Necessity of gibberellin for stimulatory effect of KAR1 on germination of dormant Avena fatua L. caryopses

Avena fatua L. florets (caryopses enclosed by lemma and palea) were partially dormant at 10-20 degrees C and did not germinate at temperatures outside this range. Afterripening florets at 25 degrees C for 12 weeks completely removed dormancy. Caryopses (florets without lemma and palea) were able to germinate totally at 20 degrees C. Karrikinolide (KAR(1)) and gibberellic acid (GA(3)) applied at 10-25 degrees C partially or markedly induced germination of dormant florets and caryopses, respectively. Both florets and caryopses were more sensitive to KAR(1) than to GA(3). To obtain similar effects, 1,000 to 10,000 times lower concentrations of KAR(1) than GA(3) were required. After-ripening with time gradually increased sensitivity of caryopses to these regulators. Likewise, after-ripened, non-dormant caryopses were sensitive to KAR(1) and GA(3). Inhibitors of gibberellin biosynthesis, ancymidol, paclobutrazol and flurprimidol inhibited the effect of KAR(1). This inhibition was reversed by GA(3). Caryopses pre-incubated in water with ancymidol or paclobutrazol in the presence or absence of KAR(1) germinated completely but with different rates after transfer to GA(3). KAR(1) probably requires gibberellin biosynthesis to stimulate germination of dormant Avena fatua L. caryopses. Both KAR(1) and GA(3) increased alpha-amylase, beta-amylase and dehydrogenases activities during imbibition before visible germination occurred.