Northern Prairie Wildlife Research Center
Vegetative branching is sympodial. A main stem is first to reach the surface; several rhizome-derived stems reach the surface several weeks later. Stem length is determined by water depth and movement.
Branching is dense (Figure), but some growth forms lack the densely branched canopy near the water surface. Biomass of these growth forms can be greatly reduced (Kautsky 1987).
Leaves may grow up to 35 cm long, 0.25-2.5 mm wide, and 0.18-1.07 mm thick. Leaf tips can be sharply pointed, gradually tapered, or variable on a single older plant (Mason 1969), and other leaf characteristics can also vary greatly on a single plant. Leaves are covered with a waterproof cuticle 0.1 micro-m thick, greater than found in many other submersed macrophytes (Sharpe and Denny 1976). It has been hypothesized that the fine linear form of leaves, such as found on sago and other submersed species that live in waters with fluctuating salinity, is an adaptation whereby the area of epidermal cells assumed to provide two-way transport of electrolytes is great compared to leaf volume, thus aiding osmoregulation (Verhoeven 1979).
The 3-7 veined leaves have ligules 4-12 mm long and tightly clasping sheaths 8-30 mm long. The number of veins and leaf width have been observed to increase in plants cultured in nutrient-rich solutions.
Growth form of sago varies considerably and may depend on a variety of factors, including shoot density, depth, light climate, bottom substrate, wave action, and waterfowl grazing, rather than genetic differences among populations (Luther 1951, cited in Van Wijk 1988). This is not to say that great genetic variability does not exist in sago populations exhibiting differences in growth form. Leaves of the early growth of sago are much longer and thicker and branches are much less numerous (var. zosteraceus) than on the narrow-leaved, profusely branched plants (var. scoparius) that form after plants reach the water surface (Howard-Williams 1978). Nearly whole plants of the large, early growth form of sago can wash ashore from deeper waters after being loosened by the feeding activities of spring migrant waterfowl, before any plants can be found in shallower nearshore areas (H. A. Kantrud, personal observation). A gigantic form up to 5.5 m long with extremely long leaves grew in Lake Cayuga, New York (Dudley 1886). Sago vegetation becomes senescent from late August to October in north temperate regions, and most decomposes or washes ashore by the time wetlands freeze. During this period, huge amounts of green sago vegetation and rhizome fragments can wash ashore as a result of disturbance from migrant waterfowl feeding.
Fruit formation in sago begins about 3 weeks after flowering. Fruits of sago are called drupelets, or, by some authorities, achenes. These are small fruits with a pulpy or leathery cover over a stony walled seed. Usually about two of the four ovaries on each flower develop into drupelets. Drupelets are 3.0-4.0 mm long, 2.5 mm wide, and tipped with a vestigial style base in the form of a short beak (Figure). Each drupelet consists of a whitish crescent-shaped cotyledon and embryo covered with a hard wall and a fleshy greenish coat on the outside. The coat turns reddish brown at maturity. A hinged opening that is unridged and obtusely apexed lies distally to the embryo and provides an exit for the cotyledon upon drupelet germination; this characteristic and the thick, large-celled wall differentiate sago drupelets from those of all others in the genus (Martin 1951). About 7 to 20 drupelets mature on a short spike.
In north temperate climates, the starchy drupelets of sago are usually mature by late July to late September. In mild climates, the fruiting phase can extend for 2 months (Ramirez and San Martin 1984). Shortly after maturity the drupelets fall to the bottom or float temporarily and wash ashore in windrows. Seed bank studies show that on some large wetlands nearly all sago drupelets are recovered close to shore (Pederson and Van der Valk .1984). Drupelet germination occurs from late March to early summer in north temperate climates. Drupelets exposed for over a year on dry shorelines will germinate in as few as 4 days if wetted.
Haag (1983) studied sago distribution and drupelet germination from sediment cores taken from a permanent Canadian lake and outlined many of the factors that limit the success of sexual reproduction. In waters < 2 m deep, physical damage, caused by semifloating masses of detached plant material and burial by litter, resulted in high mortality and prevented many seedlings from reaching maturity. Seedlings in deeper sites often lacked proper light, growing time, and nutrients, and the plants assumed a spindly growth form that rarely flowered. Thus low seedling survival in deep and shallow water becomes combined with low drupelet production to restrict sexual reproduction. Haag (1983) also thought that sago production from drupelets could be limited by relatively short dispersal distances and burial in dark sediments of low reduction-oxidation (redox) potential.
Potamogeton turions can be either dormant or nondormant. The latter does not require preconditioning before germination. Dormant propagules are called hibernaculae (Sculthorpe 1967). These require preconditioning and a specific environment, usually controlled by light and temperature, to germinate. It is unknown how dormancy is controlled and whether sago produces both turion types in tropical regions (Madsen 1986).
