Analysis of Germination Processes of Lettuce Seed by Means of Temperature and Anaerobiosis

Abstract

The physiological nature of the germination processes in Grand Rapids lettuce seeds was analyzed by applying a N atmosphere or a range of temperatures during the different phases of germination. Controls were germinated in air at 25[degree]. The preinduction phase, which took about 1.5 hours, was not influenced by placing the seeds in N, but was sensitive to temperature. Since there is a correlation between water content in the seed (roughly 40% above air-dry weight) and maximum induction of germination by red light, and since water uptake by the seed is sensitive to temperature, the temperature effect on this phase was ascribed to its effect on water uptake. The induction phase, in which the reversible reaction of the phytochrome functions maximally, was shown to be a pure photo-reaction independent of temperature and oxygen. The postinduction phase, which then took about 9 hours, started with a highly oxidative reaction immediately following photoinduction. In seeds kept under anaerobic conditions this oxidation was inhibited and the escape from inhibitory far-red light stopped, but the pigment became converted in the dark from the far-red absorbing form (Pfr) back to the red absorbing form (Pr). Under aerobic conditions both the oxidation and the escape phenomenon occurred, but the dark reversion of the pigment was difficult to detect. Evidently(Pfr)participated actively in the oxidation reaction and this reaction directly controlled the escape phenomenon. Escape was sensitive to temperature, showing an optimum at 25[degree]. Inhibition of germination caused by 35[degree] was quite different from that due to N, in that it occurred regardless of the time at which the seeds were transferred to 35 [degree] and was not reversed by a 2nd red irradiation as was the N inhibition. It is concluded that 35[degree] causes a general inhibition of the postinduction phase. Low temperature (2[degree]) substituted for red light, caused full germination in the dark on subsequent transfer to 25[degree]. However, this effect of low temperature was not reversed by far-red, so that, like gibberellin, it must act at a point other than that controlled by phytochrome. A scheme of the sequence of germination processes is presented.