A portable field rheometer reveals age-dependent changes of viscoelastic trap fluids in carnivorous plants

Abstract

Plants consist of over 80% of water, but unlike water, many plant secretions have non-Newtonian, viscoelastic properties. Examples are the trap fluids of many tropical pitcher plants (Nepenthes), where viscoelasticity has been shown to aid prey capture and retention. Measuring the viscoelasticity of pitcher fluids accurately requires in situ rheometry, in the tropical field locations where these plants grow, because the fluid properties degrade rapidly in storage. With an interdisciplinary team of biologists and engineers, we designed a portable extensional rheometer that weighs less than 1kg and can be dismantled into parts for easy transport. We used this device to characterize trap fluids of two pitcher plant species: N. rafflesiana in Brunei, Northern Borneo, and N. pervillei in the Seychelles. Viscoelastic behaviour, quantified as the relaxation time after rapid stretching of a small quantity of fluid, appeared relatively late during trap development (before opening), and dropped off sharply around two to three weeks after trap opening. Our results show that fluid properties are highly dynamic over the lifespan of a pitcher trap, with times of maximum fluid viscoelasticity coinciding with times of high attractiveness and prey capture efficiency.