ABSORPTION OF SULPHUR DIOXIDE BY ALFALFA AND ITS RELATION TO LEAF INJURY

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Abstract
Numerous SO2 fumigation exps. with alfalfa, each of either about 20 min. or about 80 min. duration, were made under different environmental conditions. In each exp. the amount of gas absorbed by the plants, its concn. in the air, and the extent of leaf destruction were measured. The amt. of leaf destruction in a fumigation of definite duration was found to be a linear function of the amount of SO2 absorbed. Appreciable quantities of gas, depending on the rate of absorption, can be taken up without causing any leaf destruction. For each degree of leaf injury, a portion of the absorbed gas, varying with the rate of absorption, is inactivated, due to oxidation and neutralization, to the much less toxic sulphate form. Assuming a uniform rate of absorption, the product of the concn. of the SO2 in the air and the duration of the fumigation, is a constant for each degree of leaf destruction. By extrapolation of the data, it is suggested that the toxic dosage of SO2 itself, uncom-plicated by the inactivation process, is about 1350 parts per million in the (dry) leaf tissue, and, assuming 50% of the dry weight of the alfalfa leaf to consist of meso-phyll, the toxic dosage approaches 2700 p.p.m. in the (dry) mesophyll cells, since the other tissues are not involved in absorption to any great extent.
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