Selenium (Se) is an essential plant micronutrient, but is toxic at high tissue concentrations. It is chemically similar to sulphur (S), an essential plant macronutrient. The interactions between Se and S nutrition were investigated in the model plant Arabidopsis thaliana (L.) Heynh. Arabidopsis plants were grown on agar containing a complete mineral complement and various concentrations of selenate and sulphate. The Se/S concentration ratio in the shoot ([Se](shoot)/[S](shoot)) showed a complex dependence on the ratio of selenate to sulphate concentration in the agar ([Se](agar)/[S](agar)). Increasing [S](agar) increased shoot fresh weight (FW) and [S](shoot), but decreased [Se](shoot). Increasing [Se](agar) increased both [Se](shoot) and [S](shoot), but reduced shoot FW. The reduction in shoot FW in the presence of Se was linearly related to the shoot Se/S concentration ratio. These data suggest (i) that Se and S enter Arabidopsis through multiple transport pathways with contrasting sulphate/selenate selectivities, whose activities vary between plants of contrasting nutritional status, (ii) that rhizosphere sulphate inhibits selenate uptake, (iii) that rhizosphere selenate promotes sulphate uptake, possibly by preventing the reduction in the abundance and/or activity of sulphate transporters by sulphate and/or its metabolites, and (iv) that Se toxicity occurs because Se and S compete for a biochemical process, such as assimilation into amino acids of essential proteins.

Interactions between selenium and sulphur nutrition in Arabidopsis thaliana

Se is an essential element for animals. In man low dietary Se intakes are associated with health disorders including oxidative stress-related conditions, reduced fertility and immune functions and an increased risk of cancers. Although the reference nutrient intakes for adult females and males in the UK are 60 and 75 μg Se/d respectively, dietary Se intakes in the UK have declined from >60 μg Se/d in the 1970s to 35 μg Se/d in the 1990s, with a concomitant decline in human Se status. This decline in Se intake and status has been attributed primarily to the replacement of milling wheat having high levels of grain Se and grown on high-Se soils in North America with UK-sourced wheat having low levels of grain Se and grown on low-Se soils. An immediate solution to low dietary Se intake and status is to enrich UK-grown food crops using Se fertilisers (agronomic biofortification). Such a strategy has been adopted with success in Finland. It may also be possible to enrich food crops in the longer term by selecting or breeding crop varieties with enhanced Se-accumulation characteristics (genetic biofortification). The present paper will review the potential for biofortification of UK food crops with Se.

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Biofortification of UK food crops with selenium

Selenium in agriculture and the environment

Factors Affecting Selenium Accumulation by Agricultural Crops

Solubility and redox criteria for the possible forms of selenium in soils

SummaryElemental selenium and a series of selenites and selenates were applied in pot and field experiments. With the elemental selenium a small increase in concentration in the plants was found in both kinds of experiments with red clover, lucerne, mustard, andsugar beet as test plants, but not with barley. In the second year an increase in the concentration in the plants was found in lucerne only (field experiment). In a pot experiment eight successive cuts of clover all had nearly the same content.All the selenites had the same effect on the concentration in the plants, and the concentration in the eight cuts of clover decreased with time at the same rate for all six selenites irrespective of solubility. The decrease was about a factor of six. In the field the effect of K2SeO3 in the second year was reduced by 50 to 80 per cent.Also the selenates gave the same concentration in plants independently of the solubility. But the concentration was 20–50 times that obtained with selenites, and the decrease in the effect with time was greater. In the eight clover cuts the effect of selenate decreased four times as much as the effect of selenite. In the field the effect of K2SeO4 decreased more from the first to the second year than the effect of BaSeO4.During a two years field experiment with mustard the total uptake as a percentage of the added selenium was 0.01% of Se°, 4% of K2SeO3 and 30% of K2SeO4 and BaSeO4. With lucerne, barley and sugar beet the uptake was one third of this or less.Determinations of water-extractable selenium in profiles from the field in the autumn showed no increase succeeding the addition of K2SeO4 in the spring while the addition of BaSeO4 increased the extractable amount in both autumns. Addition of five times more selenite increased the water-extractable as well as the total soil selenium in the upper 25 cm, and the increase in total Se was also present in the second autumn.

G. Gissel-Nielsen

Papers

The uptake of applied selenium by agricultural plants. 2. The utilization of various selenium compounds.