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Sago Pondweed (Potamogeton pectinatus L.):
A Literature Review

Growth and Production


Rate

Vegetation
Growth rate of plants is often measured by the speed at which carbon is accumulated per unit of plant weight or inhabited area. Wallentinus (1979) considered sago in a group of plants of low production rate (< 2 mg C/g dry weight per hour) when compared with several other species of vascular plants and many algal taxa of thalassic waters. Brooker and Edwards (1973), Jupp and Spence (1977b), and Kvet and Husak (1978) found that sago accumulated C at 548-1,400 mg/m2/day. Peak rates of 0.06 mg C/m2/s occur for short periods in fertile streams (Peverly 1985). Huebert and Gorham (1983) noted a seasonal periodicity in sago biomass production under standardized laboratory conditions, suggesting an internal regulation independent of obvious external signals.

Productivity is also measured by increases in weight over time. Laboratory cultures by Sheldon (1987) indicated that sago was intermediate in dry weight gain per unit of time when compared with 13 other submersed macrophytes commonly found in Minnesota lakes. Ozimek et al. (1986) found that sago in polluted and unpolluted sites in a Polish lake increased dry weight fastest early in the season, but that plants from polluted sites reached peak weight in July, whereas those from unpolluted sites continued to gain weight until October. Plants from unpolluted sites usually allocated more biomass to underground parts than plants found in polluted sites.

Sago was producing a maximum of 5.2 g/m2/day (dry weight) in early July and 2.3 g/m2/day in mid-August at time of greatest standing biomass in a fertile North Dakota wetland (Kollman and Wali 1976). Peterka and Hanson (1978) found similar (about 3.4 g/m2/day) growth rates among young sago planted in pans placed in waters obtained from a fertile North Dakota river.

Howard-Williams (1978) found that sago produced at most nearly 16 g/m2/day during the rapid branching period in February in a South African wetland; he considered this exceptionally high for any submersed hydrophyte. Purohit (1981) recorded a maximum incremental growth rate of 16.6 g/m2/day in a fertile Indian wetland. The aboveground portions of plants grown from turions added up to 223 mg/day oven-dry weight during the first 35 days and 220 mg/day during the first 21 days in the culture experiments of Otto and Enger (1960) and Ryan and Riemer (1975), respectively. An average plant cultured by Teeter (1963, 1965) added 12 mg dry weight per day during growth days 42-63.

Shoots from turions grew 0.4-0.7 cm/day over the first 10-11 days and 1.2 cm/day over the first 20 days while gaining 29 mg/day wet weight (Frank and Hodgson 1964; Devlin et al. 1972). Plants cultured from turions by Devlin and Yaklich (1971) grew 1.27 cm/day and added 3.4 mg/day dry weight.

Photoperiods from 10 to 16 h seem to have little effect on weight of cultured sago plants up to at least 8 weeks old (Spencer and Anderson 1987). Sago plants increase faster in length but slower in fresh weight under lower light intensities (Devlin and Karczmarczyk 1975). Plants grown from turions grew 2.16 cm/day for 21 days (Ryan and Riemer 1975), and young plants raised outdoors by Peterka and Hanson (1978) grew 1.2 cm/day for 45 days. Culture experiments of Spencer (1986a) showed that, at optimum growth temperatures (23-30° C), low light intensities stimulate shoot elongation, but leaf length increases with increasing temperatures at all irradiances. Plants cultured by Otto and Enger (1960) elongated < = 1.75 cm/day for 42 days in 800 ppm sediment concentrations, but they showed much slower growth in less turbid water. Purohit (1981) recorded growth of up to 13 cm/day for sago in deep (4.5-5.5 m) water when water levels were increasing.

Sago thus seems perfectly adapted to begin growth early in cool, dimly lit waters. This alone could explain sago's great success in temperate climates. The plant can also grow rapidly when water levels are rising and so is adapted to subtropical wetlands subject to monsoons as well as temperate wetlands where the greatest increases in water levels usually coincide with spring runoff and the beggining of the growing season.

Propagules

Teeter (1963, 1965) found, through culture experiments, that during growth days 42-63, sago turions gained 3.8 mg/day dry weight and by 63 days an average plant had 7 turions, 10.6 rhizome shoots, and 0.7 drupelet spikes. He also showed that plants added 0.44 rhizome shoots per day from day 63 to day 91. In culture, sago turions can form in as little as 4 weeks, and by 8 weeks individual plants can have > 12 turions which can compose 38% of plant dry weight (Spencer and Anderson 1987).

