Membrane integrity and oxidative properties of mitochondria isolated from imbibing pea seeds after priming or accelerated ageing

Germination is an energy-demanding process that requires the operation of mitochondria, which must survive desiccation in the quiescent seed and become rapidly functional after imbibition to meet the ATP demand. The relationship between germination and mitochondrial performance was addressed by analysing the properties of mitochondria isolated from control, primed and aged pea (Pisum sativum L.) seeds. Mitochondria were isolated and purified at early stages of germination (before radicle protrusion), and their oxidative properties, membrane integrity and ultrastructure were examined. Mitochondria isolated after 12 h of imbibition readily oxidized exogenous NADH and Krebs cycle substrates at high rates. However, their phosphorylation efficiency was restricted by poor membrane integrity. After 22 h from the beginning of imbibition, purified seed mitochondria had intact outer membranes and oxidized the substrates at slightly lower rates, but with higher respiratory control (improved capacity for phosphorylation). Purified seed mitochondria were always found to be deficient in endogenous NAD, although the organelles were capable of importing and retaining the cofactor. While the priming treatment appeared to slightly increase the performance of mitochondria, seed deterioration by accelerated ageing strongly affected the oxidative properties of mitochondria, which were badly impaired in ATP production. Outer and inner membrane integrity was identified as the primary target for desiccation and ageing stress. A link between mitochondrial function and seed quality was also corroborated by respiration measurements of seed fragments at the onset of imbibition.