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

Abstract

Leaf gas-exchange, carbon isotope discrimination (D) during photosynthesis, carbon isotope composition (^d13 C) of leaf dry matter, leaf carbohydrates and ‰ ^d13 C of dark respiratory CO ₂ 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 D based on the model developed by Farquhar et al. 1982, AJPP, 9, 121. On-line D measured on leaves during photosynthesis was lower in the mutants (16.5‰ 0.9) than in the WT (20.1‰ 0.6), mainly due to lower conductance to CO ₂ 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 CO ₂ was enriched in ¹³ C compared to sucrose and glucose by about 2–3 and 2.5–4‰, respectively. The enrichment in ¹³ C (about 2‰) observed in leaf metabolites and leaf organic matter in the mutants compared to the WT can be explained by differences in .during photosynthesis. However, the fractionation in the whole-leaf organic matter of both WT and mutant plants was higher (more depleted in ¹³C) than expected based on the .values obtained with on-line measurements during photosynthesis. The observed discrimination during dark respiration, releasing ¹³ C-enriched CO ₂ , may partly explain the higher fractionation in the remaining leaf organic matter compared to the overall discrimination during photosynthesis, as measured on-line.