Plants fix atmospheric carbon from photosynthesis during the day which they use for growth and respiration. They also store this carbon in their leaves as the polyglucan starch to use in nocturnal metabolism. Plants must carefully control this process to prevent starch synthesis or degradation at the wrong time in the diurnal cycle. Therefore, the regulation of the enzymes responsible for this turnover is crucial. This becomes increasingly difficult during periods of external abiotic stress, which can disproportionately affect the chloroplasts if reactive oxygen species created during the photosynthetic reactions build up. Most starch turnover enzymes are catalytically active under reduced conditions making production of starch in leaves difficult under oxidative stress conditions. This thesis will provide evidence showing plastidial starch phosphorylase (Pho1) can regulate starch turnover under oxidative stress and that oxidative activation of glucan phosphorylases is a general phenomenon that may also regulate glycogen metabolism in mammalian cells.