INFLUENCE OF LEAF DESTRUCTION BY SULPHUR DIOXIDE AND BY CLIPPING ON YIELD OF ALFALFA

Exposure to a sufficiently high concentration of sulphur dioxide for a long enough period of time always injures the leaves of alfalfa. Almost invariably the fully grown, highly functional leaves show the injury first, and are more severely injured than the rest. If the concentration is high enough, for example, to produce markings within an hour, after possibly three-fourths of this time has elapsed the leaves become noticeably stiffened, as though the cells had greatly increased in turgidity. The turgidity seems to increase until certain areas in the leaf (those along the margins and between the veins, where the intercellular air spaces are most abundant) suddenly become flaccid and take on a water-soaked aspect, giving the leaf a mottled appearance. If the fumigation be continued, additional marginal or interveinal areas on these leaves already affected, and corresponding areas on hitherto unaffected leaves, become similarly flaccid. With a few hours of sunshine after the fumigation, these flaccid areas bleach almost to an ivory color. The areas of each leaf which do not become flaccid remain green. Usually the green areas remaining after a heavy fumigation lie along the midrib and the principal veins of the leaf. They are separated sharply from the bleached areas. The marked areas of an alfalfa leaflet after any fumigation may vary from one spot the size of a pin-point to several larger ones, which may involve 50 per cent. or more of the area of the leaflet. A given fumigation may destroy more than 50 per cent. of the green tissue of some leaflets and yet leave 50 per cent. of the leaves entirely unmarked. Basal leaves are not often marked, and leaves that have not attained their growth only rarely. In all of our experience with alfalfa, a marked stem has never been found. When a majority of the green tissue of a leaflet is destroyed by sulphur dioxide, the leaflet curls and frequently drops from the plant. This type of injury is called "acute." Occasionally another type of sulphur dioxide injury on alfalfa leaves is encountered, a type that mani1 Contribution from the Department of Agricultural Research of the American Smelting & Refining Company, Salt Lake City, Utah. Grateful acknowledgment is made to MILTON R. BERNTSON, JOHN N. ABERSOLD, EVAN HARRIS, A. F. BARNEY, IVAN E. BURGOYNE, THOMAS BUNKALL, LYNN BROWN, and GEORGE GARDNER for their help in carrying forward this work.