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Gaseous hydrogen cyanide stimulated subsequent lettuce seed (Lactuca sativa L. cv. Grand Rapids) germination in darkness when applied for 1 or 22 h. Optimum concentrations were 5 × 10‐5 M and 10‐6 M, respectively. However, seeds did not germinate in the presence of HCN even at 10‐8 M. The effects of unsaturating red light (R) and HCN (1 μM) showed a slight synergism. On the other hand, there was no difference between the effects of the sequences HCN – R and R – HCN. Stimulation of lettuce germination by an HCN pulse was practically not affected by far‐red illumination, independently of the sequence of treatments. It was concluded that the primary stimulatory effect of HCN is of a regulatory character. Cyanide controls a regulatory point different from that affected by activated phytochrome.

Germination of lettuce (Lactuce sativa L. cy. Grand Rapids) seeds was promoted by red light and by pulse treatments with gibbercllie acid (GA3) or hydrogen cyanide, whereas it was inhibited by short exposure of seeds to absusic acid (ABA). The eflects of unsalLirating red light and of 10 μM GA3 on lettuce germination were completely reversed the effect of ABA (100 μM). In contrast, hydrogen cyanide did not reverse the effect of 100 μM ABA and only partly eliminated the effect of 10μM ABA, independently of the sequence of treatments. Possible interactions between HCN GA3, ABA and red light were discussed. It was concluded thai light GA3 and HCN affect different mechanisms involved in lettuce germination: ABA counteracts the stimulatory action of all these faclors. being the most effective against cvanide Additional key words ‐ Lactuca sativa, pholodormancy, phylohormoncs.

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