Effects of Some Essential Oil Vapors on the Stomata of Eupatorium and Mentha.

Abstract

Following Tyndall's observations in 1862 (8) on the great absorptive power shown by certain essential oils of plants for thermal radiations, a number of theories have been put forward concerning the possible function of such oils in the water relations of plants secreting them. Detto (2) tested the effect of menthol vapor on the rate of water loss from twigs of Syringa. He found that the rate was depressed, and that this was accompanied by the death of a large area of leaf surface. He also tested the effects of other oils on other plants, and came to the conclusion that essential oils played no part in regulating water loss from plants. Dixon (3) found that the vapors given off from leaves of Artemisia Absinthium brought about a reduction in the transpiration rate of leafy branches of Syringa and Cvtisus. He was of the opinion that such oil vapors retarded the rate of water loss either by depressing the activity of the protoplasm or by retarding the rate of diffusion of water vapor from the mesophyll cells. The author (4) compared the effects on transpira-tion by some plants and evaporation from a moist filter paper of eucalyptus, peppermint and lemon oil vapors. These were found to bring about a slight depression in the rate of evaporation, but a more pronounced decrease in the rate of transpiration. The author (5) also tested the effects of lemon, eucalyptus, peppermint and citronella oil vapors on water loss from atmometers. Vapors of fresh samples of all these oils materially reduced the rate of water loss. After a short time, however, these oils decreased in effectiveness, apparently because the more volatile fractions have been evaporated. It was suggested that essential oil vapors might cause surface tension effects leading to a reduction in the rate of evaporation. Audus and Cheatham (1) tested the effects of some essential oil vapors on the transpiration rate, taking the rate of water absorption by cut shoots of cherry laurel as a direct measure of transpiration. On exposure of shoots to air saturated with oil vapors, they failed to detect any measurable effect on the absorption rate. The authors, however, reported a small depression which they regarded as presumably due to a toxic action of the vapor. They concluded that essential oils play no part in regulating water loss. Again, in view of the conflicting reports and conclusions concerning the effects of essential oils on 1 Received revised manuscript November 26, 1957. water relations of plants, the author (6) reinvesti-gated the effects of eucalyptus, lemon, peppermint and citronella oil vapors simultaneously on transpira-tion and absorption by plants. On applying these oil vapors to cut branches of 'Mentha and Eupator-ium-onie plant which did and one which did not produce essential oils themselves-the transpiration and absorption rates were significantly depressed. On the grounds of the observation that the depressions were, however, greater in transpiration bv plants (6) than in water loss by atmometers (5), it was suggested that oil vapors might have, in addition to their physical effects on evaporation of water, a possible effect on the stomatal mechanism, and that such effect would in turn affect transpiration. The present investigation is an attempt to test this suggestion by studying the effects of the same oil vapors on the stomata of the same species of plants used in the foregoing studies (6).