Measurement of the critical negative pressure inside traps of aquatic carnivorous Utricularia species

• Critical negative pressure (CNP) for trap firing was measured in 15 Utricularia species.
• The mean CNP in all 20 species or variants tested is −0.195 ± 0.018 bar.
• High differences in CNP values appear between sections Utricularia and Pleiochasia..
• CNP values correlate negatively with trap size.
• Our results are likely to explain the different types of spontaneous trap firings.

Firing and resetting of aquatic Utricularia traps are associated with water flows and pressure changes. A negative pressure of ca. −0.16 bar is formed in reset traps, but its direct measurement is very difficult. We present a method of a gradual external application of negative pressure of −0.56 to −0.84 mbar s−1 through a fine capillary to cut off aquatic Utricularia traps to determine the critical negative pressure (CNP) at which the traps (located in air) fire and aspirate an air bubble. Using an electronic pressure sensor, we simulated the physiologically formed negative pressure needed for spontaneous trap firing in 15 aquatic Utricularia species of four generic sections. Mean CNP values ranged from −0.069 bar in giant traps of U. reflexa to −0.346 bar in U. dichotoma. The average in all 20 species or variants tested was −0.195 ± 0.018 bar, while that in 13 species or variants of the generic section Utricularia was −0.165 ± 0.015 bar and significantly differed from that of three populations of two species (U. dichotoma, U. volubilis) of the generic section Pleiochasia (−0.335 ± 0.006 bar). CNP differed significantly between giant and smaller traps of U. reflexa and young and old traps of U. vulgaris. Pooled data for 20 species or variants showed a significant negative linear correlation between trap length and CNP value. Within each species, high variability of the CNP was found: the lowest values were usually 2–3 times lower than the highest ones. This variability can represent three types of spontaneous firings described in the literature.