Zinc Nanoparticle Effects on the Green Leaf Volatiles and Phyllosphere Bacteriome in <em>Capsicum annum</em> Seedlings

The use of zinc oxide nanoparticles (ZnONPs) in agriculture has increased due to their biostimulant potential; however, their effects on plant chemical communication and associated microbial communities are still poorly understood. This study presents a multi-perspective analysis contrasting the effects of ZnONPs with those of conventional ZnO (Bulk) on Capsicum annuum seedlings grown in a substrate with concentrations of 50 and 500 mg kg⁻¹. The results reveal that, at high doses, the bulk material (B500) generated a higher foliar accumulation of zinc (128.7 mg kg⁻¹) than ZnONPs (NP500, 119.7 mg kg⁻¹), a phenomenon attributed to the agglomeration of nanoparticles in the soil matrix, which limits their root absorption. At the physiological level, a critical divergence was observed: while bulk ZnO stimulated the activity of the enzyme superoxide dismutase (SOD), ZnONPs caused severe inhibition of the same (93% reduction), compromising the enzymatic antioxidant machinery and forcing the plant to rely on non-enzymatic mechanisms, such as an increase in total phenols. The volatilomic profile revealed a specific metabolic disturbance induced by ZnONPs in the green leaf volatiles (GLV) pathway. A significant accumulation of hexanal and suppression of hexanol and hexyl acetate were detected, suggesting that the nanomaterial inhibited alcohol dehydrogenase (ADH). In addition, ZnONPs suppressed the emission of methyl salicylate (MeSA)—a key messenger in acquired systemic resistance—whereas the Bulk treatment increased its abundance to 41.7%. Finally, metagenomic analysis indicated that zinc stress restructured the phyllosphere microbiota, promoting the proliferation of Actinobacteria and eliminating sensitive taxa such as Spirochaetes. Taken together, these findings demonstrate that ZnONPs act as multifactorial stressors that not only alter internal metabolism but also silence chemical communication and remodel plant ecology.