Most amphibious herbaceous species in temperate zones are either rhizomatous or tuberous geophytes (plants with their perennating buds below ground) and rely more on vegetative propagation than seed for regeneration. Seed germination is often very low in many amphibious species and successful sexual reproduction takes place only during periods of exceptional drought when water table levels have receded long enough for seeds to germinate and produce established juvenile plants. This restricted environmental window for seedling establishment explains the small number of annual species that are found in natural wetland habitats (see also Chapter 4).
As already mentioned, shoreweed (Littorella uniflora) is among the more successful perennial amphibious species of western Europe. It is a diminutive but abundantly stoloniferous plant that typically inhabits the drawdown zone in lakes and reservoirs where it can descend to a depth of 3-4 m. It can also be found in winter-flooded depressions on cliff tops and heaths and in seasonally flooded temporary pools. In these areas shoreweed also has to endure prolonged periods in summer when these habitats dry out completely. In lakes it can form carpets, which survive under continuous immersion relying entirely on vegetative reproduction. Flowering can take place under shallow flooding (50 cm) but no seed production takes place. Successful flowering with seed production takes place only when the shoots emerge above the water level. Although there is only a limited production of viable seed this species is normally successful in having a large seed bank with the seeds remaining viable for decades (Preston, 1997).
Leaf heterophylly, the phenomenon where floating and submerged leaves on the same plant are strikingly different in morphology, has long been recognized as a feature of emergent aquatic macrophytes, and appears to play a role at the margin between submergence and emergence. Many experiments have been carried out to elucidate the control mechanisms, which determine leaf form, and to assess the physiological significance of the change in terms of growth rate, productivity and survival of macrophytes under varying submergence and emergence regimes. Shoreweed (Littorella uniflora), the small perennial, amphibious rhizomatous plant that is common along lake margins (Fig. 8.7; see also above), functions more efficiently when lake levels fall and leaves are exposed to air, even though it survives as extensive widespread colonies in many permanently submerged locations. On emergence this small plant exhibits a very rapid phenotypic adjustment which includes the production of a new set of terrestrial leaves with reduced lacunal volume and increased stomatal density. Once these leaves are formed there is a rapid increase in leaf growth rate, leading to flowering within 3-4 weeks (Robe & Griffiths, 1998). It was also found that the terrestrial plants incorporated three- to fourfold more carbon and nitrogen into above-ground biomass than submerged plants. Shoreweed is a striking example of a species which has the possibility of using two divergent survival strategies: low growth rate and vegetative reproduction when submerged, and rapid growth with flowering and seed production when water levels recede. Together these two adaptations provide a flexibility that appears to account for the success of this plant in the amphibious niche.
The heterophyllous aquatic Loddon pondweed (Potamogeton nodosus) produces morphologically distinct leaves above and beneath the surface of the water. Application of abscisic acid (ABA) for 4 hours has been found to be effective in inducing entirely submerged plants to produce leaves of the form normally found at the water surface. There appears to exist 'a window of responsiveness' to ABA in that changes in leaf morphology were evident within 2 to 3 days after the treatment and continued for only 4 to 5 days thereafter (Gee & Anderson, 1998). Conversely, terrestrial shoots of the American aquatic Ludwigia arcuata formed narrower, submerged-type leaves when treated with ACC (1-aminocyclopropane-1-carboxylic acid), a precursor to ethylene, suggesting that ethylene might also be an endogenous factor responsible for change in leaf shape (Kuwabara et al., 2001). There may also be some measure of nuclear differentiation between submerged and emergent leaves. In the heterophyllous macrophyte water chestnut (Trapa natans) cytological analyses have provided evidence for polymorphism between the genomic DNAs of floating and submerged leaves (Bitonti et al., 1996).
