Van Gogh began painting the 4th version of Sunflowers in late summer 1888 and continued into the following year. One went to decorate his friend Paul Gauguin's bedroom. The paintings show sunflowers in all stages of life, from full bloom to withering. The paintings were considered innovative for their dominant use of yellow hues, partly because newly invented pigments offered Van Gogh the possibility to exploit new vivid and bright tints. He used chrome yellow, a class of compounds usually consisting of lead chromate.
Van Gogh was no scientist, but he knew his art. His sunflowers became so iconic, that Paul Gauguin even depicted Van Gogh creating one of the paintings. What was so special about them? Van Gogh himself told his brother Theo in several missives...
The results showed that the Cr(VI) is reduced to Cr(III) in all root tissues, even at the root surface and epidermis. Castillo and his colleagues understand that this is so because cell wall components and structural biopolymers in plants reduce Cr(VI) and bind Cr(III) decreasing its toxicity and mobility.
The X-ray absorption near-edge spectroscopy data showed that the static chromium is present as Cr(III) , but the scientists couldn't figure out the form of the mobile species that moves the chromium in sunflower tissues.
The root cut-out section reveals the concentration of chromium (red is the highest concentration, blue is the lowest) in the epidermis of the plant and the xylem, especially, after 72 hours of exposing the roots to a solution with Cr(VI). Chromium is bound to oxygen from the components of the cell wall and to lignin rings in the xylems and it moves up to the leaves through the xylems and accumulates in trichomes (leaf hairs).