Considerable cumulative evidence has accrued suggesting a vital role for mitochondrial function in optimizing photosynthesis. Both pharmacological approaches using respiratory inhibitors and reverse genetic approaches have recently underscored the high degree of interconnection between photosynthesis and respiration--the major pathways of energy production which are largely confined to the plastid and mitochondria, respectively. Here recent studies into the nature of these interactions are reviewed, with particular focus on (i) the recently described link between the mitochondrial electron transport chain activity, ascorbate biosynthesis, and photosynthesis; and (ii) the contribution of mitochondrial metabolism to the photorespiratory process. Whilst there is increasing evidence of a role for ascorbate in co-ordinating the rates of respiration and photosynthesis, some data are presented here for plants grown under extreme environmental conditions that suggest that this relationship is not absolute. It thus seems likely that interactions between these compartments are perhaps more numerous and complicated than previously thought. This observation suggests that although the elucidation of the genetic bases of both photorespiration and the Wheeler-Smirnoff pathway of ascorbate biosynthesis has recently been completed, much further research is probably necessary in order to understand fully how energy metabolism is co-ordinated in the illuminated leaf.