Changes in glutamate dehydrogenase activity, glutamine synthetase activity and their amounts (GS1, GS2) in a source leaf. Plants were supplied with ample (HN, 3.75 mM) or low (LN, 0.375 mM) nitrate concentrations. These data were obtained from a selected ‘source leaf’, determined as mature at D0 (early bolting) and becoming senescent during the experiment. The activity of glutamate dehydrogenase (GDH; A) was quantified as the synthesis of glutamate and is expressed as nmol of NADH used.h−1.μg−1 proteins. The activity of glutamine synthetase (GS; B) was determined by the nmol of glutamine produced.h−1.μg−1 proteins. The GS1 and GS2 amounts (C) were quantified after western blotting with specific antibodies and the percentage of GS1 among the total GS amount was estimated. Data were observed at 0, 7 (early bolting), 14 (flower buds raised above the youngest leaves), 21 (first petals visible, but flower buds still closed) and 28 (flowering) days after the beginning of bolting (D0). Only one biological replicate remained at D28, and its value is indicated by a cross (x). In panels A and B, data are indicated as the mean value ± SE (vertical bars). Letters a, b and c represent differences in kinetics, asterisks mean significant differences between treatment and hashes represent significant differences between genotypes (n = 4 plants; p < 0.05).