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Roots and Rhizomes

Nearly 100% of the belowground biomass (roots, rhizomes, and root hairs) of wigeongrass usually lies in the upper 10 cm of the bottom substrate and nearly 90% sometimes is in the upper 5 cm. In temperate estuaries, where the plant behaves as a perennial, the dry weight of belowground parts during peak growth can vary from 76% of total plant dry weight in extremely shallow sites to only 2% in deeper sites. This may reflect different strategies for nutrient uptake or survival in dimly-lit waters. Belowground biomass averages about 30-45% of maximum seasonal biomass. The belowground biomass develops best at well-oxygenated sites in coarse-textured bottom sediments. Complete degeneration of the system can occur in very highly reduced organic bottoms.

A single wigeongrass plant can have 2-15 rooting nodes on the rhizome. Short-lived roots up to 20 cm long occur singly or in groups of up to 20 at the nodes. Roots can compose 16% of the dry weight of cultured plants 3 weeks old. A zone of dense (up to 60/mm²) relatively short (< 1 cm) root hairs occurs toward the distal end of each root. Although sheaths (coleorrhiza) partially protect wigeongrass roots from dessication and physical damage, the root system is delicate and unable to penetrate deeply into sediments. This makes the species susceptible to water turbulence.

Wigeongrass cultures easily without sediment, but roots probably serve some function because detached plant parts and the top ends of vertical stems > 1 m in length will quickly form roots. Experiments show that roots do not act independently of leaves in nutrient uptake. Roots of wigeongrass growing in highly reducing sediments probably receive oxygen through the plant's lacunar system.

The main underground body of wigeongrass is a rhizome (rootstalk) that branches from a single axis (youngest branches at the tip) with shoots originating at about 1-cm intervals. Rhizomes are anatomically similar to vertical stems except for the presence of roots instead of leaves. Rhizomes contain more starch than upright stems, are thin and pale, and usually lie only a few millimeters below the bottom surface. The vascular system in rhizomes and roots is extremely simple.

Vegetation

A wigeongrass plant can have 10-15 vegetative shoots per tenth of a square meter during the horizontal branching phase and > 20 reproductive shoots per tenth of a square meter during the flowering phase. Over 30,000 shoots per square meter sometimes occur. Shoots produced early in the growing season probably live longer than those produced later, and transition from the vegetative state to the reproductive state increases shoot lifespan. Few shoots that have normal lifespans remain vegetative. Near the end of the growing season older rhizomes degenerate, leaving ramets that die before winter. Plants subject to exposure to air in intertidal habitats have fewer shoots, a greater number of shoots flowering early in the growing season, and lower drupelet production than plants that remain submersed during the growing season.

Before and during flowering, the thin wigeongrass stems usually grow rapidly, producing numerous lateral branches that branch again, and so on, but nearly stemless plants can also occur. Stems are about 1 mm wide and up to 3 m long, but average plant height is probably about 5-20 cm in most temperate waters. Plants have little strengthening tissue and the surrounding water provides support for dense upper vegetation.

Leaves are alternate, < 1 mm wide, < 20 cm long, and held by membranous sheaths < 7 cm long with short, free tips. Leaf tips vary from obtuse and serrate (Fig. 1) to acute and entire. The proportion of leaf area at various depths reflects adjustments to differences in light regimes caused by turbidity or the presence of other plants. The structure of leaf epidermal cells and their chemical composition vary with water salinity (Jagels and Barnabas 1989).

Flowers

Wigeongrass produces huge numbers of underwater flowers about 5-6 weeks after the onset of spring growth. Two tiny (3-5 mm diameter) bisexual flowers, lacking perianth, are atop one another on a slender fleshy spike. Flowers begin development sheathed inside a pair of subfloral leaves. Each flower has two stamens and four (3-5) pistils. After 1-2 weeks, the spike is pushed out of the swollen sheath by a peduncle that grows rapidly in length. Anthers burst and release pollen, aided by gas bubbles that accumulate inside the anther sac. Some pollen is trapped within the inflorescence and some clings to the surface of the gas bubbles. Most of the eight pistils usually found in each inflorescence are thus self-pollinated, but cross-pollination occurs from the pollen-laden bubbles that rise to the water surface, transporting pollen to other wigeongrass flowers.

Peduncle form is one of the main characters taxonomists have used to split R. maritima s.l. into separate species. Annual forms have peduncles that are either short, stiff, and straight or loosely coiled, up to 3 dm long, and that are pollinated underwater. Perennials have flexuous coiled peduncles up to 10 dm long and are pollinated at the water surface. These coiled structures can pull the fertilized inflorescences back underwater.

Sexual Reproduction

Annual Ruppia taxa depend on high fecundity to increase chances of reproduction in ephemeral habitats. Important features of this reproductive pattern are rapid development, early maturity, and the allocation of much energy into many small propagules. These taxa have 100% of their biomass in reproductive material (propagules) when wetlands are dry and about 20-30% when plant weights are highest during years of good growing conditions. The propagules of annual Ruppia taxa are technically termed drupelets, but are often called "nutlets" or "seeds." Drupelets can remain viable in sediments for up to three years.

