Physiological and toxic effects of selenium on seed germination of cowpea seedlings

ABSTRACT Selenium (Se) is considered a beneficial chemical element for plants, but in high concentrations it may present symptoms of toxicity. The present study aimed to evaluate 11 concentrations of Se (0; 0.1; 0.5; 1; 5; 10; 20; 40; 80; 400; 800 mg.L-1) to determine the low and high (toxicity) critical levels to seed germination of cowpea (Vigna unguiculata). In addition, alterations in the rate of photosynthetic pigments, lipid peroxidation and sugars during the initial growth development of seedlings were analysed. Seeds exposed to 800 mg.L-1 of Se showed a decrease of 20% of seed germination index compared to the control treatment. The decrease in seedling growth reflected in the increase of total sugars and sucrose concentration in both the shoot and root in response to exposure to Se concentration. There was a decrease in the concentration of leaf chlorophyll, carotenoids and pheophytin from seedlings exposed to high Se concentration. The rate of lipid peroxidation and the hydrogen peroxide concentration in the shoot was reduced up to the concentration of 1 mg.L-1 with subsequent increase in response to Se concentration applied. In the roots, the lipid peroxidation rate increased at concentrations higher than 80 mg.L-1. The highest oxidation rate of the cellular lipid membrane in response to Se occurred in the shoot, due to oxidation reactions in the chloroplast. Degradation of photosynthetic pigments and accumulation of total sugars and sucrose can be considered efficient biomarkers to indicate the toxicity of Se in cowpea seedlings and probably in other crops.摘要 硒（Selenium, Se）被视为对植物有益的化学元素，但浓度过高时可能引发中毒症状。本研究设置11个硒浓度梯度（0、0.1、0.5、1、5、10、20、40、80、400、800 mg·L⁻¹），旨在确定豇豆（Vigna unguiculata）种子萌发所需的硒低限临界浓度与高限毒性临界浓度。此外，本研究还分析了幼苗初始生长发育过程中光合色素、脂质过氧化水平及糖类物质的变化情况。与对照组相比，暴露于800 mg·L⁻¹硒环境的种子，其种子萌发指数下降了20%。幼苗生长受抑的现象，对应着植株地上部与根系中总糖及蔗糖浓度随硒暴露浓度升高而上升的变化。当幼苗暴露于高浓度硒环境时，其叶片叶绿素、类胡萝卜素及脱镁叶绿素的浓度均出现下降。地上部的脂质过氧化速率与过氧化氢浓度在1 mg·L⁻¹硒浓度以下时呈下降趋势，之后则随施硒浓度升高而上升。而在根系中，当硒浓度高于80 mg·L⁻¹时，脂质过氧化速率会升高。受硒胁迫时，细胞脂质膜氧化速率最高的部位为地上部，这源于叶绿体中的氧化反应。光合色素降解以及总糖与蔗糖的积累，可作为有效生物标志物，用以指示豇豆幼苗（或可能适用于其他作物）所受到的硒毒性胁迫。