Antioxidants Differentially Regulate Non-Photochemical Quenching.

Non-photochemical quenching (NPQ) dissipates excess light energy as heat, limiting reactive oxygen species production during photosynthesis. Exogenous antioxidants are widely reported to improve plant performance under stress, yet their impacts on NPQ are inconsistent across studies, complicating mechanistic interpretation. Here, we systematically compared six antioxidants - ascorbate, chitosan, coenzyme M, reduced glutathione, melatonin, and N-acetyl cysteine - applied to Arabidopsis thaliana seedlings using individual concentration gradients, under standardised pH and illumination regimes. Antioxidants produced strong, divergent, and context-dependent shifts in NPQ kinetics. Coenzyme M most strongly suppressed NPQ, decreasing induction rate, maximum, and relaxation rate, whereas melatonin and N-acetyl cysteine generally enhanced NPQ maximum or relaxation and ascorbate uniquely accelerated NPQ induction. These effects changed at longer times post application. Using psbs, vde and zep null mutants, we show that short term ascorbate and coenzyme M NPQ effects require canonical NPQ components. The major contributor to their effects were opposing regulation of the xanthophyll cycle: ascorbate enhanced, while coenzyme M suppressed, light-driven violaxanthin de-epoxidation and altered xanthophyll flux during recovery. These data demonstrate two antioxidant-specific mechanisms, confirming their specificity.