Rhynchostegium is a genus of pleurocarpousmosses belonging to the family Brachytheciaceae.[2] The genus has a cosmopolitan distribution across different climatological regions except the polar regions, mostly in tropic to north temperate regions.[2][1] The genus contains both aquatic and terrestrial species.[1][3] The genus was named for their rostrate opercula.[1] The type species of this genus is Rhynchostegium confertum (Dicks.) Schimp.[1]
Terrestrial species of Rhynchostegium live in moist to wet or shaded habitats, on rock, soil, tree base, tree stem, and logs.[1][4][5]
Aquatic species live by or in running water, including streams, springs, rivers, beds of waterfalls, and seepy cliffs.[3][6]
Morphology
Gametophyte
Lamina of Rhynchostegium murale.A Close view of Rhynchostegium murale sporangium. Rostrate operculum can be seen.
Rhynchostegium are small to large mosses that form either loose tuft or extensive mats on the substrate, with irregular or regular branching.[1][3] The younger plants are generally deep green or light green; aging plants could become whitish, brownish, or paler green.[1][3] Stems are creeping and lack hyaloderm, with acute to acuminate pseudoparaphyllia.[3] Stem leaves are erectopatent or erect.[1][3] Branch leaves are similar in morphology to stem leaves but smaller and sometimes narrower.[3] Leaves are commonly straightly to homomallously arranged; subimbricate, subcomplanate, or complanate arrangement are sometimes seen, especially in branch leaves.[1] Leaf base, decurrent or not, varies from ovate to ovate-cordate, occasionally lanceolate, and the narrowing from gradual to abrupt, towards a short- or long-acuminate apex, where sometimes a differentiated long acumen or apiculus is present.[1] The leaves have a single costa that generally smoothly ends 35-75% up the leaf, and more often in branch leaves in an abaxial spine.[1] Leaf surfaces vary from flat to slightly concave and not to strongly longitudinally plicate, with little to some pores and linear laminal cells.[1][3] Leaf margins are serrate to serrulate.[1][3] Axillary hairs constitute of 3-7 cells,[3] with 1-3 upper cells.[1] Alar cells are slightly enlarged, and either undifferentiated or quadrate to elongate-rectangular.[1][3]
Sporophyte
Rhynchostegium are autoicous.[3] Covered by a naked calyptra is a rostrate to long-rostrate operculum attached to a red-brown to brown, oblong-cylindric, weakly curved capsule, which is inclined or horizontal to a red-brown, smooth seta that has abruptly contracted perichaetial leaves at the base, with acumen straight to reflexed.[1][3] An annulus separates the operculum.[3] The peristome is xerochastic and perfect, which the red to orange-red exostomes have reduced trabeculae and cross-striolae at the base of the teeth; in rare cases the exostomes are narrow and yellow.[1][3] The broadly or narrowly perforated endostomes and developed to vestigial cilia are supported by a low or high basal membrane.[1] Spore diameters range between 9-16μm.[3]
Biochemistry
Allelopathy
Allelopathy has been studied on Rhynchostegium pallidifolium, which usually form pure colonies in their natural habitat.[7][8] Methanol extract of R.pallidifolium represses the seedling of cress, alfalfa, lettuce, ryegrass, timothy,and Digitaria sanguinalis in a concentration-dependent manner.[7] A combination of ESI-MS and 1H NMR analyses identified the inhibitory chemical as 3-hydroxy-β-ionone.[7] Further study showed a minimal 3-hydroxy-β-ionone concentration of 1 μM for the inhibition of cress hypocotyl growth, and 3 μM for cress root growth, while the endogenous concentration.[8] The presence of 3-hydroxy-β-ionone in their natural substrate and the growing medium suggested secretion to the environment, which may imply an important role of 3-hydroxy-β-ionone in competition with other plants and the forming of pure colonies.[8]
Ethanolic extract of Rhynchostegium vagans showed similar effect on some Gram-negative bacteria and fungi, with performance superior to chloramphenicol and fluconazole.[10]
Applications
Freshwater monitoring
Rhynchostegium riparioides is used in monitoring of heavy metals concentration in freshwater in multiple regions around the world,[11][12][13] such as copper,[14][15] zinc.[16]R. riparioides as a neutrophilous species has been used in monitoring water acidification.[17]
List of species
The World Flora Online lists 221 species of Rhynchostegium.[18]
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