AOne of the most captivating natural events of the year in many areas throughout North America is the turning of the leaves in the fall. The colours are magnificent, but the question of exactly why some trees turn yellow or orange, and others red or purple, is something which has long puzzled scientists.
BSummer leaves are green because they are full of chlorophyll, the molecule that captures sunlight converts that energy into new building materials for the tree. As fall approaches in the northern hemisphere, the amount of solar energy available declines considerably. For many trees – evergreen conifers being an exception – the best strategy is to abandon photosynthesis* until the spring. So rather than maintaining the now redundant leaves throughout the winter, the tree saves its precious resources and discards them. But before letting its leaves go, the tree dismantles their chlorophyll molecules and ships their valuable nitrogen back into the twigs. As chlorophyll is depleted, other colours that have been dominated by it throughout the summer begin to be revealed. This unmasking explains the autumn colours of yellow and orange, but not the brilliant reds and purples of trees such as the maple or sumac.
CThe source of the red is widely known: it is created by anthocyanins, water-soluble plant pigments reflecting the red to blue range of the visible spectrum. They belong to a class of sugar-based chemical compounds also known as flavonoids. What’s puzzling is that anthocyanins are actually newly minted, made in the leaves at the same time as the tree is preparing to drop them. But why should a tree spend energy to create a whole new class of pigments, which it is soon to throw away in any case?
DThere are two main theories about why trees make anthocyanins. The first is that they act as a chemical sunscreen, protecting chlorophyll from being damaged by excess light while nitrogen is being salvaged. During autumn, low temperatures mean photosynthesis slows down, and chlorophyll is less able to handle the high levels of solar radiation, resulting in photoinhibition. The red pigments absorb excess light, allowing the tree to absorb as much nitrogen as possible before shedding its leaves.
EIt has also been proposed that trees may produce vivid red colours to convince herbivorous insects that they are healthy and robust, so that the insects might be dissuaded from laying eggs on them. This hypothesis, known as the co-evolution theory or warning signal theory, was pioneered by evolutionary biologist W.D. Hamilton. Insects such as aphids look for suitable host plants in autumn to deposit their winter eggs; trees with high nutritional value or strong chemical defences might advertise their qualities via red pigment intensity.
FThe sunscreen hypothesis, however, has more direct observational evidence. It predicts that leaves exposed to the most intense sunlight should produce the most anthocyanins. If a leaf is partly shaded by another leaf, the exposed part will turn bright red while the shaded portion remains green or yellow.
GAnother piece of evidence comes from the distribution of pigments within individual leaves. The upper surface of the leaf, which faces directly towards the sun, contains high concentrations of anthocyanins, whereas the lower surface contains very little or none at all.
HEven if you had never suspected that this is what was going on, there are clues out there in the woods that support the light screen hypothesis. One is straightforward: on many trees, the leaves that are the reddest are those on the side of the tree which gets the most sun. Not only that, but the red is brighter on the upper side of a leaf. It has also been recognised for decades that the best conditions for intense red colours are dry, sunny days and cool nights, conditions that stress chlorophyll and make a sunscreen essential. And finally, trees such as maples usually get much redder the further north you travel, into colder climates with high light intensity.
IWhat is still not fully understood, however, is why some species of trees make red pigments and others do not. The puzzle remains partially unsolved, and the exact balance between photoprotection and herbivore deterrence continues to be a fertile subject of botanical debate.
* photosynthesis: the production of new material from sunlight, water and carbon dioxide.
Reading Passage 2 has nine paragraphs, A–I. Which paragraph contains the following information? Choose the correct letter, A–I. NB: You may use any letter more than once.
Complete the notes below. Choose ONE WORD ONLY from the passage for each answer.
• The most vividly coloured red leaves are found on the side of the tree facing the .
• The surfaces of leaves contain the most red pigment.
• Red leaves are most abundant when daytime weather conditions are and sunny.
• The intensity of the red colour of leaves increases as you go further .
For questions 23–25, choose TRUE, FALSE, or NOT GIVEN. For question 26, choose the correct letter, A, B, C or D.
23. It is likely that the red pigments help to protect the leaf from freezing temperatures.
24. The ‘light screen’ hypothesis would initially seem to contradict what is known about chlorophyll.
25. Leaves which turn colours other than red are more likely to be damaged by sunlight.
26. For which of the following questions does the writer offer an explanation?