, a seedling bank was formed; 33% of hem died due to strong summer rains and the remaining developed rapidly between November and December/2005, with increased rainfall and temperature. Flowering extended from January to April 2006, with a peak in February. The flowers presented two distinct sexual phases: a male phase, lasting a few hours and a female phase lasting 31 days. Anthesis occurs early in the morning, flowers are protandric, and dehiscence of the anthers occurs around 6:00 am, with the start of the ASP process, which extends until approximately 9:00 am, when the female phase initiates. Pollen is abundant, white and of high viability, on an average, 96%. In the heads, the flowers open sequentially in groups on the first two days, with 74% of them being found in anthesis, and after 20 days all the fruit were found exposed to dispersion. Fructification peak was between March and May 2006, followed by the herbivore peak, caused by three phytophagus beetles (Chrysomelidae). A. brasilianum is self-compatible, and with no agamospermy. The results of PC (81.6% fructification) and PA (71.3% in MP and 91.2% in HB) tests demonstrated that pollinators play an important role in its reproduction, although the species has shown to be able of reproducing independently from its services, through spontaneous self-pollination (13.9%). Failete was more efficient in isolating the heads from the pollinators action being fundamental for obtaining reliable reproductive system results. Thirty-eight taxa, belonging to the order Lepidoptera (32 taxa), Hymenoptera (three) and Diptera (three) were collected. The butterflies Ithomiinae (18 species) and Danainae (one species), both Nymphalidae, and the diurnal moths Arctiidae (13 taxa) were the most representative groups in number of species. Among the Lepidoptera, Nymphalidae were the most frequent, especially Brevioleria aelia plisthenes, Episcada carcinia and Pteronymia euritea (Ithomiinae), besides Lycorea halia discreta (Danainae). The inconstant visitation of the lepdoptera was verified and was offset by its species diversity and unpredictable forage behavior, which favor xenogamy and high fructification (>70% in PA). Visitation peak occurred between 8:00 am to 11:00 am. All the Ithomiinae collected were male. As described in the literature, they seek pyrrolidizine alkaloids in the nectar, which makes them unpalatable and are precursor of pheromone biosynthesis; during mating, a part of it is transferred to the females through the spermtophores. The best seed germination condition is 25C SUP, with 85.5% rate. Dormancy was independent of the light regime, induced by temperature of 20C. Mortality and dormancy break increased with storage time. Seed longevity is short, no longer than 12 months. A head has an average of 31 seeds, 25.3 % of them imperfect, of which around 10% are parthenocarpic. Germination occurred even at low percentages under practically all the conditions tested, with such plasticity being possibly related to the fact of the plant being wild, not yet domesticated, which is advantageous for allowing germination under adverse environmental conditions. This study confirmed the existence of a codependence relation between A. brasilianum and its pollinators, although maintenance of both in MP depends on other factors, such as maintaining the habitat of A. brasilianum, its dispersers and feed sources (mainly Solanaceae leaves) for the Ithomiinae juvenile phases.

ASSUNTO(S)

sistema reprodutivo germinação de sementes ithomiinae reproductive system fenologia reprodutiva polinizadores biologia floral life history reproductive phenology seed germination botanica ithomiiame pollinators floral biology história de vida

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