EFFECTS OF FOLIAR APPLIED NITROGEN AND KINETIN ON NITROGEN REDISTRIBUTION DURING GRAIN-GROWTH IN WHEAT .3. MOBILIZATION OF NITROGEN FROM VEGETATIVE PLANT-PARTS

The nitrogen requirement of the developing grain is largely satisfied by mobilization of protein from vegetative organs. In order to better understand this redistribution of nitrogen and how this process is influenced by foliar treatments, wheat plants were grown until heading on a complete nutrient solution complemented in graduated time courses with N-15-labelled nitrogen. Qualitatively there are no differences in nitrogen remobilization in plants maintained with an adequate nitrogen supply up to maturity and those grown without an exogenous supply of nitrogen since heading. Despite the amount of grain nitrogen in both cases (+N and -N plants) most of the grain nitrogen could be attributed to flag leaf, stem and glumes. Compared with the +N plants, glumes and stem of -N plants exhibited low content of remaining nitrogen, indicating an excessive export of nitrogen combined with a low biosynthesis of insoluble nitrogen compounds. The dynamic course of N-15-nitrogen remobilization revealed remarkable differences. Depending on the time the N-15-nitrogen has been assimilated the increment and decrement of the nitrogen in the various organs seems to be correlated to the photosynthetic and biosynthetic activity of the actual organ. The nitrogen pattern was for instance in agreement with the loss of soluble protein fractions in the flag leaf and with course of dipeptidase activity in the glumes. Though the treatments of the flag leaves, especially with an AHL solution significantly retarded leaf senescence, the external nitrogen and hormone application had only little effect on grain nitrogen. The AHL solution delayed the import of nitrogen in the grain in the first 20 days after treatment of the flag leaf. When this effect decreased the translocation of nitrogen into the grain increased rapidly within a short time, resulting in the same amount of translocated N-15-nitrogen in the grain at the end of the grain filling period.