Photosystem II (PSII) photochemistry and oxidative stress of mature leaves (ML) and young leaves (YL) of 4-week-old Arabidopsis thaliana plants subjected to water deficit for 24 h was investigated. To the onset of drought, maximum quantum yield of PSII photochemistry (Fv/Fm), quantum efficiency of PSII photochemistry (ΦPSII), quantum yield for dissipation by down regulation (ΦNPQ) and electron transport rate decreased in ML more than in YL. These changes were accompanied by increased quantum yield of non-regulated energy dissipation (ΦNO) and increased excitation pressure (1 - qP) in ML more than in YL. The more excitation energy dissipated by nonphotochemical quenching (NPQ) in drought-stressed YL compared to ML seemed to be sufficient in scavenging reactive oxygen species, as it was evident by the decreased lipid peroxidation level measured as malondialdehyde content. The better PSII functioning of YL may reflect their capacity to acclimate better to the onset of drought stress, as revealed by the higher ΦPSII, the more excitation energy dissipated by NPQ and the decreased excitation pressure (1 – qP). Our results suggest that the dissipation of excess excitation energy in YL plays an important role in order to avoid possible photodamage to PSII under drought stress conditions.
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