Dehiscence is a means by which some wild plants release their seeds. In Brassicas the mature fruit or 'pod', strictly a silique, releases seed by a sometimes explosive mechanism triggered by mechanical pressure and referred to as 'shatter'. This mechanism is a problem in Brassica crop plants and results in loss of seed, and hence loss of revenue, during harvesting. This problem is further compounded by the distribution of volunteers which contaminate future crops and the environment. The post-fertilisation development of the carpel wall of a number of Brassica species has been examined including, a range of Arabidopsis ecotypes and mutants, and fruits from two other Brassicas, Brassica napus and Brassica juncea, which exhibit differences in the dehiscence characteristic. These have been studied by a combination of cytological, cytochemical and molecular techniques. Following fertilisation, dehiscence zones form at the carpel margins, separating the carpel walls from the replum and forming two valves. Cells within the dehiscence zone exhibit reduced cellular cohesion due to breakdown of the middle lamella. Differentiation of the carpel wall layers results in a thickened exocarp, a senescing mesocarp, and modification of the endocarp layers in which the inner layer Enϸ lignifies whilst Enα collapses. It is proposed that the patterns of differentiation result in the development of the dehiscence mechanism. The dehiscence mechanism and pod 'shatter' is a result of; 1) weakening of valve attachment due to reduced cell cohesion in the dehiscence zone, and, 2) tensions which develop within the carpel walls due to desiccation and shrinkage of the mesocarp which is attached to a thickened, non-shrinking endocarp.The fruits from all of the Arabidopsis ecotypes examined exhibited a similar pattern of carpel wall development and similar dehiscence characteristics. Light microscopical examination of the fruits of Brassica napus and Brassica juncea which do not shatter as easily as those of Arabidopsis showed a different pattern of endocarp development in the post fertilised fruit. Enα tangential walls thickened considerably in the post-fertilised Brassica napus and Brassica juncea fruit, prior to the collapse of this cell layer. In Indian mustard, the Brassica juncea variety which had a non-shattering phenotype, the lignified walls of En6 were surrounded by a highly pectinised layer. This deposition of pectins confers more elasticity to the carpel walls, hence reducing the tensions which normally result in dehiscence and cause pod shatter. The model of the shattering and non-shattering phenotypes described in this study suggest a number of strategies which may be used to reduce the problems of pod shatter. These include modification of the separation layer to increase cellular cohesion, and modifications to the patterns of differentiation in the carpel wall to reduce the tensions which normally develop during fruit ripening.

No description available.

Examination of the imbibition characteristics of eighty chickpea genetic lines (53 Desi and 27 Kabuli types) and eleven cowpea cultivars revealed that in both species seeds with unpigmented testae imbibed more rapidly than the pigmented seeds. Rapid imbibition was associated with high solute leakage, indicative of possible imbibition damage. Further evidence of imbibition damage was revealed in cowpea where the rapidly imbibing unpigmented seeds had poor vital staining with tetrazolium chloride after 24h imbibition. In chickpea, all the genetic lines of white Kabuli seeds had similar rapid rates of imbibition. In contrast, the pigmented Desi genetic lines revealed a range of rates of water uptake although many imbibed more slowly than the Kabuli lines. The incidence of delayed imbibers, that is, seeds which failed to imbibe until after 8h in water, was the main reason for reduced rates of imbibition in Desi lines. The restriction to water uptake by the pigmented seeds was mainly influenced by the properties of the seed coat. In cowpea seeds, the permeability of the seed coat was the most important factor limiting water uptake whereas in chickpea the permeability of the seed coat as a whole and of the micropyle as well as the degree of adherence of the seed coat to the cotyledons were important in regulating the movement of water into the seed. Investigation of the rate of imbibition during the maturation of dwarf French bean, cowpea and chickpea seeds revealed that development of slow water uptake by coloured cultivars was clearly associated with the appearance of pigmentation. In chickpea, this also coincided with the development of the adherence of the seed coat to the cotyledons. In contrast, seeds from unpigmented cultivars imbibed rapidly at all stages of maturation. Examination of the seed coat anatomy of chickpea and cowpea indicated that the pigmented seeds tended to have shorter and narrower palisade cells and also shorter tracheid bars than the unpigmented seeds. Differences in water uptake by Kabuli and Desi seeds were not evident when seeds were stored at 100% relative humidity and 40<SUP>o</SUP>C for days. All seeds deteriorated at a similar rate. However, the importance of initial seed quality in influencing deterioration during storage was emphasised by the rapid loss of germination of the Kabuli seeds in which germination and vital staining revealed poor initial quality.

