SEASONAL-CHANGES IN DISSOLVED AMINO-ACIDS AND SUGARS IN BASAL CULM INTERNODES AS PHYSIOLOGICAL INDICATORS OF THE C N-BALANCE OF PHRAGMITES-AUSTRALIS AT LITTORAL SITES OF DIFFERENT TROPHIC STATUS/

The nitrogen content of different internodes and the dissolved amino a cids and sugars of the basal culm internodes of Phragmites australis ( Cav.) Trin. ex Steud. were studied monthly in 1994 to estimate the car bon/nitrogen balance at eight sites and lakes. These stands had large differences in standing crop, with less than 400 g shoot dry weight m( -2) in a mesotrophic lake and up to 1500 g dry weight m(-2) in hypertr ophic lakes. No significant differences were found between sites in th e nitrate and ammonium concentration of the interstitial water because of very high variability in space and time. The relative nitrogen con tent (% of dry weight) of different leaves and culm internodes showed significant differences but only between the most different sites. The amount and composition of free amino acids and sugars of the basal cu lm internodes showed a clear association with the maximum standing cro p of the sites. During the later shoot growth phase (July to September ), plants of the high productivity sites exhibited a high content of d issolved amino acids in the basal culm internodes with asparagine as t he main transport and storage compound, but a very low sugar content w ith sucrose as the main transport component in summer. At low producti vity sites of mesotrophic to moderate eutrophic lakes, this relationsh ip was inverse. The mean of the proportions of total sugars to total a mino acids and asparagine to sucrose, respectively, were significantly correlated with the maximum standing crop of the eight sites. Alanine and serine were present in high concentration in early spring before outgrowth of the tiller without pressure induced ventilation of the su btending vertical rhizome parts. The results are discussed with respec t to the indicatory value of the amino acid and sugar content of the b asal shoot internodes to the productivity of the different stands and their limitation by the nitrogen availability of the plants. (C) 1998 Elsevier Science B.V.