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

Quantification of Emboli by Visualization of Air Filled Xylem Vessels

Between harvest and vase life the cut surface of most cut flowers is exposed to air for a longer or shorter period. It was hypothesized that under normal harvest and transport conditions air only enters the cut open vessels and does not move to non-cut vessels. The vessel length distribution of chrysanthemum stems was analyzed with the latex particle method and compared to the distribution of air embolisms in 5øw/w) desiccated stems, visualized using cryo-scanning electron microscopy. It was concluded that by moderate desiccation all cut open vessels are completely air-filled and that intact vessels are not embolized

Oxide-derived Silver Nanowires for CO₂ Electrocatalytic Reduction to CO

Silver electrocatalysts offer the possibility to produce CO by converting CO2, enabling the use of a greenhouse gas as chemical building block. Compared to nanoparticles, silver nanowires show an enhanced selectivity towards CO. Recent publications proved that oxide-derived electrocatalysts can exhibit better catalytic performance than the pristine metal phase, but oxide-derived silver nanowires have not been investigated. In this work, we report for the first time the electrocatalytic properties of silver nanowires, synthesized via the polyol method, and pretreated by electrochemical oxidation in basic electrolyte. By increasing the oxidation potential, both the percentage of AgxO and the surface roughness of the catalyst were progressively increased. The most oxidized sample showed a remarkably improved CO selectivity (−294.2 mA m−2Ag), producing a 3.3-fold larger CO partial current density than the pristine sample (−89.4 mA m−2Ag), normalized by electrochemically active silver surface area. This work demonstrates the beneficial effect of the controlled oxidation treatment even on highly selective nanostructures such as silver nanowires.</p

Oxide-derived Silver Nanowires for CO₂ Electrocatalytic Reduction to CO

Silver electrocatalysts offer the possibility to produce CO by converting CO2, enabling the use of a greenhouse gas as chemical building block. Compared to nanoparticles, silver nanowires show an enhanced selectivity towards CO. Recent publications proved that oxide-derived electrocatalysts can exhibit better catalytic performance than the pristine metal phase, but oxide-derived silver nanowires have not been investigated. In this work, we report for the first time the electrocatalytic properties of silver nanowires, synthesized via the polyol method, and pretreated by electrochemical oxidation in basic electrolyte. By increasing the oxidation potential, both the percentage of AgxO and the surface roughness of the catalyst were progressively increased. The most oxidized sample showed a remarkably improved CO selectivity (−294.2 mA m−2Ag), producing a 3.3-fold larger CO partial current density than the pristine sample (−89.4 mA m−2Ag), normalized by electrochemically active silver surface area. This work demonstrates the beneficial effect of the controlled oxidation treatment even on highly selective nanostructures such as silver nanowires.</p

Processes and xylem anatomical properties involved in rehydration dynamics of cut flowers

In cut flowers, which are harvested in air and transported dry, all cut xylem vessels in the basal part of the stem contain air instead of water. These air-emboli initially block water transport at the start of vase life, but usually (partly) disappear during the first hours of vase life, resulting in rehydration of the flower. However, in some cases flowers are not able to sufficiently remove these air blockages, resulting in a poor water status expressed by wilting. Differences in rehydration ability are present between cultivars, but also between different lots of flowers within one cultivar as result of growing conditions. Using chrysanthemum cut flowers, investigations are focussed on the dynamics of the flower water status during the first hours of vase life after air entrance in the xylem vessels via the cut surface. Role of xylem anatomy in the process related to the establishment of a good or bad rehydration are studied by means of cryo-SEM and other microscopic techniques, dynamic measurements of stem hydraulic resistance and 1H NMR imaging. Modeling techniques are use to explore theoretical concepts and to integrate experimental results obtained by the different experimental techniques

Influence of Ag particle size and Ag: Al2O3 surface ratio in catalysts for the chloride-promoted ethylene epoxidation

Ethylene epoxidation is catalyzed by α-alumina supported silver catalysts. The influence of silver particle size has been a topic of debate, and was typically investigated without the industrially essential chloride promoter. We studied the catalyst behavior in the presence of chloride. Transient behavior was observed in the first tens of hours on stream, not as a result of particle growth, but due to the gradual change in the nature of the active silver site in the presence of chloride. Different strategies were used to tune the particle size: either varying the silver loading or varying the decomposition atmosphere. Increasing the particle size from 13 to 50 nm by changing the Ag loading from 2 to 15 wt% increased the selectivity from 35 to 80%. However, increasing the 15 wt% Ag particle size from 48 to 184 nm by varying the heat treatment led to a decrease in selectivity from 80 to 50%. Changing the Ag particle size with both strategies also changes the Ag: Al2O3 surface ratio. The ethylene oxide selectivity is actually correlated to the Ag: Al2O3 surface ratio, rather than to the particle size in this size range. This can be explained by its influence on the probability of a formed ethylene oxide molecule to subsequently further react over support surface groups

Inverse problems with partial data for a magnetic Schr\"odinger operator in an infinite slab and on a bounded domain

In this paper we study inverse boundary value problems with partial data for the magnetic Schr\"odinger operator. In the case of an infinite slab in $R^n$, $n\ge 3$, we establish that the magnetic field and the electric potential can be determined uniquely, when the Dirichlet and Neumann data are given either on the different boundary hyperplanes of the slab or on the same hyperplane. This is a generalization of the results of [41], obtained for the Schr\"odinger operator without magnetic potentials. In the case of a bounded domain in $R^n$, $n\ge 3$, extending the results of [2], we show the unique determination of the magnetic field and electric potential from the Dirichlet and Neumann data, given on two arbitrary open subsets of the boundary, provided that the magnetic and electric potentials are known in a neighborhood of the boundary. Generalizing the results of [31], we also obtain uniqueness results for the magnetic Schr\"odinger operator, when the Dirichlet and Neumann data are known on the same part of the boundary, assuming that the inaccessible part of the boundary is a part of a hyperplane

Oxide-derived Silver Nanowires for CO2 Electrocatalytic Reduction to CO

Silver electrocatalysts offer the possibility to produce CO by converting CO2, enabling the use of a greenhouse gas as chemical building block. Compared to nanoparticles, silver nanowires show an enhanced selectivity towards CO. Recent publications proved that oxide-derived electrocatalysts can exhibit better catalytic performance than the pristine metal phase, but oxide-derived silver nanowires have not been investigated. In this work, we report for the first time the electrocatalytic properties of silver nanowires, synthesized via the polyol method, and pretreated by electrochemical oxidation in basic electrolyte. By increasing the oxidation potential, both the percentage of AgxO and the surface roughness of the catalyst were progressively increased. The most oxidized sample showed a remarkably improved CO selectivity (−294.2 mA m−2Ag), producing a 3.3-fold larger CO partial current density than the pristine sample (−89.4 mA m−2Ag), normalized by electrochemically active silver surface area. This work demonstrates the beneficial effect of the controlled oxidation treatment even on highly selective nanostructures such as silver nanowires