Factors controlling the hormesis response in irradiated seed

Ionizing radiation at very low doses frequently has a stimulating or hormetic effect on the growth of organisms. Irradiation of seed before planting can stimulate early plant growth, leading to advanced maturity and increased yield. The unreliability of this response has limited its application. However, the technique has been extensively studied and now is practiced on a large scale on some farms in certain countries. Our research has addressed various factors that may determine the stimulation response with the goal of developing an ability to predict the occurrence of increased economic yield. In this paper, we discuss several of these factors in light of our data and data from other field studies in Canada. A hormesis response was observed for most species studied, but varied among cultivars and among seed lots within a cultivar. Seed condition may be the underlying factor in these effects. The response was most frequently evident at very early stages of growth and was often masked in subsequent growth. This suggests that the hormesis phenomenon in seeds may be quite common and is not reliably manifest in advanced maturity or yield increases because of environmental effects. Storage time after irradiation must be minimized to gain the greatest response. There is little evidence to suggest that the photon energy or dose rate of the radiation source is an important factor.