Effects of Nutrient Concentration on Anatomy, Metabolism, and Bud Abscission of Sweet Pea

In the fall and early winter of 1934, sweet pea plants were grown under usual commercial conditions of temperature and humidity. All of the cultures received a complete nutrient solution which was continuously percolated through the white quartz sand of the self-draining porcelain culture jars. Each group of plants received an abundant supply of nitrogen as nitrate and the same ratio of nutrient salts. Some of the plants were supplied with relatively dilute nutrient solution at concentrations of one half and one atmosphere respectively, others with comparatively concentrated solution at two and three atmospheres respectively. 1. The plants supplied with the more dilute nutrient solutions grew vigorously and were relatively succulent; the leaf blades were thin, somewhat circular in shape, and dark green. A comparatively high percentage of flower buds abscissed. 2. The cultures supplied with the more concentrated nutrient solutions grew less vigorously and were less succulent; the leaf blades were thick, oval or eliptical in shape, and light green. A comparatively low percentage of flower buds abscissed. 3. The roots and tops of the plants grown in the less concentrated solutions were high in proportion of young active cells containing dense protoplasm. All tissues differentiated and matured slowly. Carbohydrates were relatively low and organic nitrogen high. A large proportion of the elaborated nitrogen was in the form of amide and amino nitrogen. Nitrate was present in abundance. 4. All organs of the cultures supplied with the more concentrated solutions were relatively low in proportion of young cells with dense protoplasmic contents. Tissues differentiated and matured rapidly. There were present strongly developed fibers and other mechanical elements. Carbohydrates were high and organic nitrogen relatively low. A large proportion of the elaborated nitrogen was in the form of complex proteins. Nitrate was present in abundance. 5. With a deficiency of carbohydrates and slow maturation of tissues of the low concentration cultures, there persisted at the junction point of pedicel and peduncle several tiers of cells which failed to mature and remained meristematic in appearance. These tiers of cells were approximately at right angles to the main axis of the pedicel. Ultimately the middle lamellae of one or more tiers became gelatinized, accompanying which the cells separated and the flower bud abscissed. This mode of derivation of the abscission layer is in contrast to the abscission through the non-succulent pedicels of mature fruits, such as the apple, or abscission through the comparatively woody petioles of mature leaves, in which case mature cells apparently become active and thus give rise to the tiers of meristematic cells (29) forming the abscission layer. 6. It is a well recognized fact that the amount of water absorbed from a concentrated solution of salts is much less than from a more dilute solution (16). As already recorded, the plants of the cultures supplied with concentrated nutrient were lacking in succulence and otherwise xerophytic in character. 7. Although the percentage of nitrate was fairly high in the plants of all the nutrient series, an effect of a high concentration of nutrient salts was practically that of low nitrogen nutrition, or more correctly, low protein nutrition. 8. Only comparatively young cells containing abundant protoplasm were capable of synthesis of proteins from nitrate. 9. As the plants of the high concentration cultures had relatively few young active cells, they exhibited only limited reduction and assimilation of nitrate and there was accordingly much less oxidation of carbohydrates or their derivatives. This fact would seem to account, at least in part, for the marked accumulation of carbohydrates. 10. Relationships of soil moisture content and salt concentrations in the soil are discussed and it is pointed out that, long before the wilting point is reached, the salt concentration of the soil solution may bring about early maturation of tissues. 11. Mature, optically empty tissues being low or entirely lacking in ability to synthesize proteins from nitrate, there is brought about a condition of protein deficiency incorrectly called nitrogen deficiency, since under the conditions mentioned, nitrate may be present in abundance in the nutrient medium and in the plants. The fact is also emphasized that such effects are especially severe in case of fruit trees, some grasses, and other perennials, because such plants carry on the initial phases of nitrate assimilation and protein synthesis almost exclusively in the fine succulent rootlets. These organs mature and lose ability to manufacture protein very rapidly when exposed to a relatively concentrated solution of salts. The roots, however, can absorb nitrate and other nutrients freely if they are present in the solution of the nutrient medium.