Root structural aspects associated with copper toxicity in oregano ( Origanum vulgare subsp. hirtum)

The effects of toxic Cu on the roots of oregano comprised a dramatic decrease of their volume and length as well as extensive malformations (local swellings). Roots cut at the swellings revealed a destroyed epidermis and a cortex of large cells with folded walls. Cortical cells did not ultra-structurally exhibit any signs of disintegration, as observed in other species. The most prominent cytological alteration of the cortical cells was a metamorphosis of the amyloplasts into leucoplasts containing one or two dark globular inclusions and many small droplets of an electron translucent substance. In the root vascular cylinder, the diameter of the xylem vessels increased by 52.6% with no increase of the wall thickness. Sieve tubes did not undergo any remarkable change in diameter, but their wall thickness increased by 66.7%. Atomic absorption analyses of roots grown in soils with increasing concentrations of Cu showed that roots contained higher amounts of Cu than stems, leaves and inflorescences for all soil Cu-contents. Soil Cu was further found to affect positively the bio-accumulation in roots of Cu, Fe, and Mn and negatively for Zn, K and Ca.