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Leaf Anthocyanin can Affect Tree Sap Flow

Anthocyanin is a natural chemical compound produced in plant tissues. In flowers, anthocyanin gives colours such as purples and blues; and anthocyanin in fruit provides the reds of apples and wine grapes. Anthocyanin also occurs in leaves often providing the spectacular colours of autumn.

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Anthocyanin is not just visually pleasing but is functionally important in plants. In fruits, anthocyanin attracts pollinators and dispersers (particularly humans!) In leaves, anthocyanin acts as a sunscreen and protects the delicate photosynthetic systems from excessive sunlight.

Eucalyptus saligna (Sydney Blue Gum) is a fast growing gum tree commonly used in forestry operations. At a Sydney Blue Gum plantation in western Sydney, New South Wales, Australia, a number of trees suddenly developed a large amount of anthocyanin in their leaves. Anthocyanin can occur in young leaves of eucalypts, but it is not as common for the compound to suddenly appear in mature leaves as occurred here. The anthocyanin appeared following warm weather and the sudden onset of cold weather (Figure 1). Although the exact cause was never determined, it was believed that the anthocyanin appeared during the cold to protect the leaves from photo-damage.

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Not all the trees in the plantation produced anthocyanin. About half of the trees remained “normal” with green leaves. Observing the difference between trees with anthocyanin and without anthocyanin, Dr Michael Forster, then working for the University of Western Sydney, hypothesised that anthocyanin would decrease photosynthesis, growth and stomatal conductance. Consequently, the water use of anthocyanin, or red, trees would be lower than green trees and this decrease could be measured with sap flow equipment.
The SHB Stem Heat Balance instrument, using the Dynagage sensor, was installed on red and green trees. Figure 2 shows example data from approximately three days of measurement. As hypothesised, the green trees showed greater rates of sap flow and tree water use than red trees, particularly around the middle of the day.

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Although anthocyanin can provide many protective and beneficial features for plants, Figure 2 demonstrates that the compound can have ramifications for other plant physiological functions.

 

This case study demonstrates that plant responses to the environment are complex and continuous monitoring of their physiology provides excellent insights.