Ultrastructural localisation of chromium in Ocimum basilicum

Ocimum basilicum L. seedlings were exposed to different concentrations of Cr (0-8 mg L^(-1) of to Cr^(3+) evaluate the cellular localisation of chromium, plant accumulating capacity, and potential cellular defence mechanisms against Cr stress. Dried plant tissues (roots and shoots) were exposed to X-ray microanalysis for element determination (including Cr, Cu, Zn, Co, Ni, and Mo). Results showed that the highest amount of Cr had accumulated in the roots of the plants treated with 8 mg L^(-1) of Cr^(3+), while less Cr was transported to the shoots. TEM micrographs of Ocimum basilicum root cortex cells exposed to 4, 6, and 8 mg L^(-1) of Cr^(3+) revealed dense granular metal deposits in the periplasmic zone along the cell walls; such deposits were not observed in leaf mesophyll cells. Root cortical cells of the control seedlings grown in the absence of Cr^(3+) exposures were void of such granular metal deposits. While Cr was the predominant element, lower amounts of Cu, Zn, Co, Ni, and Mo were also detected in the chromium bearing deposits analysed by X-ray microanalysis. Formation of Cr bearig deposits in the root cells of Ocimum basilicum may have the effect of maintaining relatively low cytoplasmic concentration of the element and possibly reduce the toxic effects of chromium on cellular metabolism as a detoxification mechanism.