Reactions of Cloned Poplars to Air Pollution: Premature Leaf Loss and Investigations of the Nitrogen Metabolism

Young poplar cuttings (Populus nigra L. cv. Loenen and P. maximowiczii Henry x P. nigra L. cv. Rochester) were exposed for six weeks in open-top chambers to realistic concentrations o f pollutant mixtures: 1) control; 2) SO ,/N O x; 3) 0 3/N 0 x and 4) S 0 2/ 0 3/N 0 x. In this sequence o f fumigation variants, the degree o f influence o f the various parameters o f the nitrogen me­ tabolism and o f premature leaf drop increased very frequently compared to the control plants, P. nigra L. proving to be the more sensitive species. The elevated Kjeldahl nitrogen content o f the fumigated leaves was accompanied by either an increase in free amino acids or in total protein or, in the case o f particularly large rises (SO ,/ 0 3/N 0 x variants), by increases in both substance groups. Proteolytic processes as a cause o f the elevated content o f free amino acids could be excluded to a large extent. A diminished de novo synthesis o f proteins obviously led to a shift in the amino acid/protein relationship. In the younger fumigated leaves, the total concentration o f free amino acids exceeded the values o f the older leaves. The elevated amino acid content o f the fumigated leaves was produced to a high degree by the glycolate pathway and the Krebs cycle. The increased turnover o f the car­ bon skeletons was connected with a drastic starch degradation, especially in the older leaves. The interaction o f the amino acid and carbohydrate metabolisms is probably an important regulator in the promotion o f rapid growth o f young leaves in order to compensate premature leaf loss.