Respiratory carbon loss in plant tissues under environmental stress

Crop productivity is a balance between carbon gain by photosynthetic assimilation of CO2 and the release of fixed carbon as CO2 via respiration. Respiration is the process by which carbohydrates are oxidised to produce ATP to fuel biochemical reactions, whilst simultaneously releasing CO2 as a by-product; therefore, increased demand for ATP or decreased efficiency of ATP production by uncoupling of the mitochondrial electron transport chain results in greater CO2 production. ATP produced by respiration is either used to support processes involved in growth or to power cell maintenance processes, such as macromolecule turnover or maintenance of membrane ion gradients. Respiration increases when plants are exposed to high temperatures; a factor that will become increasingly important as we try to maximise food production as the global climate changes. However, it is unknown if increased respiration at high temperature is necessary to provide energy to support growth, is a consequence of increased ATP consumption for maintenance processes or is due to increased mitochondrial uncoupling at high temperature. Flux measurements showed that CO2 production by excised Arabidopsis thaliana roots increases with temperature up to 37°C. Although growth also increased up to 37°C resulting in increased respiration associated with growth processes, the majority of overall CO2 production at high temperatures could be accounted for by non-growth respiration. An analysis of ATP-consuming processes demonstrated that protein turnover and maintenance of ion gradients collectively account for the majority of maintenance respiration, but that ATP consumption for the maintenance of ion gradients is quantitatively more important than protein turnover at high temperature. Furthermore, a decrease in in vivo P/O ratio at high temperature was demonstrated; the results presented suggest that this is most likely due to increased basal proton leak across the inner mitochondrial membrane. It can be concluded that increased CO2 production at high temperature results from a combination of increased ATP consumption for the maintenance of ion gradients and a decrease in coupling of the mitochondrial electron transport chain through a common mechanism of increased membrane fluidity and ion leak.This thesis is not currently available via ORA