Treatment of seeds by cold ambient air plasma: combining impedance measurements with water sorption modeling to understand the impact of seed hydration

Abstract

In this article, we focus on the plasma seed interaction and more specifically-on the feedback exerted by the seeds on the plasma properties. Dormant Arabidopsis seeds with different water contents (WC), namely 3 %DW, 10 %DW and 30%DW were exposed to cold ambient air plasma (C2AP) generated in a dielectric barrier device (DBD). It is found that increasing WC enhances the capacitive current of the DBD, generates a greater number of low energy streamers (characterized by current peaks lower than 10 mA) that preferentially interplay with the seeds. Since the resistive and capacitive components of the seeds modify the C2AP electrical properties, impedance measurements (also called LCRmetry) have been carried out to measure their main dielectric parameters before/after plasma exposure (seeds resistance, capacitance, complex relative permittivity, tangent loss and conductivity). It appears that WC significantly changes dielectric losses at low frequencies (< 1 kHz) due to polarization relaxation of the polar molecules (i.e. water). LCRmetry further reveals that C2AP does not substantially alter seeds dielectric parameters, i.e. it neither adds or removes significant amounts of new materials, meaning that the relative starch, protein and lipid contents remain essentially unaffected. However, it cannot be discounted that some bulk properties of the Arabidopsis seeds may be modified, especially regarding their porosity. This characteristic could facilitate penetration of plasma-generated reactive oxygen species into the internal seed tissues, leading to the grafting of oxygenated groups. To corroborate this theory, water sorption isotherms have been achieved on Arabidopsis seeds and fitted with four thermodynamic models, including the Brunauer-Emmett-Teller model and the Generalized D’Arcy and Watt model. It is demonstrated that C2AP primarily strengthens water-seed affinity by modifying molecular interactions rather than changing the seed’s moisture layer. This occurs despite a potential decrease in the number of adsorption sites, indicating a significant increase in overall seed hydrophilicity after plasma treatment.