Lettuce seed germination: A phytochrome-mediated increase in the growth rate of lettuce seed radicles.

The rate of straight growth in mannitol of the axial portion of 2 different samples of photosensitive lettuce seeds was followed after red and far-red irradiation. If the growing half-seeds were classified into proportions of the sample that had undergone a growth increment greater than a certain, selected value, growth rate was found to be enhanced by red light and this enhancement paralleled the phytochrome-mediated increase in the rate of appearance of a geotropic curvature in seed axes, an effect which had been previously reported. The increase in the rate of growth of the radicle by red light was limited to early growth. Once extension has exceeded a certain value the rate of further growth is independent of the state of phytochrome.