An insect trap adjusting to weather conditions: Nepenthes rafflesiana plants control the fluid level in their pitchers to maximise prey capture

Background and Aims
Many carnivorous plants rely on open fluid pools or droplets to trap and digest arthropod prey. Here we investigate for Nepenthes rafflesiana pitcher plants how they maintain functional fluid pools inside their traps while growing in open sites exposed to changing weather conditions. We hypothesised that very low or high fluid levels reduce the pitcher's trapping success and that pitchers possess mechanisms to minimise fluctuations of the fluid level.

Methodology
Natural fluid levels of N. rafflesiana pitchers were monitored in the field. Fluid level effects on prey capture rate and efficiency were quantified with field and laboratory experiments. To test the capacity of plants to respond to changes in fluid level, we experimentally simulated flooding by adding water to pitchers, and evaporation by replacing the contents with a smaller volume of concentrated pitcher fluid.

Key Results
Freshly opened N. rafflesiana pitchers were approximately halfway filled with fluid. Over a five-week observation period, daily fluctuations of pitcher fluid levels were significantly lower than those of water-filled control vials. Pitchers possess canopy-like lids, but this did not eliminate rainwater influx into pitchers. Both low and very high fluid levels were detrimental to prey capture, with intermediate fluid levels yielding the highest trapping rate. Experimentally flooded pitchers returned to intermediate fluid levels within 2-3 days. Pitchers responded to simulated evaporation by secreting fluid, restoring intermediate fluid levels within 2 days. These homeostatic responses may be triggered by fluid volume, or water potential gradients resulting from concentration changes.

Conclusions
N. rafflesiana pitchers regulate their fluid level and remain efficient insect traps under fluctuating weather conditions. This active control is a previously unrecognised pitcher plant adaptation to their exposed habitats; understanding it is important for predicting ability of these plants to withstand extreme weather conditions enhanced by climate change.