Ethylene and carbon dioxide as mediators in the response of the bean hypocotyl hook to light and auxins.

Ethylene inhibits hook opening in the bean hypocotyl and at high concentrations induces closure of the hook. Indoleacetic acid and 2,4-dichlorophenoxyacetic acid, whose inhibitory effect on hook opening resembles that of ethylene, stimulate ethylene production from the hook tissue, and this ethylene production is physiologically active in inhibiting hook opening. It is concluded that the inhibition of opening by auxin is due at least in a major part to auxin-induced ethylene production by the hook tissue.Carbon dioxide promotes hook opening, apparently by antagonizing the action of endogenous ethylene. The concentration of respiratory CO2 in the internal gas space of the hook tissue is high enough to play a role in the regulation of hook opening.Red light causes a decrease in ethylene production and an increase in CO2 evolution from the hook tissue. These effects are partially reversible by far-red light. It is concluded that both ethylene and CO2 serve as natural growth regulators which mediate the hypocotyl hook-opening response to light in bean seedlings.