Snacking helps Drosera’s appetite

A paper by Pavlovič et al. has caught my eye this week. Feeding on prey increases photosynthetic efficiency in the carnivorous sundew Drosera capensis has moved into Free Access. I’m used to the idea that carnivorous plants trap insects to get Nitrogen, but it is a bit more complicated than that.

Pavlovič et al set up an experiment to follow the feeding cycle of D. capensis. D. capensis is a sundew, it eats insects by trapping them on sticky leaves, coiling round them and then secreting enzymes to digest the insect. Its home is South Africa, but it’s commonly cultivated around the world now.

Pavlovič et al. were testing a simple idea. What benefit does a plant get from feeding, and how can you measure it? Givnish et al. said feeding led to increased photosynthetic efficiency. So the experiment looked at gas exchange and chlorophyll a fluorescence. They also examined what enzymes get released by the sundew to see what it is that the plant is most eager to get. They also tried another experiment which worked, but didn’t get spectacular results.

D. capensis has another form. If you don’t want red tentacles on your plant you can now buy Drosera capensis alba, a plant with white tentacles. Pavlovič et al. wondered if red, found in the wild, was a signal to attract flies. The experiment they did to find out is the simplest. Get some fruitflies, put them into tanks with sundews with the two varieties and then see which plants catch most.

What they found was there was no significant difference. This doesn’t mean the experiment failed, instead it tells us that there is no preference and whatever reason there is for the red, it’s not to attract insects. It might sound dull, but it means something odd is happening. It’s not just sundews that are red. Various forms of Nepenthes and Sarracenia also grow red forms. Yet it doesn’t seem that these use red to attract insects either, so why are various carnivorous plants coming to the same colour?

As far as digestion goes, Pavlovič et al. found that all it takes for sundews to release enzymes is some mechanical stimulus, and they found this when they used polystyrene balls as well as flies. However, to really get digestion happening the plant seems to need more poking, like from a live insect and some chemical feedback. Obviously they weren’t getting this from the polystyrene balls.

When it came to seeing what the leaves were pulling out of the insect there was a mild surprise. Nitrogen and Phosphorus were obvious grabbed along with Potassium. They did not see absorption of Calcium or Magnesium, despite other people finding Mg take-up in other experiments. Pavlovič et al. think that their plants may have already had a relatively high Mg concentration.

The key input for Drosera was Phosphorus. Pavlovič et al. found their unfed plants were P-limited, meaning that it was a lack of Phosphorus that stopped them from growing as well as they could. Phosphorus is essential for making ATP, Adenosine triphosphate, which powers plant cells and is a key part of respiration. Without Phosphorus, plants would not be able to photosynthesise, so while it’s a small part of plant’s chemical make-up it is still very necessary.

The paper is good for what it finds, I like seeing how the nutrients are tracked so you can see the plant using them, but the referencing is important. This isn’t isolated research, it builds on other work and it’s contributing to a conversation. The authors don’t just include references to support them. The lack of Mg absorption by the sundew leaves is a puzzle, but Pavlovič et al. point to the papers that show this in order to put their own results in context. That is the best way of showing how new research can help expand the field more.

Pavlovic A., Krausko M., Libiakova M. & Adamec L. (2013). Feeding on prey increases photosynthetic efficiency in the carnivorous sundew Drosera capensis, Annals of Botany, 113 (1) 69-78. DOI: http://dx.doi.org/10.1093/aob/mct254