Space-flown spearmint (Mentha spicata L.) seeds potentially tolerate nitro-oxidative stress from plasma-activated water.

Establishing fresh food sources during exploratory missions requires the identification of compatible plant species and accompanying technologies needed to support their growth in the space environment. Here, we investigated the use of plasma-activated water (PAW), generated via cold atmospheric pressure plasma, as a non-chemical fertilizer to aid the cultivation of Mentha spicata L. (spearmint) seeds previously flown aboard the International Space Station for 6 months. This study showed that PAW generated at a 10-min discharge (PAW10) may become inhibitory to plant development due to its complex chemistry, including the excessive amounts of nitrates (NO3 -) introduced during the long-duration treatment. This was reflected in the reduced germination and growth characteristics observed in PAW10-treated seeds following a 14-day germination assay. These results were not observed when PAW was activated for only 5 min (PAW5), potentially indicating a dose-dependent response to the plasma-induced reactive oxygen and nitrogen species (RONS) in the water treatment. However, GC-EI/MS revealed an alteration in the non-enzymatic antioxidant response in space-flown seeds treated with PAW10, where a decrease in 2,4-di-tert-butylphenol (2,4-DTBP), a phenolic defense compound, was seen. We speculate that, if not through non-enzymatic mechanisms, the apparent resistance against PAW10 toxicity may have risen from a shift towards enzymatic defenses against nitro-oxidative stress, a common phenomenon reported in other plants. Further study is required to confirm this hypothesis. Overall, the present work raises spearmint as a promising candidate for space cultivation, or at the very least, a model to study the plant stress response in space. Notably, prior space exposure conferred a greater tolerance against nitro-oxidative stress in spearmint, mitigating the adverse effects associated with excessively activated PAW.