Niche-differentiation has been proposed to explain the coexistence of large numbers of tree species in tropical forests. Previous studies on established individuals have found little evidence for this. However, niche-differentiation may be apparent at the seed and seedling stages. This study investigated the effects of environmental variation in germination and seedling establishment for a range of Central Panamanian species. Firstly, topography has a large impact on soil water availability; slopes are wetter than nearby plateaux. The potential impact on seedling emergence and survival was studied by monitoring seedling emergence and survival over 15 months. This revealed that small-seeded species are unlikely to establish on plateaux, presumably because of the rider conditions. Secondly, the germination of four pioneer species in response to four environmental variables, related to canopy gap size, was investigated. Species exhibited consistent patterns of response suggesting adaptation for germination in particular gap sizes; Piper marginatum responded to nitrate, had a low base potential for germination and tolerated large temperature fluctuations, suggesting adaptation for germination in comparatively large gaps. Conversely, Pipe peltatum exhibited responses, which suggested adaptations for germination in small gaps. Finally, germination and seedling mortality for four pioneer species in two sizes of canopy gap was predicted using computer modelling. This indicated that large-seeded pioneers can germinate and establish in large gaps. Conversely, percentage germination and survival of small-seeded species was low in large gaps. However, small-seeded species produce greater quantities of seed than large-seeded species. Consequently they may "win by default" in large gaps, because of the absence of seeds of larger species. These results indicate that there are a number of axes of environmental variation along with nice-differentiation can occur. Furthermore, the response is variable between species. Thus niche-differentiation is important for fostering species coexistence, although density-dependent mortality and dispersal-limitation also contribute.

The food supply to a growing world population is based on grain crops, which are harvested for their seeds. Therefore, understanding the process of seed development and its regulation has been important to increase production. This has become even more relevant and important as grain production reaches a plateau post green revolution and food security for an ever-increasing population becomes more pressing. This study focuses on altering seed size via manipulation of the endosperm, an important component of the seed that not only nourishes the developing embryo and provides the majority of our food. A classic study done by crossing different ploidies of Arabidopsis thaliana (A. thaliana) results in changes to endosperm-led growth as a consequence of altered parental genome ratios in the endosperm specifically. Increased paternal contribution results in an enlarged endosperm, and a heavier seed, whereas increased maternal contribution has the opposite effect. Whole genome transcript profiling using microarray analysis of siliques generated by interploidy crosses identified A. thaliana genes namely: PHE1, PHE2, AGL40, AGL62, AGL28, AGL45, CKX2, MAPK10, E2L2, GA1, CYCD4;1, CYCD4;2, GA20OX5, GA-regulated and AT5G46950, that are positively associated with endosperm overgrowth. In order to verify the role of these genes in endosperm proliferation, knock in (KI) which causes gene over-expression and knock out (KO) which results in gene inactivation, strategies were used. Constitutive over-expression of CKX2, MAPK10, and E2L2 showed abnormal vegetative and floral phenotypes, whereas targeted endosperm-specific over-expression of PHE2 and GA1 showed an increase in seed size and/or fertility. KI plants of AGL40, AGL62, AGL45, CYCD4;1, CYCD4;2, GA20OX5, GA-regulated and AT5G46950 did not result in any obvious phenotypic effects under normal growth conditions. KO mutant plants namely: phe1, phe2, agl40, agl62, agl28 and agl45, as single individual mutants were also indistinguishable to wild type plants in non-seed phenotypes. Embryo sac area, individual seed weight and total seed yield data obtained from a phe1 line showed a smaller embryo sac area, lighter seed and reduced total seed yield. Loss of function of agl62 showed precocious endosperm cellularisation and seed lethality. It is nearly impossible to generate phe1/phe2 double mutants due to a very tight linkage between these genes (on the chromosome physically the two genes lie next to each other) and hence doubles, triples and quadruple mutants where appropriate were made with either phe1 or phe2 in combination with other mutant lines of the MADS family of transcription factors. None of the double mutant combination tested had obvious developmental defects. However, the triple mutant phe1/phe1::agl40/agl40 ::agl45/agl45 and the quadruple mutant phe2/phe2::agl40/agl40::agl45/agl45 ::agl28/agl28 showed abnormal embryo and endosperm development which resulted to seed death. This indicates a functional redundancy among these MADS box genes. However, it is unclear how PHE1, PHE2, AGL40, AGL45 and AGL28 act to affect embryo and endosperm development. This result confirms that these genes are positively associated with endosperm over-proliferation as growth promoters and function in some cases as singly or as participants in a complex to control seed development. Thus manipulating expression of genes involved in endosperm development in A. thaliana and the potential use of this knowledge in crop plants provides us with a route to improvement of crop yields.

Thesis (Ph. D.)--University of Alberta, 2009. / Title from pdf file main screen (viewed on Jan. 14, 2010). A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Plant Science, [Department of] Agriculture, Food and Nutritional Sciences, University of Alberta. Includes bibliographical references.

Thesis (Ph. D.)--Rutgers University, 2008. / Includes bibliographical references.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

No description available.

No description available.