Heavy metal toxicity and metal pair interactions to Lemna minor (Common duckweed)

Abstract

A search of the literature for evidence concerning theeffects of pH on duckweeds has revealed little information.One-third strength Hutner's medium which was reported mostextensively in the literature for culturing duckweeds waschosen to investigate the effects of both daily regulated andbuffered pH action on the growth of Lemna minor. Daily frondcounts were made to detect their growth responses and theresults were evaluatod as percentage frond number increase perday.Hesults on daily regulated media confirmed that Lemnaminor did not favor alkaline conditions for growth.Consequently, stoadily increasing growth rates were observedbetween pH 4.0-8.0 .Growth was most successful 011 bufferedmedia at pH levels 6.3-8.0 and Lemna minor had an optimum atabout pH 6.8. Nutrient renewal of buffered media in the middleof the test period rosulted in highor growth rates whencompared with thH non-renewed media.In Ule second part of Ulis study, Lemna minor wassuggested as a promising indicator of aquatic toxicity tests.The effects of cadmium, zinc, cobalt, copper and the coexistingmetals such as c8.dmium-zinc, cadmium-copper, copper-cobalt,zinc-cobalt and z.inc-copper 011 the growth and accumulationefficiency of Lemn3 minor were searched on Jacob's media.efficiency of Lernna minor were searched on Jacob's rncc!i.:1.Relative growth rate was defined as the percentage of freshweight increase per day and the accumulation of the metalswere reported in terms of metal incorporated into the frond ondry weight basis. The uptake of metals were determined byAtomic Absorption Spectroscopy after a suitable digestionprocess.Among the metals tested, cadmium was found to be themost toxic to Lemna minor. Depending on the types of metalpairs tested they showed differing inhibitory and stimulatoryeffects on the uptake yield by the plant. The presence of copperin the growth medium decreased the uptake of cobalt andcadmium in their separate coexistence. Therefore, antagonismappeared when copper was combined with cobalt and cadmiumat equal concentration levels in different treatment sets.However, copper stimulated the uptake of zinc indicaiing thatthere were synernistic effects when the two were combined.Zinc depressed the uptake of cobalt thus an antagonism wasevident. Cadmium increased the uptake of zinc and this resultwas a syneigistic type effect.