lack of mitochondrial complex I in a CMSII mutant of Nicotiana sylvestris increases photorespiration through an increased internal resistance to CO₂ diffusion

The cytoplasmic male sterile II (CMSII) mutant lacking complex I of the mitochondrial electron transport chain has a lower photosynthetic activity but exhibits higher rates of excess electron transport than the wild type (WT) when grown at high light intensity. In order to examine the cause of the lower photosynthetic activity and to determine whether excess electrons are consumed by photorespiration, light, and intercellular CO₂, molar fraction (ci) response curves of carbon assimilation were measured at varying oxygen molar fractions. While oxygen is the major acceptor for excess electrons in CMSII and WT leaves, electron flux to photorespiration is favoured in the mutant as compared with the WT leaves. Isotopic mass spectrometry measurements showed that leaf internal conductance to CO₂ diffusion (gm) in mutant leaves was half that of WT leaves, thus decreasing the cc and favouring photorespiration in the mutant. The specificity factor of Rubisco did not differ significantly between both types of leaves. Furthermore, carbon assimilation as a function of electrons used for carboxylation processes/electrons used for oxygenation processes (JC/JO) and as a function of the calculated chloroplastic CO₂ molar fraction (cc) values was similar in WT and mutant leaves. Enhanced rates of photorespiration also explain the consumption of excess electrons in CMSII plants and agreed with potential ATP consumption. Furthermore, the lower initial Rubisco activity in CMSII as compared with WT leaves resulted from the lower cc in ambient air, since initial Rubisco activity on the basis of equal cc values was similar in WT and mutant leaves. The retarded growth and the lower photosynthetic activity of the mutant were largely overcome when plants were grown in high CO₂ concentrations, showing that limiting CO₂ supply for photosynthesis was a major cause of the lower growth rate and photosynthetic activity in CMSII.