Regulation of pitcher fluid volume and properties in six ecologically distinct Bornean Nepenthes species

Carnivorous Nepenthes plants capture and digest prey in fluid-filled pitchers, whose fluid level is subject to fluctuations caused by rain and evaporation. It was found that N. rafflesiana pitchers can reduce these fluctuations by regulating their fluid volume and composition. Here, we investigate whether and to what extent this ability occurs in other Nepenthes species varying in morphology, trapping mechanisms, habitat, lifespan and nutrient sources. We hypothesised that species more exposed to fluid level fluctuations or relying more on fluid for prey capture would exhibit more intensive regulation. In six Bornean Nepenthes species, we quantified the effect of pitcher fluid level on prey-capture efficiency, the natural fluctuations in fluid volume, and the plants response to manipulations of the fluids volume, concentration and pH. The prey capture of Nepenthes species depended to varying degrees on the pitcher fluid level: in species with waxy inner walls, capture efficiency decreased at high fluid levels, as prey could escape easily. In contrast, species with wax-free walls and steep peristomes retained prey efficiently, regardless of fluid level. Although all Nepenthes species were able to secrete and absorb water and electrolytes, the regulation rate varied considerably. The rates of the various secretion and absorption processes (concentration/volume-dependent rates of electrolyte/water transport) were positively correlated with one another, suggesting that they all depend on the activity of the pitcher glandular epithelium. While differences and similarities in fluid regulation may be partly due to phylogeny, the observed species differences can also be explained by the pitchers trapping mechanisms and ecological factors. We found weaker regulation in Nepenthes species growing in sheltered understorey habitats, and in species utilising alternative nutrient sources, which may reduce their reliance on pitcher fluid. Our study highlights the diversity of ecological strategies in pitcher plants and demonstrates how these are influenced by environmental conditions. Lay SummaryNepenthes pitcher plants have fluid-filled pitchers to capture and digest prey. Can they control the volume and composition of their fluids? We found that all six species studied regulated their fluids, but to varying degrees. Species differences can be explained by the plants habitat, pitcher morphology and nutrient acquisition strategies.