Hydrilla

Abstract: Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royle grown on 40 different sediments from 17 geographically widespread North American lakes indicated 10— and 20—fold declines in growth with increasing sediment organic matter up to a concentration of °20% dry sediment mass. Poor growth was obtained also on inorganic sediments with a sand fraction exceeding 75% dry sediment mass. Diminished growth on organic sediments occurred at low values of sediment density, and on sands, at high values of sediment density. Differential centrifugation of organic sediment, facilitating an increase in sediment density with no change in organic matter content, stimulated Hydrilla growth, indicating that sediment density rather than organic matter content was most influential in regulating growth. Macrophyte growth and nutrient mass accumulation in shoots were closely correlated, but essentially unrelated to concentrations of nutrients in shoots. The magnitude (r value) and statistical significance of correlations between nutrients in macrophyte shoots and sediments varied appreciably, depending on the form of shoot nutrient data (concentration or accumulation), and the type (interstitial water or total), and basis (mass or volume) of sediment nutrient data. Growth and nutrient accumulation were highly correlated with sediment nutrient concentrations based on volume, indicating interrelationships among growth, nutrition, and sediment density. Additions of P and Fe in combination to organic sediments with N supplied in the overlying solution resulted in significant growth increases in Hydrilla, suggesting that diminished growth on unfavorable sediments was caused by multiple nutrient limitation. Nutrient uptake on low—density organic sediments was apparently limited by long diffusion distances. Limited rates of nutrient diffusion and exchange in coarse—textured sediments, in addition to low nutrient status, may have contributed to their poor ability to support macrophyte growth. Thus, mechanisms of growth limitation on both sands and organic sediments appear to involve nutrition.

Abstract: Light- and CO(2)-saturated photosynthetic rates of the submersed aquatic plants Hydrilla verticillata, Ceratophyllum demersum, and Myriophyllum spicatum were 50 to 60 mumol O(2)/mg Chl.hr at 30 C. At air levels of CO(2), the rates were less than 5% of those achieved by terrestrial C(3) plants. The low photosynthetic rates correlated with low activities of the carboxylation enzymes. In each species, ribulose 1,5-diphosphate carboxylase was the predominant carboxylation enzyme. The apparent K(m)(CO(2)) values for photosynthesis were 150 to 170 mum at pH 4, and 75 to 95 mum at pH 8. The K(m)(CO(2)) of Hydrilla ribulose 1,5-diphosphate carboxylase was 45 mum at pH 8. Optimum temperatures for the photosynthesis of Hydrilla, Myriophyllum, and Ceratophyllum were 36.5, 35.0, and 28.5 C, respectively. The apparent ability of each species to use HCO(3) (-) ions for photosynthesis was similar, but at saturating free CO(2) levels, there was no indication of HCO(3) (-) use. Increasing the pH from 3.1 to 9.2 affected the photosynthetic rate indirectly, by decreasing the free CO(2). With saturating free CO(2) (0.5 mm), the maximum photosynthetic rates were similar at pH 4 and 8. Carbonic anhydrase activity, although much lower than in terrestrial C(3) plants, was still in excess of that required to support HCO(3) (-) utilization.Hydrilla and Ceratophyllum had CO(2) compensation points of 44 and 41 mul/l, respectively, whereas the value for Myriophyllum was 19. Relatively high CO(2) compensation points under 1% O(2) indicated that some "dark" respiration occurred in the light. The inhibition of photosynthesis by O(2) was less than with terrestrial C(3) plants. Glycolate oxidase activity was 12.3 to 27.5 mumol O(2)/mg Chl.hr, as compared to 78.4 for spinach. Light saturation of photosynthesis occurred at 600 to 700 mueinsteins/m(2).sec in each species grown under full sunlight. Hydrilla had the lowest light compensation point, and required the least irradiance to achieve the half-maximal photosynthetic rate.Field measurements in a Hydrilla mat indicated that in the afternoon, free CO(2) dropped to zero, and O(2) rose to over 200% air saturation. Most photosynthetic activity occurred in the morning when the free CO(2) was highest and O(2) and solar radiation lowest. The low light requirement of Hydrilla probably provides a competitive advantage under these field conditions.

Abstract: Growth rates of 10 aquatic macrophytes in various salinities under greenhouse conditions varied widely. Salt concentrations of 1.66'S and 2.50%, were toxic to Pistia stratiotes L. and Eichlornia crassipes (Mart.) Solms, respectively. Salinities of 16.65Sc, or higher were toxic to Lemna minor L., but growth of Lemna was increased by salt concentra- tions of 0.83?, 1.66S(, 2.506/c, and 3.33%, as compared to other Lemna plants grown in fertilized pondwater. Other species studied, Hydrilla v'erticillata Royle, Myriophlyllumn spica- tum L., Najas quadalupensis (Spreng.) Magnus, Vallisneria amnericana Michx., Azolla caro- liniana Willd., and Salvinia rotundifolia Willd., gradually declined in growth as salinity in- creased. Transpiration of the emersed growth form of Myriophyllumn brasiliense Camb. decreased with increasing levels of salinity, but root growth was stimulated by salt concen- trations of 0.83(,-3.33%,, presumably a response of the plant to overcome an internal water deficit resulting from the saline solutions.