Oxygen, the terminal electron acceptor for mitochondrial electron transport, is vital for plants because of its role in the production of ATP by oxidative phosphorylation. While photosynthetic oxygen production contributes to the oxygen supply in leaves, reducing the risk of oxygen limitation of mitochondrial metabolism under most conditions, root tissues often suffer oxygen deprivation during normal development due to the lack of an endogenous supply and isolation from atmospheric oxygen. Since changes in oxygen concentration have multiple effects on metabolism and energy production (Geigenberger, 2003), tight control of oxygen consumption and homeostasis is likely to be particularly important in underground tissues such as roots. Nitric oxide (NO) is involved in many plant processes (Mur et al., 2013) and, under hypoxia, there is good evidence that nitric oxide (NO) contributes to the recycling of NADH (Stoimenova et al., 2007), the synthesis of ATP (Stoimenova et al., 2007), and the regulation of oxygen consumption (Borisjuk et al., 2007). The involvement of NO in the metabolic response to low oxygen is consistent with increased NO production during oxygen deprivation (Borisjuk et al., 2007), but the extent to which NO might also play a role in the energy metabolism of roots under normal aerobic conditions is unknown. Mitochondria, whose functions are central to aerobic metabolism, are the major source of NO in plants, and potential targets for NO include cytochrome c oxidase in the mitochondrial electron transport chain (Gupta et al., 2011). Thus, NO could influence oxygen consumption under normal aerobic conditions in roots, and it is this specific function that is assessed here.