Mitochondria play essential roles in plant growth and development as they host the oxidative phosphorylation pathways, tricarboxylic acid cycle and other important metabolisms. Disruption of mitochondrial functions frequently leads to embryo lethality. Moreover, mitochondria play roles in programmed cell death, pathogen and stress responses in plants. In contrast to animal mitochondria, plant mitochondria possess an additional electron transport pathway, the cyanide-resistant alternative pathway catalyzed by a single alternative oxidase (AOX). Unlike cytochrome pathway that is coupled to oxidative phosphorylation via proton translocation, electron transport from ubiquinol to AOX is non-phosphorylating. It releases the energy as heat. Chlorolab II liquid-phase oxygen electrode (Hansatech) is a high-precise Clark type oxygen electrode, which is equipped with the powerful WINDOWS software and could record the oxygen changes in real time. Its electrode disc comprises a central platinum cathode and a concentric silver anode. The electrode disc is connected to an electrode control unit which applies a small polarising voltage between the platinum and silver electrodes. In the presence of oxygen, a small current is generated proportional to oxygen content in the sample. It could respond sensitively and rapidly to small changes of oxygen content in the sample. This protocol describes how to measure the mitochondrial total respiration rate, cytochrome pathway capacity as well as alternative pathway capacity in maize leaves with chlorolab II oxygen electrode.
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