Effect of the dielectric barrier discharge plasma on the growth and physicochemical properties of wheat seeds

Seed quality is often required for high germination and seedling vigor for the yield, particularly with the increasing global annual consumption of wheat. Low-temperature plasma can be expected for the germination, growth, and stress resistance of wheat seeds. However, it is lacking in the correlation between the active components in plasma and the treatment. This work aims to establish the relationship between representative active species and the modification. Jimai 22 wheat seeds were treated using dielectric barrier discharge (DBD) plasma. Different gas environments included He, He/O₂, and He/air. The electrical and optical properties were characterized using optical emission spectroscopy. The results showed that a higher proportion of He in the environment enhanced the uniformity, stability, and intensity of the discharge, due to Penning ionization. Five representative active species included HeI, N₂, N⁺₂, H_α, and OI. Among them, the H_α line originated either by the ionization of water migrating from the seeds into the discharge region, or by the detachment of the lipid layer and the exposure of starch granules on the seed surface under the impact of energetic particles. A comparison was made on the surface modification, physicochemical properties, germination, and growth characteristics of the seeds before and after treatment. Plasma treatment caused cracking and blocky spalling of the seed coat, due to the impact of energetic particles. In addition, there was the pit-like damage caused by local overheating at concentrated discharge sites after He/O₂ plasma treatment. Hydrophilicity was improved after plasma treatment, where the water contact angle (WCA) significantly decreased from about 130° to around 80°. The reactive oxygen species (ROS) were introduced into the functional groups on the surface, and then the altered surface microtopography was subjected to energetic particle impact. Furthermore, plasma treatment significantly increased the germination rate to over 90%. Both root length and shoot length of the seedlings showed a slight increase after five days of cultivation. The hydrophilicity enhanced the cracks on the seed surface for the water uptake and seed activity. Antioxidant indexes exhibited different trends in different gas environments. He and He/air plasma treatments further enhanced antioxidant activity. In contrast, He/O₂ plasma treatment increased superoxide dismutase (SOD) activity, but the DPPH (2,2-diphenyl-1-picrylhydrazyl) free radical scavenging decreased gradually, as the proportion of He decreased. The Pearson correlation coefficient was used to reveal the correlation between the content of active species and the modification. There was a relatively strong correlation between OI radicals and the modification, indicating the ROS for the seed activity. The germination rate showed a positive correlation with particle concentrations, whereas the WCA showed a negative correlation. Besides OI, nitrogen-containing species were represented by excited N₂ and N⁺₂ ions. The HeI and OI also caused antioxidant activity, due to simultaneous damage to the SOD structure and oxidative stress in seed cells. In conclusion, the best performance was achieved in the two discharged environments with He:air ratios of 8:2 and 7:3. The finding can provide a strong reference to regulate plasma parameters in applications.