Photocatalysis of nanoparticles mediates the response of plants towards nitric oxide in air.

Nitric oxide (NO), a signaling molecule involved in plant growth and metabolism, can be synthesized endogenously or assimilated from the atmosphere. Although NO can be oxidized to nitrate or NO2 by phototactically induced hot charge carriers over nanoparticles (NPs) under insolation, the specific role of NP-mediated NO transformation on foliage surfaces in stimulating plant enzymes remains unclear. In this research, Trident (T) and Willow (W) types of water spinach (Ipomoea aquatica Forsk) were employed to probe the physiological alteration by NO transformation and oxidants over photocatalytic NPs. The biomass of T was significantly enhanced by the 240-ppb NO stress or NPs, while W was immune to either condition. According to in-situ diffuse-reflectance-infrared-Fourier-transform spectroscopy, foliar ZnO or TiO2 (2 mg per plant) stimulated the oxidation of NO to NO3- and the production of •OH. NPs enhanced the activity of antioxidant enzymes and NR (nitrate reductase [NAD(P)H]) in T with low SOD (superoxide dismutase, 32 ± 7 Umin-1g-1), NR activity (reaching 188 ± 4 nmol h-1g-1) was positively correlated with a 46.39 % increase in biomass. Conversely, W, endowed with ample SOD (502 ± 30 Umin-1g-1) to offset the stress caused by NO or NPs, displayed negligible growth or NR alterations. Our findings indicate that a high [SOD] could counterbalance the oxidizing NO stress, while the photocatalytic NO conversion into nitrate could boost the NR production in plant with low [SOD] and under NO stress. This research contributes to understanding the impact of NPs application and plant responses to pollutant gas stress.