Harmless nectar source or deadly trap:Nepenthespitchers are activated by rain, condensation and nectar

Abstract

The leaves of Nepenthes pitcher plants are specialized pitfall traps which capture and digest arthropod prey. In many species, insects become trapped by 'aquaplaning' on the wet pitcher rim (peristome). Here we investigate the ecological implications of this capture mechanism in Nepenthes rafflesiana var. typica. We combine meteorological data and continuous field measurements of peristome wetness using electrical conductance with experimental assessments of the pitchers' capture efficiency. Our results demonstrate that pitchers can be highly effective traps with capture rates as high as 80% but completely ineffective at other times. These dramatic changes are due to the wetting condition of the peristome. Variation of peristome wetness and capture efficiency was perfectly synchronous, and caused by rain, condensation and nectar secreted from peristome nectaries. The presence of nectar on the peristome increased surface wetness mainly indirectly by its hygroscopic properties. Experiments confirmed that pitchers with removed peristome nectaries remained generally drier and captured prey less efficiently than untreated controls. This role of nectar in prey capture represents a novel function of plant nectar. We propose that the intermittent and unpredictable activation of Nepenthes pitcher traps facilitates ant recruitment and constitutes a strategy to maximize prey capture.

‘Insect aquaplaning’ on a superhydrophilic hairy surface: howHeliamphora nutansBenth. pitcher plants capture prey

“…This interpretation is supported by the roughly similar dimensions for both structures: 11.5 + 0.4 mm in H. nutans compared with 36.75 + 1.4 mm across 61 Nepenthes species [26]. Further experiments are needed to quantify the wettability of the trichomes and the underlying cuticle, and to elucidate the role of the nectar for wetting [20].…”

“…By contrast, the peristome of Nepenthes pitchers is only intermittently wet and slippery, and its trapping efficiency is linked to environmental factors such as rain and air humidity [20]. While the intermittent activity of Nepenthes traps might represent an adaptive strategy to capture ants [20], uncoupling this link might be better for H. nutans that grows in a more seasonal climate than most Nepenthes. We propose that by using the fluid depot inside the pitcher for surface wetting, the plant is able to extend 'active' trapping periods beyond the times of actual rainfalls and stay continuously slippery during the wet (growth) season, thereby maximizing nutrient acquisition when it is most needed.…”

Incorporating ecological context: a revised protocol for the preservation of <I>Nepenthes</I> pitcher plant specimens (<I>Nepenthaceae</I>)

“…Furthermore, it has become apparent to ecologists that both the development and extent of the wax zone inside the pitchers, and the microstructure of the inner surfaces of the peristome, can play important roles in prey capture (Bohn & Federle 2004, Bauer et al 2008. Unfortunately, the conversion of a markedly three-dimensional structure (such as a pitcher) to a two dimensional one by pressing herbarium material, causes much of this potentially valuable information to be lost.…”

Superhydrophobic and superhydrophilic plant surfaces: an inspiration for biomimetic materials

“…The microstructure of the peristome shows ridges, which mostly extend into tooth-like structures at the inner edge and where the nectaries (nectar-secreting glands) are situated. Water droplets spread rapidly on the peristome and form thin films, which make the peristome extremely slippery for insects (Bauer et al 2008). The leaves of Maranta leuconeura and Calathea zebrina show a relatively homogeneous structure of conical epidermal cells (figure 8e).…”