PEG-simulated drought stress decreases cadmium accumulation in castor bean by altering root morphology

This study aimed to test the hypothesis that root morphology and transpiration may have a crucial role in drought-induced change in cadmium (Cd) uptake and accumulation in plants. The biomass, Cd accumulation, spectral reflectance, gas exchange, and root morphology of castor bean were determined under different polyethylene glycols (PEG) concentrations (0%, 2.5%, 5%, and 10%) along with 0.2 μM CdCl2 through a hydroponic experiment. All tested morpho-physiological parameters of the castor bean were unaffected upon exposure to 0.2 μM CdCl2. PEG-induced drought stress repressed root growth, inhibited leaf gas-exchange, and reduced Cd uptake and accumulation in plants, but increased the photosynthetic pigment contents, as indicated by spectral reflectance indices. Cd accumulation in the roots and shoots, as well as the total Cd in the castor bean plants, positively correlated with plant growth, root/shoot ratio, total root length, surface area, root volume, root tips, and root length with the diameter classes of 0-0.2 mm, 0.2-0.4 mm, and 0.4-0.6 mm. No significant correlation was found between Cd accumulation and transpiration rate. Therefore, the drought-induced reduction of Cd uptake in castor plants may have resulted from the inhibition of root growth and alteration of root morphology, rather than from the decreased transpiration.