Annals of Botany

No O spells death for bladdered prey

Carnivory has evolved several times in the plant  kingdom, giving rise to the development of an amazing array of traps including pitchers, sticky glands and rapidly closing fly traps. However, spectacular as these may be, they cannot compete for complexity with the much smaller  traps possessed by the bladderworts (Utricularia spp.), as discussed by Lubomír Adamec at Třeboň, Czech  Republic (pp. 849–856). In general, Utricularia species are rootless wetland plants that feed on small aquatic organisms.  The prey are trapped when they trigger sensitive hairs around the trap-doors  of tiny (1–4 mm long and 2–6 mm in diameter)  water-filled bladders or utricles. These are maintained under negative  pressure in relation to the surrounding medium so that when the trap-door  is opened the prey animal, such as a protozoan or a small crustacean,  is swept in by the inrush of water. Once inside, the prey dies but exactly  how this happens is a mystery. One hypothesis is that prey are killed  by anoxia and this has led the author carry out a very elegant series  of experiments measuring oxygen concentrations in traps, using sensors  linked to microprobes that were able to penetrate the bladder walls. Oxygen  concentrations in traps were very low (up to 4·7 µm) and many were below the detection limit of the sensor (1·3 µm). By contrast, the aquatic medium exhibited O₂  concentrations of between 0·35 and 0·9 m m,  so that when a trap was triggered the O₂ concentration inside  the trap rose dramatically. However, the plant was able, within 20–100 min, to reduce this back to the very low values, presumably by  a resumption of aerobic respiration. This has two implications. Firstly,  the cells lining the traps must be tolerant of anoxic conditions, as must  the organisms living commensally in the traps. Secondly, these data are  consistent with the hypothesis under test, that the prey organisms die  from lack of oxygen.