Effects of light-dependent herbicides on growth, antioxidant enzymes, and chlorophyll fluorescence in Salvia nemorosa

Many species of the Salvia genus (Lamiaceae) are vital medicinal plants, the commercial yields of which, can be significantly impacted by competition from weed species. However, to date, no herbicides have been registered for selective weed control in the commercial production of Salvia . This study investigated the effects of two different herbicide treatments on Salvia nemorosa , a commonly cultivated species, in a controlled, glasshouse pot experiment. The herbicides included a photosynthetic inhibitor mixture (Phenmedipham + Desmedipham) and a Protoporphyrinogen Oxidase (PPO) inhibitor (Oxadiargyl). The results revealed that Salvia nemorosa exhibited greater susceptibility to the Phenmedipham + Desmedipham herbicide mixture, with significant damage observed, especially at higher concentrations. The quantum efficiency of Photosystem II (FV/Fm) was significantly reduced by the Phenmedipham + Desmedipham treatment across all concentrations, leading to a 62% decrease in FV/Fm at the recommended dose compared to the control. In contrast, Oxadiargyl treatment had a minimal impact on FV/Fm, with only a 3% reduction at the recommended dose. Both herbicide treatments resulted in increased malondialdehyde (MDA) levels, indicating lipid peroxidation and oxidative stress. However, Salvia nemorosa exhibited higher antioxidant enzyme activities (catalase, ascorbate peroxidase, and guaiacol peroxidase) in response to the Oxadiargyl treatment, suggesting a stronger defense mechanism against oxidative stress compared to the Phenmedipham + Desmedipham mixture. Furthermore, both herbicide treatments caused reductions in plant height and dry weight of shoots and roots, with the Phenmedipham + Desmedipham treatment showing the most significant decrease. Notably, the highest ascorbate peroxidase (APX) activity was observed at the recommended dose of Phenmedipham + Desmedipham, showing a 290% increase compared to other doses, while Oxadiargyl had a more pronounced effect on guaiacol peroxidase (GPX) activity, particularly at the recommended dose. These findings suggest that Salvia nemorosa demonstrates varying levels of sensitivity to different herbicides, with antioxidant enzyme activity playing a critical role in mitigating oxidative damage.