Oxygen consumption during leaf nitrate assimilation in a C₃ and C₄ plant: the role of mitochondrial respiration

Measurements of net fluxes of CO 2 and O 2 from leaves and chlorophyll a fluorescence were used to determine the role of mitochondrial respiration during nitrate (NO 3 – ) assimilation in both a C 3 (wheat) and a C 4 (maize) plant. Changes in the assimilatory quotient (net CO 2 consumed over net O 2 evolved) when the nitrogen source was shifted from NO 3 – to NH 4 + (Δ AQ ) provided a measure of shoot NO 3 – assimilation. According to this measure, elevated CO 2 inhibited NO 3 – assimilation in wheat but not maize. Net O 2 exchange under ambient CO 2 concentrations increased in wheat plants receiving NO 3 – instead of NH 4 + , but gross O 2 evolution from the photosynthetic apparatus ( J O2 ) was insensitive to nitrogen source. Therefore, O 2 consumption within wheat photosynthetic tissue (ΔΟ 2 ), the difference between J O2 and net O 2 exchange, decreased during NO 3 – assimilation. In maize, NO 3 – assimilation was insensitive to changes in intercellular CO 2 concentration ( C i ); nonetheless, ΔΟ 2 at low C i values was significantly higher in NO 3 – ‐fed than in NH 4 + ‐fed plants. Changes in O 2 consumption during NO 3 – assimilation may involve one or more of the following processes: ( a ) Mehler ascorbate peroxidase (MAP) reactions; ( b ) photorespiration; or ( c ) mitochondrial respiration. The data presented here indicates that in wheat, the last process, mitochondrial respiration, is decreased during NO 3 – assimilation. In maize, NO 3 – assimilation appears to stimulate mitochondrial respiration when photosynthetic rates are limiting.