Production of underground turions begins when the main stem of an older plant sends out a horizontal rhizome near the surface of the bottom substrate. The rhizome then penetrates the substrate and forms branches at every other node. Specialized tissue at the tips of the branches forms the turions. In favorable habitat subterranean turions can far outnumber mature drupelets and can be the only reproductive structure on some plants. Turions can also form in leaf axils at the tips of leafy shoots above the bottom surface. Production of this type of turion likely indicates sago populations geared to the relatively short growing season of plants with an annual life cycle. Some authorities claim turions are formed during most of the growing season, whereas others state turion formation begins after peak plant biomass is attained.
Turions (Figure) consist of two swollen internodes. The thickened starchy portion can reach 1.5 cm long. Fresh weight of underground turions is 0.9-1,001 mg. Dry weight is about 30% of fresh weight. Young turions are noticeably smaller than the old. Glasshouse experiments indicate that heavier turions produce larger plants that reach the water surface earlier and produce more shoots (Spencer 1986b). Subterranean turions occur singly or in simple or branched chains of up to five. Chains are up to 8 cm long. Axillary turions are smaller and occur singly or in chains of two. These can serve as a dispersal mechanism (Kautsky 1987). The occurrence of multiple turions may be caused by genetic differences in populations.
Turions can be found to 47 cm below the surface of the bottom substrate. Studies are inconclusive as to whether turion number varies with depth below bottom substrate, at least for depths > 7.5 cm. Mean turion weight may be greater in sago populations exhibiting perennial, versus annual, life cycles. Larger turions tend to occur deeper in the subtrate, at least in irrigation canals. Some studies indicate that this occurs because smaller axillary turions separate from parent plants and drop to the bottom as the fall dormant period begins. Others attribute the phenomenon to the feeding activities of waterfowl, or to the possibility that it represents an important adaptation that enables sago to survive desiccation. Small (< 10 mg fresh weight) turions planted shallowly (< 10 cm) produce plants with reduced growth rates and number of ramets per plant compared to those grown from larger turions. Turions of all weight classes planted at 20 cm produce plants with reduced growth rates. Smaller turions and turions planted deeper show reduced emergence. Thus turion size and depth distribution in sediments may be important environmental factors that regulate sago growth.
Turion development peaks in late summer or early fall in north temperate regions. Turions in storage can remain viable for several years (F. Nibling, U.S. Bureau of Reclamation, personal communication); they can survive in exposed mud for a year in temperate climates (Van der Valk and Davis 1978). Published studies do not report how long turions remain viable in nature, but data of the U.S. Bureau of Reclamation (Garrison Diversion Unit Refuge Monitoring Annual Reports, Bismarck, North Dakota, 1987, 1988, 1989, unpublished) suggest that turion density accumulates from more than 1 year's growth.
Most sago reproduction in nature is from turions that simultaneously send up shoots and develop extensive subterranean systems of rhizomes that send up additional shoots in great abundance. Production of leafy shoots also occurs through axillary buds, stem or rhizome fragments, and thick woody rootstalks that can be buried more than 15 cm in bottom soils. At the beginning of the growing season in thalassic Swedish waters, nearly 100% of existing sago biomass in exposed sands can be in the form of turions, whereas in sheltered muds, 75% of the biomass can consist of overwintering shoots (Kautsky 1987).
As early as late March in the Northern Hemisphere, overwintering turions begin to develop a long shoot bud that develops leaves at the tip and rhizomes and roots on lower nodes. A dormant bud develops if the main bud is severed. Not all turions germinate each spring, at least in temperate waters (M. G. Anderson, personal communication). Turions have highest germination rates during certain months and germinate regardless of light climate, even though light may aid germination, especially of the shoot portion. Light intensity seems unimportant.
Maximum germination and growth of turions occurs over a broad temperature range--15-26° C--and germination temperatures as low as 5.5° C have been recorded in the field. Turions from temperate climates require cold preconditioning (stratification) for good germination. Temperatures exceeding 30° C may damage proteins and inhibit germination.
Turions have been experimentally germinated while suspended in the water column at depths of 10 m, but best germination occurred at 1.0 m, where, after 24 days of growth, means of 8.8 leaves per plant and 4.9 roots per plant were recorded; at 10 m (where pressure was in excess of 1 atm.), leaf and root production was 90% lower. Turions can grow normally when suspended at 5 m depth even though Secchi depth is only 82 cm. In the laboratory, turions can be germinated in the dark, which suggests that excess pressure, rather than low light conditions, inhibits sago colonization beyond 5 m depths.
After 30 days of growth, a turion can have developed a main stem bearing 2 rhizomes that have a total of 11 shoots. Laboratory tests show the carbohydrate reserves in turions are exhausted in 3 weeks. Turions can continue growth for several years. Young, small turions germinate within 10 days in culture, but old, large turions can require a resting period of up to 110 days. Surface water or ice cover is not necessary for overwinter turion survival. They are, however, sensitive to desiccation, as up to 60% of turions exposed for 2 weeks to sediment moistures < 23% failed to germinate.
Some investigators have found that quiescent stems and bristly apical shoots serve as vegetative propagules in sago, whereas others could not detect the ability of the plant to grow from detached fragments that bore buds or nodes.