Effects of Ambient Humidity on Plant Growth Enhancement by Atmospheric Air Plasma Irradiation to Plant Seeds

Humidity is an important factor for plasma-bio applications because composition of species generated by atmospheric pressure plasmas significantly depends on the humidity [1]. Here we have examined effects of humidity on the growth enhancement to study the mechanism. Experiments were carried out with a scalable DBD device [2]. 10 seeds of \textit{Raphanus sativus L}. were set for x$=$ 5 mm and y$=$ 3 mm below the electrodes. The humidity H$_{\mathrm{air}}$ was 10 - 90 {\%}Rh. The ratio of length of plants with plasma irradiation to that of control increases from 1.2 for H$_{\mathrm{air}}=$ 10 {\%}Rh to 2.5 for H$_{\mathrm{air}}=$ 50 {\%}Rh. The ratio is 2.5 for H$_{\mathrm{air}}=$ 50-90 {\%}Rh. This humidity dependence is similar to the humidity dependence of O$_{2}^{+}$-H$_{2}$O, H$_{3}$O*, NO$_{2}^{-}$-H$_{2}$O and NO$_{3}^{-}$-H$_{2}$O densities, whereas it is different from that of other species such as O$_{3}$, NO, and so on [1]. The similarity gives information on key species for the growth enhancement.\\[4pt] [1] T. Murakami, et al., Plasma Sources Sci. Technol. 22, 015003 (2013).\\[0pt] [2] S. Kitazaki, et al., Curr. Appl. Phys., 14, S149 (2014).