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We investigated flavonoid accumulation and lipid peroxidation in young leaves (YL) and mature leaves (ML) of Arabidopsis thaliana 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 PSII ), to heat dissipation by regulated non-photochemical energy loss (Φ NPQ ) and to non-regulated energy dissipated in PSII (Φ NO ). 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 p ). Young leaves at MoDS possessed lower 1 - q p 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 NO , suggesting a lower singlet oxygen ( 1 O 2 ) 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 Arabidopsis thaliana may not show a dose dependent DS response.

We evaluated photosystem II (PSII) functionality in potato plants ( Solanum tuberosum L.) before and after a 15 min feeding by the leaf miner Tuta absoluta using chlorophyll a fluorescence imaging analysis combined with reactive oxygen species (ROS) detection. Fifteen minutes after feeding, we observed at the feeding zone and at the whole leaf a decrease in the effective quantum yield of photosystem II (PSII) photochemistry (Φ PSII ). While at the feeding zone the quantum yield of regulated non-photochemical energy loss in PSII (Φ NPQ ) did not change, at the whole leaf level there was a significant increase. As a result, at the feeding zone a significant increase in the quantum yield of non-regulated energy loss in PSII (Φ NO ) occurred, but there was no change at the whole leaf level compared to that before feeding, indicating no change in singlet oxygen ( 1 O 2 ) formation. The decreased Φ PSII after feeding was due to a decreased fraction of open reaction centers (q p ), since the efficiency of open PSII reaction centers to utilize the light energy (F v '/F m ') did not differ before and after feeding. The decreased fraction of open reaction centers resulted in increased excess excitation energy (EXC) at the feeding zone and at the whole leaf level, while hydrogen peroxide (H 2 O 2 ) production was detected only at the feeding zone. Although the whole leaf PSII efficiency decreased compared to that before feeding, the maximum efficiency of PSII photochemistry (F v /F m ), and the efficiency of the water-splitting complex on the donor side of PSII (F v /F o ), did not differ to that before feeding, thus they cannot be considered as sensitive parameters to monitor biotic stress effects. Chlorophyll fluorescence imaging analysis proved to be a good indicator to monitor even short-term impacts of insect herbivory on photosynthetic function, and among the studied parameters, the reduction status of the plastoquinone pool (q p ) was the most sensitive and suitable indicator to probe photosynthetic function under biotic stress.