Ozimek et al. (1986) showed that young (small) turions germinate very quickly, but that the plants grown from them add weight to both above- and belowground parts at a much smaller monthly rate than do plants cultured from old (large) turions. They also observed that, compared with plants grown from old turions, plants grown from young turions added weight to belowground plant parts at a much faster monthly rate than to aboveground parts.


Yield

Vegetation
Sago forms dense monotypic stands of up to 840 individual plants per square meter or 1,000 shoots per square meter (Sheldon and Boylen 1977; Howard-Williams 1981; Purohit 1981). Plants from densest aggregations at eutrophic sites in a Polish lake formed 100 shoots per square meter where total above- and belowground biomass was 43.6 g/m2 dry weight (Ozimek et al. 1986). Greatest sago biomasses (> 1,500 g/m2 dry weight) were found in Africa (Zaky 1960; Aleem and Samaan 1969b; Howard-Williams 1978; Appendix A).

Although some leaves are naturally abscissed or weight is otherwise lost during the growing season, seasonal maximum biomass of sago (Bmax) is not much less than total annual production (P). P/Bmax ratios typically are between 1.1 (Purohit 1981; Berendsen and Van der Kruis 1986, cited in Van Wijk 1988) and 1.3 (Howard-Williams 1978), although these likely were slightly underestimated because of the methods used (Carpenter 1980). Madsen (1986) considered P/Bmax ratios > 2.0 reasonable for streams or areas of heavy wave action.

In nature, the belowground biomass of sago can vary from 4% to 78% of total plant weight (Howard-Williams 1981; Kautsky 1987; Van Wijk 1988). The proportions of plant weight found above and below ground at various times during the year depend greatly on whether the habitat creates conditions suitable for the perennial or annual life cycle of sago and whether or not the site is grazed (Van Wijk 1988). Culture experiments of Teeter (1963, 1965) showed that turions alone can make up 17% of the total weight of 9-week-old plants.

Ozimek et al. (1986) noted that heavier sago plants seemed to allocate less biomass to underground parts than lighter plants in mesotrophic to hypereutrophic waters. They found as little as 3.0% or as much as 41.4% of the dry weight of sago below ground. Kautsky (1987) observed that aboveground sago biomass can vary from 96% of total plant weight on soft bottoms in sheltered sites to 76% on exposed sands and gravels.

Madsen (1986), working in a polluted Wisconsin stream, found variance of sago shoot biomass nominal until the maximum was attained and flowering occurred. After that, senescence began and biomass varied greatly for the rest of the growing season, as clones of overlying shoots sequentially flowered and senesced.

Maximum standing crop or total biomass of sago < 200 g/m2 dry weight might indicate the presence of several factors that limit growth (Appendix A). However, reproductive strategy may also be important in this regard--of 11 sago-dominated European wetlands studied by Van Wijk (1988), the 5 with the highest peak biomass had sago populations that exhibited a perennial life cycle, with both vegetative parts and propagules surviving winter and contributing to biomass production over a relatively long growing season.

Propagules

Propagule densities and yields under various field conditions are shown in Table 2. Direct comparisons between studies are unwarranted because environmental conditions and genetic populations differ greatly. Comparisons within studies are also subject to some error, as where waterfowl exclosures result in greater organic matter inside because of reduced wave action. Nevertheless, likely factors resulting in high turion production are sparse carp and waterfowl populations, brackish or oligosaline water salinity, and maintenance of sufficient water depth during the growing season. These factors, as well as adequate protection from wave action, also seem to result in greater drupelet production.

Mean turion densities in California irrigation canals were 0.2-4.1 per 100 cm3, and the densities were not significantly related to the depth of sediment where the turions were collected (Spencer 1987). The U.S. Bureau of Reclamation (Garrison Diversion Unit Refuge Monitoring Annual Report 1987, unpublished) found little relation between turion density and depth or sediment characteristics on various water bodies along the James River, North Dakota and South Dakota, and concluded that both factors needed to be looked at simultaneously before their effects could be understood.

Propagule production from single sago propagules can be astounding. In a single-season culture experiment, Yeo (1965) grew 36,000 subterranean turions, 800 axillary turions, and 6,000 drupelets from 1 turion and 63,300 drupelets, and 15,000 subterranean turions from a single drupelet. Densities reached during this experiment were 3,308 subterranean turions per square meter from a single turion and 8,624 drupelets per square meter from a single drupelet. In nature, wind drift can deposit > 4,000 drupelets per square meter on bottoms away from areas of greatest sago production (Aleem and Samaan 1969b).


