Interaction of phloem-translocated amino compounds with nitrate net uptake by the roots of beech (Fagus sylvatica) seedlings

In the present study two experimental approaches were used to investigate the influence of changes in the allocation of amino compounds in the phloem of beech (Fagus sylvatica L.) seedlings on nitrate net uptake by the roots. In a first set of experiments Gin or Asp were directly fed into the phloem of the epicotyl via bark flaps. These compounds were previously found to be allocated in the phloem of adult beech trees and were shown to inhibit nitrate net uptake when supplied to beech roots. Feeding of solutions containing 100 mM of Gin or Asp plus 10 mM EDTA into the phloem resulted in a significant enrichment of the fine root tissue with the amino compound fed as compared to the roots of control plants supplied with amino acid-free EDTA solutions. Nitrate net uptake by the roots decreased by 61% (Gin) and 79% (Asp) as compared to the controls. In a second approach, shoots of young beech seedlings were exposed to 40g NH3 m^-3. NH3 uptake by shoots, nitrate net uptake by roots, and the contents and composition of total soluble non-protein nitrogen (TSNN) in leaves, phloem, and fine roots were determined and were compared to results gained with control plants exposed to charcoal-filtered air. NH3 fumigation of the shoots of beech seedlings resulted in a 35% reduction of nitrate net uptake by the roots as compared to controls. TSNN contents in leaves and phloem exudate of NH3-fumigated plants increased by 56% and 37%, respectively. This enrichment was mainly due to Arg and Glu in the leaves and Asp, Asn, Glu, and Gin, but not to Arg, in phloem exudate. The TSNN content of the fine roots was not changed by NH3 fumigation, but a significant increase in the Gin content was observed. From these results it is concluded that phloem transport of amino compounds, especially of Gin and Asp, from the shoot to the roots mediates regulation of nitrate net uptake by the roots of beech trees in order to adapt this process to the nitrogen demand of the whole plant.