Production and survivorship of the functional stolons of giant cutgrass, Zizaniopsis miliacea (Poaceae).

Giant cutgrass [Zizaniopsis miliacea], a tall emergent grass native to the southeastern United States, was studied in two Florida lakes. In Lake Seminole (15 176 ha) giant cutgrass forms large expanding stands, but in Lake Alice (9 ha) it is confined to a stable narrow fringe. By monitoring individual plants in Lake Seminole, it was found that an average decumbent flowering stem produced three flowers and ten nodes, 80% of which became rooted in the substrate. Such flowering stem development could potentially result in stand expansion of 2.2-2.7 m/yr, depending upon water levels and rates of node rooting. Once flowering stems became decumbent in Lake Alice, they typically broke, producing no more than two flowers with four nodes in a growing season. While still attached to the parent plant, few of these nodes were able to become rooted in the substrate, limiting the rate of stand expansion in Lake Alice. Sections of flowering stems bearing axillary shoots that were detached from the parent plant and free-floating could become rooted on reaching shallow water and produce robust, new, flowering plants. This interesting mode of population dispersal and spread has important implications for the distribution and management of giant cutgrass.

Comparison of the photosynthetic characteristics of three submersed aquatic plants.

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.