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

Measurements of net fluxes of CO2 and O-2 from leaves and chlorophyll a fluorescence were used to determine the role of mitochondrial respiration during nitrate (NO3-) assimilation in both a C-3 (wheat) and a C-4 (maize) plant. Changes in the assimilatory quotient (net CO2 consumed over net O-2 evolved) when the nitrogen source was shifted from NO3- to NH4+ (DeltaAQ) provided a measure of shoot NO3- assimilation. According to this measure, elevated CO2 inhibited NO3- assimilation in wheat but not maize. Net O-2 exchange under ambient CO2 concentrations increased in wheat plants receiving NO3- instead of NH4+, but gross O-2 evolution from the photosynthetic apparatus (J(O2)) was insensitive to nitrogen source. Therefore, O-2 consumption within wheat photosynthetic tissue (DeltaOmega(2)), the difference between J(O2) and net O-2 exchange, decreased during NO3- assimilation. In maize, NO3- assimilation was insensitive to changes in intercellular CO2 concentration (C-i); nonetheless, DeltaOmega(2) at low C-i values was significantly higher in NO3--fed than in NH4+-fed plants. Changes in O-2 consumption during NO3- 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 NO3- assimilation. In maize, NO3- assimilation appears to stimulate mitochondrial respiration when photosynthetic rates are limiting.