The redox reaction between cystein and cystine is studied using water vapor plasma produced by the RF discharge concerning the growth control of plants. The stress reaction of plants has been achieved, when the water vapor plasma is irradiated to the seeds or stem, leaf of plants such as radish sprout. The mechanism of the growth control is investigated by analyzing amino acids of cystein and cystine. The water vapor plasma can oxidize and reduce the cystein and cystine, respectively by choosing the plasma parameters. The redox reaction of cystein and cystine in thioredoxin would be the mechanism of the plant control.