2-Naphthoic acid induces systemic phytotoxicity and impairs photosynthetic performance in maize

The discovery of phytotoxic compounds remains essential for the development of next-generation herbicides. Here, we investigated the physiological effects of 2-naphthoic acid on maize, a model species with C4 photosynthesis, and complemented this analysis with germination assays in maize and four agronomically relevant weed species to assess interspecific variation in sensitivity and its potential as a lead compound for herbicide discovery. Maize plants were cultivated for 14 days under hydroponic conditions, with 2-naphthoic acid supplied through the nutrient solution. Exposure severely impaired plant growth, particularly root development and architecture, and reduced photosynthetic performance, including carbon dioxide assimilation, apparent quantum efficiency, photosystem II photochemical function, and electron transport. Chlorophyll a fluorescence analyses, including fast fluorescence transients, confirmed impaired photochemical performance, whereas maximum photosystem II quantum efficiency and the contents of chlorophyll a, chlorophyll b, and carotenoids remained unchanged. At the cellular level, 2-naphthoic acid induced redox imbalance, characterized by increased reactive oxygen species in leaves, enhanced conjugated-diene formation, and reduced antioxidant enzyme activities in leaves and roots. Acute in vitro assays revealed pronounced sensitivity of catalase to 2-naphthoic acid, whereas superoxide dismutase and peroxidase were not significantly affected. The compound was also associated with changes in mineral nutrient composition. Germination responses were species-dependent, with Digitaria insularis showing the strongest inhibitory response. Collectively, these findings identify 2-naphthoic acid as a potent phytotoxic compound and support its further evaluation for herbicide discovery.