A sequential response to growth substances in coleoptiles from γ-irradiated wheat.

γ-irradiated wheat seed (500 kr) produces coleoptiles that grow without cell division or DNA synthesis. Apart from an initial 24-hr delay in growth, intact γ coleoptiles have a pattern of cell elongation similar to normal coleoptiles. The elongation of coleoptiles excised at a size of 2 mm, when the cells are small and just prior to entering a rapid elongation phase, is promoted by kinetin and gibberellic acid (GA3). Elongation of coleoptiles excised at 8 mm, when the cells are larger and in the rapid elongation phase, is promoted by indoleacetic acid (IAA). This sequential response to growth substances in γ coleoptiles is remarkably similar to that in normal coleoptiles. The GA3 response in excised γ coleoptiles is not inhibited by FUDR, confirming that DNA synthesis is not required for GA3-induced elongation in γ coleoptiles.