Plasma-activated water seed treatment modulates osmotic stress responses in pea through morphological, ionomic, hormonal, and antioxidant changes

Abstract

Plasma-activated water (PAW) seed treatment is an emerging green technology that enhances plant germination and growth; however, its role in drought stress tolerance remains unclear. In this study, pea (Pisum sativum L.) seeds were treated with PAW (10 mg L⁻¹ nitrite, 30 mg L⁻¹ nitrate, pH 3.9), Zinc (Zn)-enriched PAW (PA(W+Zn); 6 mg L⁻¹ hydrogen peroxide (H₂O₂), 19 mg L⁻¹ nitrite, 20 mg L⁻¹ nitrate, 14 mg L⁻¹ Zn, pH 4.7), distilled water (DW), or Zn-enriched DW. Subsequently, three-week-old plants were subjected to osmotic stress using polyethylene glycol 8000 (PEG8000) (20% w/v). PAW induced partial hilum opening and mild seed coat damage, while PA(W+Zn) increased nitric oxide (NO) accumulation. Both treatments enhanced nutrient levels and shootward translocation, particularly of Zn and iron (Fe). Under stress, PAW promoted the formation of larger but thinner leaves, indicating adaptive morphological responses. However, stomatal conductance, relative water content, photosynthesis, and starch accumulation remained unaffected. Unchanged proline levels and reduced soluble sugars suggest that classical osmotic acclimation was not induced by PAW treatments. Instead, both PAW and PA(W+Zn) increased abscisic acid (ABA) levels and PsDHN2 expression, indicating activation of ABA-dependent stress signalling, likely mediated by nitrate-, nitrite-, and H₂O₂-derived signals. Tissue-specific cytokinin (CK) responses and enhanced antioxidant enzyme activities (superoxide dismutase [SOD], catalase [CAT]) further suggest the establishment of a primed state. Overall, PAW-based seed treatments induce complex physiological and molecular responses involving ABA signalling, selected ion translocation, and antioxidant enzymes without significantly improved water status, stomatal regulation, or photosynthetic performance.