The early growth of tiller buds in barley is determined by application of mineral nutrient solution to the growing medium. The aim of this research was to determine whether the effect of mineral nutrient supply is exerted through an effect on the levels of endogenous growth regulators, particularly cytokinins. Three approaches were used to examine the problem, namely to determine the effects of exogenously applied growth substances on growth of tiller buds, to observe bud growth under treatment expected to modify the levels of endogenous cytokinina, and to measure the cytokinin activity in plants supplied with minerals. Exogenously applied cytokinins not only increased lateral bud growth in plants supplied with minerals but also promoted growth of buds arrested by mineral nutrient stress. Application of auxin or gibberellin increased dry matter in buds released from apical dominance by either mineral nutrient or cytokinin application, but had no effect on arrested buds. Root removal treatments, expected to modify levels of endogenous cytokinins, released growth of tiller buds under conditions of lack of mineral supply. Both root removal experiments and treatments with exogenous growth regulators confirmed the idea of an important role for growth substances, particularly cytokinins in the release of buds from apical dominance. Preliminary results thowed the presence of a high level of a cytokinin-like substance in plants given minerals nutrient solution, which was absent in the deficient condition, also favouring the initial idea. It is proposed that growth of exillary buds in vegetative barley plants is controlled by interactions between nutritional factors and endogenous growth substances. It is considered that mineral nutrient availability determines the levels of endogenous cytokinin which in turn determines whether or not buds are released. Mineral nutrients, other metabolites and growth substances are important in controlling subsequent growth of the tiller bud.

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This thesis reports the results of studios using cultured vascular endothelial and smooth muscle cells derived from porcine aorta to gain an understanding of their physiological roles, particularly in the control of vascular permeability and vascular tone. Cultured vascular cells are identified in primary culture and subculture by light and electron microscopy and their ultrastructure is compared with that of aortic tissue. Porcine vascular cells are reported to show the same morphology and ultrastructural features as vascular cells from other sites and species. Endothelial cells are specifically identified from smooth muscle cells using immunofluoresoent staining for angiotensin converting enzyme. After positive identification, homogenous populations of vascular cells are used for biochemical studies. The regulation of cyclic AMP metabolism during the growth of vascular cells and the effect of exogenously applied vasoactive agents on intracellular cyclic AMP are studied. The roles of cyclic AMP in the control of cell growth and the regulation of cell contractility are discussed. Cyclic AMP is measured using a binding protein and the assay is validated using W138 fibroblasts, which are known to respond to prostaglandine and amines via increased cyclic AMP. The role of endogenous prostaglandin production in the modulation of intracellular cyclic AMP of vascular cells is discussed. The release of prostaglandin I2 and E2 by aortic tissue and vascular cells in culture is measured by radioimmunoassay. The rates of prostaglandin production by vascular tissue and vascular cells are expressed on a cellular basis and compared. The effects of environmental variables such as buffer composition and flow conditions are studied and the stimulatory effect of serum compared with that of exogenously supplied-arachidonic acid. Prostaglandin production is characterised using classical inhibitors such as aspirin; the potency of aspirin in tissues and cells is discussed. Tranyylcypromine and isobutylmethylxanthine are both reported to inhibit vascular prostaglandin production.

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