R. maritima s.s. produces enormous numbers of drupelets about two weeks after first flowering because the many inflorescences are efficiently self-pollinated. The dark brown or black drupelets are 0.5-4 mm long, and vary from obliquely ovoid or rounded to asymmetrical, flattened, and beaked. Salinity may control drupelet size and shape, and drupelets produced in early summer can have a thicker coat than those from the same plant in early fall. Drupelets are attached to slender pedicels that vary in form depending on water conditions. Populations in shallower, more saline waters typically have nearly straight pedicels up to 4 cm long; populations in deeper, fresher water generally have curved, longer pedicels. Pedicels are always longer than the drupelets. Healthy drupelets average 1-7 mg dry weight (Gore 1965; Prevost et al. 1978). An elliptic or triangular perforation occurs near the base of the beak. A fully-pollinated and mature inflorescence usually consists of eight pedicellate drupelets atop a straight or coiled peduncle. A Ruppia taxa with up to 12 sessile drupelets occurs in hypersaline Australian wetlands (Brock 1982a).

Ripe drupelets are transported short distances in floating vegetation, considerable distances by wind and in the guts of fishes, and long distances in the digestive tracts of waterfowl. Wigeongrass drupelets mix with small amounts of other plant material, forming compact balls up to the size of small watermelons - these are often found along the beaches of saline lakes in windy locations. Such balls presumably form by wave action (field notes of F.P. Metcalf in McAtee 1925; Essig 1948; Swanson and Springer 1972; Gerbeaux and Ward 1986).

Water permanency, water depth, depth distribution of drupelets in sediment, sediment chemistry, and water column chemistry can influence drupelet distribution and germination and interact with temperature effects. In temperate climates, drupelets usually lie dormant underwater or on dessicated bottoms until the following spring. Most drupelets are found in the upper 5 cm of bottom sediment, but they can occur as deep as 25 cm. Drupelets buried > 10 cm in sediment probably do not germinate under natural conditions. Drupelets do not germinate on moist soil, but will germinate under as little as 4 cm of water indoors and 5-10 cm outdoors. However, little or no drupelet production occurs from plants germinated and grown at these shallow depths. Germination of R. maritima s.l. drupelets in Europe begins when water temperatures exceed the minima-maxima interval of 10-15 degrees C for about 10 days; previous dessication may stimulate germination. For European R. maritima s.s., stratification for 2 months at 4 degrees C increases germination. Temperature at germination usually is about 15-30 degrees C. Drupelets germinate in as few as 8 or as many as 30 days. Drupelets from habitats subject to prolonged drought probably take longer to germinate than those from more permanent water bodies.

Germination of wigeongrass drupelets is greatly reduced where upper layers of sediments contain > 1-2% soluble salts or where NaCl (sodium chloride) concentrations in the water exceed 15 g/L. However, drupelets that will not germinate because of higher salinities can recover and germinate after about 2 weeks in fresh water. Germination rate of drupelets kept in fresher ( < 3.5 g/L) waters at higher temperatures is lower than for those kept at lower temperatures in waters where salinity ranges up to 26 g/L. Drupelets of R. maritima s.s. germinate well in water salinities up to 43.4 g/L if an optimum water temperature of 28 degrees C is maintained. These drupelets are very drought-resistant.

Experiments on the germination and growth of wigeongrass from mild climates illustrate the plant's rather complicated life strategy there. Some drupelets germinate at relatively low temperatures (16 degrees C) and the plants grown from them produce flowers and fruit in as little as 8 weeks, whereas plants from drupelets that germinate at a higher rate under optimum temperature (28 degrees C) take up to five months to yield fruit. Lack of oxygen - as indicated by low redox potential of -300 mV - retards germination. Thus, in nature, drupelets from plants produced from drupelets that germinate at low spring temperatures probably will easily germinate during the summer in places with sufficient oxygen because habitat temperatures will then be near optimum. Drupelets that settle in poorly-oxygenated bottoms will lie dormant until the following year. However, drupelets that germinate when optimum temperatures are reached produce plants that do not mature until winter; drupelets from these plants go into winter dormancy, but they germinate in early spring at relatively low temperatures, starting a new cycle.

Animals also influence germination. Agami and Waisel (1988) found that the hard-seeded drupelets germinated at high rates after passing through the digestive tracts of tilapia (Oreochromis sp.) and grass carp (Ctenopharyngodon idella). However, nearly all drupelets eaten by common carp (Cyprinus carpio) were digested.

I conclude that, although drupelet germination in wigeongrass occurs under a rather narrow range of water levels, drupelets are easily dispersed and adapted to survive and germinate in a wide range of salinity-temperature regimes common to drought-prone environments.

Asexual Reproduction

Ruppia maritima s.l. also colonizes by rhizomes. Rapid growth of rhizomes on overwintering plants begins about the same time as drupelet germination and, like germination, is probably temperature controlled. Colonies reach maximum development during July or August in temperate climates. Spring and fall growth peaks occur in subtropical polyhaline estuaries. Recolonization of sediments denuded of wigeongrass by a boat propeller proceeded at about 0.25 m/year (Orth and Moore 1982). Floating fragments of wigeongrass grow roots freely at the nodes, sink, and attach to the bottom. Haag and Noton (1981b) suggested that reproduction of R. occidentalis in Alberta wetlands is low under high water conditions when rhizome growth predominates and shoots are short with long leaves. However, they also suggest that lower water levels cause shoots to increase in length and form vegetative propagules (undescribed) that are easily torn from parent plants. Turions (asexual, carbohydrate-rich perennating organs) or turionlike structures have been described on some Australian Ruppia taxa by Brock (1982b). These structures are about 2.5 mm long and form terminally on the rhizomes or at the junction of rhizome and leaf-sheath. These structures are unknown for R. maritima s.s.