| Summary: | We investigated flavonoid accumulation and lipid peroxidation in young leaves (YL) and mature leaves (ML) of <i>Arabidopsis thaliana</i> plants, whose watering stopped 24 h before sampling, characterized as onset of drought stress (OnDS), six days before sampling, characterized as mild drought stress (MiDS), and ten days before sampling, characterized as moderate drought stress (MoDS). The response to drought stress (DS) of photosystem II (PSII) photochemistry, in both leaf types, was evaluated by estimating the allocation of absorbed light to photochemistry (Φ<i><sub>PSII</sub></i>), to heat dissipation by regulated non-photochemical energy loss (Φ<i><sub>NPQ</sub></i>) and to non-regulated energy dissipated in PSII (Φ<i><sub>NO</sub></i>). Young leaves were better protected at MoDS than ML leaves, by having higher concentration of flavonoids that promote acclimation of YL PSII photochemistry to MoDS, showing lower lipid peroxidation and excitation pressure (1 − q<sub><i>p</i></sub>). Young leaves at MoDS possessed lower 1 − q<sub><i>p</i></sub> values and lower excess excitation energy (EXC), not only compared to MoDS ML, but even to MiDS YL. They also possessed a higher capacity to maintain low Φ<i><sub>NO</sub></i>, suggesting a lower singlet oxygen (<sup>1</sup>O<sub>2</sub>) generation. Our results highlight that leaves of different developmental stage may display different responses to DS, due to differential accumulation of metabolites, and imply that PSII photochemistry in <i>Arabidopsis thaliana</i> may not show a dose dependent DS response.
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