Ammonium and Nitrate Nutrition of Dormant Delicious Apple Trees at 48⚬F

Abstract

1. Dormant 2-year-old nitrogen deficient Delicious apple trees were subjected to a constant temperature of 48⚬F. and continuous darkness. They were grown in sand, and the solutions of each culture underwent renewal at the rate of 36 liters every 24 hours. As a result, after passing through the sand, the solution did not vary more than 0.1 pH from that initially supplied. Trees were also grown with the same nutrient treatments but in water instead of in sand. The water cultures were not only renewed but were constantly stirred with a mechanical agitator. 2. The sole source of nitrogen for some of the trees was a complete nutrient solution containing ammonium sulphate supplied at pH 6.0; for others, calcium nitrate in complete nutrient solution at pH 4.5. Some cultures were also grown with no external nitrogen supply but with other essential elements at pH 5.0. 3. During the 16-day period of these experiments while subjected to a constant temperature of 48⚬F., buds expanded only slightly but there was vigorous and rapid development of roots in both the sand and the water cultures. 4. Simple proteins were synthesized in the fine fibrous roots of the nitrogen supplied trees. They accumulated apparently exclusively in these organs and were present in much higher concentrations than in the fine fibrous roots of the series grown with no nitrogen in the nutrient solution. 5. The trees which received ammonium sulphate, however, synthesized amino acids and asparagine more rapidly and in greater quantity than did the trees of the calcium nitrate cultures. 6. In the fine fibrous roots of the trees with no external nitrogen supply carbohydrates increased, undoubtedly owing to translocation. But in those cases where ammonium and nitrate nitrogen were assimilated, there necessarily occurred considerable decrease in carbohydrates, especially in the case of the ammonium supplied trees which were synthesizing organic nitrogen more rapidly than the trees of the nitrate series. 7. On shifting to 70⚬F., a large part of the newly synthesized amino acids and asparagine was translocated from the fine roots to the buds, which expanded rapidly at the higher temperature. 8. Although the nutrient solutions after passing through the sand did not vary more than 0.1 pH from that initially supplied, the absorbing surfaces of the fine roots of the sand cultures supplied with ammonium sulphate at pH 6.0 were relatively acid (pH 4.2). Those of the calcium nitrate series at pH 4.5 were relatively less acid (pH 5.6). This was presumably due in each case to comparatively rapid absorption of the nitrogen containing ion, with consequent increase or decrease in acidity due respectively to the residual sulphate and calcium ions. 9. From these results it appears that there is not directly required a widely different pH value of the nutrient medium for ammonium as compared with that for nitrate assimilation, provided the absorbing surfaces of the roots are not subjected to extremely acid or alkaline conditions. Practically, however, it would be inadvisable continuously to apply ammonium sulphate to an acid soil, or sodium or calcium nitrate to a very alkaline soil. 10. In a few preliminary experiments in sand culture where higher concentrations of ammonium sulphate were used, there resulted an extremely acid condition of the root surface and outer cortical cells (pH 2.8-3.0). Associated with the acid condition of the cortex many of the root primordia failed to emerge, which resulted in roots that were stubby and bulbous in appearance. 11. In water cultures where the solutions were constantly stirred, similar pH changes occurred but they were much less extreme. Under these conditions where extreme changes in the pH at the root surfaces were eliminated, the roots grew vigorously and assimilated ammonium rapidly, even when present in concentrations four times as high as that in the regular nutrient solutions. 12. Except in the case just described in which injury occurred, the pH of the internal tissues did not vary with that of the nutrient solution. The pH values of respective tissues were essentially the same for all nutrient treatments. 13. The contents of the cells of the cortex of the fine fibrous roots varied in pH from 5.2 to 4.4, or even 4.2 in occasional cells, but the two lower figures were invariably associated with heavy deposition of starch (as in the roots of minus-nitrogen cultures), not with the pH of the nutrient medium. 14. The phloem and cambium were relatively less acid (pH 5.6-5.8) except in case of the phloem rays which were about pH 4.6 when starch was present. Mature xylem appeared to have a H-ion concentration of about 4.8-5.2. 15. Limited examination of old roots and tops also indicated that starch storing cells were relatively acid and meristematic tissues, as the cambium, comparatively alkaline.