Colour Cycle of a Maple Tree

Discovering why leaves change from green in late summer to deep red in autumn

Written by Guy Fernando

Last modified November 2019

A leaf appears green because it contain an abundance of chlorophyll which is a green pigment. A green summer leaf contain so much chlorophyll that other coloured pigments in the leaf are masked out. Sunlight controls the amount of chlorophyll produced, so as autumn days grow shorter less chlorophyll is produced. This gradual reduction of chlorophyll allows other pigments in leaf to be revealed. During the autumn levels of chlorophyll decreases, as well as increased sugar concentration which cause an increase production of anthocyanin. Anthocyanin a redish pigment which gives leaves their red autumnal colour.

The spectrum graphs below show transmission plots, which is the measurement of light of varying wavelength that is able to pass through the sample. This is typically the inverse of absorption plots, which is the measurement of light that is absorbed by the sample. Also shown below are CIE colour space chromaticity diagrams which plot the actual perceived colour of the sample.

In plants there are two types of Chlorophyll, namely Chlorophyll-A and Chlorophyll-B. Primarily it is Chlorophyll-A that is responsible for performing photosynthesis, the role of converting light energy to chemical energy such as the production of sugars.

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autumn


Two samples of leaves from the maple tree were taken. One set in early October when the leaves were still green, and the other set in mid November when the leaves were a deep red. Both leaf samples were crushed and placed in a solution of water and acetone, and optically tested using the PIC Optical Spectrum Analyser (POSA-1)
.

Early-October Leaf Sample


Chlorophyll-A chemical formula is \( \begin{aligned} C _{55} H _{72} O _{5} N _{4} Mg \end{aligned} \)
Chlorophyll-B chemical formula is \( \begin{aligned} C _{55} H _{70} O _{6} N _{4} Mg \end{aligned} \)

The spectrum graph below shows that a combination of both Chlorophyll-A and Chlorophyll-B have absorption wavelengths (the troughs) at 390nm, 440nm, 620nm and 680nm. These wavelengths correspond to the blue and red parts of the spectrum, respectively. This accounts for why plants require both red and blue light to grow healthily.

Chlorphyll Spectrum

The CIE plot below shows the grey dot in the yellow/green area which is the characteristic green colour of the chlorophyll sample.

Chlorophyll CIE Colour

Mid-November Leaf Sample



Anthocyanin chemical formula is \( \begin{aligned} C _{15} H _{11} O ^{+} \end{aligned} \)

The spectrum graph below shows anthocyanin peak wavelength (the peak) at 610nm. This wavelength correspond to orange part of the spectrum. The actual colour of anthocyanin can vary between purple, blue, orange and red depending on the overall pH of the plant. In fact paper dyed using anthocyanin can be used like litmus paper. In this case the anthocyanin is orange/red colour.

Anthocyanin Spectrum

The CIE plot below shows the grey dot in the orange area which is in this case the orange/red colour of the anthocyanin sample.

Anthocyanin CIE Colour

It is certainly possible through experiment to see two distinguishing spectra taken using samples from maple leaves in later summer and autumn. Both spectra characterising the distinctive chlorophyll and anthocyanin chemical signatures.