Differential changes in respiratory activity in plant

Seed germination is a critical step in the plant life cycle. When quiescent dry seeds imbibe water, their oxygen uptake increases and energy metabolism becomes very active, allowing for the biosynthesis of macromolecules and solute transport required to complete germination. The metabolic source of ATP requires the operation of mitochondrial oxidative phosphorylation at the earliest stage of seed imbibition. The reactivation of mitochondrial metabolism also provides an important source of reactive oxygen species (ROS). Under normoxic conditions, approximately 2-3% of the oxygen used by mitochondria can be converted into superoxide and H2O2. Changes in ROS homeostasis result from a modification in the balance between ROS-producing and ROS-scavenging processes. Plants are provided with soluble antioxidant compounds, such as ascorbic acid, vitamin E and glutathione and a battery of protective and repair enzymes that can efficiently destroy superoxide radicals and hydrogen peroxide. These enzymes include superoxide dismutase (SOD), catalase (CAT), peroxidases (POD), thioredoxin system enzymes and ascorbate glutathione cycle enzymes.