Natural variation in correlations between cadmium and micronutrients in potato tubers

Highlights

• Variations in Cd, Zn, Fe, Mn, and Cu in tubers of ten potato varieties.

• Relationships among variations in cadmium and the selected micronutrients contents.

• Positive correlation between Cd accumulation in tubers and Zn, Cu or Mn.

• A variety with low Cd accumulation and high Fe accumulation in tubers identified.

Abstract

Tuber cadmium (Cd) accumulation was evaluated in ten potato cultivars grown in the same field with an average soil Cd level of 0.06 mg kg⁻¹ dry weight (DW). Analysis of tuber metal levels revealed a significant difference in mean Cd concentrations among different cultivars, which varied from 0.05 to 0.21 mg kg⁻¹ DW. No cultivars exceeded the maximum permissible concentration (MPC) for Cd in potato. Correlation analysis showed a strong positive relationship between bioaccumulation of Cd and zinc (Zn), copper (Cu) or manganese (Mn). There was also a negative correlation between Cd and iron (Fe) accumulation which was significant only within three cultivars including Summer Delight, Russet Burbank and Agria. Summer Delight, with the lowest Cd and highest Fe content, was identified as a low Cd accumulating cultivar as well as a Fe-enriched tuber vegetable making it of significant interest. Laura, Yukon Gold and Purple Passion were identified as strong Cd accumulators. One of the implications is that growing these three potato cultivars in soils with Cd levels greater than 0.06 mg kg⁻¹ DW might be of human health concern.

Introduction

Cadmium (Cd) is a non-essential and potentially toxic trace metal found in soils. It can arise naturally but its increase in the environment is usually a consequence of anthropogenic activities. Cd is known as a food-chain contaminant which can be easily taken up from soils by the roots of crop plants and translocated to the other organs such as the shoot, leaf and tuber (Li et al., 2012, Reid et al., 2003). Cd can pose adverse effects to human health, even at very low concentrations. Chronic Cd toxicity can significantly affect kidney function, increase the risk of osteoporosis, bone fractures (Itai-Itai disease) and cancer (Chen et al., 2007, Pan et al., 2010). The World Health Organisation (WHO) recommended a tolerable daily Cd intake of 1 μg per kg body weight (World Health Organisation, 1992). Hence, caution is needed with respect to the consumption of staple crops capable of taking up and accumulating relatively high levels of Cd. Of particular significance In terms of dietary Cd exposure from consumption of potatoes (Solanum tuberosum L.), Cd may contribute to more than 50 percent of human dietary Cd intake (Dunbar et al., 2003, Reid et al., 2003).

Previous studies revealed that Cd accumulating ability is cultivar-specific in plants. For example, Cd concentrations varied among different cultivars in rice (Zhang et al., 2014), durum wheat (Cieslinski et al., 1996), barley (Chen et al., 2007), peanut (Su et al., 2013) and Chinese cabbage (Liu et al., 2010). However, our understanding of Cd accumulation variation in Solanaceae is limited and the outcomes of the few studies to date are contradictory. For instance, a significant variation in Cd uptake was observed among different Nicotiana species, but not varieties of the same species (Doroszewska and Berbec, 2004). In potato, Harris et al. (1981) found no correlation between cultivar and Cd accumulation in the tubers grown in a contaminated field but in contrast, McLaughlin et al. (1997) reported considerable differences in tuber Cd concentrations of the 14 most common Australian cultivars. Likewise, Dunbar et al. (2003) found different Cd contents in the tubers of varieties Wilwash and Kennebec. A similar finding was obtained for sixteen potato cultivars grown in Turkey (Ozturk et al., 2011).

In recent years, the concept of cadmium safe crop cultivars (CSCs) and their identification have been discussed (Liu et al., 2010, Zhang et al., 2014, Ashrafzadeh and Leung, 2016). Uptake of Cd in CSCs may also influence the uptake of other trace elements particularly those with implications for food nutritional value. Interaction between the uptake of Cd and other trace elements has been noted previously in plants. Cataldo et al. (1983) found an antagonistic correlation in accumulation of Cd and Fe, Zn, Cu, Mn in soybean suggesting that a common transport site or process might be involved in the uptake of these trace elements. Chen et al. (2007) compared the bioaccumulation patterns of Cd and Fe, Zn, Cu, Mn in barley cultivars concluding that only Mn uptake was correlated significantly with that of Cd. Zhang et al. (2002) found that correlation between uptake and translocation of Cd and some other trace elements was cultivar-dependent in wheat. A synergistic correlation was observed between Cd and Zn uptake in tomato plant (Smith and Brennan, 1983). Likewise, in potato tubers of two Australian cultivars, Ca, Mg, Mn, Fe, Cu and Zn contents showed positive relationships with Cd level (Dunbar et al., 2003). An important part of food composition analysis in evaluation of crop cultivars for the purpose of identifying CSCs should therefore not only include an assessment of bioaccumulation of Cd but also other micronutrients, particularly those known to be beneficial to human health. The aim of the present study was to evaluate the food safety and nutritional status of potato, which is one of the four most consumed food crops in the world, in relation to the bioaccumulation of Cd and four other micronutrients in the tubers of ten different cultivars.