## 4 Discussion
In the CD reactor, some excited species from air molecules (oxygen and dinitrogen) were observed in the emission spectrum, although the reactor was closed by a Petri dish. A strong argon emission, in addition to hydroxyl radicals, and weak emission of dinitrogen, nitrogen oxide and singlet oxygen was detected. Some of those reactive species possess known decontaminating properties and can oxidize lipids, proteins and nucleic acids and ultimately lead to pathogen destruction (e.g., [43]). More specific, inactivation mechanisms can encompass erosion the surface of microbial cells through etching (e.g., [44]), oxidative damage of intracellular macromolecules, such as membrane lipids, proteins, and DNA, and a reduction in intracellular pH from diffusion into the microbial cells disrupting pH homeostasis (e.g., [45]). Comparing the inactivation efficiency detected in our study to previously investigated and published indirect CAPP treatment using plasma processed air (PPA), PPA proved to be more efficient in inactivation of _B. atrophaeus_ spores with mean reduction of 3 log\({}_{10}\) CFU/mL for barley, which is equal to ~99.99% decontamination, 2.56 \(\pm\) 0.61 log\({}_{10}\) CFU/mL in rapeseed and 2.44 \(\pm\) 0.52 log\({}_{10}\) CFU/mL in lupine (~99.9%) [35]. In contrast to the volume-DBD and CD, PPA is enriched in RNS, including nitride oxide and its possible reaction compounds nitrogen dioxide, nitrous acid and nitric acid. Those components form an acid environment known for favoring the inactivation of spores [46]. Although showing efficient spore reduction, PPA can lead to a reduction in seed vitality, as observed and published for rapeseed, depending on the applied process parameters [35]. In contrast to this, no significant reduction in maximum germination was detected in our study using direct CAPP treatment at any exposure time. Direct CAPP treatment is also known for its stimulating effect on seed germination potential, germination speed and seedling development (e.g., [47; 48; 49; 50]), which has been confirmed and previously published for the CAPP reactors presented in our study as well [32; 33]. Moreover, direct CAPP treatment has been shown to change the physicochemical properties of seed surfaces and to increase seed surface hydrophilicity by means of attachment of polar groups such as hydroxyl-, carboxyl- and carbonyl- groups which lead to an increased interaction with water via dipole-dipole intermolecular forces (e.g., [50; 51; 52; 53; 54]). Ultimately, water contact angle, a proxy for hydrophilicity of a surface, is significantly decreased below 90\({}^{\circ}\) after direct CAPP treatment, indicating facilitated water uptake by the seed. In some cases, stimulated water uptake which is accompanied by reduced WCA has also been reported (e.g., [50; 55; 56]), while there was no effect on water uptake by seeds even though WCA was reduced after direct CAPP treatment (e.g., [57; 58]). Indirect CAPP treatment does not alter the WCA (e.g., [35; 57; 59]). This effect of elevated hydrophilicity was also detected in all crop species tested in our study. In wheat, significant differences occurred between the two tested plasma sources, as well as for the different exposure times, with a significantly lower and more homogeneous contact angle after treatment with the volume-DBD. Inhomogeneous treatment was observed, especially for the CD reactor, resulting in large spans of data ranging from, e.g., 30\({}^{\circ}\) to 108\({}^{\circ}\) in barley for 10 min exposure. In the CD source, plasma filaments are created, possibly leading to a treatment of mainly one side of the seed or an object with restricted treatment of the opposing side. As no shaking and turning of the seeds took place during treatment with the CD reactor, different WCA values for each side of the seed can occur and can lead to a higher span because it cannot be assured that the upper side of the treated seeds is also used in WCA determination.