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

Abstract

Utricularia intermedia Hayne, U. ochroleuca R.W. Hartm., U. stygia Thor and U. bremii Heer ex Kölliker (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, oligo-mesotrophic 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. bremii is partly a stenotopic species preferring only slightly acidic to neutral (pH 6–7), very soft to slightly hard, oligo-mesotrophic waters. Yet it can grow well both in strongly dystrophic and clear waters, in peat bogs as well as sand-pits over peaty soil and clayish sand. Long-term, very low water levels in combination with habitat eutrophication, whatever the reason, leading to peat bog and fen infilling, are the most common and unfavourable ecological threads at the most sites of the four rare Utricularia species. However, ecological consequences of high-water level at the sites can be ambiguous for the populations: it reduces the strongly competitive cyperoid and graminoid species but can speed up site eutrophication. All four species are considered (critically) threatened in European countries and are usually under official species protection or their sites are protected. Regeneration of infilled fens or peat bogs and creation of shallow fen pools and canals in these mires, combined with (re)-introductions of these species have shown to be a very successful and efficient measure to protect the natural populations for many decades. Old shallow sand-pit pools have become outstanding substitution habitats for the protection of U. bremii.