Requirement for Ethylene Synthesis and Action during Relief of Thermoinhibition of Lettuce Seed Germination by Combinations of Gibberellic Acid, Kinetin, and Carbon Dioxide

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Abstract
Application of exogenous ethylene in combination with gibberellic acid (GA3), kinetin (KIN), and/or CO2 has been reported to induce germination of lettuce seeds at supraoptimal temperatures. However, it is not clear whether endogenous ethylene also plays a mediatory role when germination under these conditions is induced by treatment regimes that do not include ethylene. Therefore, possible involvement of endogenous ethylene during the relief of thermoinhibition of lettuce (Lactuca sativa L. cv Grand Rapids) seed germination at 32°C was investigated. Combinations of GA3 (0.5 millimolar), KIN (0.05 millimolar), and CO2 (10%) were used to induce germination. Little germination occurred in controls or upon treatment with ethylene, KIN, or CO2. Neither KIN nor CO2 affected the rate of ethylene production by seeds. Both germination and ethylene production were slightly promoted by GA3. Treatments with GA3+CO2, GA3+KIN, or GA3+CO2+KIN resulted in approximately 10-to 40-fold increases in ethylene production and 50 to 100% promotion of germination as compared to controls. Initial ethylene evolution from the treated seeds was greater than from the controls and a major surge in ethylene evolution occurred at the time of visible germination. Application of 1 millimolar 2-aminoethoxyvinyl glycine (AVG), an inhibitor of ethylene synthesis, in combination with any of above three treatments inhibited the ethylene production to below control levels. This was accompanied by a marked decline in germination percentage. Germination was also inhibited by 2,5-norbornadiene (0.25-2 milliliters per liter), a competitive inhibitor of ethylene action. Application of exogenous ethylene (1-100 microliters per liter) overcame the inhibitory effects of AVG and 2,5-norbornadiene on germination. The results demonstrate that endogenous ethylene synthesis and action are essential for the alleviation of thermoinhibition of lettuce seeds by combinations of GA3, KIN, and CO2. It also appears that these treatment combinations do not act exclusively via promotion of ethylene evolution as the application of exogenous ethylene alone did not promote germination.