Growth and biomass partitioning in zinc‐toxic bush beans

Bush bean plants (Phaseolus vulgaris L.) were grown during the exponential growth phase in nutrient solution containing 2, 6.6, 9.6 or 13.5 μM Zn. At six different harvest intervals samples were taken for dry weight and Zn content determinations. The primary growth data were fitted to exponential curves, from which the relative growth rates (RGR) and the net assimilation rates (NAR) were derived. With the RGR data the biomass partitioning coefficients of the different organs were calculated. Zinc toxicity generally decreased both the NAR and the growth of all organs, and increased their Zn contents. The relative root expansion rates (RER) and the relative root dry weight growth rates were the most affected parameters, while the leaf expansion rates and the stem dry weight growth rates were hardly affected. The biomass partitioning coefficients indicate that after long‐term exposure to toxic Zn levels the root growth is more affected than the photosynthetate production. This leads to a relative increase of the shoot growth, which may be due to either the decrease of the sink force of the roots or the Zn‐induced decrease of assimilate translation