Biological flora of Central Europe: Utricularia intermedia Hayne, U. ochroleuca RW Hartm., U. stygia Thor and U. bremii Heer ex Kolliker

摘要

Utricularia intermedia Hayne, U. ochroleuca R.W. Hartm., U. stygia Thor and U. bremii Heer ex Kolliker (Lentibulariaceae, Lamiales) are the four rarest and critically endangered European Utricularia (bladderwort) species from the generic section Utricularia. They are aquatic, submerged or amphibious carnivorous plants with suction traps which grow in very shallow, standing dystrophic (humic) waters such as pools in peat bogs and fens (also pools after peat or fen extraction), shores of peaty lakes and fishponds; U. bremii also grows in pools in old shallow sand-pits. These Utricularia species with boreal circumpolar distribution (except for U. bremii) are still commonly growing in northern parts of Europe (Scandinavia, Karelia) but their recent distribution in Central Europe is scarce to very rare following a marked population decline over the last 120 years. All species have very thin linear shoots with short narrow to filamentous leaves bearing carnivorous traps (bladders, utricles) 1-5 mm large. The first three species form distinctly dimorphic shoots differentiated into pale carnivorous ones bearing most or all traps, and green photosynthetic shoots with only a few (or without) traps, while the last species usually forms non-differentiated (monomorphic) or slightly differentiated shoots. The plants exhibit a marked physiological polarity along their linear shoots with rapid apical shoot growth. Their very high relative growth rate is in harmony with the record-high net photosynthetic rate of their photosynthetic shoots. Flowering of these species is common under favourable conditions and is stimulated by high temperatures but only U. intermedia sets seeds; the other species are sterile due to pollen malformation. Some molecular-taxonomic studies indicate that U. ochroleuca and U. stygia might be hybrids between U. intermedia and U. minor. All species propagate mainly vegetatively by regular branching and reach high relative growth rates under favourable conditions. All species form spherical dormant winter buds (turions). Suction traps actively form negative pressures of ca. -0.22 to -0.25 bar. The traps are physiologically very active organs with intensive metabolism: as a result of the presence of abundant glands inside the traps, which secrete digestive enzymes and absorb nutrients from captured prey carcasses (quadrifid glands) or take part in pumping water out of the traps and producing negative pressure (bifid glands), their aerobic respiration rate is ca. 2-3 times higher (per unit biomass) than that of leaves. Although oxygen concentrations inside reset traps are (almost) zero, traps are inhabited by many microscopic organisms (bacteria, euglens, algae, ciliates, rotifers, fungi). These commensal communities create a functional food web and in traps with captured macroscopic prey, they act as digestive mutualists and facilite prey digestion. Traps secrete a great amount of organic substances (sugars, organic acids, aminoacids) to support these commensals ('gardening'). Yet the nutritional role of commensals in prey-free traps is still unclear. Quadrifid glands can also serve in the reliable determination of three species. Ecological requirements of U. intermedia, U. ochroleuca and U. stygia are very similar and include very shallow dystrophic waters (0-30 cm deep) with highly variable levels of dystrophy, common mild water level fluctuations, oligomesotrophic to slighly eutrophic waters, optimal pH values from 5.5 to 7.0 but always high free-CO2 concentrations of 0.8-1.5 mM. Limited data indicate that U.