To assess the availability of bracken and the factors influencing frond yields, this thesis investigated the growth and phenology of bracken populations across Scotland in relation to site characteristics.  The change in elements in fronds and litter from April to November was analysed, along with the concentrations of the carcinogen Ptaquiloside (Pta).  Frond dry matter yields increased to maximum in August/September, falling to those in November.  In September/October, frond yields ranged from 4.70-15.9 DM t ha^-1.  Frond concentrations of phosphorus and potassium reduced, and concentrations of calcium and magnesium increased from May to November.  Frond Pta concentrations reduced during this period also, with the highest concentration recorded of 5792 mg kg^-1 in May 2002 at Loch Grannoch, SW Scotland. This thesis also investigated the effects of harvesting of fronds, and the burning and clearing of litter on the yield of frond dry matter, yield and content of ash and on Pta concentrations in fronds.  The concentration of twenty-one elements, the CaCO₃ equivalence and pH of this ash were determined, and the effect of harvesting of fronds on subsequent yields was assessed.  Harvesting during the growing season reduced the preceding yield of fronds, but not if harvested after senescence.  A positive regression was found between frond DM and ash yields.  The maximum yield of K in ash was harvested in August/September, at around 180 kg ha^-1.  Calculations of the value of bracken ash using August 2001 K concentrations and the cost of other organic K sources came to between £219-£980 t^-1.  The ash produced had a mean pH of 11.9, and mean calcium carbonate equivalence of 52%.  Lifting litter increased the density of fronds, and burning litter increased the soil available Mg concentrations and Ca concentrations in the ash.  Pta concentrations in fronds that emerged in reaction to harvesting were higher compared to those fronds first harvested at the same time.

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The toxicity of plant lectins to insects after oral ingestion was investigated by incorporating the proteins into insect diets. Bioassays carried out using an artificial diet system demonstrated that kidney bean (Phaseolus vulgaris) lectin (PHA) caused a significant decrease in survival of larvae of the tomato moth, Lacanobia oleracea. Jackfruit (Artocarpus integrifolia) lectin (jacalin) and black mulberry (Morus nigra) lectin both caused a significant decrease in growth of the peach potato aphid (Myzus persicae) when compared to controls in an artificial diet based bioassay. Interactions of lectins with insect gut tissues in vivo were studied by immunolocalisation. Binding of the snowdrop lectin (Galanthus nivalis agglutinin;GNA) and jack bean (Canavalia ensifomiis) lectin (Concanavalin A; Con-A) to the digestive tract of L oleracea larvae was observed and localised at the electron microscope level after oral ingestion of the proteins. GNA was also observed to bind to the midgut of the two-spot ladybird Adalia bipunctata. No disruption of the brush border membrane of either L oleracea or A. bipunctata was observed. Binding of GNA to the peritrophic membrane of L. oleracea was observed by fluorescence microscopy. Histological evidence of lectin binding to insect guts in vivo was corroborated by in vitro studies, which showed that the lectins GNA and Con- A bind to sections of the digestive tract of L. oleracea larvae. Binding of Con-A to proteins from brush border membranes, solubilised brush border membranes and peritrophic membranes was also observed. The use of confocal microscopy showed that GNA bound to the midgut and haemocytes of the peach potato aphid Myzus persicae, both when incubated with isolated tissues and cells and when fed orally to live insects, providing evidence for transport of GNA across the gut wall. Larvae of L.oleracea fed the lectins GNA and PHA showed a significant increase in polyphenoloxidase levels within the haemolymph, suggesting that the lectins were causing systemic responses in the insects. A partial sequence for leucine aminopeptidase a potential receptor for lectin binding was obtained from a cDNA library constructed from the midgut of the tomato moth larvae.

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Rice accounts for 21% of the total energy content of the world's food and about 40% of the world's population depend on it. Nitrogen fertilization, mainly as urea, has played a major role in increasing rice production. But the fertilizer use efficiency is low. Therefore, the present investigation was undertaken to understand the effect of floodwater algae and green manure on the transformations of labelled urea applied to floodwater. The pertinent literature was reviewed. The experiments were carried out in the growth chamber using 4 to 5 cm soil columns covered with 4 to 5 cm floodwater. Algae growing in floodwater increased the pH of floodwater during the day and it cane back to its original level during the night. The magnitude of the daily increase in pH was influenced by the alkalinity and the pH of the floodwater. The application of green manure depressed this daily increase in floodwater pH. The production of 02 during photosynthetic activity increased the depth of the aerobic soil layer under the soil-water interface during the early period of algal growth but it was eliminated by subsequent settling of algae on the soil surface. The application of green manure eliminated the aerobic soil layer even in the presence of algae. Algae growing in floodwater increased urea hydrolysis. Large quantities of applied N assimilated by growing algae resulted in lower total gaseous losses of applied N. The application of urea in floodwater where algae was already growing enhanced NH3 volatilization and denitrification. A large portion of N assimilated by algae can mineralize and be available to the same crop and this mineralization is further enhanced by the exclusion of light by the crop canopy. When urea was applied 17 days after incorporation of green manure it increased urea hydrolysis significantly but urea hydrolysis was not effected when urea was applied immediately after incorporation. Application of green manure caused less nitrification and subsequent denitrification which encouraged NH3 volatilization. Further investigation is needed to confirm these findings when growing rice plants are present.

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