Seasonal variation of leaf glutamine synthetase isoforms in temperate deciduous trees strongly suggests different functions for the enzymes

Changes in the activities of leaf glutamine synthetase (GS) isoforms were followed in four temperate deciduous trees from full leaf expansion to senescence (May to November). In the early part of the season, total GS activity was high in all species, with values ranging from 90 to 200 μmol h−1 g−1 fw. During this early period this activity comprised only the activity of the chloroplastic (GS2) isoform in all species. These high GS2 activities are consistent with the role of GS2 in the re-assimilation of photorespired ammonia. The early high values also coincided with high nitrate reductase activity in one of the species, the highly nitrophilous species Sambucus nigra, with values of up to 16μmol h−1 g−1 fw. This indicates that GS2 is also important in the assimilation of ammonia produced from nitrate reduction. From mid- to late-season, the cytosolic isoform (GS1) was detected in all four species and became increasingly more active in comparison to GS2. By the time of senescence it was the dominant enzyme of the two forms in both S. nigra and Carpinus betulus. The results provide strong support for recent findings that GS1 is an important enzyme for the mobilization of nitrogen for translocation or storage.