Chemical and Caloric Content

Vegetation
Dry matter constitutes 7.0-16.7% of the fresh weight of sago (Edwards and Owens 1960; Oborn 1964; Hannan and Dorris 1970; Paullin 1973; Purohit 1981; Chapman et al. 1987). Penuelas et al. (1988) reported that the dry weight of leaves was 11.8% of fresh weight and that of stems was 13.6%; this was markedly higher than for several other submersed angiosperms. Purohit (1981) found that highest dry matter content existed in sago at time of peak biomass. Air-dried sago is about 92.8% dry matter (Linn et al. 1975).

Ash content varies widely (10.3-56% of dry weight; Rich 1966; Hannan and Dorris 1970; Sugden 1973; Kollman and Wali 1976; Kvet and Husak 1978; Katanskaya 1986; Petrova 1986). Ash content is directly related to marl encrustations at time of collection, and levels depend greatly on treatment of the collected material (Westlake 1965; Kollman and Wali 1976; Petrova 1986). Data compiled by Katanskaya (1986) suggest that ash content in sago rises with increases in water salinity and total hardness.

Organic matter composes about 83% of the oven-dry weight of cultured sago shoots (Huebert and Gorham 1983) and 58-90% of material gathered in nature (Carpenter 1980; Purohit 1981; Katanskaya 1986). Organic C content is about 30-39% of dry weight (Owens and Edwards 1962; Hannan and Dorris 1970; Neel et al. 1973; Hill 1979) and 48% of the ash-free dry weight (Edwards and Owens 1960).

As percent dry weight, the aboveground biomass of whole sago plants or sago foliage contains 10.2-17.1% protein, 14.7-40.3% crude fiber, 0.5-2.7% crude fat, and 16.1-57.8% soluble carbohydrates (Sugden 1973; Linn et al. 1975; Anderson and Low 1976). Paullin (1973) compared the nutritional qualities of sago with seven other submersed vascular hydrophytes and found that sago was lowest in protein and fat. Sugden (1973) listed 17 amino compounds found in sago foliage. Linn et al. (1975) analyzed fiber from sago and found that it contained 51% neutral detergent fiber, 42% acid detergent fiber, and 6% acid detergent lignin.

Caloric content of the aboveground parts of sago is 2.0-3.7 Kcal/g ash-free dry weight, and likely is greatest in tissue 3 weeks old (Sugden 1973; Kollman and Wali 1976; Handoo et al. 1988). Handoo et al. (1988) found that the caloric content of sago was slightly lower than the mean for seven other submersed macrophytes tested. In subtropical climates, whole plants can have up to 4.7 Kcal/g ash-free dry weight at time of fruiting, but another, smaller peak occurs in winter when plants are known to accumulate more lipids (Purohit 1981).

Comparison of the elemental composition of sago (Table 3) with data presented by Hutchinson (1975) indicated that the plant is higher in Ca, iron, K, lithium, Mg, Na, and several micronutrients than the average hydrophyte. However, the higher values could merely be related to external carbonate encrustations that are often included in analyses (Kelly and Ehlmann 1980). Ho (1979) found that concentrations of Ca and Mg in sago tissue were negatively correlated with amounts of these two elements in the water column, and attributed this to the formation of carbonate encrustations on the plants. Encrustations could also have caused seasonal lows in these elements in sago tissue--lows that occurred around the time of peak sago biomass in a polluted Wisconsin stream (Madsen 1986). Gopal and Kulshreshtha (1980) calculated that a 1-m2 bed of sago at peak biomass would contain 27 g of Ca, 9.6 g of Mg, 27 g of N, and 0.7 g of P.

Kollman and Wali (1976) found that Na, K, and C content in sago tissue rose during the rapid growth period, declined during reproduction, and rose again during plant decline in a mixosaline, relatively unpolluted North Dakota lake. Amounts of Fe and Mg peaked at the end of the reproductive period. Many other elements (aluminum, Ca, Cu, nickel, silicon, Sr, zinc) increased during the fruiting period and decreased thereafter. Concentrations of manganese remained fairly constant until the decomposition period when an increase occurred. Complex interactions among plant uptake from sediments and water column, marl formation and disintegration, hydrology, and the ecosystem made it impossible to attribute these trends to any single factor. Hill (1979) found that N content of sago in a Minnesota lake reached maximum in late May, whereas organic C and P content peaked in mid-June.

Ho (1979) sampled sago shoots during their period of maximum growth in a lake heavily polluted with domestic sewage and found increasing concentrations of Ca, P, and Mg and decreasing levels of N. Potassium and sodium attained minima and maxima, respectively, during the middle of the period, while Fe levels initially fell and then stabilized. Significant positive correlations were found between tissue N and concentrations of inorganic N in the water column and between tissue P and PO4 content of the water. Levels of N and P in sago can be well above that required for growth, indicating the presence of luxury consumption (Jupp and Spence 1977b; Ho 1979; Madsen 1986).

