Transgenic tobacco, exhibiting transformed ascorbate oxidase (AO) activity, was employed to examine the role of cell wall-localised ascorbate (ASCapo) in mediating ozone (03) tolerance. Plants exhibiting reduced ASCapo content developed typical visible symptoms of 0 3 sensitivity, but plants in which the ASCapo pool had been substantially reduced were not as 'sensitive' to 03induced oxidative stress as predicted. This study supported the growing view that AO mediated shifts in ASCapo constitute an important regulatory mechanism in the orchestration ofplant responses under stressful conditions. A reduction of apoplast pH by an average 0.5 units when common stinging nettle plants were fed with an ~+ based solution did not appear to alter ASCapo content in accordance to the 'ion trapping theory' of ASC partitioning between sub-cellular compartments. This finding casts doubt an assumption within the Simulated Ozone Detoxification in the Apoplast (SODA) model developed to estimate 0 3 impinging on the plasmalemma by taking into account potential 03 detoxification by ASC. Laboratory and field-based measurements on the flag leaf of spring wheat facilitated the. derivation of multiplicative algorithm describing the manner in which key environmental variables modulate stomatal conductance, and thus stomatal 03 uptake. Utilising SODA it was possible - for the first time - to formulate a preliminary multiplicative algorithm describing the manner in which environmental parameters drive shifts in detoxification (i.e. mesophyll conductance) and may in future provide a better predictor of flux-response relationships than traditional approaches employing an empirically-derived cutoff flux threshold. Consistently throughout each study, modification of ASCapo and/or redox status was reflected in changes in stomatal conductance thus supporting the growing body of evidence that ASCapo and/or redox status plays a role in modulating stomatal conductance.