Management impacts on the quiescence and sprouting of subterranean turions of dioecious hydrilla (Hydrilla verticillata (L.f.) Royle)

A greater understanding of the factors influencing sprouting and the population dynamics of subterranean turions (tubers) of hydrilla is critical to developing improved management programs for this invasive, exotic aquatic plant. Vertical distribution of hydrilla tubers in lake hydrosoils was determined; however, subsequent studies indicated that sprouting was not influenced by location of tubers in the sediment. Mesocosm studies (900 L tanks) indicated that simply removing the vegetative canopy of hydrilla does not impact tuber sprouting; however, control methods that kill the root system increased sprouting rates by 20 to 48 percent (independent of tuber age). Tuber sprouting was much greater in sand than in organic or loam sediments following treatment. Changes that occur in the microenvironment where roots and tubers are closely associated, likely stimulated sprouting in mesocosm studies. Studies of tuber population dynamics over a 30-month period in research ponds in North Florida showed no difference in sprouting between untreated control ponds and treated (vegetation removed) ponds. Sprouting rates generally remained below 3 percent, with peaks (5--7%) noted in the fall. Limited tuber production in untreated systems was attributed to reduced rootcrown density (loci for tuber production) due to intraspecific competition. When management was stopped at 27 months, tubers were replenished to near pretreatment densities within 3 months. Laboratory studies show that once a tuber is disturbed following its removal from the sediment, the likelihood of sprouting increases linearly with time through 48 hrs. Use of disturbed tubers in laboratory studies may confound results depending on the length of time the tuber has been removed from the sediment. Laboratory evaluations suggested that exogenous application of abscisic acid at concentrations as low as 0.05 to 1.0 muM strongly inhibited tuber sprouting under both aerobic and anoxic conditions; however, this effect was partially overcome by addition of GA3 (15--150 muM). Inhibitors of ethylene action and synthesis, as well as ethanol did not impact tuber sprouting at physiological concentrations. Carbon dioxide at concentrations of 1 to 14 atm also inhibited tuber sprouting. Results suggest that drawdowns remain the only management tool currently available that will significantly stimulate sprouting of hydrilla tubers.