Analysis of photoprotection and apparent non-photochemical quenching of chlorophyll fluorescence in Tradescantia leaves based on the rate of irradiance-induced changes in optical transparence

The kinetics of irradiation-induced changes in leaf optical transparence (Δ T ) and non-photochemical quenching (NPQ) of chlorophyll fluorescence in Tradescantia fluminensis and T. sillamontana leaves adapted to different irradiance in nature was analyzed. Characteristic times of a photoinduced increase and a dark decline of Δ T in these species were 12 and 20 min, respectively. The Δ T was not confirmed to be the main contributor to the observed middle phase of NPQ relaxation kinetics (τ = 10-28 min). Comparison of rate of photoinduced increase in Δ T and photosystem II quantum yield recovery showed that the former did not affect the tolerance of the photosynthetic apparatus (PSA) to irradiances up to 150 μmol PAR·m –2 ·s –1 . Irradiance tolerance correlated with the rate of “apparent NPQ” induction. Considering that the induction of apparent NPQ involves processes significantly faster than Δ T , we suggest that the photoprotective mechanism induction rate is crucial for tolerance of the PSA to moderate irradiance during the initial stage of light acclimation (first several minutes upon the onset of illumination).