Cold tolerance of the axillary turions of two biotypes of Hydrilla and northern watermilfoil

A two-step assay was developed to test the cold tolerance ranges of axillary turions of dioecious and monoecious Hydrilla (Hydrilla verticillata [L.F] Royle) and Myriophyllum sibiricum (Kamarov). In phase one, cold treatments of 0, 28, 63, and 105 d duration were used to test overwintering effects on mortality rates. Phase two consisted of a test of survivorship over 10 weeks in treatments simulating early growing season. These early growing season treatments included rapid and slow temperature increases and long and short daylengths. Survivorship, shoot dry weight, branch number, and root development were measured to determine responses to different temperature increases and daylengths. As overwintering period (phase I) increased from 0 to 105 d, mortality increased from 8% to 98% for dioecious Hydrilla and 0% to 48% for monoecious Hydrilla. M. sibiricum had low percent mortality across all overwintering treatments (0 to 105 d) and no deaths at 0 and 105 d. Survivorship decreased during the early growing season (phase II) for both Hydrilla biotypes and was affected by overwintering period. No dioecious Hydrilla turions survived the early growing season after overwintering periods of 63 and 105 d. Monoecious Hydrilla turions survival ranged from 67% (63 d) to 42% (105 d). M. sibiricum had variable rates of survival in the early growing season after all overwintering treatments, ranging from a minimum of 8% at 0, 28, and 105 d, to a maximum of 67% at 63 d. Shoot dry weight and branching increased with more rapid changes in early growing season temperatures. Using a two-step assay to determine cold tolerance suggests that monoecious Hydrilla will likely overwinter in northern latitudes more successfully than dioecious Hydrilla.