This thesis describes investigations which aim to increase understanding of the effect of temperature on root system development and longevity and to consider the role played by an important symbiotic fungus known as Arbuscular Mycorrhizae in influencing plant root demography. Mycorrhizal fungi (Order Glomales) are an important group of soil borne microorganisms which colonise the roots of most plants. The production and mortality of roots is an important factor determining nutrient and carbon fluxes in terrestrial ecosystems. The first experiment, carried out in soil and utilising <i>in-situ</i> camera techniques, minirhizotrons and image analysis software, establishes that in fact there is a temperature effect on root longevity, with roots living for shorter periods of time at higher temperatures. As this experiment was carried out in unsterile conditions the effect could have been due to a direct effect of temperature on the plant itself or due to other indirect effects such as the mineralisation of nutrients and fluctuating organic and inorganic N concentrations brought about by microbial activity. The remaining experiments utilise a microcosm system which was developed to enable visualisation of the whole root system <i>in situ</i>, whilst allowing definable levels of nutrients to be given to the plant. The system allows temporal and spatial root measurements to be made simultaneously and non-destructively. Results indicate that the altered longevity obtained in experiment one is not solely due to nutritional factors. Temperature has an impact on many aspects of root development including increased branching rates at higher temperatures, resulting in an increase in the number of higher order roots, as well as roots becoming longer and thinner with increasing temperature. These effects are likely to influence root longevity, as higher order roots are likely to survive for less time. AMF had the effect of increasing root:shoot ratios, thus, will also have implications in terms of nutrient cycling. In conclusion, understanding the dynamics of the plant root system is crucial to understanding ecosystem function, as temperature effects are significant, implications of climate change should be considered.

This project aimed to examine the relationship between spikelet primordium size at the double ridge stage and the final grain weight in two row barley. Earlier work in this laboratory had suggested that the size of primordia at the double ridge stage influenced final grain size. This work was repeated to confirm the finding and, whilst the relationship held, the results were quantitatively different due to differing environmental conditions during the later stages of grain development. Subsequent work sought to manipulate primordial width at double ridge stage by altering both temperature and nitrogen supply in order to determine the generality of the apparent primordium width/grain weight relationship. Results from these experiments led to investigation of the effect of tillers and the application of a growth retardent on the size of spikelets and grain. Lowered nitrogen supply for the first 40 days after sowing reduced the size of spikelet primordia at the double ridge stage and this resulted in the production of smaller grain. Reduced nitrogen supply for only 20 days after sowing led to reduced grain weight in <i>cv</i>. Maris Mink but not in <i>cv</i> Proctor. Proctor had larger central florets and set larger grain than Maris Mink. Reduced nitrogen supply during the vegetative stage of mainstem apex development had no effect on spikelet size in Proctor but marginally increased spikelet size in Maris Mink. Final grain size was reduced in Proctor but increased in Maris Mink. Lowering the temperature in which plants were grown increased grain size and number but had no effect on spikelet size.

The advantages of windbreak in terms of reducing soil erosion and providing tree products for farmers in the Sahel are well known. There is less certainty about the effect of shelter on crop growth, the degree of competition for water and light between windbreak and crop, and almost no information on the amount of water a windbreak transpires over a year. Field measurements of millet (<i>Pennisetum typhoides</i>) growth and microclimate behind a young double row neem (<i>Azardirachta indica)</i> windbreak at the ICRISAT Sahelian Centre, Sadore, Niger, were carried out in 1988 and 1989. Measurements of tree transpiration were made using a commercial sap flow meter, along with measurements of tree heights, stem basal areas and leaf areas. The reduction in wind speed produced by the windbreak changed over the season, as a result of the reduction of the porosity of the lower half of the windbreak by the growth of the millet crop on the windward side of the windbreak. Maximum shelter was found at 6<i>h</i> at the beginning of the cropping season and 3<i>h</i> during the middle and end of the season, where <i>h</i> is the height of the windbreak. Factors influencing the magnitude of shelter are discussed.

The usefulness of mathematical models in identifying efficient management strategies under uncertain conditions is, however limited by the theories used in models as well as by the availability and quality of field data that can be used in the calibration and validation of these models. Many models have been developed and used to simulate water and solute flux in the crop rootzone. This thesis describes the development and application of two different models, the WAVE and UNSATCHEM models to simulate water and solute transport in the vadose zone and their effect on crop transpiration and yield. The WAVE model was modified to include the effect of salinity on crop transpiration, and used to simulate soil water balances, to investigate long-term salinity build-up in the root zone, and in conjunction with a crop yield response model to assess their effect on crop yield. The practicality of the modelling approach in the establishment of optimal irrigation and drainage practices is considered through application to the Makhtaaral region of South Kazakhstan. The impact of several irrigation and drainage scenarios was evaluated.  Optimal irrigation and drainage strategies for sustainable crop production have been derived. The application of the UNSATCHEM model as a multi-species model to the Makhtaaral region is also demonstrated for the evaluation of the current irrigation and drainage practices. For the problem considered in this study, the WAVE model along with the crop yield response model can be used as a tool for assessing the impact of different irrigation and drainage scenarios on crop yield. The results demonstrate that the modelling approach is robust and applicable under arid and semi-arid conditions and to a wide range of water shortage and salinity.

No description available.

Windbreaks have been promoted as a means of arresting degradation of agricultural lands in the Sahel resulting from deforestation, but competition for water between windbreak trees and adjacent crops must be minimised if windbreaks are to be successfully established in areas where crop production is threatened by loss of trees. Research was done, consequently, between 1991 and 1993 in southern Niger, in West Africa, to aid development of strategies for the deployment of windbreaks in the Sahel which minimise competition for water. The objectives of this research were to: (1) quantify water use by windbreak trees; (2) determine the source of water utilised by windbreaks; and (3) identify the environmental and physiological variables controlling transpiration by windbreak trees. The heat-pulse method was used to measure water use during the cropping season by three tree species growing in windbreaks and patterns of water extraction from soil adjacent to windbreaks of each species were assessed from soil moisture contents measured by neutron attenuation. <I>Azadirachta indica </I>transpired less water than either <I>Acacia holosericea </I>or <I>Acacia nilotica </I>and extracted less water from the rooting zone of nearby crops, and was thus shown to be the least competitive of these tree species. Naturally-occurring variations in the ratio of the stable isotopes of oxygen (<SUP>18</SUP>O/<SUP>16</SUP>O) in water were utilised to trace the sources of water transpired by <I>Azadirachta indica </I>trees in windbreaks and adjacent crops of pearl millet (<I>Pennisetum glaucum</I>) at two locations with contrasting water table levels. Where groundwater could be accessed by the trees, they obtained large proportions of their water from surface layers of the soil only when water there was plentiful, for example after rain. Where groundwater was not accessible, the trees fulfilled their requirements for water from the surface layers of the soil profile throughout the year. Thus, competition for water between windbreaks and crops is more severe at locations where trees cannot utilise groundwater.

No description available.

No description available.

No description available.

No description available.