Metabolic changes during natural ageing in sunflower (Helianthus annuus) leaves:: expression and activity of glutamine synthetase isoforms are regulated differently during senescence

Analysis of the changes in the carbon and nitrogen metabolite, photosynthetic pigment, soluble protein and total RNA contents, and in the transpiration and carbon dioxide fixation rates during sunflower leaf development, indicated that senescence initiates and progressively advances in primary leaves aged between 28 and 42 days. Leaf senescence was associated with significant metabolic changes. A loss of pigments and photosynthetic activity was observed, and chlorophylls were more susceptible to degradation than carotenoids during senescence. Soluble protein and total RNA levels also decreased. Concentrations of soluble sugars increased during leaf ageing, whereas a decrease in the starch content was found. The highest ammonia concentrations were observed in young leaves aged 16 days and senescent leaves aged 42 days. The content of glutamate, the major amino acid in sunflower leaves, and the (Glu + Asp)/(Gln + Asn) ratio progressively decreased during ageing, suggesting that nitrogen-rich amino acids are stored to be exported from senescent leaves. Total glutamine synthetase (GS) activity declined during leaf senescence, due to a marked decrease in both transcripts and enzyme activity of the chloroplastic isoform GS2. In contrast, cytosolic GS1 activity increased with leaf age. Thus, the GS2/GS1 ratio may be used as a marker of senescence in sunflower leaves. The senescence process was also associated with an increase in the oxidative stress, as revealed by hydrogen peroxide accumulation, lipid peroxidation and antioxidant enzyme activities. Oxidative stress caused a strong inhibition of GS2 activity, but the GS1 isoform was much more resistant to oxidative damage.