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 investig ate the influence of changes in the allocation of amino compounds in t he phloem of beech (Fagus sylvatica L.) seedlings on nitrate net uptak e 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 w ere previously found to be allocated in the phloem of adult beech tree s 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 c ontrol plants supplied with amino acid-free EDTA solutions. Nitrate ne t uptake by the roots decreased by 61% (Gln) and 79% (Asp) as compared to the controls. In a second approach, shoots of young beech seedling s were exposed to 40 mu g NH3 m(-3). NH3 uptake by shoots, nitrate net uptake by roots, and the contents and composition of total soluble no n-protein nitrogen (TSNN) in leaves, phloem, and fine roots were deter mined and were compared to results gained with control plants exposed to charcoal-filtered air. NH, fumigation of the shoots of beech seedli ngs resulted in a 35% reduction of nitrate net uptake by the roots as compared to controls. TSNN contents in leaves and phloem exudate of NH 3-fumigated plants increased by 56% and 37%, respectively. This enrich ment was mainly due to Arg and Glu in the leaves and Asp, Asn, Glu, an d Gin, but not to Arg, in phloem exudate. The TSNN content of the fine roots was not changed by NH, fumigation, but a significant increase i n the Gin content was observed. From these results it is concluded tha t phloem transport of amino compounds, especially of Gin and Asp, from the shoot to the roots mediates regulation of nitrate net uptake by t he roots of beech trees in order to adapt this process to the nitrogen demand of the whole plant.