Foliar uptake of radiocaesium from irrigation water by paddy rice (Oryza sativa): an overlooked pathway in contaminated environments

Abstract

Flooded (paddy) rice (Oryza sativa) can take up ions from the irrigation water by foliar uptake via the exposed stem base. We hypothesised that the stem base uptake of radiocaesium (RCs) is a pathway for rice grown in RCs-contaminated environments. We developed a bi-compartmental device which discriminates the stem base from root RCs uptake from solutions, thereby using RCs isotopes ( Cs and Cs) with< 2% solution leak between the compartments. Radiocaesium uptake was linear over time (0-24 h). Radiocaesium uptake to the entire plant, expressed per dry weight of the exposed parts, was sixfold higher for the roots than for the exposed stem base. At equal RCs concentrations in both compartments, the exposed stem base and root uptake contributed almost equally to the total shoot RCs concentrations. Reducing potassium supply to the roots not only increased the root RCs uptake but also increased RCs uptake by the stem base. This study was the first to experimentally demonstrate active and internally regulated RCs uptake by the stem base of rice. Scenario calculations for the Fukushima-affected area predict that RCs in irrigation water could be an important source of RCs in rice as indirectly suggested from field data.

Characterization of rice KT/HAK/KUP potassium transporters and K⁺ uptake by HAK1 from <i>Oryza sativa</i>

“…Recently, OsHAK1 has been indicated to be the main pathway to absorb cesium (Rai et al 2017). Radiocesium uptake activity by the stem base was detected in flooded rice and reducing the K + supply to the roots increased the uptake activity by the stem base (Uematsu et al 2017). Since OsHAK1 expression is induced in shoots under the K + starved condition ( Figure 1A), the foliar K + uptake activity of OsHAK1 might be important for the plants to adapt to changing environmental conditions.…”

“…The expression of OsHAK1 was induced not only in the roots but also in the shoots (Figure 1). Recent reports showed that cesium uptake by rice roots largely depends upon OsHAK1 (Rai et al 2017) and flooded rice can take up radiocesium by foliar uptake via the exposed stem base (Uematsu et al 2017). Taking those reports into consideration, we tested the K + uptake activity in shoots using leaf blades of oshak1 mutants and wild-type plants.…”