Proteolytic activity and nitrogen remobilisation in senescing leaves of phenological forms of Fagus sylvatica

Leaf senescence allows plants to remobilise and use the same nitrogen repeatedly and is closely linked to autumn phenology. The timing of leaf senescence affects the growth rate and survival of trees due to the association between senescence and the remobilisation of nutrients, particularly nitrogen. The present study compares protein degradation dynamics and nitrogen remobilisation in early, intermediate and late phenological forms of beech trees (Fagus sylvatica L.). Specimens of phenological forms were marked and examined in 2005 and 2008. Leaf samples were collected from August to October during each of these years, and a biochemical analysis and a determination of proteolytic enzyme activity were conducted. The early phenological form showed protein degradation with three clearly indicated phases, whereas in the late form, protein degradation was stable with a constant decrease. The phenological forms differed significantly in their C/N ratios, which increased from approximately 20 in August to 37.5, 35 and 32 for the early, intermediate and late forms, respectively, at the end of leaf senescence. The date of the sudden drop in temperature had a decisive effect on the beginning of leaf senescence. Temperature has a greater effect on protein degradation and the protein and nitrogen resorption efficiency in the early form than in the late form. The trees that began to senesce the earliest exhibited the highest resorption of nitrogen compounds. Senescence led to an increase in proteolytic activity. Aminopeptidase activity was highest at the beginning of senescence, while endo- and carboxypeptidase activity was highest in the middle of senescence. The early form had the highest activity levels for all peptidase types. These results indicate that beech trees that differ in their autumn senescence timing display different nitrogen remobilisation efficiencies. This efficiency depended on the length of leaf senescence, peptidase activity and the sensitivity of particular phenological forms to temperature changes.