This thesis describes work undertaken to determine the effect of imazethapyr on the morphology and physiology of stored potato tubers. Imazethapyr is an effective sprout suppressant, providing control at least equivalent to industry standards, and the molecule has a very favourable toxicological profile. However, some treated and stored tubers have displayed increased incidence of vascular discolouration. This work identifies links between imazethapyr treatment and vascular discolouration and the conditions under which it may occur. Imazethapyr treatment was found to cause an increase in tuber respiration, increases in polyphenol oxidase activity, inhibition of mitotic division, disruption of amyloplast integrity and electrolyte leakage. These effects were also examined when imazethapyr was applied to tubers in adverse conditions. In many cases synergistic relationships were detected when imazethapyr was applied at low temperature, acidic pH, with an ethylene-releaser and with immersion in water. In some cases the formulation of imazethapyr could be contributing to vascular discolouration more than the active ingredient itself. A novel method was developed to trace ¹⁴C-imazethapyr through the treated tuber. This study has illustrated the rapid movement of imazethapyr in the tuber periderm and has demonstrated that only a small percentage of applied imazethapyr enters the tuber cortex. The activity detected in the tuber cortex increased when imazethapyr was applied in acidic conditions, suggesting that the increases in vascular discolouration could be due to increased penetration of imazethapyr into the cortex under stress conditions. It is concluded that imazethapyr, in the present formulation, can cause increased incidence of vascular discolouration, particularly when applied under adverse conditions.