The role of abscisic acid in germination of light-sensitive and light-insensitive lettuce seeds

The role of abscisic acid (ABA) in seed germination of two cultivars of let tuce (Lactuca sativa L.; light-sensitive Ritsa and light-insensitive Strada ) was investigated. The inhibition of Ritsa seed germination by exogenous A BA was higher than that of Strada seeds, the extent of inhibition of both c ultivars being reduced by a short light break. At 25 degrees C the sensitiv ity of both cultivars to exogenous ABA was higher than at 15 degrees C. The endogenous level of ABA was similar in dry seeds of both cultivars, increa sing temporarily in Ritsa seeds during the first 4 h of imbibition in darkn ess but not in Strada seeds, nor in Ritsa seeds exposed to a short light br eak. The transitory increase of ABA content in Ritsa seeds imbibed in darkn ess was accompanied by increased expression of the gene responsive to ABA ( Rab) under these conditions. Zorial (Norflurazone), an ABA-biosynthesis inh ibitor, decreased ABA content and allowed dark germination of the light-req uiring Ritsa seeds. A short light break induced germination of Ritsa seeds when applied at 24 and 48 h after imbibition onset, i.e. after the transito ry increase of ABA. GA(3), on the other hand was effective when applied at the beginning of imbibition. It seems that light induces germination of the photoblastic Ritsa seeds by both inhibiting ABA synthesis and decreasing s eed sensitivity to ABA and inhibitory processes induced by it.