The sight of woods in autumn is exhilarating to most people lucky enough to live in temperate regions. Accompanied by crisp air and the smell of decaying leaves, the explosion of red, orange and yellow hues temporarily pulls thousands of people away from their cities.
“Autumnal senescence may attract more attention from the public, but less from scientists, than any other plant developmental process.”, wrote Johanna Keskitalo, a molecular biologist working in Sweden.
Decades ago, a Time-Life book on plants described a simple experiment which provides an ideal introduction into the chemistry of leaves. It involved apple skins, but the pigments in red apples are of the same type as those in red maples. Known as anthocyanins, they are pH sensitive, and they are not found together with other pigments like carotenoids and chlorophyll in chloroplasts. Instead they are located in cell vacuoles. And their location is tied into the odd fact that if you add boiling water to autumn maple leaves, the water turns brownish, not red.
Why does boiling leaves cause the anthocyanins to undergo a chemical change? Dilution on its own cannot account for the required increase in pH.
Acid maintains the red color of pigments in the vacuoles The vacuole is in a sense the storage attic where excess water, acid and anthocyanins are kept. But boiling disrupts cell membranes and cell walls and bursts vacuoles. The rest of the plant cell is slightly alkaline, neutralizing the vacuolar acid, and modifying the network of multiple conjugated chemical bonds in the anthocyanin. This translates into a different energy requirement for the transition from the highest occupied molecular orbital to the lowest unoccupied one. In short, a different wavelength of light will be absorbed , and we see a different residual color.
The change is not irreversible. If you add vinegar to the extract to simulate the original acidic conditions of the vacuole, an approximation of the original reddish color of the maple leaf is restored.
So what are the anthocyanins doing in cell attics? In the same way that not everything stored in the attic is useless, anthocyanins play a protective role in leaves. Although chlorophyll levels start to decrease in late summer and early autumn, activity is maintained thanks to anthocyanins’ shield against intense light, a stress that becomes more pronounced in plants when bright light is accompanied by the low temperatures of autumn.
Here is the evidence for the popular and actually correct idea that cold and sunny days make the reds of an autumn display more spectacular. The plot is that of anthocyanin concentration in Eurasian aspens (Populus tremula) monitored over a period of about 45 days.
However, once anthocyanin accumulation had been initiated, further accumulation seemed to be strongly dependent on excess light conditions, i.e. cold and sunny weather. The clear, cold days on September 19 and 20 induced a huge accumulation of anthocyanins, more than one-half of which disappeared in the milder and rainy week that followed, while frost and sun on September 30 resulted in a new accumulation, followed by degradation during the mild and rainy days at the beginning of October. Finally, from October 4 onward, the weather was clear and cold, resulting in a very high accumulation of anthocyanins.
Keskitalo J et al. Plant Physiol. 2005;139:1635-1648
Recent research has confirmed the hypothesis that anthocyanin “bodyguards”are especially needed in the fall when leaves are busy reabsorbing nutrients like nitrogen and phosphorus. By comparing anthocyanin-deficient mutant trees to three normal species, they observed that the latter continue to photosynthesize in autumn, but the mutants’ leaves dropped while still green and showed signs of irreversible damage from photo oxidation.
What about those trees that just turn yellow and orange? These are the colors of carotenoids such as xanthophylls, neoxanthin, leutin and beta carotene. With chlorophyll, they are found in chloroplasts and serve to relay energy not directly absorbed by green pigments. Understandably, there is a quicker breakdown of chlorophyll compared to that of carotenoid accessory pigments. The former contain nitrogen and magnesium which are more scarce and so have to reabsorbed. Instead hydrogen, carbon and oxygen, the only elemental ingredients of carotenoids are dispensable because they can easily be reabsorbed in the following growing season from carbon dioxide and water.
It’s probably not a coincidence that the top cherry-leaves, those most exposed to direct sunlight, have been the first to produce anthocyanins.