Diversity Of Behaviors Of Touch-sensitive Stigmas In Flowering Plants And Its Adaptive Significance

Touch-sensitive stigma of many species in Angiosperm closes in response to touch by an animal pollinator. Compared with rapid leaf movements in "sensitive" plants such as Mimosa pudica or Dionaea muscipula, research of touch-sensitive stigma is still scarce. However, these limited results reveal the rich variety of touch-sensitive stigma and its importance to plant reproduction hence, intensive research remains to be developed. In this research, we focused on the following five aspects and carried out in-depth exploration:(1) phylogeny distribution of touch-sensitive stigma and its diversity in behavior pattern, (2) the response of touch-sensitive stigma to different visitors with different pollination efficiency, (3) the influence of different co-flowering plants to the reproductive success of plants with touch-sensitive stigma, (4) the relationship between stigma behaviors and breeding system of plants with touch-sensitive stigma, (5) the relationship between the number and density of stigma papillae and stigma sensitivity. The outcome of this paper is aimed to fill some gaps in the study of touch-sensitive stigma and expand the knowledge to this floral phenomenon.So far, no system summary has been carried out, or does the exposition of phylogyny distribution for all the touch-sensitive stigma plants. To address this, we took a survey of touch-sensitive stigma plants which revealed that all the families including Acanthaceae, Bignoniaceae, Lentibulariaceae, Linderniaceae, Martyniaceae, Mazaceae, Phrymaceae, and Rehmannia (family belonging is unclear) and Plantaginaceae, belong to Lamiales. Most of the families concentrate on higher core Lamiales except Plantaginaceae. With regard to their stigma shape, we divided the touch-sensitive stigmas into 3 types of touch-sensitive stigma:the first type is the most common 2-lobed broad touch-sensitive stigma in the rest 7 families; the second type has a sensitive linear branch of bifid stigma with a reduced posterior lobe lying on the lower lip of the corolla with shorter anthers in Strobilanthes of Acanthaceae; the third type has a long, wet and motile lower lobe which rises immediately through an arc of 180° or more after being mechanically stimulated, but does not close against the short and rigid upper lobe in Utricularia of Lentibulariaceae. All the three types of touch-sensitive stigma are located in the pollinator entrance, where they can easily receive the outcross pollen. They are all "closed" when the pollinator gets out of the floral tube to avoid receiving self pollen. Besides, we also comprehensively summarized the biological significance of touch-sensitive stigmas.Many flowering plants may have many pollinators with different visit efficiency. Although the knowledge about the relationship between pollinator visit efficiency and evolution of floral traits has been achieved, there are still limited studies for detecting the relationship between touch-sensitive stigma behavior and pollinator visit efficiency. In this experiment, in order to detect the adaptive strategies of touch-sensitive stigma to different pollinator visit efficiency, we chose Mazus miquelii with touch-sensitive stigma as the study material. Two experiments were simultaneously conducted:(i) the single visit efficiency experiment with four main visitors:Halictus spp., Lasioglossum spp., Anthophorini spp., and Osmia spp. of Mazus miquelii, and (ii) the artificial touch experiment. The results showed that the touch-sensitive stigma reopened quickly when pollinated by pollinators with low visit efficiency, because the pollen loading was not enough; while the touch-sensitive stigma reopened slowly when pollinated by pollinators with high visit efficiency, which we supposed was in order to protect the already loaded pollen. In addition, the stigma of M. miquelii can reopen repeatedly to increase the chance to obtain the maximization pollen loading and will close permanent with enough pollen loads. To our knowledge, this is the first study to prove the adaptive strategies of touch-sensitive stigma to reopen with regard different pollinator visit efficiency and the touch-sensitive stigma with the capacity to reopen repeatedly by combining the single visit efficiency experiment and the artificial touch experiment.When co-occurring plant species overlap in flowering phases and share the major pollinator fauna, pollination interactions among them may occur. The plant-plant pollination interaction has long been of concern to evolutionary ecologists. However, to date, little study has focused on the pollination interaction between plants with touch-sensitive stigma and their co-flowering plants.In this experiment, to detecting the effects of co-flowering plants to the reproduction success of plants with touch-sensitive stigma, we selected M. miquelii as the focal plant, and separately selected Glechoma longituba, Ajuga decumbens, and Muscari botryoides as co-flowering plants. The results showed that there was no hetero-pollen interference among M. miquelii and its co-flowering plants. In addition, G. longituba reduced the seed set of M. miquelii by attracting pollinators (Halictus spp.) with low visit efficiency; A. decumbens promoted the reproductive success of M. miquelii by attracting pollinators (Osmia spp.) with high visit efficiency; M.botryoides did not change the reproductive success of M. miquelii because it did not affect the pollinator composition of M. miquelii. To our knowledge, this is the first study to test the effects of co-flowering plants to the reproduction success of plants with touch-sensitive stigma. We found that the co-flowering plants can influence the pollinator composition of M. miquelii hence its reproductive success; contrarily hetero-pollen interference was not important for the reproductive success of M. miquelii.Touch-sensitive stigma has been recognized to facilitate outcrossing. We hypothesized that species with different levels of sensitivity may have corresponding differences in components of their breeding system. In this study, three Mazus species with bilobed stigmas were used to test the hypothesis. We explored stigma behaviors of the species in reaction time, recovery time, permanent closing time, and the minimum pollen load causing permanent closure. We investigated floral traits, pollinator type and behavior, pollination intensity, and natural schedule of pollen deposition on stigma. Moreover, we evaluated the mating system of the species by checking seed set after controlled pollination treatments, namely; natural flowers with open pollination, enclosed flowers without pollination, and enclosed flowers with self and outcross hand pollination. Results indicated that stigma of M. pumilus (N. L. Burman) Steenis was not sensitive, whereas stigmas of M. miquelii Makino and M. stachydifolius (Turcz.) Maxim closed and reopened quickly in response to pollination. Accordingly, hand pollination treatments revealed that seed set of self-spontaneous pollination in M. pumilus was similar to the other hand pollination treatments. For M. miquelii, outcross pollen resulted in significantly higher seed set than self-pollen. M. stachydifolius was self-incompatible. Additionally, the corresponding characteristics in other components of the breeding system for each species were found. Our study indicated that the sensitivity of bilobed stigma may be linked to the mating system in a given species. Touch-sensitive stigma may be regarded as an evolutionary mechanism for enhancement of outcrossing.In order to explore the relationship between the papillae number and the papillae density on the reception surface and stigma sensitivity, scanning electron microscope was used to detect the reception surface of stigma in M. pumilus, M. miquelii, and M. stachydifolius. We found papillae in three Mazus species. Since all the papillae are longer than pollen diameter, therefore pollen is sandwiched among them. Therefore, we supposed that the function of papillae is to hold pollen grains. In addition, the total number of papilla on the stigma of M. pumilus is more than M. miquelii and M. stachydifolius while papillae density of M. miquelii and M. pumilus is more than M. stachydifolius. Besides, the result in chapter 5 revealed that the stigma sensitivity of M. miquelii is the highest, followed by M. stachydifolius while the stigma sensitivity of M. pumilus is the lowest. Therefore, we could not confirm the relationship between stigma sensitivity and the total number of papilla and the papilla density on the stigma. However, further studies are required to reveal this relationship.