Photosystem II- and photosystem I-inhibitor herbicides-driven changes in the dynamics of photosynthetic energy dissipation of Conyza spp

The quick detection and knowledge on dynamic dissipation of photosynthetic energy processes may improve weed management. This work aimed to evaluate the dynamic dissipation of photosynthetic energy of fleabane ( Conyza spp.) upon PSII- and PSI-inhibitor (photosystem I and II) herbicides application. Treatments were comprised of the application of metribuzin (PSII) and paraquat (PSI), following recommended doses for fleabane plants (40 days after germination: 10 to 12 leaves), and an untreated check. Chlorophyll a fluorescence transient was performed at 0.5, 1, 2, 4, 8, 24, 48 and 96 h after application (HAA) and gas exchange analysis was performed at 2, 4 and 96 HAA. Injury symptoms analysis was performed at all previously mentioned time points plus 7, 14, 21, and 28 days after application (DAA). Injury symptoms were observed at 7 DAA and 2 HAA of PSII- and PSI-inhibitors, respectively, while changes in the dynamic dissipation of photosynthetic energy were observed in both cases at 0.5 HAA. At 0.5 and 1 HAA, plants showed a decrease in Q A − (quinone) re-oxidation per reaction center and reduction in electron transport yield from Q A − to the end electron acceptor of the PSI, for both herbicides. Moreover, plants showed a 90% decline in the performance index and an increase in energy dissipation as heat and fluorescence. Fleabane showed a significant decrease in net assimilation rate (CO 2 ), transpiration rate and stomatal conductance within 1 HAA, for both herbicides. Therefore, PSII- and PSI-inhibitor herbicides presented a rapid negative effect on photosynthetic energy dynamics that can be monitored before the appearance of injury symptoms, which demonstrates the potential use of chlorophyll fluorescence in the rapid screening of metabolic perturbations caused by herbicides.