GROWTH OF DUCKWEED AND LABELLING IT WITH 15N FOR USE IN ISOTOPE DILUTION STUDIES

Abstract

Two consecutive laboratory experiments, each lasted for about two weeks, were designed to investigate the optimum concentrations of macro- and micro-nutrients in a synthetic growth medium for the growth of duckweed and whether the growth would respond differently to different N sources at the same N concentration. Results were used as a basis for selecting a medium for producing ¹⁵N-labelled duckweed as substrate in ¹⁵N studies. In the first experiment, two duckweed (Spirodela punctata) strains (‘big fronds’ and ‘small fronds’) were grown on a medium containing MgSO₄.7H₂O, Ca(NO₃)₂.4H₂O, KH₂PO₄, KNO₃, micronutrients and Fe-EDTA at different concentrations (either ‘1.0’, ‘0.5’ or ‘0.25’ strength). In the second experiment, the growth was tested on the ‘0.25 strength’ medium in which the KNO₃ was replaced with either CO(NH₂)₂ or (NH4)₂Cl providing approximately 25 mg N/l. The growth of duckweed was measured from the appearance of new fronds. The exponential equation and time (d) taken for the fronds to double in number (doubling time) were obtained. At the end of experiment, the strength of the medium did not effect the growth of the ‘big fronds’ strain, but in the ‘small fronds’ strain the ‘0.25 strength’ solution produced more fronds than that of ‘1.0 strength’. At the same medium concentration, the ‘big fronds’ strain produced more fronds than the ‘small fronds’ one. The N source at 0,25 mg N/l did not affect the growth of either duckweed strains. The ‘0.25 strength’ solution was finally adopted as the growth medium to grow and label duckweed with ¹⁵N, with a typical ¹⁵N abundance of the duckweed produced was 2 – 3 %.