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The possibility of an excess of IAA as being the germination preventing mechanism of light sensitive lettuce seeds was investigated. No IAA was found in the dry seeds. Growth promoting substances appear in seedlings, only when germination proceeds. Two acidic growth inhibiting substances were the only growth active substances present in the dry seeds. These substances apparently disappear when germination proceeds. A variety of other active substances appears during germination. The chemical nature of the growth inhibiting substances in the dry seeds, also the reaction of these substances to light are still not known. After this paper had been submitted to the press, a paper by Wareing P. E. and Foda V. entitled "Possible Role of Growth Inhibitors in the Dormancy of Xanthium and Lettuce". Nature (in press) was brought to our notice. The behaviour of the growth inhibitors found by these authors is similar to those found by us. The inhibitors are probably not identical due to differences in isolation procedure. Further investigations by one of us (S. GB) have shown that some of the acid substances extracted from lettuce seeds inhibit their germination. [ABSTRACT FROM AUTHOR]

The effects of germination stimulating red light and germination inhibiting infrared light were studied in photoblastic lettuce seeds var. Grand Rapids after different periods of storage. The following results were obtained: (1) After a shorter period of storage: QO2 and QCO2 of seeds illuminated with infrared light were almost identical with those of seeds kept in darkness. Seeds treated with red light have a significantly higher QO2 and QCO2 root emergence begins than seeds kept in darkness. This effect is already evident one hour after illumination. The highest QO2 and QCO2 were found in decoated seeds kept in darkness. Time curves of respiration rates show more or less the same pattern in all treatments, RQ's remained at the same level and no significant difference could be found between those of seeds under different treatments. (2) After a longer period of storage: The effect of red and infrared light on respiration changes, as red light only raises QCO2 in comparison wild darkness, whereas QO2 is not affected, while infrared light lowers QO2 only. Seeds exposed first to red and afterwards to infrared light have rates intermediate between those of seeds treated only with red or infrared light. The time curves of respiration rates differ from those after short periods of storage. The RQ's do not remain at the same level and there are differences between those of different treatments. It is suggested that in young seeds a respirational block exists which may be responsible for the germination behaviour of light sensitive seeds in darkness. This block may be overcome to various degrees by red light, by storage or by decoating the seeds. [ABSTRACT FROM AUTHOR]