Specific turion yields of different clones of Spirodela polyrhiza depend on external phosphate thresholds

Turions play an important role in the survival strategy of the duckweed Spirodela polyrhiza. Therefore, factors influencing the formation of these survival organs were studied. Phosphate deficiency is the main natural factor inducing turion formation and the specific turion yield (SY), i.e. the number of turions formed per frond, varied widely for five different clones from different climate zones. The concentrations of phosphate and nitrate in the nutrient media were investigated at the onset of turion formation, with SY ranging from 0.22 to 5.9. Tissue P and N content was also investigated in vegetative fronds at the onset of turion formation and in newly formed turions. The clones were selected to test possible correlations between SY and threshold nutrient concentration for turion formation. Only one correlation, between SY and external phosphate concentration, was significant: clones with high SY started turion formation at higher external phosphate concentrations. Turion formation is thus mainly induced by the external phosphate concentration, below a defined, clone-dependent threshold. We propose the following mechanism: a switch of the developmental programme of frond primordia from vegetative frond to turion formation at a higher phosphate threshold saves more phosphate for turion formation instead of using it for a further vegetative growth. However, the period of growth preceding turion formation does not depend on this threshold value, but rather on the growth rate of the vegetative fronds, which actually produce the phosphate shortage by taking it up from the surrounding medium.