Environmental and hormonal control of turion formation in Myriophyllum verticillatum

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Abstract
The turions of Myriophyllum verticillatum, an aquatic vascular plant, develop in the fall and function in propagation and dispersal as well as in over-wintering. Experiments with controlled evnironments indicate that both temperature and photoperiod regulate turion formation. Turions can be induced at 15°C or lower, but not at 20°C. At 15°C, turions form in both 8- and 12-hr days, but not in 16-hr days. Plants collected in early spring do not form turions readily in response to short days unless previously exposed to long days; thus, turion formation is a long-day-short-day response. This combination of photoperiod and temperature requirements probably prevents turion development in early spring when the temperature and photoperiod are similar to those in the fall. Treatment of plants with ABA (10−5M) enhances turion development under marginally inductive conditions (12-hr days at 15°C) but cannot induce it under long days. On the other hand, the cytokinin benzyladenine (10−5M) blocks turion formation. GA3 (10−5M) and AMO-1618 (10−5M) exert only small qualitative effects on turion development, while IAA (10−5M) retards it. During turion development, the level of ABAlike activity and of one or two unidentified inhibitors increases. Cytokinin activity decreases at the start of turion formation, increases during development, then decreases at abscission. Thus two lines of evidence suggest that a decrease in cytokinin activity and an increase in acidic inhibitor activity play important roles in turion induction.