Suppression of mitochondrial dehydrogenases accompanying post-glyoxylate cycle activation of gluconeogenesis and reduced lipid peroxidation events during dormancy breakage of walnut kernels by moist chilling

• Moist chilling suppressed walnut kernel pyruvate- and succinate-dehydrogenases.
• Cold conditions activate succinate oxidase and phosphoenolpyruvate carboxykinase.
• Declined kernel respiration favors post-glyoxylate cycle gluconeogenesis activation.
• Buildup of lipid hydroperoxides only occurred in warm incubated kernels.
• Moist chilling attenuates kernel peroxidative events thus promotes germination.

Cold stratification supposedly allows dormant seeds to germinate by removing their metabolic blocks. Nothing is known about the role of mitochondrial enzymes in metabolic regulation of seeds during dormancy release. As cold stratification removes walnut (Juglans regia L.) kernel dormancy, the activities of three mitochondrial enzymes i.e. pyruvate dehydrogenase complex (PDH), NAD+-isocitrate dehydrogenase (NAD+-IDH; EC 1.1.1.41), succinate dehydrogenase (SDH; EC 1.3.99.1) and the extra-mitochondrial glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) were compared in both cold-stratified (5 °C; 60 days) and warm-incubated (27 °C; 20 days) kernels. The kernel gluconeogenic competence was also assessed by measuring the activity of phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.49). These were then correlated to kernel lipid hydroperoxide contents and lipoxygenase (EC 1.13.11.12) activity. Lipid hydroperoxides and lipoxygenase activity were significantly greater in warm-incubated kernels. The mitochondrial NAD+-IDH activity was indifferent between cold-stratified and warm-incubated kernels. The activities of PDH and SDH though undetectable in cold-stratified kernels, were relatively high under warm conditions. Succinate oxidation in cold stratified kernels however, was achieved by a glyoxysomal succinate oxidase activity formerly reported in monocotyledons. Furthermore, the kernel succinate oxidase, PEPCK and G6PDH activities were significantly greater at cold conditions. It was concluded that cold induced dormancy release of walnut kernels is associated with suppression of kernel mitochondrial respiration and oxidative stress and meanwhile it allows activation of oxidative pentose phosphate (OPP) and post-glyoxylate cycle gluconeogenesis pathways. The build-up of oxidative stress and toxic lipid hydroperoxide derivatives however, probably result in the compromised germination of warm-incubated walnut kernels.