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This Thesis aims to increase the understanding of the way in which herbicides inhibit plant photosynthesis by interference at the site of photosystem II. Two structurally dissimilar series of compounds, provided by Schering Agrochemicals (Saffron Walden, U.K.), one rigid, one flexible, both series showing varying herbicidal activity, were submitted to X-ray crystallography, NMR spectroscopy and molecular modelling studies. As a result of these investigations, an explanation is given for the observed trend in activity of both series of compounds. A template for the herbicide binding site is also presented. The most active molecules in the two series of' compounds were compared using molecular modelling. A model is presented which suggests that both molecules may interact with the same binding site at the photosystem II reaction centre.

The study was undertaken to improve phosphorus (P), iron (Fe), zinc (Zn) and manganese (Mn) availability and crop uptake in Kuwaiti calcareous soils. Field, glass-house and laboratory studies investigated the effectiveness of i) pyrophosphate (PP) mixed with orthophosphate (OP) fertilizers and ii) elemental sulphur (S') as a soil acidulent. In the field PP was incorporated with OP at a PP: OP P ratio of 0: 100,5: 95 and 10: 90 and applied at recommended agronomic P rates to Zea mays (corn) and Triticum spp (wheat) with micronutrients as subtreatments. Crop yields did not differ for all treatments receiving P probably due to high P residual levels in the soil but plant P uptake and P-tissue concentrations were significantly higher with PP treatment and Zn-tissue concentrations increased above the control treatments by 17% and 34% for the corn and wheat respectively, whereas with OP only treatments there was a slight decline in Zntissue concentrations. A further two experiments were conducted in the glass-house utilising a similar soil and experimental design to grow Zea mays and Avena sativa (oat) under distilled or brackish water irrigation regimes. PP amended treatments increased plant yields in the glass-house experiments by 15-18% and 20-25% for the corn and oats, respectively, while plant P-uptake increased by 83% and 32% when PP was incorporated with OP for the corn and oats respectively. Similarly, PP treatments stimulated higher plant Zn-uptake. In the glass-house Zn concentrations in plant tissues were higher by 24-40% for treatments with PP than with OP only. These findings were supported by NaHC03 P and DTPA-Zn soil extraction, where levels moved from "marginal" to "adequate" ranges with the inclusion of PP with OP. Generally there were no significant improvements when PP/OP ratio increased from 5 to 10%. Furthermore, brackish water irrigation had no direct influence on PP soil reaction or hydrolysis. Elemental sulphur (S') was applied in quantities that could theoretically neutralise 0,25,50,100 and 200% of the soil CaCO3 with two rates of P and micronutrient combinations in a split-split plot design. In the field experiment corn and wheat were grown as test crops, while in the glass-house corn was grown with distilled or brackish water after the soil/S' mixture was incubated at 30'C and approximate field moisture capacity for six months. The oxidation of S' to H 2SO4 did not proceed in a linear fashion; there were lower rates of S' oxidation at the high S' application, better monitored by the generated SO42 than the decline in either soil pH or CaCO3 content. Soil pH decline was moderate until soil CaCO3 content dipped below 6%. At these levels soils became acidic. The dissolution of soil CaCO3 was proportional to the S' applied, but its effective size distribution changed from coarse clay/fine silt to that of coarse silt/fine sand with the highest S' rates. This would have profound effects on its surface area and activity. The S' treatment also resulted in higher soil salinity; EC increased from 3-4 to as high as 12 dSm-1. Simultaneously there was a build up of gypsum which under SEM examination revealed that soil particle surfaces were shielded by gypsum crystals. Plants did not respond favourably to S' inclusion, and yield declined by 35% with the highest S' rates. However, at moderate rates the yield was similar to the control. Soil-P increased (NaHCO3 P extraction) at moderate levels of S' and then declined at higher rates. Extractable soil-Fe (DTPA) significantly increased only at higher S' rates, while Zn (DTPA) did not have a clear response and DTPA-Mn steadily increased even with the lowest S' rate. Plant tissue concentration and element uptake were difficult to assess in the light of declining plant growth and changed soil nutrient availability. Nevertheless S' treatment equivalent to 25% CaCO3 neutralisation maintained yield, P and Zn uptake while significantly increasing Fe and Mn uptake by 63 and 12% respectively for the field grown wheat. In none of the studies was there consistent response to micronutrient fertilisation. Furthermore, brackish irrigation did not interfere with the soil nutrient extractions or plant uptake.