Stamen development

In this thesis the development of the stamen is investigated, using structural and histochemical observations, micromanipulation and in-vitro culture.Formation and exposure of pollen are the two main goals of the developing stamen. The main processes of the pollen formation were already known. In this thesis they are critically evaluated and some additional functions of the observed structures and processes are proposed. Moreover, the pollen formation process appears to have direct connections with the processes leading to pollen exposure, including anther dehiscence. Although the latter process was investigated by some researchers around the turn of our century, it was in the main unknown until now. In this thesis the old works were reviewed and completed with additional research to a description of the entire anther dehiscence process. Also the development of the filament had been hardly described until now and appears to have specialized relationships with pollen exposure.In chapter 1 the changes in the water content of the developing anther tissues are described. The most striking changes occur at anthesis when the dehydration of the locule wall causes anther dehiscence. This is immediately followed by the dehydration of the pollen, indicating that both these dehydrations are due to evaporation. In case the relative humidity in the flower bud is artificially decreased in different stages of its development, the evaporation ability of the anthers appears to be better shortly before anthesis than in younger stages. This indicates any preparation on dehydration in the oldest stages in favour of dehiscence.In chapter 2 the process of anther dehiscence is analysed. It appears to consist of 4 major steps.1. The enzymatical opening of the tissue (septum) between each two adjacent locules.2. The mechanical rupture of the tapetal membranes that are bordering these sites.3. The inward bending of the locule walls, due to the expansion of the epidermis and endothecium cells and the rigidity of the inner tangential endothecium wall. This movement dissociates the epidermis cells of the stomium mechanically and keeps the thus opened anther closed, preventing premature loss of pollen.4. The outward bending of the locule walls, due to the dehydration of the epidermis and endothecium cells and the rigidity of the inner tangential endothecium wall. This mechanism works contrary to the former movement and is caused by evaporation, in most species due to the entrance of relatively dry air in the flower bud after anthesis.After dehiscence the pollen grains of most animal pollinated species remain stuck on the inside-out bent locule wall by means of the tapetum-derived pollenkitt.Chapter 3 describes the synthesis of this pollenkitt in the tapetum cells. Next this substance is transferred by capillary action to the locule, after the expanding pollen grains have pressed themselves into the tapetum cells. This sticks the pollen grains to the locule wall, from where they can be picked up after dehiscence by a pollinator. As after meiosis the cell walls on the border between the locule wall and the pollen grains change from hydrophylic to hydrophobic and the pollenkitt is also hydrophobic, this sticking complex is resistant against moisture. This prevents an undesired loss of pollen after dehiscence. If the expansion of the pollen grains in prevented artificially, the pollenkitt stays inside the tapetum cells, turning the plant into a wind pollinator.In chapter 4 the degeneration of the microspores in a male sterile species is related to accompanying deviations as well as normal processes in the locule wall. The premature degeneration prevents the expansion of the pollen grains in this species and also in this case the pollenkitt remains Inside the tapetum cells. which agrees with the proces described in the previous chapter. The tapetum. develops in a normal way and appears to be independent upon the presence of developing pollen grains, as is most of the development of both the epidermis and the endothecium. However, in the regions of the anther where the locular fluid is sucked away in an early stage, due to the male sterility, dehiscence does not take place, indicating that the needed expansion of the epidermis and the endothecium for this process (ch.2) may be due to water retraction from the locular cavity.In chapter 5 an ultrastructural analysis of the developing locule tissues is presented. Ultrastructural changes in the epidermis and the endothecium can be related to the dehiscence processes of chapter 2. The changes on the border between the locule wall and the pollen grains from hydrophylic to hydrophobic properties, outlined in chapter 3, appear to be due to tapetal activity.The meiotic callose walls equalize the size and shape of the future pollen grains, excluding any influence of these factors during pollination.The developing pollen grains are polarized cells from their formation upon meiosis. This may result in the deviating differentiation of the generative cell, firstly by the exclusion of plastids, secondly by the isolation of this cell from both the vegetative cell and the locule. This isolation is a result of the presence of a callose wall between both cells and the position of the generative cell in relation to the site of the colpus.In chapter 6 an impression is given of the filament development. This organ shows a developmental gradient from its tip to its base. Premature degeneration proceeds from the tip to the base and is accompanied by the thickening of both the outer tangential epidermal wall and the cuticle and the enlargement of the surface of the latter. This supports our idea that these epidermal changes improve the rigidity. The presence of a cuticle in some of the intercellular spaces of the filament indicates gas transport, possibly towards the drying locule that was described in chapter 1.In chapter 7 structural differences in the filament tip and the connective tissue of three different species are related to their speed of dehydration and anther dehiscence. Closing the stomata of an anther appears to retard dehiscence, thus indicating their important role in this process. This idea is supported by the slow dehiscence of a stomata-less anther. Apart from this acceleration of dehiscence, the stomata may play a role in the drying of the locule (ch.1), which was also related to the intercellular spaces of the filament (ch.6). In some species the filament tip dehydrates and shrivels together with the dehydration of the dehiscing anther, which enables the latter to dangle on the filament, probably in favour of an optimal collaboration with the pollinators. Any role of this shriveling filament tip in the prevention of water supply to the anther is unlikely, as the tracheary elements remain open in this zone, which is demonstrated in chapter 6.In chapter 8 the reactions of explanted tetrads in older anthers or in vitro indicate a selective influence of the callose on the transfer of substances to the microspores. This supports the already existing theory about the role of this wall, apart from its shaping function that we described in chapter 5. The polarity of the microspores, an described in chapter 5, disappears after the explantations, indicating a great influence of the environment on this phenomenon