Effects of repetitive acid immersion, red light, and gibberellin A3 treatments on phytochrome-mediated germination control in skotodormant lettuce seeds

Dry lettuce seeds (Lactuca sativa L. cv. Grand Rapids), which received 5 min far-red light (FR) 0.5 h after the onset of water imbibition, showed 17% and 50% germination without and with acid immersion treatement (pH 0.1) for 1 h and rinsing with water, respectively. The acid treatment caused only 6% germination or less in FR-treated seeds held for 10 to 30 d in dark storage. The 10 to 30 d skotodormant seeds did not respond to red light (R) or gibberellin A3 (GA3) singly, but showed 84% or higher percentage germination if 1 h acid immersion was given before R or GA3. The 20 d skotodormant seeds, which received R treatment at day 10 but remained dormant showed 89% germination with only acid treatment. Similar values were obtained with 30 d skotodormant seeds which received one or two R treatments at day 10 or 20, i.e. the only requirement for these R-treated dormant seeds was an acid immersion. This releases the skotodormancy and renders the seeds more sensitive to R or GA3, but the skotodormancy was initiated again if no light or hormone treatments were given immediately. The repetitive R or GA3 treatments, which did not cause skotodormant seeds to germinate, lessened the degree of skotodormancy. The germination of these skotodormant seeds can only be induced by the synergistic action of R and GA3. In this study, GA3 caused higher germination percentages in R-treated skotodormant seeds than R stimulated in GA3-treated seeds. It is suggested that (i) repetitive R or Ga3 treatments maintain a high endogenous level of the far-red-absorbing form of phytochrome (Pfr) and GA activity, respectively, (ii) the accumulated stable intermediates of phytochrome persist in fully-imbibed skotodormant seeds for up to 20 d, without phytochrome expressing its function until the seeds are acidified and (iii) a model is formulated to interpret the results of acidification, growth promoters and R effects on germination of light-sensitive lettuce seeds.