Colourful trees
Leaves on the trees that change colours are one of the most beautiful features of the autumn landscape. The golden Polish autumn is yellow, orange, red and brown, but also burgundy, purple and even deep purple. However, few people wonder how it happens that leaves change colours.
The process of colour change is related to the transformation of chloroplasts, i.e. plastids that are present in the cells of green leaves, into chromoplasts, characteristic of colourful leaves
– explains Dr hab. Katarzyna Popłońska, Professor Emeritus of the University of Lodz from the Department of Cytophysiology at the University of Lodz Faculty of Biology and Environmental Protection.
Chloroplasts contain a very large amount of chlorophyll, a green pigment that is responsible for the green colour of leaves. However, they also contain other pigments – carotenoids such as carotenes (orange) or xanthophylls (yellowish). From spring to early autumn, there is so much chlorophyll in the leaves that it masks other pigments. When green leaves change colour, additional synthesis of carotenoids and xanthophylls takes place, while chlorophyll and the structures in which it is located are degraded.
Red, yellow and orange colours are present in the leaves all the time, but they are invisible due to the dominance of chlorophyll. In autumn, the internal structure of the chloroplast is transformed. This happens due to changes in light intensity, temperature and humidity, but the annual life cycle of plants is also important.
– adds Dr hab. Agnieszka Wojtczak, Department of Cytophysiology at the University of Lodz Faculty of Biology and Environmental Protection.
Fruits and vegetables – a treasure trove of antioxidants
However, autumn colours are not only about colourful leaves.
Fruits and vegetables also delight with their colours. Does the same colour change process occur in them as in leaves? – Not always. In fruits and vegetables, dyes, apart from chloroplasts, are found, in a completely different place – explains Dr hab. Katarzyna Popłońska. – They are present in a vacuole, which is located in the centre of a mature cell and occupies almost its entire space. This is where anthocyanins are found, i.e. pigments that are responsible for the yellow, purple, bluish and pink colours of fruits, vegetables, flower petals and even seeds. Anthocyanins are found primarily in dark fruits: grapes, chokeberries, elderberries and blueberries, but also in raspberries and pomegranates. They are also found in vegetables: eggplants, purple sweet potatoes, purple carrots, black beans, red cabbage, and purple corn.
Why is it so important?
Anthocyanins are strong antioxidants that scavenge free radicals from cells. It can be said that thanks to them we age slower. Anthocyanins have a significant impact on our health. They are important in our daily diet because they protect, among others: against diabetes, heart diseases and cancer
– explains Dr hab. Agnieszka Wojtczak.
Anthocyanins absorb light of the same wavelength as chlorophyll b, which helps protect plant tissues. Due to their strong antioxidant properties, anthocyanin pigments also help protect plants from UV damage. In vitro studies have shown that they can also inhibit the multiplication of viruses such as parainfluenza virus, herpes simplex virus, adenovirus and rotavirus.
Temperature, light and pH of the environment
Due to their beneficial effects on health, we should include products rich in anthocyanins in our diet. However, it should be remembered what factors influence whether there are more or less of them in fruits and vegetables.
Temperature plays an important role in the synthesis of anthocyanins.
The largest amount of the pigments can be found in plant cells when it’s cold, i.e. in autumn. Heat causes a low level of anthocyanins, while cold causes their rapid accumulation in the fruit skin. And it is important for us that the level of these compounds is as high as possible. Climate change also affects the level of anthocyanins in fruits and vegetables – explains Dr hab. Katarzyna Popłońska.
The pH of the environment is also important.
In an acidic environment, the colours of anthocyanins present in vacuoles are more red, purple and pink, while in an alkaline environment they will be more blueish – adds the biologist.
The influence of light
Light also has an influence on the amount of anthocyanins. The more of it, the better for the fruits.
Scientists will help
Anthocyanins are becoming increasingly popular due to their health benefits and attractive colours. Customers are increasingly willing to pay more for healthier natural products with high nutritional values. It is therefore not surprising that scientists are also trying various methods to retain anthocyanins in fruits and vegetables, and even use other organisms to "produce" these compounds.
In biotechnology, organisms such as microorganisms or baker's yeast are used for this purpose
– explains Dr hab. Katarzyna Popłońska.
The most important thing for consumers will be to choose products rich in anthocyanins in autumn, when the infection season begins, preferably those picked straight from the bush.
Source: Faculty of Biology and Environmental Protection, University of Lodz
Text: Justyna Kowalewska (3PR)
Photos: Justyna Kowalewska, Karolina Wilczyńska