Both atmospheric and low-pressure plasma irradiation to seeds is effective for the growth enhancement of radish sprouts. On the other hand, there is no relation between the UV light and the growth enhancement of radish sprouts. In excimer UV irradiation, oxygen atoms and ozone are generated by excimer UV light wavelength of 172 nm and 222 nm. But in plasma irradiation, ions O⁺, O₂⁺, oxygen atoms O(¹D), O(³P) and excited oxygen molecules ¹Σ_g⁺ are produced. The ions are not excluded from factor of growth enhancement because of its short life. From this difference, the growth enhancement is due to reactive oxygen species generated by plasma irradiation.