Interactions among plant hormones, carbon dioxide and light in the relief of thermoinhibition of lettuce seed germination: Studies in a flow-throught gaseous system

The effects of gibberellin A3 (GA3), kinetin, C2H4, CO2, and (or) red light (5-min single exposure) on the germination of lettuce (Lactuca sativa L. cv. Grand Rapids) seeds at 32 °C were studied in a continuous-flow system. This system permitted strict control of the gaseous environment, which has not been possible in previous experimental set-ups. Except for a minor effect of light (10% enhancement), none of the treatments applied singly had any effect on the germination of these thermo-inhibited seeds. The action of any one hormone (GA3, kinetin, C2H4) required the presence of at least one more hormone or CO2 or light. Complete alleviation of thermoinhibition required a minimum of three factors in the following combinations: GA3 with two of kinetin, C2H4, or CO2, and kinetin either with GA3 + light or C2H4 + light. Given in combination with light, kinetin, GA3 + C2H4, GA3 + CO2, or C2H4 + CO2 induced above 75% germination. Approximately half maximal germination was obtained with GA3 + kinetin, GA3 + CO2, and C2H4 + light. Other treatment combinations showed either no or only a weak interaction. Results showed that the action of hormones, particularly that of GA3, was markedly influenced by CO2. Carbon dioxide was also able to substitute for a number of other treatments that interacted with GA3. In addition, the effects of several interactive treatments were further accentuated by CO2