We often think of carnivorous plants as predators, feeding on insects and other prey captured by their specialised leaves, but that’s not the only role carnivorous plants play. It might sound strange, but carnivorous plants often provide food for other organisms, whether it be as a lure to capture more prey with their specialised leaves, or as a reward for pollinators visiting their flowers. These floral rewards often come in the form of nectar or pollen, but some plants also produce nutrient-rich floral structures such as edible trichomes (aka hairs).
Pinguicula, a sticky-leaved genus of carnivorous plants in the family Lentibulariaceae, have distinctive flowers which are covered in multicellular trichomes (hairs, if plants had hair, see Figure 1) which are thought to maybe be edible rewards for pollinators. Different Pinguicula species are pollinated by all sorts of organisms, from bees (mellitophily) and flies (myophily), to butterflies (psychophily) and birds (ornithophily), so it would make sense that they provide something tasty for the pollinators to nibble on and encourage pollination, right?
But the question is: are these Pinguicula trichomes actually edible? Do they contain food reserves, like starch, proteins, or lipid droplets, and could they therefore be floral rewards for pollinators? A recent study by Lustofin and colleagues set out to find the answer.
Using fresh flowers and herbarium material, the authors examined 17 species of Pinguicula (within three subgenera: Temnoceras, Pinguicula and Isoloba) using light microscopy, scanning and transmission electron microscopy, and histochemistry. Here’s what they found:
Some, but not all, of the bee/fly-pollinated species had starch grains in their trichome cells.
The authors detected starch in the floral trichomes of several Pinguicula species pollinated by bees or flies (including P. agnata, P. albida, P. ibarrae, P. filifolia, and P. gigantea, see Figure 2), and concluded that “these peculiar trichomes contain food reserves and probably function as edible trichomes”.
However, not all of the bee- or fly-pollinated Pinguicula species had starch in their trichomes (see P. alpina and P. vulgaris in Figure 2), and thus the authors suggested that instead of being ‘feeding hairs’, “these trichomes may play a tactile role and act as guides [for pollinators] or they might mimic the edible trichomes of other species”.
The trichomes of butterfly-pollinated, bird-pollinated, and self-pollinated species didn’t contain anything edible
Starch grains were not detected in the trichomes of the butterfly- or bird-pollinated Pinguicula species (including P. moranensis, P. hemiepiphytica, P. rectifolia, P. emarginata, P. esseriana, P. laueana, and P. moctezumae, see Figure 2). It’s also worth noting that there was no evidence for protein bodies or lipid droplets in the floral trichomes of any of the examined Pinguicula species.
So why has starch only been detected in the trichomes of bee-pollinated species? From an evolutionary perspective, the authors suggest that “the edible trichomes are derived and are possibly a specialization for fly and bee pollinators that act as a food reward for these visitors”. The authors also compare the edible trichomes of Pinguicula to those of orchids, which are also formed specifically for bee-pollination, although they add that “the edible trichomes of orchids are more diverse in the types of food material compared with Pinguicula”.
How do Pinguicula floral rewards compare to other related carnivorous plants?
Alongside Pinguicula in the family Lentibulariaceae, there are two other genera of carnivorous plants: Utricularia (with suctioning bladder-traps) and Genlisea (with winding corkscrew traps). The authors write that the reward for pollinators in both of these two genera is nectar, though they note that Utricularia flowers do have some interesting floral structures.
“In some species (U. antennifera, U. capilliflora, U. dunlopii, U. dunstaniae and U. lowriei), the spur is significantly reduced and the corolla forms filiform appendages. In U. dunlopii, the glandular trichomes (osmophores) are densely distributed on the modified floral appendages, and therefore their scent is most probably the attractant for visiting insects. Although there are yellow non-glandular trichomes in the flower throats of U. multifida and U. tenella, they do not play the role of edible trichomes.”
So what’s next?
The authors recommend that there is still more work to be done to better understand the floral rewards produced by Pinguicula, including further research on their edible trichomes as well as their nectar production.
“Field observations are needed to answer the question of whether insects consume ‘starch’ trichomes of Pinguicula flowers and thus whether these structures can be regarded as pollinators’ rewards,” the authors wrote. “Most researchers accept that in Pinguicula the reward for pollinators is generally nectar because of the occurrence of a spur with glandular trichomes; however, actual observations of nectar secretion and nectar analysis are rare. Although edible trichomes may act as a reward in addition to nectar, a detailed study of nectar production and secretion in Pinguicula is required to be absolutely certain that all Pinguicula species produce nectar and in what quantities.”