Thursday, November 3, 2016

Why leaves change color in the fall

Leaves are green because they contain lots of chlorophyll, the molecule that is responsible for photosynthesis. Chlorophyll is green because it reflects the green wavelengths of sunlight. It absorbs the blue and red portions of the visible spectrum and uses the energy of those light waves to manufacture carbohydrates. But chlorophyll is not the only pigmented substance in a leaf. There are also yellow and/or orange colored molecules in a leaf. The orange color
is due to carotenoids, named after carotene, the pigment that makes carrots orange. The yellow pigment is due to molecules called xanthophylls. Together, the xanthophylls and carotenoids absorb the green wavelengths of light and pass that energy on to the chlorophyll molecule. In that way they act to increase the efficiency of photosynthesis. Leaves appear green because they contain so much more chlorophyll than carotenoids and xanthophylls.

As autumn approaches leaves begin a process called senescense in preparation for winter. (See Why do trees drop their leaves) In this process the chlorophyll in the leaf is broken down and the products reabsorbed by the tree. As the chlorophyll is removed from the leaf the orange and yellow pigments are revealed. The leaf begins to turn yellow/orange. The exact color depends on the relative amount of orange and yellow pigment. Different tree species vary in these amounts. Some have no orange at all while others skimp on the yellow. (The brown color seen in some leaves is due to a substance called tannin, a brown colored molecule. Tannin is used as a defense against insect herbivores. Tannins are also present in some unripe fruits, like persimmons, and give them a bitter, astringent taste. This discourages animals from eating the fruit before it is ripe and the seeds mature.) 

All these pigments and colors have been present in the leaf throughout summer. Their presence has just been masked by the abundance of chlorophyll. 

The red coloration appears in some tree leaves in the fall and it is caused by yet another kind of pigmented molecule – anthocyanin (pronounced: ann-tho-SIGH-ah-nin). Unlike the other colors, anthocyanins are not present in the leaf during the summer. Instead they are made from sugars in the leaf in the fall when the chlorophyll is being broken down and reabsorbed. 

It has long been a mystery why a tree should use valuable carbohydrates to make red pigments in a leaf that is going to be discarded, especially at a time when it is recapturing valuable resources from the leaf. There have been many hypotheses offered to explain this conundrum, but the "sunscreen hypothesis" seems to have gathered the most convincing support. It suggests that the function of anthocyanins is to protect the cellular mechanisms that are disassembling the photosynthetic machinery in the leaf. Sunlight, as anyone who has forgotten to wear sunscreen at the beach knows, can be very damaging to living tissues. A sunscreen enables the leaf to continue photosynthesis with the remaining chlorophyll even as other chlorophyll molecules are being taken apart and sent back to the tree. Especially important is the recovery of substances that contain phosphorus and nitrogen. These are often scarce or limiting to the growth of plants and it is important to avoid their loss. The sunscreen hypothesis suggests that the red pigment, by absorbing some of the wavelengths of light that chlorophyll uses, shields the chlorophyll from light-induced damage at a time when chlorophyll cannot be replaced.

The sunscreen hypothesis may be plausible, but it is of little value if it cannot be tested. A few years ago a group of researchers devised a test. They used three woody plant species whose leaves turned red in the fall. Each of these plants had mutant forms that could not produce anthocyanin. The sunscreen hypothesis predicts that the leaves with red pigment will reabsorb more material from the leaf than the mutant plants can. The amount reabsorbed was measured by the leaf nitrogen content before and after senescence. If the hypothesis is true you would expect to find less nitrogen in the red-leaf plants than in the mutant-leaf plants. (Because plants with red leaves would be able to reabsorb more nitrogen than mutant plants before the leaf fell off.) Furthermore, if you expose both kinds of plants to intense sunlight the difference in the nitrogen content in the senesced leaves should be even greater. Both of these predictions were observed, so the sunscreen hypothesis was confirmed.

But what about the trees that don't turn red in the fall? The same study that tested the sunscreen hypothesis also looked at Paper birch, a tree that does not manufacture anthocyanins in the fall. They found that Paper birch was just as efficient in recovering nitrogen from its leaves as the three species that produced anthocyanins. The implication of this result is that non-anthocyanin producers must have another mechanism for nutrient recovery from senescing leaves. So there is still work to be done!

There is another phenomenon where the sunscreen hypothesis might be applied. In several species the newly emerging leaves have a reddish-pink blush, just as the leaf is starting to produce chlorophyll. Examples are White oaks and the shrub, Red Tip Photinia. Is this early production of anthocyanin protective?

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