Increased Yield through Correction of Sulphur Deficiency in Ryegrass exposed to Sulphur Dioxide

Abstract

SULPHUR dioxide (SO2) in the air at high concentrations may be injurious to higher plants: at low concentrations it is not injurious. On the basis of experimental exposures1, a concentration of about 429 µg SO2 m−3 (0.15 p.p.m.) has been widely accepted as a threshold below which a range of higher plants would not be injured even during prolonged exposure. The dividing line between injurious and non-injurious concentrations is not well defined because plants vary in their susceptibility to injury according to the species, stage of growth, environmental factors, such as light intensity, humidity, water supply, and the duration of the exposure at a given concentration2. The injurious effects of SO2 have been investigated extensively but much less attention has been given to its effects at non-injurious concentrations. This aspect deserves further study because in some circumstances the effects of SO2 are known to be beneficial. Thus, it has been observed that (i) the rate of photosynthesis in lucerne, as measured by the uptake of carbon dioxide, was increased by exposing the plants to air containing 486 µg SO2 m−3 for 66 h (ref. 3) and (ii) the yields of dry matter of several species growing in solution culture were enhanced when the shoots were exposed for 9 to 20 days in air containing SO2, labelled with 35S, at 200 and 500 µg m−3; 35S appeared in both organic and inorganic fractions of the plant sulphur4. The fact that SO2 can be utilized after absorption through the shoots may be of widespread significance in crop nutrition in industrialized countries. Indeed, we have found recently5 that when ryegrass was grown on sixteen soils from England and Wales in a controlled environment cabinet with air filtered to remove SO2, the plants developed sulphur deficiency on all soils except one; deficiency has not been encountered in crops growing on these soils in the field. The inputs of sulphur from the air and rainfall to plants and soils under field conditions are not known with certainty, but in rural areas in Britain the air near the ground contains on average 50 µg SO2 m−3 (ref. 6) and for urban areas the average is slightly above 100 µg m−3 (ref. 7).