Influence of Sulphur Deficiency on Metabolism of the Sunflower

The visible effects of sulphur deficiency in the sunflower were similar to those noted in previous work on the soybean: the yellow-green color of the leaves, the smaller leaves, and the thinner stems as compared with the plus-sulphur plants. Stem elongation was a prominent feature. 2. The soluble organic nitrogen fractions and nitrates accumulated in the minus-sulphur stems as compared with the plus-sulphur. This was also true of the soybean. Soluble sulphur compounds were also higher in the sulphur-deficient sunflower stems. These were not determined in the case of the soybean. 3. While the minus-sulphur sunflower stems were somewhat higher in starch and acid-hydrolyzable carbohydrates than the plus-sulphur, they were so much lower in sugars that they were to be considered low-carbohydrate stems. On the other hand, although the sulphur-deficient soybean stems were lower in sugar than the plus-sulphur, they were so much higher in the polysaccharides that they were to be regarded as high carbohydrate. All forms of carbohydrates accumulate in a minus-sulphur tomato stem. So plants vary in this regard. 4. The accumulation of nitrates and carbohydrates in minus-sulphur plants seems to be due to low nitrate assimilation, which results from the low reducase content of the plants. The low nitrate assimilation also no doubt accounts for the yellow color of the sulphur-deficient leaves, as is true of other deficiencies; for example, potassium, calcium, and phosphorus. Accumulation of soluble organic nitrogen fractions and soluble sulphur compounds seems to be due mainly to proteolysis. Proteolysis and reutilization seem to be more prominent features of sulphur deficiency than is true of most other deficiencies. 5. Part of the accumulation of the soluble organic nitrogen fraction as a result of sulphur deficiency may be due to the lack of sulphur-containing amino acids for the completion of protein synthesis. 6. It is not known why low protein synthesis results in the accumulation in the tomato of all forms of carbohydrates but in the soybean and sunflower of only starch and acid-hydrolyzable carbohydrates. 7. Since the gradients of the nitrogen and sulphur fractions are negative upward rather than positive upward, they offer no support for the theory of proteolysis in the lower levels of the sulphur-deficient stems and transport of the resulting materials to the tips to be used in stem elongation. 8. Since only one analysis was made (and that rather late in the vegetative period), the different tissues were not analyzed separately, and no ringing experiments were performed, perhaps the nature of the gradients should not necessarily be regarded as evidence against upward transport. 9. It may be, however, that proteolysis in the minus-sulphur stems goes on at or near the place where the resulting materials are found, with transfer at the most through only short distances. 10. The gradients of nitrogenous and sulphur compounds were in the same direction as that of the metabolically active cells, namely, positive downward. It seems likely that the gradients result from the metabolic activities of these cells: proteolysis in the case of the sulphur-deficient stems, synthesis in the case of the plus-sulphur. 11. The positive downward gradients of starch and reducing sugars may also result from synthesis in living cells, although in this case the gradients may be partly translocatory. 12. The gradients in themselves do not afford evidence for the theory of proteolysis and reutilization. But the accumulation in the minus-sulphur stems of soluble organic nitrogen fractions, of which amide forms an abnormal percentage, indicates that rapid proteolysis is going on in these stems. This situation usually prevails in proteolysis. 13. Proteolysis in a sulphur-deficient plant seems abnormal in that it proceeds even though the plant is high in carbohydrates. Usually proteolysis is associated with a reduction in carbohydrates. But it would seem that carbohydrate content is not the controlling factor in proteolysis. The process may go on in detached leaves, for example, under conditions of either high or low carbohydrate content. The same may be said for minus-sulphur plants. The sulphur-deficient tomato and soybean stems may be regarded as high carbohydrate, although sulphur deficiency results in a lower sugar content of the soybean stems. On the other hand, a minus-sulphur sunflower stem is to be considered low in carbohydrates although slightly higher than the plus-sulphur in starch and acid-hydrolyzable carbohydrates. From this standpoint proteolysis resulting from sulphur deficiency should not be considered so abnormal. 14. While the starch and reducing sugar gradients of the sunflower stem are in the direction of the metabolically active cells, positive downward, the acid-hydrolyzable gradient is in the reverse direction, that of the nonliving cells. This may be regarded as evidence that under the conditions of this experiment the acid-hydrolyzable carbohydrates are not so important in metabolism as starch and reducing sugars. 15. No categorical statement can be made as to the importance of the acid-hydrolyzable carbohydrates in metabolism of plants. Their usability may vary with the kind of plant, carbohydrate, climatic conditions during growth, with the amount of other more available carbohydrates present, and perhaps other factors.