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This thesis is concerned with an analysis of swimming of oomycete zoospores, particularly in relation to the hypothesis that electrotaxis mediated host root colonisation. A correlation was found between the electrotactic behaviour of zoospores of <I>Phytophthora palmivora </I>and <I>Pythium aphanidermatum</I> and their localisation relative to anodic or cathodic regions of the majority of non-host roots. Cathodotropic <I>P. aphanidermatum </I>zoospores were found to be attracted to the cathode generated at the wound site on monocotyledonous and dicotyledonous plant roots studied. Zoospores of <I>P. aphanidermatum</I> were also found to become gradually less attracted to these wound sites as they dissipated over time. Anodotropic <I>P. palmivora </I>zoospores were found to be repelled by the cathodic wound sites on roots of all plants investigated, with the exception of <I>Petunia hybrida. </I>In addition to this zoospores of <I>P. aphanidermatum</I> were found not exhibit chemotaxis or encystment in gradients of the wound-specific metabolite acetosyringone. This suggests that electrotaxis and chemotaxis both operate in directing zoospore accumulation around roots. This reduction of the conductivity of the bathing medium by the addition of different concentrations of sodium chloride salts did not appear to affect the accumulation of both zoospore species around roots of rye grass. The results of this study supports the hypothesis that zoospores use electrotaxis as one means to locate new plant hosts in the rhizosphere. However, chemotaxis may still augment the regulation of zoospore colonisation and encystment.

Phytostabilization offers a less expensive alternative to traditional cap and plant methods for containing metalliferous mine tailings to prevent wind erosion and contamination of nearby communities and the environment. However, plant establishment during phytostabilization of pyritic legacy mine tailings in semiarid regions is challenging due to particularly extreme conditions including low pH, low organic carbon, low nutrients, and high toxic metal(loid) concentrations. Microorganisms drive major biogeochemical cycles in soils, however, the roles microorganisms play at the root – soil interface during phytostabilization, particularly in relation to plant health and metal immobilization, are not yet fully understood. The aims of this dissertation are to focus on bacterial communities associated with the roots of buffalo grass used in the phytostabilization of pyritic metalliferous mine tailings to: i) characterize bacterial diversity and communities of rhizosphere and bulk substrate, ii) delineate associations between rhizoplane bacterial colonization patterns and environmental and plant status parameters, and iii) develop an in situ method to visually assess associations between roots, bacteria, and metals. Key findings indicate that after addition of a compost amendment to alleviate the plant-growth inhibiting characteristics of mine tailings, rhizosphere and bulk substrate contain a diverse, plant-growth supporting bacterial community. As substrate re-acidifies due to compost erosion, an emergence of an iron (Fe)- and sulfur (S)-oxidizer and Fe-reducer dominated, less diverse community develops in the bulk and rhizosphere substrate, thus posing a threat to successful plant establishment. However, even at low pH, some plant-growth-promoting bacteria are still evident in the rhizosphere. On the rhizoplane (root surface), the relative abundance of metabolically active bacteria was positively correlated with plant health, verifying the strong association between plant health and bacteria. Furthermore, pH showed a strong association with the relative abundance of Alphaproteobacteria and Gammaproteobacteria on the rhizoplane. In relation to microbe-metal interactions on the root surface, results showed that Actinobacteria and Alphaproteobacteria colocalized with Fe-plaque and arsenic (As) contaminant on the root surface, indicating their potential role in adsorbing or cycling of these metal(loid)s. Developing a more thorough understanding of bacteria-root-metal interactions in relation to plant health and metal immobilization can help to improve phytostabilization efforts and success.

Sunflower plants (Helianthus annus, var. Russian mammoth) were grown in Hogland nutrient solution. The roots (after being subjected to treatments with either respiratory inhibitors or respiratory stimulators) were used to measure the flux of water Jw, flux of solute Js , and the rate of respiration Jo. The thermodynamic theory of irreversible processes was used to examine the interaction between fluxes, and the changes in conductivity under different treatments. The rate equations for a root membrane of unit thickness were developed as: Jw = LwwVw∆p + LwsRT ln C1s/C2s +LwoRT ln C1o/C2o Js = LswVx∆p + LssRT ln C1s/C2s + LsoRT ln C1o/C2o Jo = LowVw∆p + Los RT ln C1s/C2s + LooRT ln C1o/C2o Where: Lww, Lss, Loo are the direct transfer coefficients for water, solute , and oxygen; and Lws, Lsw, Lwo, Low, Lso, Los are the interaction or linked transfer coefficients; Vw partial molal volume (or specific volume) of water , ∆p the difference in pressure between the external solution and xylem: C1s and C2s , C1o and C2o are the salt and oxygen concentration in external solution and xylem respectively. The results showed that: 1. The nonlinearity of the flux of water through the root system of sunflower is due to causes associated with the membrane (mainly the permeability). 2. The increase in respiration did not increase the permeability of the membrane. 3. The uptake of water due to solute potential under transpiring conditions is small, but important. 4. The uptake of solute in normal root systems is by active process fromsolutions to the zylem and then moves passively to the leaves. 5. An increase in passive water uptake might cause an increase of respiration of the root.

The relationships that exist between soil and plant are very complicated and have aroused man's curiosity for centuries. Many studies have been done in order to understand and clarify this relation. Among these is the hypothesis that plant roots exhibit cation exchange capacity (CEC) and that this is in some way responsible for differential cation uptake . The CEC of the roots was defined by Helmy (l958a) as the total cations which can be exchanged or replaced from the root surface under a given set of conditions and is usually expressed as milli-equivalents per 100 grams (me/100 g) of dry roots. The CEC hypothesis of cation uptake proposes that the uptake of cations from soil by plants is in some extent controlled by the CEC of the plant roots and the valence of the cations. The CEC may therefore account for the differences between species in taking up different amounts of nutrients from the same soil.

A yellowing which develops in some plants growing on naturally calcareous soils is called lime-induced chlorosis. The problem is complex, as indicated by Brown and Holmes (1956) and Porter and Thorne (1955). Species and varieties of plants differ in their iron requirements, susceptibility to lime- induced chlorosis, and inter acting soil f actors which affect iron supply (Thorne et al 1950). Chlorosis of plants does not appear, therefore, to stem from a common causative factor. At least a part of this difference has been found to be associated with the plant roots. Thus, through the use of resistant root stock Wann (1941) was able to produce non-chlorotic grapes. These grapes grown under similar conditions without the resistant root stock would have been high1y chlorotic. Certain citrus root stocks have also been used on calcareous soils because they give citrus trees resistance to chlorosis.