As summer gives way to autumn, it brings beautiful displays of colour. Leaves change from shades of green to hues of yellow, oranges, reds, and purples, but what causes this colour change, and what causes them to fall?
Throughout the warm and bright spring and summer months, leaves appear green due to the high levels of a pigment called chlorophyll. Chlorophyll is critical for plants to convert sunlight into energy through a process called photosynthesis, but it takes a lot of energy and water for plants to maintain their leaves and constantly replace this pigment. In autumn, the leaves stop producing chlorophyll and the existing chlorophyll begins to break down, thus the leaves lose their greenness. The autumnal colours are a combination of pigments that are present in the leaves year-round (but in much lower levels and are therefore masked by chlorophyll) and pigments that are made during the autumn months.
The yellow and orange colours are caused by a family of pigments called carotenoids; the yellow colours are produced by xanthophylls, while carotenes produce the orange shades. Carotenoids are present in the leaves throughout the year, but only when the chlorophyll fades do these colours become more noticeable.
Anthocyanins are members of the flavonoid family of chemicals, and these pigments appear red, purple, blue or black depending on their pH. Anthocyanins are not present in the leaves year-round but are produced in the autumn when it is bright but cold. As autumn progresses, the plant cuts contact with the leaves and the remaining sugars that are trapped in the leaves promote the formation of the anthocyanins, which is why the red colours only begin to appear later in autumn.
Part of the wonder of autumn is the varying colours of leaves, from crimson to golden to just plain brown – what causes this? Similar to blending different coloured paints to create a final colour, the proportion of the pigments, amount of sugars left in the leaves, and amount of chlorophyll left before the plant cuts contact with the leaf determines the colour. If a good proportion of chlorophyll remained in the leaf and conditions were optimal for anthocyanin production, the leaves will turn a brown colour when still attached to the rest of the plant.
Before the plant cuts contact with the leaf, the essential elements and nutrients are shunted out of the leaves and into storage tissues in the stem/branches of the plant. Here they remain over winter until the plant is ready to recycle these nutrients into developing leaves the following spring. When the nutrients are transferred and stored, the plant seals the leaf off through a process called abscission.
Where the base of the petiole (the stem of the leaf) joins the rest of the plant, there is an area called the abscission zone. The small parenchyma cells within this zone have very thin cell walls, and there are no fibre cells surrounding the vascular tissues. A thin layer of cork forms on the branch side of the abscission layer, which creates a protective barrier between the main body of the plant and the atmosphere, protecting it from any pathogens when the leaf is no longer attached.
Abscission is controlled by a change in the balance of the plant hormones ethylene and auxin. While auxin inhibits leaf abscission and encourages the leaf to stay attached, ethylene promotes abscission and leaf detachment. An ageing leaf produces less auxin, making the cells in the abscission layer more sensitive to ethylene. This starts a self-perpetuating cycle, as the cells in the abscission layer become more sensitive to ethylene they also begin producing ethylene, further inhibiting auxin production. As ethylene concentrations increase, cells in the abscission layer produce enzymes that hydrolyse polysaccharides in the cell walls, breaking down cellulose and other components and further weakening the abscission layer. Eventually, the weight of the leaf and help from a gust of wind causes the leaf to break the weakened abscission layer and the leaf falls.
Some years the autumn leaves appear especially colourful, and this change in colour is influenced by a combination of both weather conditions and chemical processes.
- Dry weather causes the sugars to become more concentrated in the leaves, enhancing anthocyanin production and deepening the red colours seen later in autumn
- Cold but not freezing temperatures cause chlorophyll to be degraded and anthocyanin production to be enhanced. If temperatures dip below freezing this has a detrimental effect on anthocyanin production so fewer reds will be seen, so it is best for temperatures to be low but above freezing
- Bright sunny conditions means photosynthesis can still occur using the remaining chlorophyll in the leaves. This uses up the remaining chlorophyll pigments while increasing sugar production, and therefore the vibrancy of the red hues
For an especially colourful autumn, cool, dry, sunny conditions are best.
| “Autumn, the years last loveliest smile” – William Cullen Bryant |