Prof. Dr. Paul Struik

Contaminants can be present in potatoes at alarming concentrations. In general, contamination of potato tubers occurs through various pathways. Such pathways include uptake from contaminated soil in which the crop is grown, either directly or through the roots, uptake of contaminated (in)organic fertilizers or polluted irrigation water followed by internal transport of the contaminants, and uptake of atmospheric deposition, either through the canopy, the roots, or the tubers themselves.
Van Gogh’s potato eaters had many worries. Microplastics were not among them. For the 21st-century human they are a growing health concern. Nowadays, microplastics have been found at the bottom of the deep ocean, at the peaks of the Himalaya, in human brains, other parts of our bodies, and mother milk, and yes, also in the potato tubers we eat. How do the microplastics get into the plant? A key pathway appears to be endocytosis, i.e., an energy-dependent process through which cells internalize extracellular materials via vesicle formation. Moreover, transport of microplastics from roots to the shoot can occur through the apoplast, aided by transpiration. Finally, entry through cracks is also possible, and quite often efficient. Microplastics accumulate more in skin than in the internal tissues of the tubers, so washing and peeling helps reduce the contamination to a considerable extent. Cultivars differ greatly in microplastic accumulation, but more research is needed here. How microplastics affect the physiology of the potato plant is still a big question. Research should focus on their effects on photosynthetic efficiency and oxidative stress.

Heavy metals, such as cadmium, lead, and arsenic, are toxic, even at trace concentrations. They can easily accumulate in potato tubers. Contamination with heavy metals in potato often surpasses the health standards of the World Health Organization. Several reports, from Peru and China, indicated that the accumulation of heavy metals strongly depends on altitude, with more accumulation reported for lower altitudes. Irrigation and quality of irrigation water are essential influencing factors. This also applies to fertilizer sources. Soil type, especially its pH, plays a crucial factor in the uptake of heavy metals by the roots. Moreover, modern varieties show stronger accumulation than native varieties, according to the publication in Potato Research, with significant variation among modern varieties as well. Other publications report higher concentrations in native varieties. Some modern varieties can be stronger accumulators of specific heavy metals than other varieties. Microplastics can serve as carriers enhancing the uptake of heavy metals, creating synergy between different types of contaminants.
Concentrations of polycyclic aromatic hydrocarbons (PAHs) can be high in potato, even in organically grown potato crops. Fortunately, not all PAHs are carcinogenic. Tubers produced in urban environments show higher concentrations than tubers produced in rural areas, because of stronger soil contamination, air pollution, and atmospheric depositions in and around cities. Concentrations are significantly higher in skins than in internal tissues: the skin is directly exposed to (the source of) PAHs and there is limited translocation from skin to internal tissues. There can be large differences in concentration among cultivars, and variation in skin characteristics (thickness, permeabi­lity, waxiness) might be responsible for that. Washing, peeling, and boiling reduce PAH contents to a great extent. However, PAH concentrations in processed food can be significantly higher than in raw potatoes. Food processing is therefore an important factor to include in any research on how to reduce PAH contamination.
In short: the Potato Research publication warns for novel contaminants, but proper washing and peeling significantly reduce the problem; breeding for reduced uptake and accumulation of contaminants may also help on the long run. ●