The arsenic now spreads to the so-called mesophyll, which makes up most of the leaf. "This means that the capacity of the epidermis is exhausted and the plant can no longer get rid of the toxin, and that's when things start to get serious," says Küpper. Only when the concentration rises to five micromols per litre, a level that the plants are unable to withstand for prolonged periods, is arsenic also found in the vacuole and thus more or less throughout the entire cell. "Surprisingly enough, we found that arsenic initially accumulates in the cell nuclei," Küpper reports. The plant first deposits the toxin in its outer layer, the epidermis. "This allowed us to localise the arsenic more precisely within the cell – after all, it makes a difference whether it is in the cell wall, for example, or in the vacuole."Ī concentration of one micromol of arsenic per litre of water is still tolerated by the plant. "Thanks to PETRA III we could peer into individual cells of the plant for the first time," reports Küpper. The scientists exposed the plant under investigation to arsenic concentrations between one and five micromols per litre, and then shone a narrowly focused X-ray beam from PETRA III through the leaves. Credit: Mishra et al., adapted from JExpBot CC-BY-3.0 /licenses/by/3.0/Īccording to the researchers, the rigid hornwort is a kind of indicator plant for metals, and experiments with this species can mostly be transferred to other species as well. At low concentrations, arsenic accumulates predominantly in the nuclei of epidermal cells, while at high concentrations, it floods the whole cell. "We wanted to know what happens at ecologically and physiologically relevant concentrations." Up to 33 micromols arsenic per litre were reported in contaminated areas in irrigation water and soil solution.ĭistribution of arsenic within leafs of rigid hornwort at concentrations of 1 micromol (75 micrograms, top) arsenic per litre water and 5 micromols (375 micrograms, bottom) arsenic per litre water. "Whereas concentrations of just one micromol, corresponding to 75 micrograms, per litre are already relevant in terms of plant physiology, it was not unusual to work with concentrations of up to 250 micromols per litre – concentrations at which completely different things are going on," explains the biologist. The results could help researchers to breed plants that absorb less arsenic.Īccording to Küpper, previous research on plants has usually been carried out with arsenic concentrations that were far too high. "By carrying out our analysis, we wanted to find out exactly how arsenic poisoning occurs in plants," adds co-author Gerald Falkenberg, who is in charge of the P06 beamline at DESY, where the experiments were performed. "On top of this, humans eat plants, of course, and feed them to their livestock, so that arsenic accumulates in these and eventually ends up in human beings," explains Küpper. It is taken up by the same transport mechanism as phosphorus, an element that is essential to plants, and even at levels far beneath the lethal concentration, it impairs plant growth and therefore reduces the size of crops. In humans, arsenic can cause cancer, necrosis, or acute renal and circulatory failure. Although arsenic is a natural component of soils there, well drilling and other human impacts on the ground have in recent decades mobilised it, leaching it into the drinking water. The concentration of arsenic in the soil is increasing as a result of human activities, and in many countries – especially on the Indian subcontinent – the concentration of arsenic in the groundwater has become a problem. Arsenic is highly toxic and poses a growing environmental and health problem all over the world.