• 14C-translocation;
  • chlorophyll a fluorescence;
  • electron transport rate (ETR);
  • imaging;
  • leaf development;
  • Nicotiana tabacum (tobacco);
  • quantum efficiency of photosystem II;
  • sink–source transition


  •  The sink–source transition of developing Nicotiana tabacum (tobacco) leaves was studied here using chlorophyll fluorescence imaging.
  •  In accordance with leaf development, the quantum efficiency of PSII, showed a steep gradient across the leaf with increasing values towards the tip.
  •  The linear electron transport rate (ETR) saturated at higher CO2 concentrations in the younger, than in the mature, part of the leaf, probably due to a lower Rubisco activity or a higher CO2 diffusion resistance.
  •  The induction of ETR at CO2 concentrations near the compensation point after long-term dark adaptation of the young leaf, showed distinct responses; ETR rose rapidly in the basal but more slowly in the apical regions. There was a correlation between fast induction and carbohydrate import, as measured by 14C-translocation. In the basal regions, larger pools of metabolic intermediates are expected due to imported carbohydrates. These might be used in the Calvin cycle directly after dark–light transition providing the electron acceptors for the faster induction of ETR. Additionally, a higher mitochondrial respiration can provide CO2 for the Calvin cycle in these regions.