Light gradients in a plant canopy. (A) Varying light intensities are depicted as grey triangle, light quality gradients are given as red triangle. Under saturating light the photosynthetic electron transport chain (including PQH₂ and TRX) is highly reduced and feedback-de-excitation mechanisms restore the redox-poise in the photosynthetic light reaction. Enrichment of farred wavelengths can be observed under light limiting conditions in plant canopies which preferentially excite PSI leading to a redox imbalance between the photosystems and oxidation of the PQ-pool. By this means a signal is generated which leads to an adjustment of photosystem stoichiometry, thylakoid structure and metabolism as a consequence of the LTR For further details see text. (B) Fs/Fm parameter as a marker of LTR under laboratory and field conditions. Plants acclimated to either PSI-light or shade of a canopy (can) were shifted for 2–3 days to PSII-light or a neutral (neut) shade and vice versa. The plants show a comparable acclimation pattern with similar high Fs/Fm values under PSI-light and canopy shade, respectively, whereas plants acclimated to PSII-light or neutral shade exhibit drastically lower values. PAR under all light conditions was set to ∼20 µmol photons*m⁻²*s⁻¹.