Changes in primary metabolites and volatile organic compounds in cotton seedling leaves exposed to silver ions and silver nanoparticles revealed by metabolomic analysis

In the area of climate change, nanotechnology provides handy tools for improving crop production and assuring sustainability in global agricultural system. Due to excellent physiological and biochemical properties, silver nanoparticles (AgNPs) have been widely studied for potential use in agriculture. However, there are concerns about the mechanism of the toxic effects of the accumulation of AgNPs on crop growth and development. In this study, the impacts of AgNPs on cotton (Gossypium hirsutum) seedlings were evaluated by integrating physiological and comprehensive metabolomic analyses. Potting-soil-grown, two-week-old cotton seedlings were foliar-exposed to 5 mg/plant AgNP or 0.02 mg/plant Ag(+) (equivalent to the free Ag(+) released from AgNPs). Primary metabolites and volatile organic compounds (VOCs) were identified by gas chromatography–mass spectrometry (GC-MS) and solid-phase microextraction (SPME) GC-MS, respectively. AgNPs inhibited the photosynthetic capacity of the cotton leaves. The metabolic spectrum analysis identified and quantified 73 primary metabolites and 45 VOCs in cotton leaves. Both treatments significantly changed the metabolite profiles of plant leaves. Among the primary metabolites, AgNPs induced marked changes in amino acids, sugars and sugar alcohols. Among the VOCs, 13 volatiles, mainly aldehydes, alkanes and terpenoids, were specifically altered only in response to AgNPs. In summary, our study showed that the comprehensive influence of AgNPs on primary metabolites and VOCs was not merely attributed to the released Ag(+) but was caused by AgNP-specific effects on cotton leaves. These results provide important knowledge about the physiological and chemical changes in cotton leaves upon exposure to AgNPs and offer a new insight for supporting the sustainable use of AgNPs in agriculture.