Effects of praseodymium on root growth and morphological characteristics of rice seedlings under cadmium stress
Rare earth elements (REEs) have positive effects on the growth and development of plants and have been used as a fertil-izer additive for decades in China. In order to explore the ameliorating effects of praseodymium (Pr) on the toxicity of cadmium (Cd) to rice roots, root fresh weight, morphological characteristics and root activity of rice seedlings were studied with roots exposed to different Pr concentration and 50 μmol·L~(-1) Cd by hydroponic culture. The results showed that root growth was inhibited significantly by 50 μmol·L~(-1) Cd treatment. Fresh weight, total root length, root surface-area, root volume and root activity were decreased obvi-ously. And the inhibition becomes more serious with prolongation of treatment time. However, no marked change was noted for root average diameter. The inhibition of root fresh weight, root length, root surface-area, root volume and root activity were alleviated by the treatment of 23～184 μmol·L~(-1) Pr. And the alleviation of Pr on Cd toxicity was more evident with prolongation of treatment time. After 5 d' s Pr treatment, root fresh weight, root length, root surface-area, root volume and root activity increased by 0.92%～14.8%, 1.61%～16.3%, 2.57%～20.3%, 1.52%～17.2% and 3.71%～32.8%, respectively, and those increased by 2.55%～31.2%, 1.44%～21.4%, 1.78%～27.2%, 2.16%～23.4% and 8.14%～52.9%, respectively, after 10 d' s Pr treatment. Roots length and root surface-area with root diameter (D)≤1.0 mm and root volume with root diameter 0.5<D≤ 1.0 mm were significantly affected by 23～184 μmol·L~(-1) Pr, which improved by 0.9 %～24.3 %, 1.9 %～32.6 % and 0.4 %～33.2 %. Generally, the optimum Pr concentration to Cd for rice was 92 μmol·L~(-1). However, 230 μmol·L~(-1) Pr could aggravate the toxic effect of Cd on rice roots.
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