The interaction of oxygen, radiation exposure and seed water content on γ-irradiated barley seeds

Abstract

The roles of oxygen concentration, seed water content, and their interaction in the γ-ray-induced damage to dry barley seeds were investigated. Himalaya (C.I. 620) barley seeds were adjusted to water contents ranging from 2 to 10%, irradiated with 60Co γ-rays, and soaked at 0°C in distilled water bubbled with oxygen-nitrogen gas mixtures containing 0.0, 3.1, 6.25, 12.5, 25, 50 and 100% oxygen. Biological effects of the treatments were recorded as M 1 seedling injury. Essentially no oxygen enhancement of biological damage was obtained with an oxygen concentration of 3.1% in the gas phase of the soaking solution. The minimum OCHG needed to cause an oxygen enhancement of biological damage (3.1% OCHG) increased with increasing seed water content between 1.8 and 10.0%, and decreased as the radiation dose increased, suggesting a triple factor interaction. For moderate levels of injury (between 20 and 60%), a nearly linear increase in seedling injury was obtained when the OCHG was increased in an exponential fashion. For greater seedling injury, the response tended toward a sigmoid shaped curve, probably due to limitations of the biological parameter. The same types of response were obtained when the results were analyzed in terms of oxygen enhancement ratios (OER) obtained from seedling injury data. Decreases in oxygen-independent radiosensitivity, determined by the increase in radiation exposure required to induce 40% injury with anaerobic soakings, were obtained as the seed water content was increased from 2.0 to 9.9%. The same pattern of radiosensitivity was observed with aerobic soakings and was more pronounced at intermediate levels of OCHG. The change in radiosensitivity of seeds between seed water contents of 2.0 and 7.7% was consistent with published EPR data. The increase in response at intermediate OCHG was in part a reflection of the influence of seed water content on the level of OCHG needed to produce an oxygen enhancement of damage. Cooling seeds of 6.1% water content to dry ice temperatures immediately after irradiation at 0°C decreased both oxygen-dependent and oxygen-independent damage. The decreases in damage were greater at 20 krad than at 10 krad and tended to be greater at the higher OCHG.