Relation of embryo structure, node position, tillering and depth of planting to the effects of X-rays in barley

Abstract

Biological, especially genetic, effects of X-rays in barley are influenced by embryo development, node position in M 1 plants, depth of planting and plant tillering. Bud or tillerinitial viability was determined with the vital stain, tetrazolium chloride. Both deeply planted and irradiated seeds exhibited reduced tillering from the bud in the axil of the coleoptile. Mutation rates (frequencies of chlorophyll-deficient M 2 seedlings) were calculated by the M 1 plant, M 1 spike, or M 2 seedling methods. Mutation rates were also determined by each method for apical spikes, primary and secondary axillary spikes, and each node position. These latter rates differed less from one another when scored by the M 1 spike or M 2 seedling methods than when scored by the M 1 plant method. The M 1 spike mutation curve approached closest to linearity for plants with all primary axillary and apical spikes present and with all duplicate mutants excluded. Reliable mutation rates can be calculated from the apical spike alone. Node position on the M 1 plants, with the exception of the fourth node, did not appear to affect mutation rates. Spikes above the fourth node and from secondary axillary tillers frequently had as many or more mutations than the primary axillary spikes. In fact, a higher mutation rate occurred in plants with higher numbers of tillers. Summarized mutation rates of the primaries at the upper nodes were higher than at the lower nodes. The converse was true for the secondaries which also had lower rates than the primaries. Fluctuations in rates were related to variations in embryo structure and to a decrease in the number of nodes (with tillers), spikes, and seedlings per plant. Therefore, the nearly linear mutation rates with increasing dose are probably related to high numbers of tillers which would permit mutant cells to be detected.