EFFECTS OF SULPHUR DEFICIENCY ON METABOLISM IN TOMATO

Abstract

Symptoms of S deficiency developed slowly in plants which lacked an external sulphate supply: the plants looked as though they had been gradually but not completely deprived of N, the lower leaves were yellowish green, the stems were hard and woody, the roots were extensive, and roots and stems were slender. These characteristics may also be exhibited by tomato plants deficient respectively in N, P, or K; but the S-deficient plants had a remarkable capacity for stem elongation, and although the sterns were woody and thin they increased in length, but not in diam., as rapidly as the stems of the complete-nutrient plants. The S-deficient plants were extremely high in carbohydrates, and contained much more nitrate than the plants which received the complete nutrient solution. Carbohydrates and nitrate accumulated in the sulphate-deficient plants because reduction of nitrates and oxidation of sugars were slow. Digestion of starch and translocation of sugars took place freely in sulphate-deficient plants. Cell wall thickness in tomato plants seems to be more intimately associated with carbohydrate content than with any other single factor of nutrition. The high-carbohydrate, minus-sulphate plants had thick cell walls and a high proportion of fibers and lignified tissue. Protoplasm of the S-deficient plant looked much like that of plants lacking N; i.e., it was limited in amount but not noticeably injured as in .deficiency of P, Ca, or K. Cysteine specifically, probably glutathione, and possibly other S-H compounds were present in meristematic tissue and were especially high when complete-nutrient plants contained an abundance of sulphate and nitrate. Associated with low content of S-H sulphur in the minus-sulphate plants there was practically no active cambium. Roots and stems were slender, but they increased in length as a result of cell division in the apical meristems of stems and roots. The meristematic tissues contained faint traces of S-H compounds. Proteolysis is usually accompanied by decrease in reserve carbohydrates. The minus-sulphate plants were at all times extremely high in carbohydrates, yet complex proteins were rapidly broken down to soluble organic compounds of S, to amino acids, to asparagine, and to ammonia. This proteolytic activity resulted, however, in little if any S-H sulphur. Although the plants lacking an external sulphate supply were not very low in percentage of total organic S, much of it was water-soluble; whereas the organic sulphur of the complete-nutrient plants was mainly in a complex protein form. The soluble organic S in the minus-S plants contained practically no cysteine, glutathione or cystine. Limited data give some evidence that sulphate and ammonium may be formed proteolytically. It is known that phosphatides break down in part to phosphates and probably ammonium. In tomato, sulphate is reduced to sulphite and apparently to S-H sulphur in the comparatively alkaline phloem region of roots and tops. This is also true of the reduction of nitrates to nitrites and ammonium. In apple, narcissus, and asparagus, the region of nitrate reduction is largely confined to the fibrous roots. Sulphate reduction in these plants takes place in the roots to some extent, but mainly in the tops.