The ¹³C/¹²C isotopic signal of day-respired CO₂ in variegated leaves of Pelargonium × hortorum

In leaves, although it is accepted that CO₂ evolved by dark respiration after illumination is naturally ¹³C-enriched compared to organic matter or substrate sucrose, much uncertainty remains on whether day respiration produces ¹³C-depleted or ¹³C-enriched CO₂. Here, we applied equations described previously for mesocosm CO₂ exchange to investigate the carbon isotope composition of CO₂ respired by autotrophic and heterotrophic tissues of Pelargonium×hortorum leaves, taking advantage of leaf variegation. Day-respired CO₂ was slightly ¹³C-depleted compared to organic matter both under 21% O₂ and 2% O₂. Furthermore, most, if not all CO₂ molecules evolved in the light came from carbon atoms that had been fixed previously before the experiments, in both variegated and green leaves. We conclude that the usual definition of day respiratory fractionation, that assumes carbon fixed by current net photosynthesis is the respiratory substrate, is not valid in Pelargonium leaves under our conditions. In variegated leaves, total organic matter was slightly ¹³C-depleted in white areas and so were most primary metabolites. This small isotopic difference between white and green areas probably came from the small contribution of photosynthetic CO₂ refixation and the specific nitrogen metabolism in white leaf areas.