Occurrence of nutrients and plant hormones (cytokinins and IAA) in the water fern Salvinia molesta during growth and composting

The aquatic fern Salvinia molesta is classified as an invasive, noxious weed in southern Africa. It has rapid growth and forms a thick mat in slow moving water bodies. It is hypothesized that S. molesta is a suitable candidate to promote as an organic fertilizer. The aims of this study were to determine the mineral content as well as the cytokinin and auxin content in S. molesta. The release of these compounds into the culture medium and upon decomposition of the air-dried plant matter was also determined. Minerals detected in the fresh plant material were N > K > P > Ca > Mg > Fe > Na > Cu > Zn while Cd, Pb and Ni were below the level of detection. The mineral concentrations changed after 14 days decomposition with there being an increase in Ca, Mg and Fe and a decrease in K. Sixteen isoprenoid cytokinins were detected in the plant material (total concentration 81.62 pmol g(-1) DW) while only four cytokinins occurred at a greatly reduced concentration in the culture medium (4.31 pmol 500 ml(-1) medium). The material that had been composted for 14 days showed a 17% reduction in cytokinin concentration (67.96 pmol g(-1) DW) compared to the plant material. Run-off, termed leachate, collected during the composting experiment also had high amounts of cytokinin-like activity measured using the soybean callus bioassay. This activity peaked on day 7 of the composting experiment, indicating a rapid release of cytokinin-like compounds. IAA occurred in much higher concentrations than the cytokinins in the plant material (393.6 pmol g(-1) DW) while very little IAA was detected in the culture medium (30.8 pmol g(-1) DW). IAA concentrations were much lower in the composted material (148.1 pmol g(-1) DW) with a 62% loss being recorded compared to the plant material. No auxin-like activity was measured using the mungbean rooting bioassay in the leachate collected at the start of the composting experiment. However, the leachate collected on days 7 and 14 had biological activity indicating that auxin-like compounds were released from S. molesta upon decomposition. (C) 2007 Elsevier B.V. All rights reserved.