Beschreibung:
<p> • 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 CO<sub>2</sub> concentrations in the younger, than in the mature, part of the leaf, probably due to a lower Rubisco activity or a higher CO<sub>2</sub> diffusion resistance. • The induction of ETR at CO<sub>2</sub> 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 <sup>14</sup> C-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 CO<sub>2</sub> for the Calvin cycle in these regions. </p>