RELATION OF SULPHUR DIOXIDE IN THE ATMOSPHERE TO PHOTOSYNTHESIS AND RESPIRATION OF ALFALFA

Abstract

The effects of SO2 fumigations of alfalfa were evaluated by measuring the rates of photosynthesis and respiration of a treated plot in comparison with a similar check plot. The treatments ranged from fumigations of high concn. which produced almost complete foliar destruction to long-continued sublethal dosages of the gas. Extensive leaf destruction was followed by the rapid growth of new leaves and a corresponding restoration of photo-synthetic activity. Fumigations of sufficiently high concn. reduced the rate of assimilation while the gas was present, the amount of reduction depending on the severity of the treatment. Following such fumigations, the rate returned more or less quickly to a normal or greater than normal level, if leaf destruction was not extensive. The data suggest that a slight stimulation of photosynthetic activity may occur following such treatments. In intermittent fumigations with 0.35 p.p.m. to 1 p.p.m. SO2 the decreased assimilation during the fumigation was practically offset by increased assimilation following the treatment. The continuous application of 0.20-0.25 p.pjn. SOa did not affect the rate of assimilation for several days; 0.14 p.pjn. SO2 was without effect for over a month. No significant effect on respiration was observed. Most of the absorbed SOa was oxidized to sulphate and tended to remain in the leaves. When the S content of the leaf reached about 2%, the leaf became chlorotic. With smaller amounts of S the leaf appeared to be normal. Chlorophyll analyses are given.