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Botanical Buzz - Autumn chemistry

Thursday, March 26, 2015

Autumn is the perfect time to have a picnic in the Dubbo Regional Botanic Garden. The beautiful surrounds and milder weather encourage us to stop, relax and ponder the deeper questions of life…. like why do the leaves of some trees change colour in autumn?

Leaves contain a number of different coloured compounds. Healthy leaves are usually green because they contain more chlorophyll (which is green) than any other coloured substance. 

Chlorophyll is in almost every plant on earth because it is good at absorbing light from the sun.

Plants need energy from light to help them produce food so that they can grow, flower and produce seed.

However, chlorophyll is destroyed by bright sunlight so during summer plants have to continuously regenerate it. This process requires sunlight and warm temperatures.

Another compound found in the leaves of many plants is carotene which is yellow. Carotene promotes the absorption of light by chlorophyll.

Some leaves also contain anthocyanins. These are responsible for the red skin of ripe apples and the purple of ripe grapes. The formation of anthocyanins requires light and a high concentration of sugar in the sap. The former requisite is why apples often appear red on one side and green on the other; the red side was in the sun and the green side was in shade.

As autumn progresses nutrient flow to the leaves reduces and chlorophyll regeneration declines. If the leaves contain carotene, as do the leaves of the ginkgo bilobas growing in Shoyoen, they start to turn yellow.

The brightest autumn colours are produced when dry, sunny days are followed by cool, dry nights. Bright sunshine destroys chlorophyll and cool temperatures prevent its regeneration. In addition, dry weather (by increasing sugar concentration in the sap) and bright sunshine enhance anthocyanin production creating vivid red and purple colours.

Kick back and watch chemistry, biology and physics at work in the Dubbo Regional Botanic Garden.