Review: Why do some plants have leaves with red or purple undersides?

Red or purple coloration of abaxial (lower) leaf surfaces is a taxonomically-widespread phenomenon that has evolved independently in multiple plant lineages. The trait has long been associated with deeply-shaded forest understory habitats, but also occurs in many taxa adapted to high light, including aquatic plants with floating leaves, and herbs that thrive in sun-exposed meadows, rocks, or cliff faces. We surveyed the literature for information related to the taxonomic and environmental distribution of plants displaying abaxial leaf reddening, the chemical structures and anatomical locations of pigments associated with abaxial leaf reddening (if known), and the physiological and ecological (e.g., defensive– anti-herbivory and anti-pathogenic) hypotheses proposed to explain their occurrence. We report examples of abaxial leaf reddening in more than 50 angiosperm families and 100 genera, as well as analogous coloration in some basal plant lineages (e.g., selaginellins in ventral surfaces of Selaginella microphylls, auronidins in ventral surfaces of liverwort thalli). Leaf-warming was one of the first hypotheses proposed for abaxial leaf reddening (19th century); while slight (1–2 °C) warming effects were reported, few studies have since successfully corroborated these early results. Lee et al. (1979) proposed that abaxial red pigments back-scatter red-photons, maximizing light capture efficiency of the leaf in light-limited environments. This hypothesis was largely refuted by later studies. The photoprotection hypothesis is currently the most well-supported explanation for abaxial leaf reddening, whereby red pigments function to attenuate excess photons either directly incident on the abaxial surface of inclined/inverted leaves, or transmitted through adaxial cell layers. This function is beneficial in understory environments exposed to frequent high-light events (e.g., sunflecks, sungaps), as well as in high light environments or seasons. 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, leaf camouflage, undermining insect camouflage, defense from fungi, interference competition) have not been fully tested.