Photocontrol of Lepidium Seed Germination.

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Abstract
Germination studies were conducted with seed of L. virginicum and L. densiflorum to extend knowledge of the reversible photoreaction that has been found to control seed germination of several species, floral initiation of Xanthium saccharatum and other plant responses. Action spectra for the germination of both species showed maximum sensitivity for inhibition to be near 7400 A, as previously found for lettuce seed germination and other responses. Sensitivity decreases with wave length by about 100 fold from 7400 A to 8500 A. Maximum sensitivity for promotion is near 6400 A and this decreases by 100[degree] fold to 5260 A. The seeds were unresponsive to inhibition or promotion of germination by radiation of wave lengths between 4000 A and 5260 A at 50 fold the energy required for 50% germination at 6400 A. A single shift in temperature of imbibed seeds of L. virginicum from 15[degree] to 25[degree]C following exposure to red radiation enhanced the population of germinating seeds without a change of variance. A shift from 25[degree] to 15[degree] was without effect on germination. Interaction of the photoreaction with nitrate and thiourea are coupled in such a way as to cause opposite changes in sensitivity for promotion and inhibition of germination. This indicates that a molecule other than the pigment is involved in the photoreaction. A possible course of the reactions involved is presented.

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