Carbon isotope discrimination during photosynthesis and dark respiration in intact leaves of Nicotiana sylvestris:: comparisons between wild type and mitochondrial mutant plants

Leaf gas-exchange, carbon isotope discrimination (Delta) during photosynthesis, carbon isotope composition (delta C-13) of leaf dry matter, leaf carbohydrates and delta C-13 of dark respiratory CO2 were measured both in wild type (WT) and in a respiratory mutant of Nicotiana sylvestris Spegazz. plants. The mutation caused a dysfunction of complex I of the respiratory chain which has been described in detail by Gutierres et al. 1997, PNAS, 94, 3436. The aim of this work was to verify if this mutation has an influence on carbon isotope discrimination during photosynthesis and dark respiration. Another objective was to study the possible effect of respiratory fractionation on the isotopic composition of dry matter and on the discrimination measured on-line, in comparison with the expected Delta based on the model developed by Farquhar et al. 1982, AJPP, 9, 121. On-line Delta measured on leaves during photosynthesis was lower in the mutants (16.5 parts per thousand +/- 0.9) than in the WT (20.1 parts per thousand +/- 0.6), mainly due to lower conductance to CO2 diffusion (both across stomatal pores and in the gaseous and liquid phases across the mesophyll) in the mutants. No statistically significant difference in the fractionation during dark respiration was observed between WT and mutant plants. However, respiratory CO2 was enriched in delta C-13 compared to sucrose and glucose by about 2-3 and 2.5-4 parts per thousand, respectively. The enrichment in C-13 (about 2 parts per thousand) observed in leaf metabolites and leaf organic matter in the mutants compared to the WT can be explained by differences in Delta during photosynthesis. However, the fractionation in the whole-leaf organic matter of both WT and mutant plants was higher (more depleted in C-13) than expected based on the values obtained with on-line measurements during photosynthesis. The observed discrimination during dark respiration, releasing C-13-enriched CO2, may partly explain the higher fractionation in the remaining leaf organic matter compared to the overall discrimination during photosynthesis, as measured on-line.