RHS EXPLAINS
Why do leaves change colour? Laurel Emms has the answer
AS the year begins to draw to a close and there is a feeling of crispness in the air, our expectations are of a rich display of vibrant autumn leaf colour to finish the gardening year. Although some evergreens and shrubs change colour at this time, it is deciduous trees that put on a show. They begin the process of shedding leaves in late summer, and colours develop as the trees activate plant hormones, shut down chlorophyll production and expose other pigments.
Role of chlorophyll
As plants grow through spring and summer, chlorophyll – which gives leaves their green colour – is constantly replaced in the leaves. Water and carbon dioxide are converted into carbohydrates and oxygen using the energy of sunlight, which is harnessed by chlorophyll in plant leaves during the process known as photosynthesis.
Process of shutting down
Loss of sunshine and low night temperatures in autumn will cause chlorophyll to be broken down until it disappears completely. This allows yellow pigments (xanthophyll) and orange pigments (carotenoids) previously masked by the chlorophyll to become visible, as they are broken down more slowly. It also promotes the formation of more red and purple pigments (anthocyanins) manufactured from the sugars that are trapped
in the leaf.
Pigment variations
The combination of these pigments in the leaves determine the trees’ autumn colour. Some, including alders, show little change while other such as Betula, Fagus, Liriodendron and Robinia have yellowish shades. Trees with the largest amounts of anthocyanins include North American maples and Liquidambar which have, perhaps, the most vibrant colour. Once leaves have finished producing food, a process of shutting down and sealing off the leaf begins.
How and why leaves fall
Leaves have a finite life. Senescence is the period when deciduous plants prepare for leaf fall and winter dormancy. During leaf senescence, nutrients such as nitrogen and residual carbohydrates, are transported from leaves to woody stems and branches, and stored over winter. Trees depend on this food store for their burst of new growth in the spring.
Hormones will inhibit senescence during spring and summer while growth regulators, acting against them, promote the aging process. Seasonal changes in temperature and day length shift the balance between the sets of hormones. Leaf fall is initiated when days and nights are approximately equal in length.
It is believed that changes in the pigment phytochrome (which regulates aspects of plant growth), brought about by day length, may be linked to production of high levels of the hormone plant growth regulators auxins and gibberellins. Short days produce low levels of these growth regulators and high levels of inhibitors, such as the hormone abscisic acid, which stimulates the formation of the abscission zone.
Leaf separation
Leaf abscission (separation) is the process by which deciduous trees and shrubs shed leaves. The abscission zone is a narrow band of corky cells at the base of a leaf petiole where it connects to a stem. The cell walls in this zone weaken during leaf senescence, then close off and prevent the flow of nutrients and water into the leaves. Once supply is blocked by these abscission zone cells, a tear line forms, then the leaf falls off or blows away.
In evergreen plants there is gradual abscission of leaves; the older leaves fall while new leaves develop continuously during the growing season.
Other triggers for leaf abscission can be defensive and occur at different times of year. Examples include diseased or weather-damaged leaves. Evergreens can shed alarming amounts of leaves in spring after winter frost damage or summer sun scald during heatwaves.
Leaf retention
Leaf marcescence (leaf retention) affects many juvenile trees and tree seedlings, especially if early frost kills the leaves before the abscission zone develops or completes development. It is not clear why some deciduous trees retain spent leaves until mid-winter or later – perhaps for cold protection or for defending winter twigs against herbivores, or to draw on recycled nutrients during the later leaf fall. Some trees such as beech, although deciduous, will keep dead leaves through winter, making ideal hedges as juvenile foliage promoted by pruning will remain.
effects of environmental change
Climate and environmental conditions influence seasonal changes in plants. Traditionally, autumn in the UK has been influenced by cool, damp, overcast conditions that are not usually associated with vibrant leaf colour. However, our gardens are undergoing a period of significant change with warmer, often wetter winters and hotter, drier summers, and weather conditions can be unpredictable from year to year.
Drought and heat during the growing season, as seen in many parts of the country this year, resulted in poor tree growth and scorched foliage. This stress can cause the abscission layer to form early, with leaves dropping before changing colour.
Strong winds or rain can also cause leaves to fall before they fully develop colour. Overcast conditions and a mild autumn result in pale autumn leaf colour. Early frosts destroy the leaves’ ability to manufacture red and purple pigments and end the colourful foliage.