Effect of nitrogen nutrition on the export of sulphur from leaves in soybean

Soybean plants were grown in complete solution for 33 days and then transferred to medium containing inadequate sulphur (5 t M) and nitrogen at 15, 7.5, 2 or 0.25 mt M. In mature leaves (L1 and L2), and leaves that were 70% expanded at day 33 (L3), the net loss of sulphur over the ensuing 25 days was inversely related to the level of nitrogen nutrition. Leaf 5, which formed during the study period, exhibited complementary characteristics; the increase in the sulphur content was inversely related to the level of nitrogen nutrition even though low nitrogen nutrition supported less growth. L4, which was 31% expanded at day 33, exhibited intermediate characteristics. 35S-Labelled sulphate was supplied to all of the plants for 48 h at day 31 and was distributed principally to L3 at day 33. During early development, L5 became heavily labelled but, at low nitrogen nutrition, the massive import of total sulphur into L5 during the late stages of development was not accompanied by a commensurate increase in 35S-label, indicating that redistribution of soluble sulphur from mature leaves was not involved. The loss of sulphur from mature leaves was parallelled by similar changes in nitrogen at all levels of nitrogen nutrition. Collectively, the data suggest that a common mechanism, presumably proteolysis, is involved in the export of sulphur and nitrogen from mature leaves and that this process is inhibited at high levels of nitrogen nutrition, even under conditions of sulphur deficiency.