There’s a paper that’s moved into free access at Annals of Botany. Uptake of ant-derived nitrogen in the myrmecophytic orchid Caularthron bilamellatum by Gegenbauer et al. It describes an odd orchid.
Caularthron bilamellatum is a Myrmecophyte. A Myrmecophyte is a plant that has evolved to live with ants. It’s a facultative mutualism which means that C. bilamellatum doesn’t need ants to survive, but that it’s very happy to use them. If the plant can survive without ants then how closely are they connected to the insects?
You’ll find C. bilamellatum in the forests of Central America. They’re an epiphyte, which means you’ll find them growing on other plants, but for support and not as parasites. They lack roots, so they have to store water and to do this many epiphytes have pseudobulbs. These are swellings used as storage organs. However, C. bilamellatum has hollow pseudobulbs. It doesn’t just store water. It stores ants.
If you’re an ant, then C. bilamellatum is heaven. The orchid doesn’t just produce nectar in flowers, it produces nectar in many places around the plant. There’s nectar slightly further from the flower at the pedicel, the seedpods, developing shoots and even leaf bases. Gegenbauer et al say this means ants can get nectar around the year, and it’s not a trivial amount. For some colonies the orchid provides half the food the colony needs. This is a staggering investment for the plant in making ants happy. Why?
One reason could be protection. Providing lunch for the ants could help prevent the orchid being lunch for something else. It takes a dedicated herbivore to carrying munching with a mouth full of angry ants. But Gegenbauer et al have found another thing in the literature that they thought they could test. The pseudobulbs with ants in them produced more flowers and fruits than the empty pseudobulbs. It’s obvious ants take nutrition from the plant, but could they be giving something back with droppings or debris that worked as fertiliser? This is what the paper tests, in particular the movement of N from the ants to the plant.
First you need to find your plants. Gegenbauer’s team found them in Panama at the Barro Colorado Nature Monument. et al. Here the orchids grow in the canopy of the forest. The orchids grow Annona glabra which usually reach just 7m, so the canopy is accessible. The team grabbed a number plants of inhabited and empty pseudobulbs at various stages of growth.
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In the lab the team used a heavy isotope of Nitrogen 15N to make Ammonium Chloride. They injected this into the cavities of the pseudobulbs and grew the plants to see if other parts of the plant started showing they had 15N in them.
In the wild things were different. To test that the ants were fertilising the plants, they had to get the 15N into the insects. To do this they had to set a honey trap, literally. They added honey with some heavy Ammonium Chloride in it. The ants would take the food back to the nest and into the pseudobulb. If their waste was fertilising the plant then the 15N would move from the honey via the ants to the orchid.
In both cases the tell-tale 15N moved to other parts of the plant. Gegenbauer et al showed that it wasn’t just possible that C. bilamellatum got nitrogen fron ants, they could show it happening in the wild. They discuss similarities to carnivorous plants. Carnivorous plants capture insects because they live in nutrient-poor soils. This orchid doesn’t trap ants, but it does also have a nutrient problem. Gegenbauer et al compare it to the protocarnivorous plant Roridula gorgonias.
Roridula gorgonias can’t digest prey, but it can trap it for another insect. The plant then feeds on the nutrients in the droppings. Caularthron bilamellatum isn’t trapping anything, but the digestive process is similar.
When writing about orchids, Charles Darwin said: “In my examination of Orchids, hardly any fact has so much struck me as the endless diversity of structure… for gaining the very same end, namely, the fertilization of one flower by the pollen of another…” It seems that C. bilamellatum as another sort of fertiliser.