The root meristem of Allium sativum has been described from longitudinal and transverse sections of the root apex. No evidence for the presence of a quiescent centre can be gained from a Norper-kappe analysis of cell patterns. The presence at the pole of the stake of a quiescent centre comprising 30-50 cells has been demonstrated by autoradiographs of roots fed with tritiated thymidine. Rates of mitosis in different regions of the root meristem have been measured by three independent methods. A) metaphase accumulation b) continuous labelling c) pulse labelling The results of the three methods are consistent and show that the rate of mitosis in the quiescent centre is of the order of ten times that in other parts of the meristem. The duration of the different phases of the mitotic cycle have been measured for various regions of the meristem by a pulse labelling for various regions of the meristem by a pulse labelling technique. It has been found that the time spent in mitosis and the later stages of interphase is approximately similar in all regions of the meristem. The extent of the mitotic cycle in quiescent centre cells is due to the length of time spent in G1 the period of interphase prior to DNA synthesis. Quiescent centre cells contain the 2G amount of DNA during approximately 90% of their cell cycle. It has been found that during recovery from colohicine treatment the mitotic index in the quiescent centre rises while it falls in other regions of the meristem. The cells forming the primordium from which the root regenerates after colochicine treatment are derivatives of quiescent centre cells. Using the number of cells carrying micronuclei as an estimate of radiation damage the radiosensitivity of different regions of the root meristem of Allium Sativium have been investigated after various doses of X-rays. It has been found that the cap initials are the most radiosenstive and quiescent centre cells by far the least sensitive. The duration of G1 is shorter in the cap initials than elsewhere in the meristem while it is longest in the quiescent centre. Cells from the two regions of the stele are approximately equally sensitive, the average values of G1 in these two regions are the same. Rates of mitosis in the different regions of the meristem have been measured by metaphase accumulation at various times after irradiation with 250 rads of X-rays. It the quiescent centre three or four days after irradiation the rate of mitosis is greater than in the other regions of the root being ten times that found in the quiescent centre of unirradiated control roots. Average rates of micosis in the rest of the meristem are considerably reduced.

This study was designed to throw some light upon the nutritional requirements of certain commercially important species of the western coniferous forests of Canada. Five species, coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii), interior Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. glauca), Engelmann spruce (Picea engelmannii (Parry) Engelm.), ponderosa pine (Pinus ponderosa Dougl.) and western white pine (Pinus monticola Dougl.) were grown in sand cultures in a greenhouse and artificially subjected to macro-nutrient deficiency for a period of nineteen months. The quantitative effects of such deficiencies on shoot and root growth, shoot/root ratios, chlorophyll content, vigor, specific gravity of the wood, foliar content of reducing sugars and foliar content of copper and zinc were studied and described. Chlorotic and necrotic symptoms were compared with the use of a coded color chart and keys to the various deficiencies constructed on the basis of these color comparisons. The foliar symptoms of deficiency were completely documented by color photography. A concurrent study of the growth of the same five species on four different soil types was also undertaken. Apart from affording useful information on the suitability of the soils for the growth of the species studied, this portion of the experiment confirmed the fact that the sand culture technique is an acceptable experimental method for growing conifers. The concept of tolerance was discussed and a method proposed for its objective determination. In essence, then, this study established a dependence between five coniferous species and an adequate supply of macrometabolic elements for normal development and growth. The imposition of deficiencies of any one of six elements studied resulted in reduced growth and arrested development. Quantitative and qualitative confirmation is given of the reality of these effects. / Graduate and Postdoctoral Studies / Graduate

Inaug.-Dissert. Kgl. Bayer. Julius-Maximilians-Universität Würzburg. 17 Feb. 1904.

Thèse--Université de Paris. / "Index bibliographique": p. [159]-160.

Thèse--Université de Paris. / "Index bibliographique": p. [159]-160.

Inaug.-Dissert. Kgl. Bayer. Julius-Maximilians-Universität Würzburg. 17 Feb. 1904.

The floral transition is a developmental process through which some plants commit to flowering and stop producing leaves. This is controlled by changes in gene expression in the shoot apical meristem (SAM). Many of the genes involved are known, but their interactions are usually only studied one by one, or in small sets. While it might be necessary to properly ascertain the existence of regulatory interactions from a biological standpoint, it cannot really provide insight in the functioning of the floral-transition process as a whole. For this reason, a modelling approach has been used to integrate knowledge from multiple studies. Several approaches were applied, starting with ordinary differential equation (ODE) models. It revealed in two cases – one on rice and one on Arabidopsis thaliana – that the currently available data were not sufficient to build data-driven ODE models. The main issues were the low temporal resolution of the time series, the low spatial resolution of the sampling methods used on meristematic tissue, and the lack of gene expression measurements in studies of factors affecting the floral transition. These issues made the available gene expression time series of little use to infer the regulatory mechanisms involved. Therefore, another approach based on qualitative data was investigated. It relies on data extracted from published in situ hybridization (ISH) studies, and Boolean modelling. The ISH data clearly showed that shoot apical meristems (SAM) are not homogeneous and contain multiple spatial domains corresponding to coexisting steady-states of the same regulatory network. Using genetic programming, Boolean models with the right steady-states were successfully generated. Finally, the third modelling approach builds upon one of the generated Boolean models and implements its logic into a 3D tissue of SAM. As Boolean models cannot represent quantitative spatio-temporal phenomena such as passive transport, the model had to be translated into ODEs. This model successfully reproduced the patterning of SAM genes in a static tissue structure. The main biological conclusions of this thesis are that the spatial organization of gene expression in the SAM is a crucial part of the floral transition and of the development of inflorescences, and it is mediated by the transport of mobile proteins and hormones. On the modelling front, this work shows that quantitative ODE models, despite their popularity, cannot be applied to all situations. When the data are insufficient, simpler approaches like Boolean models and ODE models with qualitatively selected parameters can provide suitable alternatives and facilitate large-scale explorations of the space of possible models, due to their low computational cost.

Understanding root development and patterning is important for both nutrient and water uptake. The Arabidopsis thaliana primary root has a di-arch vascular pattern consisting of a central xylem axis, perpendicular phloem poles and intervening procambial cells. Governance of this pattern involves a dynamic, antagonistic interaction between domains of auxin and cytokinin signalling bias. Here, one element of this auxin-cytokinin relationship; cytokinin’s indirect transcriptional regulation of the auxin PIN-FORMED 7 (PIN7) efflux transporter, has been investigated. Two complementary strategies were employed; transcriptomic profiling of an Type-B ARABIDOPSIS RESPONSE REGULATOR (ARR) response (the last known components in the core cytokinin signalling machinery) via an inducible glucocorticoid system, and an EMS mutagenesis based forward genetic screen of reduced PIN7::PIN7:GFP expression and subsequent genomic resequencing to identify potential causative agents. Both workflows produced novel candidate PIN7 regulators and the ensuing candidate validation revealed ETHYLENE RESPONSE FACTOR 104 (ERF104), CYTOKININ OXIDASE/DEHYDROGENASE 5 (CKX5) and the ECA1-like AT5G36520 from its vascular over-expressor DOUBLE PROTOXYLEM (DPX) phenotype, in particular as strong contenders for components involved in the regulation of PIN7 and patterning of the vascular cylinder.

No description available.

Call number: LD2668 .T4 1961 P35