Through the course of evolution, carotenoids, thank to their unique physical and chemical properties, have proven to be highly versatile molecules, being able to perform many functions in many different organisms, like plants.
Carotenoids in photosynthesis
Carotenoids, in the early stages of the emergence of single-celled photosynthetic organisms, are probably been used for light harvesting at wavelengths different from those covered by chlorophyll. Therefore carotenoids, acting as light absorbing accessory pigments, have allowed to expand the range of solar radiation absorbed and so utilized for photosynthesis, energy that is then transferred to chlorophyll itself.
The major carotenoids involved in light harvesting, that accumulate in green plant tissues, are beta-carotene, lutein, neoxanthin, and violaxanthin, that absorb light energy in the 400- to 500-nm range.
Moreover, they protect chlorophyll from photooxidation (in humans, they may contribute to the protection of photo-oxidative damage caused by UV rays, thus acting as a endogenous photo-protective agents).
Carotenoids and autumn leaf color
Leaf color of deciduous plants in different seasons, green, yellow, orange or red, is due to the presence in them of natural pigments.
In spring and summer, the predominant pigment present in the leaf is chlorophyll, and therefore the color is green.
During the fall, the color changes from green to yellow, orange or red, depending on the type of plant: this is a consequence of the change, both qualitative and quantitative, in the pigment content.
In fact, as a result of the decrease of the temperature and daylight hours, the production of chlorophyll is interrupted and that already present is demolished into colorless metabolites. In this way the predominant pigments become carotenoids (yellow-orange), molecules much more stable than the chlorophyll, which remain in the leaf coloring it (it do not seem to be synthesized de novo), and anthocyanins (red-purple), which, unlike carotenoids, are not present during the growing season, but are synthesized in autumn, just before leaf fall. Therefore, it can be concluded that the red-purple color assumed from the leaves of certain plants is not a side effect of leaf senescence but results from anthocyanins de-novo synthesis.
Depending on the prevalence of carotenoids or anthocyanins, leaf color changes from green to yellow/orange, as in Ginkgo biloba (yellow), or red-purple as in some maples.
And plants with non green leaves?
Their color is not due to the absence of chlorophyll but the presence of very high amounts of other pigments, typically carotenoids and anthocyanins, that “cover” the chlorophyll, determining the color of the leaf.
Some functions of apocarotenoids in plants and foods
These oxygenated carotenoids, containing fewer than 40 carbon atoms, have many functions in plants and animals and are also important for the aroma and flavor of foods.
Some of their main functions include the following.
- Apocarotenoids have significant roles in the response signals involved in the development and in the response to the environment (for example abscisic acid).
- They can act as visual or volatile signals to attract pollinators.
- They are important in the defense mechanisms of plants.
- They have a role in regulating plant architecture.
- An apocarotenal, trans-beta-apo-8′-carotenal, found in citrus fruits and spinach, with a low provitamin A activity, is used in pharmaceuticals and cosmetics, and is also a food additive (E160e) legalized by the European Commission for human consumption.
- Apocarotenoids make an important contribution to the nutritional quality and flavor of many types of foods such as fruits, wine and tea. Two natural apocarotenoids, crocetin and bixina, have economic importance as they are used as pigments and aroma in foods.
- Finally, a broad range of apocarotenals derive from oxidative reactions that occur in food processing; these molecules are intermediates in the formation of smaller molecules, important for the color and flavor of the food.
References
- Archetti, M., Döring T.F., Hagen S.B., Hughes N.M., Leather S.R., Lee D.W., Lev-Yadun S., Manetas Y., Ougham H.J. Unravelling the evolution of autumn colours: an interdisciplinary approach. Trends Ecol Evol 2009;24(3):166-73. doi:10.1016/j.tree.2008.10.006
- de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010