Growth and morphology of two U.S. hydrilla biotypes grown under 10- and 16-h photoperiods in outdoor aquaria were investigated. The dioecious hydrilla biotype elongated extensively under both photoperiods and reached the water surface within 2 to 3 weeks after planting. In contrast, the monoecious biotype grew more densely near the sediment, primarily by producing a high number of horizontal stems, root crowns, and higher shoot densities. Monoecious hydrilla grown from tubers produced new tubers after 4 weeks of exposure to the 10-h photoperiod, and the number of tubers produced was five- to sevenfold higher in the monoecious than in the dioecious biotype. Furthermore, monoecious hydrilla produced tubers under both 10- and 16-h photoperiods, with tuber production much higher under the shorter photoperiod. Dioecious hydrilla grown from tubers under similar conditions produced new tubers after 8 weeks only under the 10-h photoperiod. No tubers were produced by the dioecious biotype throughout a 10-week exposure to the 16-h photoperiod.
