Lettuce Seed Germination: Evidence for a Reversible Light-Induced Increase in Growth Potential and for Phytochrome Mediation of the Low Temperature Effect

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Abstract
With the aid of a new technique employing the radicle half of the seed of lettuce (Lactuca sativa), variety Grand Rapids, irradiation with red light and treatment with gibberellin were shown to increase the growth potential of the embryonic axis. The differential effect of red and far-red light was evident in expansion in water, but was greatly magnified if the axes were allowed to expand in 0.46 [image] D-mannitol. The results with red irradiation showed directly that the contribution of phytochrome (far-red-absorbing form) to germination consisted in an increase of the growth potential of the embryo which enabled the latter to overcome the mechanical restraint against expansion imposed by the endosperm. The response to gibberellic acid (GA3) was probably achieved in a different manner than that to irradiation, but both appeared to be confined to the radicle. The response was not elicited by kinetin. The light response in the half-seed did not depend on the presence of the endosperm. This eliminates the possibility that a simple chemical inhibition of growth by this tissue is responsible for the light requirement. The promotive effect of low temperature on germination was lost if cold incubation was preceded by a short exposure to high temperature. Far-red light, normally germination-inhibiting, reversed the action of high temperature, restoring the capacity for promotion by low temperature. The promotive effect of low temperature on dark germination was most prabably a result of prevention or delay of transformation of physiologically active phytochrome to an inactive form.