Uptake of radiocesium in paddy rice via the stem: Did we overlook a pathway?

The Fukushima nuclear power plant accident on March 2011 caused a large-scale soil contamination with ¹³⁴Cs and ¹³⁷Cs. Paddy rice (Oryza sativa L.) is a dominant staple crop in Japan and is grown in flooded conditions. The lower part of the stem of rice is in contact with the surface water, which may contain radiocesium (rCs). The soil-to-plant pathway of rCs transfer via root uptake has been regarded as a dominant pathway. In contrast, little is known about the rCs uptake from the surface water via the stem. The objective of this study is to measure the relative contribution of rCs uptake by rice via the stem to that via the root. 21-day-old hydroponically grown rice plants, were used for the rCs uptake studies. The root was soaked in the nutrient solution labelled with ¹³⁷Cs, while the stem was submerged with another solution labelled with ¹³⁴Cs. After 24 h of contact, the plants were divided into shoots and roots and the ¹³⁴Cs and ¹³⁷Cs concentrations in both plant parts were determined by a Ge-detector. The uptake rate to the shoots via the stem during 1 day was calculated (as ¹³⁴Cs in shoot (Bq kg^-1) / ¹³⁴Cs in solution (Bq L^-1) per dry weight of stem (kg)) and that via the root (as ¹³⁷Cs in shoot (Bq kg^-1) / ¹³⁷Cs in solution (Bq L^-1) per dry weight of root (kg)). The root uptake rate to the shoot showed its highest rate (1.60 × 10⁶ L kg^-1 d^-1 kg^-1) when the K concentration was 0.025 mM. This rate significantly decreased to 3.69 × 10⁵ L kg^-1 d^-1 kg^-1 when the K concentration was 2.5 mM. In contrast, the stem uptake rate of rCs to the shoot was not affected by different K concentrations in the root solution (P > 0.05, ANOVA), and was on average 1.57 × 10⁶ L kg^-1 d^-1 kg^-1. Further analysis of the results will contribute to identifying the cases in the field where the rCs uptake via the stem will be a dominant uptake pathway to rice compared to the root uptake.