Mark Cocker’s Country diary piece in the Guardian this week, 1 June 2021 (‘the most seductive shade of green‘) got me thinking about leaves, trees and colours. As Mark reminded us, leaves look green because chlorophyll molecules absorb the red end of the visible light spectrum in photosynthesis, and the unused green light is reflected. Counterintuitively perhaps, they are green because they don’t use green light!

But many leaves don’t start off green. The fresh, young leaves of many plants, like these lovely sycamore leaves (below) are red, or at least reddish, to begin with.

Leaving aside autumn leaf colours for the moment (for another blog perhaps), why are the young (new) leaves of many plants red? Well it turns out that they red for a short time because of the accumulation of anthocyanins. The redness is transient and disappears as the leaves mature. Small sycamore seedlings are red at the top, but the leaves soon turn green as they pass down the growing plant. The delayed greening of young leaves is a world-wide phenomenon and is also very common in the tropics (Gong et al., 2020).

Anthocyanins are almost the opposite of chlorophyll – they are in fact complementary – because they absorb light in the blue-green wavelengths, allowing the red wavelengths to be scattered by the leaves, which is why they look red to us. There’s a good video here, which it explains it nicely.

There are two main theories as to why plants produce these reddish pigments in young leaves: 1) because they have a sunscreen photoprotective function, which shields the leaves against excess visible light and 2) that they are a sort of signal to insects not to waste their time eating them! Put another way, the red coloration of young leaves, which contain high concentrations of tannins and anthocyanins, may function as an anti-herbivore defence strategy (Gong et al., 2020). There seems to be evidence in favour of both theories!

Some recent research suggests that the anthocyanin coating is photoprotective (Zhang et al., 2016). Here is what the chemical structure of an anthocyanin molecule looks like (below).

Anthocyanins are mainly found in cell vacuoles, in or just below the adaxial epidermis (upper part of the leaf). See below.

Other researchers have found that young red oak (Quercus coccifera) leaves are attacked less by insect consumers than young green leaves (Karageorgou and Manetas, 2006). It’s not altogether clear why the insects were put off by the red leaves. It may be that they are just more cued into searching for green leaves (tasty!), and they just don’t recognise the red ones as food, or it may be that the red leaves just don’t taste as nice as the green ones!

Some leaves start off copper coloured and stay that way, including cultivars such as Acer platanoides Crimson King Norway maple (below) and copper beech of course. They do seem to stay relatively insect free, but there is probably a trade off in terms of loss of photosynthesis?

Like all simple questions – such as why are leaves green?; why are young leaves red? – the answers turn out to be complex, interesting and often uncertain. Scientists – even those who have studied these questions for years! – are not entirely sure what the right answers are, other than in general terms! Science does not provide certainty (turn to religion for that) but it does give us a fascinating insight into the depth of biology and the wonder of nature.

References

Gong, W. C., Liu, Y. H., Wang, C. M., Chen, Y. Q., Martin, K., & Meng, L. Z. (2020). Why are there so many plant species that transiently flush young leaves red in the tropics?. Frontiers in plant science, 11, 83.

Karageorgou, P., & Manetas, Y. (2006). The importance of being red when young: anthocyanins and the protection of young leaves of Quercus coccifera from insect herbivory and excess light. Tree Physiology, 26(5), 613-621.

Merzlyak, M. N., Chivkunova, O. B., Solovchenko, A. E., & Naqvi, K. R. (2008). Light absorption by anthocyanins in juvenile, stressed, and senescing leaves. Journal of Experimental Botany, 59(14), 3903-3911.

Pena‐Novas, I., & Archetti, M. (2020). A comparative analysis of the photoprotection hypothesis for the evolution of autumn colours. Journal of Evolutionary Biology, 33(12), 1669-1676.

Zhang, T. J., Chow, W. S., Liu, X. T., Zhang, P., Liu, N., & Peng, C. L. (2016). A magic red coat on the surface of young leaves: anthocyanins distributed in trichome layer protect Castanopsis fissa leaves from photoinhibition. Tree physiology, 36(10), 1296-1306.