Zinc Nanoparticle Effects on the Green Leaf Volatiles and Phyllosphere Bacteriome in Capsicum annum Seedling

The application of zinc oxide nanoparticles (ZnONPs) in agriculture is expanding due to their biostimulant potential; however, their influence on plant chemical communication and associated microbial communities remains not fully characterized. This study presents a multi-perspective analysis contrasting the effects of ZnONPs with those of conventional microparticulate ZnO (Bulk) on Capsicum annuum seedlings grown in substrate at 50 and 500 mg kg−1. Results indicate that, at high doses, the bulk material (B500) led to higher foliar zinc accumulation (128.7 mg kg−1) compared to ZnONPs (NP500, 119.7 mg kg−1), a difference potentially linked to nanoparticle aggregation in the soil matrix limiting root uptake. At the physiological level, a distinct response was observed: while Bulk ZnO stimulated superoxide dismutase (SOD) activity, ZnONPs resulted in a marked reduction (93%), suggesting a shift in the antioxidant strategy toward non-enzymatic mechanisms, such as increased total phenol content. Regarding the volatilomic profile, ZnONPs induced specific metabolic alterations in the green leaf volatile (GLV) pathway, characterized by hexanal accumulation and reduced levels of hexanol and hexyl acetate. Additionally, ZnONPs were associated with lower methyl salicylate (MeSA) emissions, whereas the Bulk treatment increased its relative abundance to 41.7%. Finally, metagenomic analysis revealed that zinc treatments modulated the phyllosphere microbiota, favoring the proliferation of Actinobacteria while decreasing the abundance of sensitive taxa, such as Spirochaetes. Taken together, these findings suggest that ZnONPs act as a distinct metabolic modulator, altering internal physiology and chemical signaling.