Northern Prairie Wildlife Research Center
Moderate salinities may benefit S. maritimus. In studies of Mercado et al. (1971), young plants grown under increasing sodium chloride concentrations (as much as about 7 mS/cm in the soil extract), generally produced larger plants, more shoots, and more big corms per plant than plants grown in fresher water (1.4-4.6 mS/cm). Mercado et al. (1971) considered the plant a salt lover, albeit not a true halophyte whose existence depends on saline habitat. Similarly, Kaushik (1963) showed that S. maritimus plants cultured in water of about 5.8 g/L salinity produce more achenes than plants grown at lower salinities. He listed symptoms of excessive salinity that appear in plants when salinities are raised to about 7.7 g/L. These include necrotic mottling and chlorosis (yellowing) of leaves, pale green color, curling and discoloration of leaf tips, dwarfing, and reduced root and root-hair development. Except for chlorosis and dwarfing, these symptoms are less evident in plants grown outdoors (Kaushik 1963).
Concentrations of chlorine and sodium ions in culms and roots of S. robustus reflect the levels in the root zone, but when levels there are very low, plants concentrate these ions in culms and roots (Palmisano 1970). The effects of salinity on the physiology of S. robustus were investigated more thoroughly by Pearcy et al. (1982) and Pearcy and Ustin (1984). Relative growth rate is more strongly inhibited than photosynthesis, even at moderate (10 g/L) salinities, and may be a primary factor decreasing productivity in saline habitats. Productivity is more restricted by reduced photosynthetic leaf area than by decreased rates of photosynthesis. Plants increase allocations to belowground structures and reduce rates of leaf elongation and leaf length at high salinities. Plants exclude mineral salts and use organic compounds for osmotic adjustment. These adaptations to high salinities may be less efficient than those possessed by true halophytes. However, S. robustus may have greater salt tolerance than indicated by indoor culture experiments because plants can attain large leaf areas early in the growing season when water salinities are lower. The timing of salinity stress is important. Plants that develop under lower salinity but whose growth is curtailed by higher salinity later in the season may still shade out possible competitors.
Methods of osmoregulation in 16 European halophytic angiosperms were studied by Briens and Larher (1982). Scirpus maritimus was included in a group of emergent species that accumulate little water and low concentrations of inorganic ions in the shoots and relatively large amounts of water in the roots. Soluble carbohydrates, especially sucrose, found in relatively large amounts in the leaves and rhizomes, seem to be the organic solutes that have significant osmotic function in this plant. The carbohydrate reserves are probably also responsible for the ability of the buried rhizomes of this species to survive long periods of anoxia and still send shoots through to the surface (Barclay and Crawford 1982). Clevering et al. (1995) found that during the period of submerged growth, S. maritimus can take up ample oxygen from the water. They also found that during this period relative growth rate is independently affected by light availability and corm size. After this period, plants grown from small corms (8.9 + 2.6 g fresh weight) in darkness can weigh only 67% of plants grown from large corms (16.2 + 2.8 g fresh weight). Whether or not carbohydrate accumulations are involved in the selection of these plant parts by feeding waterfowl is unknown. In S. maritimus, the concentration of the amino acid proline seems too small to serve as a cytoplasmic osmoticum (Popp and Albert 1980).
Higher average temperatures generally produce taller plants and favor shoot and corm production in S. maritimus (Visperas and Vergara 1976b). The optimum indoor air temperature range for maximum CO2 uptake in S. robustus is 27-32° C; at 20° C, the plant has higher rates than three other associated hydrophytes (Pearcy et al. 1982). The photosynthetic rate peaks at leaf temperatures of about 30° C. Plants survive in high-salinity California wetlands by completing vegetative growth in spring when temperatures are cool and salinities low (Pearcy and Ustin 1984).
Scirpus maritimus produces taller plants with fewer shoots and corms under longer photoperiods, but photo-period has no effect on flowering; low light intensity drastically reduces growth and production of shoots and corms (Visperas and Vergara 1976b). High light intensity (2,000 µE/m2/s) is required for maximum photosynthesis of S. robustus indoors, and plants cannot be made to show saturation of CO2 uptake by light (Pearcy et al. 1982). Elevated CO2 concentrations increase the growth, photosynthesis, and water-use efficiency of S. maritimus (Rozema et al. 1991).
When growing in nutrient-poor sediments, S. maritimus concentrates nutrients in its aerial parts to a greater extent than several other common emergents (Dykyjova 1978). Plants cultured from corms and fertilized with ammonium sulfate increase rapidly in dry weight while taking up very little nitrogen (Visperas and Vergara 1976a). Clevering and Van der Putten (1995) grew S. maritimus seedlings indoors in experiments to determine whether large accumulations of detritus limit nutrient supplies and thus account for the poor growth of this bulrush in wetlands formerly influenced by tides but now impounded. They concluded that a combination of toxic organic compounds and anaerobiosis in roots was a more likely explanation. This species probably does not use nitrogen, phosphorus, or carbon from its belowground parts during the growing season (Hall and Yesaki 1988). Kaushik (1963) cultured S. maritimus from Utah in water with added calcium chloride and sodium chloride (1:2 ratio) and nutrients. At salinities greater than about 7.7 g/L, the plants accumulated less sodium, calcium, chloride, and magnesium but more potassium than the similarly treated freshwater species Typha latifolia and S. acutus. Only minimal increases in sodium and chloride occur in Phillipine S. maritimus shoots when potting soils contain more than about 0.8% sodium chloride (Mercado et al. 1971). When increasing amounts of potassium, nitrogen, phosphorus, and calcium are added to hydroponic culture solutions supporting growth of the freshwater subspecies (S. maritimus ssp. maritimus) from Czechoslovakia, the plants show little or no increased uptake of these ions (Dykyjova 1986). Dykyjova (1986) could not complete a similar nutrient-uptake experiment on the brackish water subspecies (S. maritimus ssp. compactus). Nevertheless, the seemingly increased ability of S. maritimus to control uptake of some of these ions probably is related to osmoregulation. High levels of magnesium, or lower levels in the absence of calcium, cause abnormal seedling development in Louisiana populations of S. robustus, but such conditions usually are not found in wetlands in that area and are probably not important in establishing stands of this species (Palmisano 1970). Scirpus robustus may have a relatively low ability to take up zinc, cadmium, nickel, lead, and chromium; all these metals as well as phosphorus and iron are found at higher concentrations in the roots than in the tops or rhizomes (Lee et al. 1976). Meiorin (1989) found highest concentrations of magnesium in leaves of S. robustus, whereas zinc was greatest in roots.
In summary, physiological and anatomical evidence indicates that S. maritimus and S. robustus are adapted to thrive in moderately saline, well-lit environments that experience high summer temperatures. Nutrient use patterns seem efficient. A selective uptake of sodium and chlorine ions and the manufacture of soluble carbohydrates may aid osmoregulation. The latter ability may also enhance survival in anoxic sediments.