A simple method to visualize the mechanism why Alnus glutinosa remains green during autumn colouration of Sorbus aucuparia

Abstract

The autumn colouration of two deciduous trees (Sorbus aucuparia and Alnus glutinosa) from northern Finland (65°N) was studied during leaf senescence in autumn 2003. Their leaves were harvested from the end of August until leaf abscission in Sorbus trees at the end of September. The leaves were extracted in ethanol and measured with a spectrophotometer at 663 nm (chlorophyll a), 644 nm (chlorophyll b), 536 (anthocyanins) and 470 nm (carotenoids). The ratios A663/644, A663/470 and A663/536 were calculated to demonstrate the degradation of chlorophyll a in relation to the other studied pigments. The most important results of the present study were as follows: (1) Rapid (within 4 days) visually observed change in Sorbus colour from green to red. (2) Simultaneously occurred significant change in the calculated absorbance ratios in the same species. (3) The distinct difference between the tree species in both variables: Alnus remained green and maintained its absorbance ratios above the threshold ratios of Sorbus throughout the experiment. Sorbus exhibited the reddish colour in a ratio of 0.4 for A663/470 and 4 for A663/536 around the middle of September. The lowest ratios for Alnus remained 0.5 and 6 for A663/470 and for A663/536 (respectively), i.e. the ratios in Alnus never reached the level in Sorbus. Alnus maintained its green colour throughout the studied period, but the Sorbus trees started to turn red in mid-September. Alnus remained green for two reasons: It maintained higher chlorophyll concentrations, and higher ratios of chlorophyll to other pigments.