A Relationship between Rates of Ethylene Production by Roots and the Promoting or Inhibiting Effects of Exogenous Ethylene and Water on Root Elongation

Abstract

The effects on elongation of applying a continuous flow of ethylene in moist air to the roots of intact seedlings of rice (Oryza sativa L. cv. «IR 20»), white mustard (Sinapis alba L.) and tomato (Lycopersicon esculentumMill., CV's «Ailsa Craig», «Moneymaker» and «Diageotropica») were studied in relation to their rates of endogenous ethylene production. Elongation was stimulated in all the species by applying 0.02 ppm of ethylene and inhibited by 1.0 ppm and above. The extent of the stimulation at 0.02 ppm varied according to the species or cultivar and was greatest in roots with low rates of endogenous ethylene production (e. g. rice) and least in those which produced ethylene at high rates (e. g. white mustard). The extent to which 1.0 ppm ethylene inhibited elongation was least in those roots with a low rate of endogenous ethylene production and greatest in roots which produced the gas at the highest rates. The roots of some species or cultivars elongated faster than others. High rates of elongation were associated with high rates of endogenous ethylene production and low growth rates with low production rates of the gas. The possibility is discussed that endogenously produced ethylene may act to promote extension growth under conditions where diffusion of the gas out of root tissue is unimpeded. It is calculated that if the loss of ethylene by diffusion from the root is restricted by the presence of water, ethylene may accumulate to concentrations which inhibit elongation in species with high rates of ethylene production (e. g. white mustard). In the roots of rice, which produce very little of the gas and grow well in wet conditions, the presence of water does not appear to increase the amount of ethylene sufficiently to inhibit extension growth.