Scavenging of Free Radicals and Radiation Protection by Nitric Oxide in Plant Seeds.

Abstract

Under anoxic conditions NO is non-toxic to living tissue. Treatment of resting barley seeds with NO at atm. pressure for several days neither induces growth inhibition nor raises the mutation rate. The reactions of NO with free radicals (with which it reacts in a 1:1 ratio) were used to show the mechanism by which H2O exerts an action on the radiation sensitivity of plant seeds, an effect consisting of a decrease of the radiation sensitivity with increasing H2O content with a min. at some value between 12 and 20% H2O. The growth inhibition induced at various H2O contents is proportional to the level of radiation after decay; this indicates that the protective action of H2O is due to an increasing recombination of free radicals. In N and air a clear min. sensitivity occurs at 12% H2O; in the presence of NO, max. protection is found at a water content below 12%. The influence of the radical scavenger,. NO, connects directly the radiation-produced free radicals with a large part of the biol. effect. Within this range of H2O content the destruction of radicals leads to a protection, either by a recombination facilitated by the presence of H2O of hydration or by reaction with a scavenger. The sensitivity-increasing effect of O does not necessarily depend on immediate reactions with radicals. At H2O contents above the min. where the radiosensitivity is higher, NO increases the radio-sensitivity in a manner similar to that of O. When the HoO content exceeds 12%, more loosely-bound H2O is present, permitting new types of reactions. Below 12% H2O, the reaction const. for radical recombinations is independent of H2O content, indicating that H2O is exclusively present as a simple complete layer of H2O of hydration. Above 12% the reaction const. increases with increasing H2O content.