FOLIAR APPLICATION OF NITRATE OR AMMONIUM AS SOLE NITROGEN SUPPLY IN RICINUS-COMMUNIS - I - CARBON AND NITROGEN UPTAKE AND INFLOWS

Following a precultivation with pedospheric nitrogen nutrition, nitrat e or ammonium solutions were supplied to the shoots of Ricinus plants by spraying (during the experimental period) resulting in an increase of biotic/organic and abiotic/inorganic particles on the surface, whic h significantly increased wetting of the leaf surfaces. The distributi on of particles on the surface of sprayed leaves, in particular crysta ls around and in stomata, indicated the possible entry of nutrients vi a thin water films through the stomatal pores in addition to diffusion through the cuticle. Ammonium was taken up more readily than nitrate by the foliage, but both at relatively low rates which caused N limita tion. Interestingly, the inorganic N, both in the form of nitrate and even ammonium, was entirely assimilated in the shoots; phloem transpor t of inorganic N to the root was negligible. The flows of malate, and the acidification of the apoplastic washing solution of leaves in ammo nium-sprayed plants pointed to the role of metabolism of malate and ex cretion of protons in maintaining pH during ammonium assimilation in t he shoot. Ammonium-sprayed plants incorporated the N in the same amoun ts in shoots and roots, only 38% of the shoot borne N being recycled i n the xylem. In nitrate-sprayed plants the root was not only favoured in N partitioning, but even a net export of previously incorporated N from the shoots occurred which reflected the N limitation. The N limit ation also affected carbon metabolism, in particular the flows of C, i ncorporation in the shoot and photosynthesis, which were decreased whe n compared with data from recent experiments with pedospheric well fed Ricinus. However, there was little difference in C flows between nitr ate and ammonium-sprayed plants with respect to respiration, C partiti oning and, most interestingly, in relative stimulation of root growth. The loss of C from dark respiration of the shoots was high on a f. wt basis as well as in relative terms, owing to exclusive N assimilation in the shoot. In general the plants invested untargeted increases in root growth as a result of N limitation irrespective of the imposed ar tificial treatment which made the shoot the site of mineral N uptake.