The higher sensitivity of PSI to ROS results in lower chilling-light tolerance of photosystems in young leaves of cucumber.

The development of PSII tolerance to stress and photoprotection mechanisms during leaf growth has been widely studied, however, knowledge about PSI photoinhibition and interaction between PSI and PSII under stress during leaf growth is still lacking. This study showed that during the chilling-light treatment, the photoinhibitions of PSI and PSII were more severe in young leaves than in fully-expanded leaves of cucumber, but the inhibition of CO2 assimilation and the accumulation of reactive oxygen species (ROS) were similar in leaves at different development stages. During the chilling-light treatment, PSII photoinhibition was positive correlated to PSI photoinhibition in leaves, however, this correlation no longer existed in leaves pretreated with DCMU, an inhibitor of electron transport from PSII to PSI. Although the photoinhibitions of PSII and PSI in young leaves were more severe, the sensitivity of PSII to excitation pressure was lower in young leaves. The above results demonstrate that, the lower chilling-light tolerance of photosystems in young leaves was due to the higher sensitivity of PSI to ROS and the higher PSII excitation pressure caused by PSI photoinhibition in young leaves, rather than the difference of ROS content and sensitivity of PSII to excitation pressure between the young and fully-expanded leaves.