Ecophysiology of aquatic carnivorous plants

About 60 species of the genera Aldrovanda and Utricularia are submersed aquatic or amphibious carnivorous plants. They all are strictly rootless and take up mineral nutrients for their growth from the ambient water and captured prey through their trap-bearing shoots. These species represent a specific ecophysiological group that are dissimilar in their principal morphological and physiological features from terrestrial carnivorous plants and from rooted and nonrooted aquatic noncarnivorous plants. I review the ecology of habitats of aquatic carnivorous plants; characteristics of their growth traits, photosynthesis, and mineral nutrition; regulation of the investment in carnivory in Utricularia; biophysical and physiological peculiarities of Utricularia traps; and turion ecophysiology. Open questions of the ecophysiology of aquatic carnivorous plants are discussed.

Enzyme production in the traps of aquatic Utricularia species

We studied the influence of habitat and increased mineral phosphorus and nitrogen loading on the extracellular activity of five selected hydrolases and pH in the trap fluid of the aquatic carnivorous plants, Utricularia vulgaris, U. australis, and U. foliosa (Lentibulariaceae). Enzyme activities in the trap fluid were determined using fluorometry. Phosphatase exhibited the highest activities in the traps of the European species as well as field-grown tropical U. foliosa. Trap enzyme production appeared to be uninfluenced by elevated dissolved mineral N or P concentrations both in the trap and ambient environment and thus, it seems to be constitutive. Enzyme activity in the trap fluid was determined by species and environmental conditions and varied significantly among sites within a single species. Trap fluid pH was between 4.2–5.1 in U. vulgaris and U. australis but between 5.7–7.3 in U. foliosa and seems to be regulated by the traps.

Respiration of turions and winter apices in aquatic carnivorous plants

Basic respiration characteristics were measured in turions of six aquatic plant species differing greatly in their ecological and overwintering characteristics both before and after overwintering, i.e., in dormant and non-dormant state: non-carnivorous Hydrocharis morsus-ranae and Caldesia parnassifolia and carnivorous Aldrovanda vesiculosa, Utricularia australis, U. ochroleuca, and U. bremii, and in non-dormant winter apices of three Australian (sub)tropical populations of Aldrovanda and of two temperate North American Utricularia species, U. purpurea and U. radiata. Respiration rate of autumnal (dormant) turions at 20°C ranged from 0.36 to 1.3 µmol O2 kg−1 (FM) s−1 and, except for U. bremii, increased by 11–114% after overwintering. However, this increase was statistically significant only in two species. Respiration Q10 in dormant turions ranged within 1.8–2.6 and within 2.3–3.4 in spring (non-dormant) turions. Turions of aquatic plants behave as typical storage, overwintering organs with low respiration rates. No relationship was found between respiration rate of turions and overwintering strategy. In spite of their low respiration rates, turions can usually survive only from one season to another, due to their limited reserves of respiratory substrates for long periods. Contrary to true turions, respiration rates in non-dormant winter apices both in Australian Aldrovanda populations and temperate U. radiata and U. purpurea, in sprouting turions, and growing shoot apices of Aldrovanda were high and ranged from 2.1 to 3.1 µmol kg−1 (FM) s−1, which is comparable to that in aquatic plant leaves or shoots.