Increased phosphate concentration, higher temperature and addition of glucose all increased the number of fronds and turions of the duckweed Spirodela polyrhiza formed under in vitro conditions. Increasing the number of turions by increasing the plant biomass does not mean that the developmental process (switch of the programme of the primordia from vegetative fronds toward resting turions) has been specifically influenced. The specific turion yield (STY; number of turions formed by one frond) and the time of onset of turion formation have been used as more specific measures of turion induction. At more than 30 µm initial phosphate the STY was increased by lower temperature (15 °C) and became independent of the phosphate concentration. Between 10 and 30 µm and at higher temperatures (25 °C) the STY was increased by lower phosphate levels. The stimulatory effect of lower temperature was more pronounced than that of lower phosphate concentrations. Decreased phosphate concentration highly accelerated the formation of the first turions. The influence of low temperature was small at lower phosphate concentration but became dominant at higher concentrations (especially in autotrophic cultures). Low phosphate levels (e.g. 10 µm) and low temperatures (e.g. 15 °C) both represent specific turion-inducing factors having significant interactive effects. In S. polyrhiza, these signals may replace the interactive effects of photoperiods and low temperature known from other hydrophytes in turion induction under natural conditions.