Did you know that when flamingos are born, they are grey and their feathers are naturally white? Then… why do they display that pinkish colour? And hydrangeas, why do they show different colours? What do flamingos and hydrangeas have in common? And about the leaves of a tree…why do they change colour during the year? Did you know that eggshells present fluorescence? And finally, what do all these questions have in common?
To give an explanation to this we have to refer to colours, this is, pigments. The main pigments in flowers and plants are chlorophylls (green), carotenoids (yellow, orange and red), anthocyanins (red and pink) and betalains (purple). Figure 1 shows examples of all of them.
Therefore, the colour of plants, flowers, fruits and vegetables comes from these pigments and from some other factors such as the light they receive, the type of soil or the effect of senescence. Some plants show their flowers in a specific colour when the sunlight is intense, whereas at the end of the day – in the sunset – the colour changes. Likewise, the colour can evolve when a flower fades. What happens when nutrients or the soil pH change? Let’s see the different pigments one by one.
CHLOROPHYLLS
Chlorophyll is a pigment that gives a green colour to leaves. Chlorophyll belongs to the called “porphyrin group”. Actually, it is a derivative of protoporphyrin IX, a pigment which can be found in eggshells and that is highly fluorescent when irradiated with ultraviolet (UV) light (Figure 2). However, chlorophyll also includes a magnesium atom in its structure.
Chlorophylls are essential for photosynthesis, a process in which plants convert carbon dioxide (CO2) and water into glucose and oxygen (O2). There are lots of different types of chlorophylls, but the common ones are a, b, c and d, which can be mainly found in plants, algae and phytoplankton. The latter is responsible for the greenish appearance of the sea in some places.
CAROTENOIDS
This type of chromophores (part of a molecule that gives the colour) displays yellowish, orangish and reddish colours on fruits and vegetables (for instance, carrot, pepper, pumpkin and orange) and on the leaves when fading. Carotenoids can split into carotenes (only carbon and hydrogen atoms, non-polar) and xanthophylls (polar, with functional groups bearing oxygen). Animals can’t synthesize carotenoids. However, some of them show a pinkish colour such as salmon, trout, or shellfish. This is possible since these animals ingest the pigment in their diet and their metabolism fixes the carotenoids. This is also the reason why flamingos are pink (Figure 3). When they are born, their feathers are grey, but their diet based on algae and shellfish dyes the feathers making them red and/or orange.
ANTHOCYANINS
They give reddish, purplish and bluish colours to plants. This pigment, which is pH sensitive, is found in some flowers like hydrangeas, in different colours. However, the change in pH does not directly modify the colour.
Firstly, we will see what pH is. pH is a measure of acidity or basicity in a substance. For example, we have lemon or vinegar which are acids and bicarbonate or milk which are bases. Some vegetables, like red cabbage, are useful pH indicators. This means the colour changes with pH modifications. In presence of acids the purple colour changes to pink or red, whereas in presence of bases, it turns to green or blue (Figure 4).
Pink hydrangeas display this colour due to an anthocyanin called 3-O-glucosyldelphinidin, which can be found in its natural form. Nonetheless, when the soil pH acidifies, the anthocyanin links aluminium, making a blue metal complex. This means a chemical reaction occurs that causes a change of colour associated with the acidification of the soil (Figure 5).
BETALAINS
This pigment is mainly found in beetroot, bringing them with a dark red almost purple colour. They are commonly used for food colouring.
Most of the plants, and more specifically tree leaves, contain chlorophill, carotenoids and anthocyanins. In spring and summer, chlorophyll prevails, showing green leaves. When autumn arrives, chlorophylls degrade, causing the fading of the green colour and the outbreak of the carotenoids (both xanthophylls and carotenes), and as a result, displaying yellowish and orangish colours. Finally, and when chlorophylls have partially downgraded, anthocyanins, which are not continuously present, get developed and leaves became redder and redder (Figure 6).
Nevertheless, plants and flowers containing anthocyanins do not have betalains and vice versa. This fact could occur since both pigments give similar colours (i.e. they absorb the same wavelengths).
As you can see, pigments imply not only colours but also curiosities which explain the surrounding world. Now you can identify the pigments in the see when it looks green, in an orange juice and in the egg you had for breakfast, in the flamingos you have seen in the zoo, in these colourful flowers you have never looked at and in the tree leaves depending on the season.
This was a piece I wrote for a contest about science outreach articles. I didn’t win, but the original article (in Spanish) was published in UBUinvestiga. You can read it here: https://ubuinvestiga.es/naturaleza-camaleonica-pigmentos/
Also, if you want to know a bit more about colours, don’t miss Red, blue, green and purple: the colours of…
References
- https://www.nationalgeographic.org/encyclopedia/chlorophyll/#:~:text=Chlorophyll%20is%20a%20pigment%20that,their%20own%20food%20through%20photosynthesis. (Visited on 29/01/2022).
- https://www.americanscientist.org/article/curious-chemistry-guides-hydrangea-colors (Visited on 30/01/2022).
- Annu. Rev. Plant Biol. 2006, 57, 761–80. DOI: 10.1146/annurev.arplant.57.032905.105248.
- Annu. Rev. Plant Biol. 2006, 57, 55–77. DOI: 10.1146/annurev.arplant.57.032905.105212.
- Biol. Rev. 2019, 95, 22-50. DOI: 10.1111/brv.12552.
- Scientific Reports, 2019, 9, 5450. DOI: 10.1038/s41598-019-41968-7.