Actions of Gibberellic and Abscisic Acids on Lettuce Seed Germination Without Actions on Nuclear DNA Synthesis

Abstract

Promotion and inhibition of DNA synthesis have been suggested as important causal events in the growth-promoting action of GA2 and the growthretarding action of ABA, respectively (11,12,15). We here report typical effects of GA and ABA during lettuce seed germination after seed irradiation that prevents detectable DNA synthesis as determined by 2 criteria: Feulgen microspectrophotometry and 3H-thymidine incorporation into nuclear DNA. Air-dry seeds of lettuce, Lactuca sativa 'New York,' were given 1.3 Mrad 60Co gamma-rays and then sown in Petri dishes containing distilled water or solutions of GA (Gibrel, Merck) or ABA (Shell Development Company) as indicated. Whereas lower radiation doses are sufficient to prevent mitosis during germination (7), the 1.3 Mrad dose was necessary to inhibit nuclear DNA synthesis, as will be shown in this report. In all cases the pH was 5.7, and the dishes were exposed to continuous illumination with 900 jUw/cm2 white light i(Sylvania GroLux) at 20°. After 5 days, germination of these irradiated seeds (as the percentage of total number of seeds sown) was 0 % in water controls, 84 % in 5 X 10-4 M GA, and 1.5 % in 5 X 10-4 M GA + 100 mg/l ABA. Thus germination of these seeds was very sensitive to GA and ABA. By Feulgen microspectrophotometry the relative amount of DNA per nucleus was measured for individual cells from ,(a) the apical 0.5 mm of the radicle of unirradiated embryos before sowing, and (lb) the protruded 1.1 mm of 5-day-old root tips from the