Cationic interactions in radiocaesium uptake from solution by spinach

Interionic effects on radiocaesium uptake in spinach (Spinacia oleracea L, cv. Subito) were measured in a solution culture experiment. In total, 15 different nutrient solutions, spiked with137Cs, were prepared in which K, NH4, Ca and Mg concentrations were varied at four different total salt concentrations between 5.3meql−1and 21.2meql−1. The range of cationic concentrations studied was: K, 0.53 to 10.4 mmoll−1; NH4, 0 to 8.47 mmoll−1; Ca, 0.15 to 5.0 mmoll−1; Mg, 0.08 to 2.0 mmoll−1. The plant/solution137Cs transfer factor (TF) varied between 41 lkg−1 and 117 lkg−1. Radiocaesium levels in the plants were poorly, but positively, correlated with the K concentration in solution. However, radiocaesium levels were significantly reduced by increasing Ca + Mg concentrations in solution. Between 10.06 and 1.06moll−1 K, radiocaesium levels in 20-day-old plants were only slightly affected if K was replaced by NH4 at almost constant Ca + Mg concentrations. The negative effect of Ca + Mg concentrations on137Cs uptake is interpreted from their effect on137Cs loading in the apoplast of the root cortex. A quantitative relationship is presented between the137Cs TF and the fractional loading of137Cs in the apoplast, which is calculated by ion exchange laws. This relationship is linear (R2 = 0.81) for the 15 different solutions studied.