Phytotoxicity of hydrocarbons

Abstract

Benzene, cyclohexene, cyclohexane, n-hexane, and hexene-1 were applied to young carrot and barley plants as vapors diluted with air, and to Anacharis shoots and root and leaf segments of barley and carrot in aqueous solutions. The tetrazolium reaction and plasmolysis were employed as vitality criteria in the aqueous-solution tests. With the vapors, toxicity (as the reciprocal of minimum lethal concentration) decreases in the order benzene, cyclohexene, cyclohexane, hexane, hexene. Minimum lethal relative saturation in water increases in the order benzene, cyclohexene, cyclohexane, hexene, hexane; but on a molar concentration basis this order is reversed. In a general way the data appear to support the Ferguson Principle, that toxicity is related more to degree of saturation in a medium than to concentration. There seems to be no relation between water solubility and toxicity of low-boiling hydrocarbons of the types investigated. Toxicity is closely related to oil-air distribution values, when plants are treated with hydrocarbon vapors, and when cottonseed and corn oils are used as models of cell lipid in calculating the distribution coefficients. Calculation of theoretical concentration of hydrocarbon in cell lipid by use of distribution coefficients gives rather constant minimum lethal values of about 2 mols per liter for barley and 3 mols per liter for carrot tissue. Thus there seems to be no great difference in the absolute toxicity of the five substances; apparent differences must be ascribed to physical factors involved in movement of molecules to and retention at the site of physiological action. The data obtained favor a mechanism of toxic action similar to that embraced by the classical Overton-Meyer theory of narcosis. The essence of this theory is that narcosis is brought about by accumulation of the narcotic in certain cell constituents, probably the lipids. All of the hydrocarbons studied are good lipid solvents. Used in sufficient amounts they act as cytolytics and are lethal to a marked degree. The site of action is probably the external limiting surface membrane of the cell, the ectoplast. This membrane is known to be rich in lipid; it is the first structure met upon absorption by the cell; and the rapid increase in permeability of cells is evidence that membrane structure has been altered.