Vivian K. Toole

Action of Light on Lettuce-Seed Germination

1. The observations of Flint and McAlister that imbibed seed of some lettuce varieties can be promoted in germination by irradiation in the red portion of the spectrum and inhibited in the infrared were verified. 2. The action spectra for promotion and inhibition were measured in detail for wave lengths greater than 4000 A. Maximum sensitivity for promotion was found in the region 6400-6700 A (red) and for inhibition in the region 7200-7500 A (infrared). 3. Absorption of radiation in the red or in the infrared region changes the effective pigment into the infrared- or the red-absorbing form, respectively. The alternation of form can be repeated many times. 4. The action spectrum for lettuce-seed germination is the same as that effective for photoperiodic control of floral initiation. The two phenomena involve the same initial photoreaction. 5. The photoreaction involves monomolecular isomerization of the effective pigment. 6. A reaction by which the pigment changes from the infrared- to the red-absorbing form occurs in darkness. This reaction was elsewhere found to be the one by which duration of darkness is measured in photoperiodic control of floral initiation. 7. Seed of one lettuce variety, Great Lakes, that did not require radiation for germination, became sensitive to radiation when held imbibed at 35⚬ C.

Photocontrol of Plant Development by the Simultaneous Excitations of Two Interconvertible Pigments. III. Control of Seed Germination and Axis Elongation

Action spectra between 4100 and 8500 A for seed germination and radicle elongation were measured for Nemophila insignis and Lactuca sativa var. Great Lakes and between 6500 and 8500 A for seed germination of Lamium amplexicaule. The responses observed are features of the continued excitation of the photomorphogenic pigment in those regions of the spectra where both pigment forms have appreciable absorptivities.