“The lower (abaxial) leaf surfaces of many plant species are brightly colored by red to purple pigments, most commonly anthocyanins. This trait has evolved multiple times independently in the plant kingdom, most prolifically in angiosperms but also in some seedless, vascular plants […], making it a textbook example of convergent evolution.” (Hughes and Lev-Yadun 2023, p. 1)
Before I go into the presumed evolutionary benefits of red-coloured leaf undersides by continuing to quote from the recently published, highly interesting review article by Nicole M. Hughes and Simcha Lev-Yadun, I would like to introduce you to a plant that displays this characteristic in a pronounced way and does really well in a closed terrarium: Strobilanthes sinuata actually grows too tall for my 5-litre container, which is why I have already had to cut it back twice. Nevertheless, it enriches the jar.
The plant was first described very briefly by C. v. Linné as Ruellia repanda (Linnaeus 1763, p. 886). Five years later, the plant appeared under the same name with an illustration in the “Flora Indica” by Nicolaas L. Burman (1768, p. 134). Christian G. D. Nees von Esenbeck (1847) put the taxon into the family Acanthaceae, introduced in 1789 by Antoine L. de Jussieu. Johannes G. Hallier (1897, p. 198 f.) assigned the plant to the genus Hemigraphis and published the most detailed description of the plant up to that time and was the first to mention the striking red-coloured undersides of the leaves. The currently valid classification and naming comes from John R.I. Wood (2014, p. 393): “Strobilanthes sinuata is a Javanese species reported as cultivated in India […]. The new epithet sinuata refers to the distinctive form of the leaf margins.”
Hallier (1897) writes about the occurrence of the plant: “According to Linné, the plant is native to Java, but I have not yet encountered it anywhere in the wild. On the other hand, it is found in cultivation in Hort. Bog. […] and is often used in the private gardens of West Java […] for framing flower beds. In October and November 1896 I also saw it […] in the botanical gardens of Singapore, Pinang, Calcutta and Peradenia.” There are actually only few documented occurrencies of this plant presumably from its original natural habitat. Among these are preserved specimens collected in 1842-1844 by H. Zollinger and in 1880 by H.O. Forbes.
Red-coloured undersides of otherwise green leaves are reported from more than 50 angiosperm families and 100 genera. So, what explanations can science provide today for this phenomenon? In brief: Many hypotheses are discussed, but surprisingly few are well supported by data. Red leaf undersides are particularely common in plants growing as the understory of moist, deeply shaded tropical and temperate forests. Although plants from other habitats also exhibit this characteristic, in the following I will focus on plants from tropical forests, as this is also the habitat of Strobilanthes sinuata. Two hypotheses seem particularly plausible for this kind of plants:
Protection of photosynthetic tissues during periods of high light stress
Excess light that cannot be utilised by leaves for phytosynthesis has the potential to damage leaf cells. The pigments responsible for the red undersides of leaves (anthocyanins) intercept photons and dissipate this energy as heat and thus protecting the tissue. But what value would photoprotection be in the deeply shaded understory? Typical for these habitats are frequent episodes of intermittent, high light intensities which occur when direct sunlight is transmitted through gaps in the canopy, resulting in sunflecks (cf. a previous post). These short-time high light intensities can be especially damaging to plants whose photosynthesis is optimized for low light intensities. Why then position the sunscreen layer on the underside of the leaf, rather than the top side? One might assume that red, light-absorbing pigments on the top side of the leaf reduce the efficiency of photosynthesis in low light. However, this could not be confirmed experimentally. Given the superior photoprotection provided by anthocyanins in the upper leaf surface, and their apparently negligible costs to carbon gain under low light, why might natural selection nevertheless consistently favor underside anthocyanins in understory habitats? Indeed, tissue with anthocynins on the top side exhibited significantly lower photosynthesis under saturating irradiance compared to tissue with anthocynins on the underside. These results were optained through experiments with variegated leaves of Colocasia esculenta var. “Mojito” (Hughes e.a. 2014). The conclusion so far is that underside anthocyanins might represent a compromise between the photosynthetic costs of anthocyanins (reduced capacity for carbon gain during low light exposure) and its benefits (providing some moderate photoprotection during high-light events) on each surface.
The authors conclude: The photoprotection hypothesis is currently the most well-supported explanation for underside leaf reddening. “However, this case is far from ‘closed’, as photoprotective effects have only been shown for a few select species, and many ecological hypotheses (e.g., aposematism [warning colouration], leaf camouflage, undermining insect camouflage, defense from fungi, interference competition) have not been fully tested.”
Addendum from 05.03.2024:
Another stunning example of red leaf undersides shown by a tropical understorey plant is the Peperomia sp. native to Ecuador included in the jar pictured below.
Hermetospheres 