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

Physiology


Sago is well suited to laboratory culture. Its physiology is known partly from studies on the uptake of herbicides tested to reduce sago's presence in irrigation canals.

Welsh and Denny (1979) showed that P moved both from and into sago shoots. Huebert and Gorham (1983) examined the ability of sago roots to mobilize nutrients from a constant small volume of sediment in the absence of one or more of the nutrients from the water column. Roots were capable of mobilizing sufficient N, P, potassium, sulfur, and micronutrients from the sediment to the shoots to meet normal growth requirements. In the absence of K from the water phase, sodium replaced it, although plant vigor suffered. Roots could not mobilize enough magnesium, calcium, or dissolved inorganic carbon from the sediment to the shoots to meet normal growth requirements. They speculated that the exclusion of sago and other species from waters of low alkalinity can be caused by the water-phase inorganic C requirement of these plants. They also suggested that water-phase Ca was necessary to prevent toxicity of other cations when they are present in the water phase (see Water Column Chemistry).

Sago uses HCO3 during photosynthesis but has a higher affinity for CO2 (Sand-Jensen 1983). Production may be reduced by the excess energy required to use HCO3 as a carbon source, but the formation of monotypic stands of sago in saline environments may result from the plant's ability to use HCO3 and simultaneously withstand osmotic stress (Kollman and Wali 1976). The lower surface of Potamogeton leaves is involved in HCO3 use, and the upper for OH release (Prins et al. 1980).

Total chlorophyll content of sago is 0.81 mg/g fresh weight (Madsen 1986). Chlorophyll a, b, and carotenoid content of sago have also been reported (Spencer 1986a; Azcon-Bieto et al. 1987; Spencer and Anderson 1987; Spencer and Ksander 1987; Penuelas et al. 1988). Pigment concentrations are probably lower in sago stems and leaves than in several other common submersed angiosperms (Azcon-Bieto et al. 1987; Spencer and Anderson 1987; Penuelas et al. 1988). This could be an adaptation to the low levels of light or high concentrations of forms of carbon other than CO2 that often characterize waters inhabited by sago.

Maximum photosynthetic rate for sago is 2.1 umol O2/mg chlorophyll per minute. This rate is maximum at pH < 7.0 at dissolved inorganic carbon concentrations found in nature, but photosynthesis can continue to pH > 10.0 (Sand-Jensen 1983). Gross (1.2 mg O2/L/h) and net (0.98 mg O2/L/h) primary productivity of a sago community in a eutrophic Wisconsin stream peaked at 1000 h; maximum respiration was 0.65 mg O2/L/h (Madsen et al. 1988) Azcon-Bieto et al. (1987) measured the dark respiration rate of sago leaves (39.4umol O2/g dry weight per hour) and stems (13.3 umol O2/g dry weight per hour) and found it markedly lower than for several other submersed angiosperms, bryophytes, and algae common to temperate climates. This could help explain sago's success in polluted, oxygen-poor environments. Jana and Choudhuri (1979, 1981, 1982a) reported that the dark respiration rate and photorespiration rate of sago is high, but apparent photosynthesis is low. Thus, the authors postulated that sago is not able to compete in tropical or subtropical waters inhabited by plants with less photorespiratory O2 demand.

Madsen (1986) found sago to have a rather modest photosynthetic rate, comparable to other Potamogetons, but much less than for one of sago's most common associates and likely competitor, Myriophyllum spicatum. He concluded that although sago does not exhibit the high photosynthetic rates of species common to warm waters, the plant is successful because it can exploit the environment early in the growing season, before plants with higher rates are at an advantage. He also noted that sago has other morphological and reproductive features that assure the plant success in various aquatic habitats. Westlake (1967) also found sago to have low photosynthetic capacity.

Sago grown from turions in N-deficient medium for 35 days was only slightly paler than control plants, but control plants averaged 10 cm longer (Devlin and Yaklich 1971). During the same experiment, P-deficient plants were similar to controls in both height and dry weight.

Ca deficiency in sago is evidenced by reduced growth; rapid paling of leaves followed by necrotic spotting, curvature, and eventual gelatinous texture; and weakening after 19 days of initial growth from turions. This is attributed to the function of Ca as a [strengthening?] constituent of cell walls in the form of calcium pectate (Devlin et al. 1972).

After 20 days of initial growth, Mg deficiency in sago grown from turions is evidenced by reduced weight and slight chlorosis; this reflects the function of Mg in the chlorophyll molecule (Devlin et al.1972).

Potassium deficiency in sago caused reduced growth, a darker green color, and shortened internodes (Devlin et al. 1972).

Experiments of Peter et al. (1979) documented the upward translocation of copper in sago, from sediments to stem apices and youngest leaves; similar translocation of lead did not occur. Jana and Choudhuri (1979, 1981, 1982a) found that cadmium, Cu, and mercury increased signs of photorespiration in sago, whereas Pb decreased them. Everard and Denny (1985) discovered that sago does not absorb Pb as fast as several other submersed and floating macrophytes; they attributed this to sago's relatively thick cuticle. Marchyulenene et al. (1978) measured uptake of nuclides of Pb and strontium in sago but did not investigate toxic effects.

In young sago plants grown from turions, chlorophyll a and b content peaked when temperatures were held at 30° C, but carotenoids reached maximum values when plants were held at 17° C (Spencer 1986a). Spencer postulated that the protection afforded the chlorophyll by the carotenoids at low temperatures can indicate an adaptation of sago for early growth in cooler water. Spencer and Anderson (1987) found little difference in chlorophyll a or carotenoid content in sago cultured under photoperiods of 10, 12, or 14 h, but these pigments were reduced in plants grown at a 16-h photoperiod.

Heavy flow of carbohydrates from turions to upper parts of the young plants occurs during the first 2-3 weeks of growth (Hodgson 1966). The relative growth rate of sago and root-to-shoot ratio was not significantly affected by photoperiod, but production of turions was enhanced at photoperiods up to 12 h (Spencer and Anderson 1987).

Jana and Choudhuri's (1982b) study of ethylene production in sago revealed that it may be geared to rapid senescence when compared to several other hydrophytes. These authors (1987) also tested the ability of various antioxidants to arrest senescence in sago leaves and found glutathione most effective.

Lakes with an abundance of rooted hydrophytes can contain substances that inhibit the growth of phytoplankton regardless of nutrient supply, illumination, or water temperature (Hogetsu et al.1960), but the source or composition of such substances remain unknown. Substances that increase turion growth and inhibit germination in seeds of other plants can be distilled from sago turions (Yeo 1965), but it is unknown whether these substances function in nature.


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