We all love the autumn displays that deciduous trees and shrubs put on, but how many of us ask ourselves why it happens?
Colour all year
Many plants have colourful leaves all year, but they tend not to be appreciated until autumn.
The green pigmentation in leaves comes from chlorophyll, a key compound in the process of photosynthesis.
Plants convert light energy into chemical energy, which is stored in compounds such as sugar that are essential to growth.
In plants that have leaves which are not green, chlorophyll is still there, as this is essential to photosynthesis, but other pigments are also present.
Anthocyanins in plants can cause red or purple colour in leaves throughout the year.
In some plants, as the concentration of sugar increases and the plants are exposed to light, anthocyanins are formed.
The colour produced by anthocyanin is sensitive to cell pH.
If quite acidic, the pigments impart a bright red colour; if the sap is less acidic, its colour is more purple.
Carotenoids give a yellow or orange colouration and are present in most leaves throughout the growing season.
This becomes more visible if the green chlorophyll levels become low.
From my horticultural experience in the United Kingdom, I find plant leaf colour much more intense in New Zealand.
I wonder if it is caused by different light levels here?
Will autumn colours be good?
An autumn of sunny days and cool, but not freezing, nights seems to bring out the best colours.
A reduction in the green chlorophyll allows the bright yellow and orange carotenoids to show through.
In some plants anthocyanin gives a bright display of scarlet.
If we get freezing nights the cells in the leaves are destroyed, causing them to brown and end another year's display. - Dylan Norfield
Dylan Norfield is the geographic collection and arboretum curator at Dunedin Botanic Garden.
