Chlorophylls are degraded and flavonoids synthesized during autumn senescence of deciduous trees. In a recent study by Mattila et al. published in AoB PLANTS, chlorophyll and flavonol contents of individual leaves of a number of deciduous tree species (rowan, Norway maple, silver birch and bird cherry) were monitored non-destructively throughout the autumn.
Loss of chlorophyll and synthesis of flavonols were not gradual. Instead, in most of the studied trees, the chlorophyll content of each individual leaf remained constant until a phase of rapid degradation commenced. In approximately one week, the leaf turns yellow and falls. The pattern was similar in birch (Betula pendula), maple (Acer platanoides) and bird cherry (Prunus padus); in rowan (Sorbus aucuparia), very slow gradual chlorophyll degradation occurred on top of the main pattern. The results may suggest that each individual tree leaf retains its photosynthetic activity, reflected by a high chlorophyll content, until a rapid phase of chlorophyll degradation and flavonoid synthesis begins. Therefore, in studies of autumn senescence, leaves whose chlorophyll content is decreasing and leaves with summertime chlorophyll content (i.e. the leaves that have not yet started to degrade chlorophyll) should be treated separately.