The effect of PAW on the growth and gene activation of healthy and tomato-mottle-mosaic-virus inoculated tomato seedlings

Non-chemically based strategies for food production and plant protection are expected to improve the microbial decontamination and the activation of self-defence mechanisms against pathogens along with the increase of biometric and nutraceutical properties of plants. Non-equilibrium Low-Temperature Plasmas (NLTP) represent an emerging technology as a source of Oxygen and Nitrogen reactive species (RONS) and UV radiation that act efficiently in the decontamination, sterilization and chemical modification of surfaces. However, plasma treatments are mainly confined to the surface of materials, and therefore not able to interact with the entire plant tissue. Thus, different strategies are required for applications, such as the production of plasma-activated media. Among them, water activation by interaction with NLTP seems to be the most cost-effective approach. Exposing water to plasma enriches it with RONS that can have positive effects on plant development. Plasma Activated Water (PAW) can be used for watering crops, soaking seeds, and spraying on foliage, being a sustainable and environmentally friendly alternative to traditional chemical fertilizers and pesticides. This work aimed to evaluate the role of PAW in enhancing early-stage growth and the activation of defence-related genes in tomato plants. We produced PAW using a multi-pin double dielectric barrier pulsed discharge applied to distilled water. To control the production of the PAW we monitored the plasma emission. By following N2 SPS and FNS band emissions the plasma temperature was estimated, while hints on the E/N were derived during the Plasma Pulse. The energy deposited in the plasma was monitored by Charge Voltage characteristics. The obtained PAW was characterized in terms of content in reactive species, i.e. H2O2, NO3−, NO2−, pH and ORP. Seedlings of tomato cv. Regina were transplanted in commercial peat moss soil at two true-leaves stages. Pots were kept in a glasshouse box at constant temperature (25 °C) and an 18-h photoperiod. Tomato plants were irrigated with distilled water or PAW. Plants grown using the standard procedure were used as control. Negative control plants were also cultivated using the conventional procedure. Half of the samples were also inoculated with Tomato Mottle Mosaic Virus (TMMV). The results on the plant growth were characterized in terms of biometric parameters such as foliar coverage, chlorophyl contents, and stem length. The leaf macro/micronutrients were characterized using micro-X-ray imaging. To evaluate possible induction of defence responses, four different defence-related genes [phenylalanine ammonia-lyase (PAL), pathogen-related protein 1 (PR-1), lipoxygenase (LOX), and catalase (CAT)] were selected and used for gene expression profiling by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) at an early stage after treatment with PAW and after artificial inoculation with the virus (TMMV). We observed that plants irrigated by PAW showed better biometric parameters and were able to resist the viral infection compared with the untreated plants, showing stem lengths almost similar to the not-inoculated ones, even though foliage coverage and macro/micro contents were lower.