Control Processes in the Induction and Relief of Thermoinhibition of Lettuce Seed Germination

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Abstract
Germination of lettuce seeds (Lactuca savita L. cv Grand Rapids) in the dark was nearly 100% at 20.degree.C but was inhibited at 27.degree.C and higher temperatures (thermoinhibition). A single 5-minute exposure to red light completely overcame the inhibition at temperatures up to 28.degree.C, above which the effectiveness of single light exposures gradually declined to reach a neglegible level at 32.degree.C. However, the promotive effect of light could be extended to 34.degree.C repeated irradiations. At any one temperature, increased frequency or irradiations increased germination percentage, and with each degree increase in temperature, increasingly frequent irradiations were necessary to elicit maximal germination. Loss of the effectiveness of single irradiations with increase in temperature may result either from acceleration of the thermal reversion of the far red-absorbing form of phytochrome or decrease in seed sensitivity toward a given percentage of the far red-absorbing form of phytochrome. Using continuous red light to induce germination, the role of endogenous C2H4 in germination at 32.degree.C was studied. Ethylene evolution from irradiated seeds began to increase 2 hours prior to radicle protrusion, whereas the dark-incubated (nongerminating) seeds produce a low, constant amount of C2H4 throughout the 24 hour incubation period. Inhibition of C2H4 synthesis with 2-aminoethoxyvinly glycine and/or inhibition of C2H4 action with 2,5-norborniadiene blocked the promotive effect of light. Exogenous C2H4 was essential for the light-induced relief of thermoinhibition of lettuce seed germination. However, light did not act exclusively via C2H4 since exogenous C2H4 alone in darkness did not promote germination.