Sago can play an important role in the P cycle in wetlands (Vermaak et al. 1981; Huebert and Gorham 1983) and release P into the water column (Vermaak et al. 1976). Howard-Williams (1981) found that dense stands of sago can remove large amounts of N and P from the water column in a single day. Vermaak et al. (1976) cultured sago in relatively high (0.3 mg/L) PO4-P concentrations and found that, within 5 days, the plants had concentrated P32 to 4,738 times the amount found in the water column. This was more than several other submersed, emerged, and free-floating hydrophytes, but only 10% of that found for the filamentous alga Oedogonium.

Purohit (1981) measured highest Ca content (12.46%) in sago from sites where productivity was greatest, highest total N content (4.29%) where productivity was lower than average, and highest total P content (0.215%) when biomass was declining.

Ozimek (1978) related the chemical content of sago to the degree of pollution from municipal sewage and found that, as loadings increased, chlorine, Fe, Mg, Na, and Zn concentrations in plants increased and Ca and K decreased, whereas Mn, N, and P remained nearly constant. One variety of sago (var. scoparius) was only found in the most heavily polluted site and was the only vascular plant present.

Madsen (1986) measured changes in the concentrations of major and minor nutrients in sago tissue collected during the growing season in a polluted stream. Peak N, K, and S concentrations tended to occur near times of peak sago biomass, whereas the opposite was true for Ca, Mg, Zn, and Cu. Manganese and boron tended to increase during the growing season, whereas concentrations of P, Fe, Na, and Al varied greatly. None of these elements except K was found below critical concentrations, and the brief period of low indicated K could have been caused by acid-washing during analysis. The author concluded that there was no limitation to macrophyte growth in this stream attributable to lack of nutrients and that water column loadings alone were more than adequate to prevent nutrient limitation to the growth of sago and other submersed macrophytes.

In natural waters, greater concentrations of N and P have been found in the upper or aboveground parts of sago than in the lower or belowground portions (Neel et al. 1973; Van Vierssen 1982b). Leaves were highest and rhizomes lowest in concentrations of both N and P in unfertilized plants studied by Howard-Williams (1981); roots concentrated N slightly more than did stems, but the difference between P in stems and roots was negligible. He found that treating the water column with extra heavy amounts of N and P would raise the concentrations of these elements in the four plant organs. Getsinger et al. (1982) found greater N and P concentrations in sago roots than in shoots in a wetland with low ambient P. Combined data of Neel et al. (1973), Getsinger et al. (1982), Van Vierssen (1982b), and Peverly (1985) indicate that Al, Ca, Mg, and Mn are more concentrated in upper or aboveground tissue than in lower or belowground tissue, whereas the reverse is true for cobalt, Cu, K, and molybdenum. Inconclusive results were found for Fe, Na, and Zn.

Propagules

Turions contain slightly higher amounts of protein and much more carbohydrate than sago foliage, but turions are much lower in fiber and ash (Anderson and Low 1976). Hodgson (1966) found that the high (74% of dry weight) carbohydrate reserves in cultured turions were exhausted 16-23 days after growth began and that the proportion of starch to sugar was greater in larger turions. He found sucrose only in unsprouted turions and sprouted turions up to 10 days old, but older turions from growing plants also contained some fructose and glucose.

Drupelet heads contain large amounts of fat (6.3-6.9% dry weight) compared to foliage (0.5-2.7%) or turions (0.9-1.1%; Anderson and Low 1976).

In summary, sago is able to greatly concentrate major nutrients, micro-nutrients, and trace elements (Table 3) as well as profoundly influence the nutrient flux in natural waters. Whether this ability is caused by a propensity to form external encrustations or, for major nutrients, to exhibit luxury consumption remains unclear. The large carbohydrate accumulation by sago turions undoubtedly explains their extensive use by migrant and wintering waterfowl.

The high uptake capacity of mineral elements by sago and other hydrophytes has led to proposals for the use of such plants in tertiary waste treatment for nutrient and waste removal from domestic or industrial effluents (Dykyjova 1979) or to reduce nutrient pollution in fairly large water bodies (Howard-Williams 1981). However, comparable numerical data on mineral uptake and fluxes are still largely unavailable because there remain many sources of error in analyses of plant tissue composition and much unexplained variability in chemical composition in plants because of plant age, phenology, and ecotype (Dykyjova 1979).


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