Effect of drought stress or soil pH on cesium accumulation in Napier grass

Radio-cesium (Cs) decontamination efficiency by plants is supposedly affected by environmental conditions. To elucidate the factors influencing the unstable ¹³⁷Cs-decontamination efficiency, we first examined the influence of drought or soil pH on Cs accumulation using cesium-133 (¹³³Cs) using Napier grass (Pennisetum purpureum Schum.) grown under pot conditions. The experiment was performed on three different conditions with 150 µM ¹³³Cs applied to soil: low pH (L-pH, pH = 5.6 ± 0.0), near-neutral pH (N-pH, pH = 6.6 ± 0.1), and the combination of low pH and drought stress (Drought). Drought stress had prominent negative effects on plant height, aboveground dry matter weight (DW_above), ¹³³Cs concentrations in aboveground or root (Cs_above or Cs_root) parts, or ¹³³Cs total content in the part aboveground (Cs-T). ¹³³Cs concentration and total content in Drought conditions were reduced by 44.1% and 60.0% aboveground and 63.6% and 66.0% in root parts compared with counterpart normal soil moisture conditions (L-pH), respectively. Conversely, there were no significant effects of soil pH on Cs_above, Cs_root, or Cs-T in L-pH and N-pH conditions. Chlorophyll fluorescence parameters (F_v/F_m, F_v/F₀) and the temperature in leaves were negatively affected by drought stress or soil pH conditions. From our results, drought strongly influenced plant growth and Cs accumulation in plants compared with soil acidity. Therefore, soil moisture appears to be a significant factor in maintaining ¹³⁷Cs-decontamination efficiency by Napier grass.