Temporal and Age-Dependent Dynamics of Photoinhibitory Responses under Diverse Light Scenarios

Plants in natural environments encounter fluctuating irradiance, whereas controlled cultivation systems typically provide constant illumination.Coping with dynamic light conditions requires flexible regulation of photoprotective mechanisms, particularly non-photochemical quenching (NPQ), to dissipate excess energy and reduce photoinhibition.Here, we investigated chlorophyll fluorescence responses of lettuce (Lactuca sativa L. cv.Top Green) under four 10-hour light scenarios differing in the frequency and rate of transition to high light exposure: alternating light/dark cycles, a single prolonged high light period with dark recovery, stepwise increasing-decreasing irradiance, and fine-grained transitions of the stepwise scenario.Fluorescence was continuously monitored using a pulse-amplitude modulation fluorometer, and parameters including Y(II), Y(NPQ), Y(NO), NPQ, electron transport rate, and qP d , an index that compares actual and theoretical Fo values to assess the extent of photoinhibition, were analyzed across leaves of different ages.Alternating cycles caused progressive loss of PSII efficiency, whereas prolonged high light induced photoinhibition that was partially recovered during darkness.Stepwise light transitions enhanced electron transport and maintained higher photosynthetic efficiency, whereas fine-grained transitions alleviated photoinhibition but were limited by lower total light input.Responses were age-dependent, as younger leaves exhibited higher NPQ induction and maintained higher qP d during illumination.However, they also showed lower Y(II) and greater declines in F v /F m after treatment.Notably, qP d and F v /F m did not always align, reflecting their complementary roles in indicating real-time photoprotection and cumulative photoinhibition, respectively.Our results demonstrate that the balance between photoprotection and photoinhibition is strongly influenced by both light scenario and plant age, suggesting that dynamic lighting strategies accounting for these factors can enhance energy efficiency and sustainability in vertical farming.