Abstract
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 hypothesized that very low or high fluid levels reduce the trapping success of the pitcher and that pitchers possess mechanisms to minimize fluctuations of the fluid level. METHODS: Natural fluid levels of N. rafflesiana pitchers were monitored in the field. Effects of the fluid level 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 simulated evaporation by replacing the contents with a smaller volume of concentrated pitcher fluid. KEY RESULTS: Freshly opened N. rafflesiana pitchers were approximately half-filled with fluid. Over a 5-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 might be triggered by fluid volume or by water potential gradients resulting from changes in concentration. CONCLUSIONS: Nepenthes rafflesiana pitchers regulate their fluid level and remain efficient insect traps in fluctuating weather conditions. This active control is a previously unrecognized pitcher plant adaptation to their exposed habitats; understanding it is important for predicting the ability of these plants to withstand extreme weather conditions enhanced by climate change.