Mitochondrial Respiration Can Support NO 3 − and NO 2 − Reduction during Photosynthesis

Abstract

Mass spectrometric analysis shows that assimilation of inorganic nitrogen (NH4 +, NO2 −, NO3 −) by N-limited cells of Selenastrum minutum (Naeg.) Collins results in a stimulation of tricarboxylic acid cycle (TCA cycle) CO2 release in both the light and dark. In a previous study we have shown that TCA cycle reductant generated during NH4 + assimilation is oxidized via the cytochrome electron transport chain, resulting in an increase in respiratory O2 consumption during photosynthesis (HG Weger, DG Birch, IR Elrifi, DH Turpin [1988] Plant Physiol 86: 688-692). NO3 − and NO2 − assimilation resulted in a larger stimulation of TCA cycle CO2 release than did NH4 +, but a much smaller stimulation of mitochondrial O2 consumption. NH4 + assimilation was the same in the light and dark and insensitive to DCMU, but was 82% inhibited by anaerobiosis in both the light and dark. NO3 − and NO2 − assimilation rates were maximal in the light, but assimilation could proceed at substantial rates in the light in the presence of DCMU and in the dark. Unlike NH4 +, NO3 − and NO2 − assimilation were relatively insensitive to anaerobiosis. These results indicated that operation of the mitochondrial electron transport chain was not required to maintain TCA cycle activity during NO3 − and NO2 − assimilation, suggesting an alternative sink for TCA cycle generated reductant. Evaluation of changes in gross O2 consumption during NO3 − and NO2 − assimilation suggest that TCA cycle reductant was exported to the chloroplast during photosynthesis and used to support NO3 − and NO2 − reduction.