A Reversible Photoreaction Controlling Seed Germination

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Abstract
The action spectrum for promotion and inhibition of germination of Grand Rapids lettuce seed was obtained with a 2-prism (glass) spectro-graph having a dispersion of about 35 A/cm. at 6000 A. The slit width was about 90 A at this wavelength and the incident energy 0.100 x 10-3 joules/cm2./sec. Lettuce seeds were placed on wet blotters in dishes and maintained in darkness at 20[degree] C for various times before irradiation. Those to be inhibited in germination were exposed to radiation between 5800 and 6600 A (which gave 98+% germination in the controls), after which they were irradiated in various wavelength regions to test for inhibition. Those to be promoted in germination were taken directly from darkness and irradiated. After irradiation all seeds were held for 2 days at 20oC in darkness before germination counts were made. The seed was equally sensitive for 4 to 16 hrs. after imbibition to promotion by red radiation (between 6530 and 6635 A) and equally sensitive for the same period of imbibition to suppression o germination by irradiation at more than 7000 A. Seeds of a temp.- sensitive white-seeded lettuce var., if held for several days in darkness at 30oC, remained dormant if returned to 20[degree] in darkness, but, if irradiated at 5800 to 6800 A before returning to 20[degree] in darkness, full germination was promoted. Preliminary studies on the action spectrum below 5200 A indicated that blue radiation could effectively promote germination.

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