8.6.2 Speciation and population zonation in relation to flooding
Some closely related amphibious species specialize in particular habitats through speciation and population zonation instead of responding phenotypically and morphologically with different types of leaves as discussed above. Such a species differentiation in habitat preferences can be seen on the closely related club-rushes. Adults of the emergent common club-rush (Schoenoplectus lacustris) and the closely related zebra rush (B. tabernaemontani) and the sea club-rush (Bolboschoenus maritimus) occur respectively along a gradient in water depth from deep to shallow water (see also Fig. 3.29). Seedlings of the two Schoenoplectus species showed their highest relative growth rate under terrestrial growth conditions, whereas Bolboschoenus maritimus grew best under submerged growth conditions. However, submerging seedlings that were established in unflooded conditions reduced growth in all three taxa. This effect became stronger with increasing age of the seedlings at the time of submergence. When transferred the other way round, from submerged to unflooded conditions, seedlings of S. tabernaemontani and B. maritimus adapted quickly to the terrestrial growth conditions, whereas the thin leaves of S. lacustris partly dried out. It was concluded that although seedling establishment of all three species will be most successful under terrestrial conditions, subsequent fluctuating water levels may act as a strong selective force which finally determines the distribution of these taxa along a gradient in water depth (Clevering et al., 1996).
The genus Eriophorum (cotton grasses) also shows differences in anoxia tolerance. In common with many graminoid species, the northern temperate species, E. vaginatum and E. angustifolium are not tolerant of prolonged anoxia as tested experimentally in an anaerobic incubator at 20-22 °C (Fig. 8.13). Two days of oxygen deprivation can cause a total kill in E. angustifolium. However, an arctic provenance of E. scheuchzeri still had a 20% survival of its stolons after 12 days of total anoxia at 20-22 °C. At the much lower temperatures that prevail during arctic winter a higher survival rate can be expected which will be almost a necessity (Figs. 8.13-8.14) as in both Greenland and Spitsbergen the arctic populations of this species may have to endure many months of ice encasement which will deny any access to oxygen.
Days of Anoxia
Fig. 8.13 Comparisons of anoxia tolerance in the genus Eriophorum based on the percentage survival of shoots compared with aerobic controls after being kept under strict anoxia in an anaerobic incubator (see Chapter ). Eriophorum scheuchzeri was of arctic origin from Spitsbergen; the other two species, E. angustifolium and E. vaginatum, were collected in Scotland. For experimental details see Crawford et al. (1994).
Other amphibious species exist as distinct populations in relation to their ability to survive winter flooding. A striking example is found in the American speedwell (Veronica peregrina). This species is a winter annual that has been successful in inhabiting shallow vernal pools in California. Plants at the centre of pools have been observed to be less variable and produce larger seeds than those at the edge as well as showing some metabolic adaptations to flooding including increased accumulations of malic acid (Linhart & Baker, 1973).
A dissected-leaved form of the creeping buttercup (Ranunculus repens) occurs in temporary limestone lakes or turloughs (Irish tuar loch, a dry lake) that can be found on limestone pavements in the Burren region in western Ireland (Fig. 8.15). Turloughs fill with ground water for up to eight months of the year and then dry out in the summer. Comparisons of the effects of experimental flooding up to ground level revealed localized populations of plants of R. repens growing within the turlough that differed both physiologically and morphologically from populations of this same species growing on neighbouring ruderal unflooded sites. Comparisons of the effects of experimental flooding up to ground level on populations of plants of R. repens collected from the turloughs as compared with ruderal locations showed no differences. However, when the plants were submersed with their shoots also under water this led to direct tissue death in the ruderal population but had no effect on the turlough population. There was no detectable difference in the proportion of aerenchyma in drained, flooded and submerged roots of plants from either population. However, the dissected leaf form in the populations that grew in the flooded turloughs demonstrated a higher rate of aerial and submerged photosynthesis than populations of the more typical broad-leaved ruderal form. The turlough populations also had higher rates of stomatal conductance and exhibited a higher stomatal index on the upper leaf surface and a lower index on the lower leaf surface than the ruderal populations. It would therefore appear that the more dissected leaf shape of the turlough population may have a thinner boundary layer and thus enhance gas exchange in submerged conditions (Lynn & Waldren, 2002, 2003).
The Alpine krummholz pine (Pinus mugo) is a very variable species and several ecologically distinct subspecies have been described. In particular the ecotype that is found in alpine bogs (P. mugo ssp. pumilio) appears to be a form that is particularly adapted to living by the water's edge (Fig. 8.16).
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