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What happens to nutrients, after a carnivorous plant has digested its prey?

While carnivorous plants eat insects, they get most of their energy from photosynthesis, like other plants. New research investigates how carnivory affects photosynthesis.

Some plants have turned to carnivory to get nitrogen and other nutrients. While botanists have studied how plants capture their prey to get these nutrients, knowing what the plants do with them once they have them has been a puzzle for many years. Sebastià Capó-Bauçà and colleagues have been taking a close look at how capturing prey affects the limits of photosynthesis for the plants.

“The effects of carnivory in plants, for example on photosynthesis, have received a lot less attention than the mechanisms employed by the various forms of trap to capture their prey. N from prey increases the production of chlorophyll and Rubisco, so enhances photosynthetic assimilation – this has been well documented in carnivorous plants. But we’re still in the dark about how this interplay of nutrient intake and photosynthesis plays out in natural systems,” said Chris Thorogood, co-author of a commentary on the paper.

Image: Canva.

Capó-Bauçà and colleagues studied Nepenthes × ventrata, a natural hybrid of N. alata and N. ventricosa, found in the Philippines. The team wished to find out how different types of nutrition, such as nutrients gained from traps, compared to nutrients taken through the roots, affected photosynthesis.

To do this, the botanists grew and then starved some pitcher plants, plugging their traps with cotton wool to avoid accidental meals. They made sure the plants were well watered with distilled water, meaning that the roots were not taking up any nutrients either. Once the plants were stressed, they were fed four different kinds of insects, wasps, ants, beetles or flies. They then measured photosynthesis using gas exchange, chlorophyll fluorescence and immunodetection of photosynthesis-related proteins. Another batch of plants got no insects, but did receive fertiliser in their water.

As expected, when plants had nutrients, they were able to invest them into their leaves. The team also found that the method of gaining the nutrients mattered. Root-fertilised plants put comparatively more of their nitrogen into photosynthetic leaves, while the insect-fed plants steered comparatively more nitrogen into their traps. The team also found that the bioavailability of N in the prey wasn’t all the same, a result that interested Chris Thorogood. 

“The N content of herbivorous insects is known to vary, but the reasons for this variation are not fully understood. Furthermore there are differences in the prey spectrum among Nepenthes in nature; one species attracts termites almost exclusively, while others specialise on ants or flying insects – and some, even on mammalian faeces (the animals defecate into the pitchers whilst feeding on the nectar). So I was interested that this new research identified differences in N bioavailability among the four different types of insects fed to the pitchers. It also highlights the importance of examining a range of prey species, to replicate those found in natural systems – or better still, carrying out work using natural systems directly.”

The team note that their research confirmed a hypothesis dating back to 1984, that carnivorous plants use prey for nitrogen to increase Rubisco, to aid photosynthesis. Despite clearing up a 35-year-old problem, Thorogood said there will still be plenty to study in plant carnivory in  the future. “Every near new species of Nepenthes are described in Southeast Asia. We know nothing about the trapping strategies or prey spectrum of these species – never mind the effects of these prey on the plants’ physiologies. Examination of a range of species, and documentation of the prey they capture, will provide a rich living library of species for plant physiologists to explore.”

“The construction and maintenance costs of traps vary considerably among species, as does the type of prey they attract in their various habitats. Scientists should explore the effects of prey and nutrient assimilation in a range of natural systems to consolidate our understanding of the effects of carnivory on photosynthesis.”