Physiological characterisation of aquatic traps in the epiphytic carnivorous plant Utricularia humboldtii

Abstract

The suction traps of aquatic Utricularia species are discoid bladders, 1–6 mm long, with their lumen isolated by a mobile trapdoor from the ambient medium. Water is pumped out of the traps to maintain the negative pressure in the reset traps. When a prey animal touches the sensory hairs on the trapdoor it opens, the ambient water along with the prey is sucked inside and the trapdoor is closed again. Utricularia humboldtii is a robust, semiaquatic–epiphytic or terrestrial species from the generic section Orchidioides from South America. The efficiency of its aquatic traps was measured based on trap firing and resetting rates as well as changes in trap thickness due to both mechanically stimulated and spontaneous firings using an electronic sensor. The U. humboldtii traps exhibited relatively low firing and resetting rates. These values are 2 to 20 times lower than those in other aquatic Utricularia species reported in the literature. These results together with the low aerobic respiration rate of sliced traps indicate that the less efficient aquatic traps in U. humboldtii are specialised for catching fine prey items in bromeliad water tanks. A marked lag-period in trap resetting was found to occur during the first 30 min after firing, contradicting the accepted concept of continuous water pumping. Spontaneous trap firings were also observed with the same magnitude as that in stimulated firings. This implies that spontaneous firings occur consistently in all the tested aquatic Utricularia species. However, although they relate to the continuous water pumping mechanism of the traps, their physiological importance has not been fully elucidated.

Keywords

• trap firing
• water pumping rate
• trap thickness
• aerobic respiration rate