Experimental and Theoretical Study of the Effects of Rare Earth Elements on Growth and Chlorophyll of Alfalfa (Medicago sativa L.) Seedling.

Kexiao Song,Xueqing He,Baiyu An,Haoyang Sun,Shuo Li,Jinzhu Gao,Tianming Hu,Yunfu Sun

doi:10.3389/fpls.2021.731838

Kexiao Song, Xueqing He + Show 6 more

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https://doi.org/10.3389/fpls.2021.731838

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Abstract

Rare earth elements (REEs) of low concentration are usually beneficial to plant growth, while they are toxic at high concentrations. The effects of treatment with lanthanum (La) (10 and 20 μM), cerium (Ce) (10 and 20 μM), and terbium (Tb) (10 and 20 μM) on seedling growth of alfalfa (Medicago sativa L.), which is one of the most important perennial leguminous forages in the world, were studied. The results showed that all three REE treatments quickened the germination of seeds. The length of shoot under La (20 μM) treatment was significantly shortened (P < 0.05). In addition, treatment with La, Ce, and Tb had a “hormesis effect” on root length. There was a significant decrease in chlorophyll content on treatment with the three REEs, and the degree of decline was in the order of La < Ce < Tb, under the same concentration. In vitro experiments and quantum chemical calculations were further performed to explain why the treatments with REEs reduced the chlorophyll content. In vitro experiments showed that La, Ce, and Tb treatments reduced the absorbance of chlorophyll, and the decrease followed in the order of La > Ce > Tb. Quantum chemical calculations predicted that the decrease in absorption intensity was caused by the reactions between La, Ce, Tb, and chlorophyll, which formed lanthanides-chlorophyll; and there were five types of stable lanthanides-chlorophyll. In conclusion, the decrease in chlorophyll content on treatment with REEs was caused by the change in chlorophyll structure.

Highlights

Rare earth elements (REEs) are divided into two parts, light rare earth elements (LREE: cerium group elements) and heavy rare earth elements (HREE: yttrium group elements) (Binnemans et al, 2013), which are important plant growth stimulants in agriculture (Cotruvo, 2019; Kovaríková et al, 2019)
The results showed that treatments with the three REEs with 10 and 20 μM concentrations could speed up the germination of alfalfa, but had no significant effect on seed germination rate for the 14 days (Figures 1A,B)
The results showed that treatments with REEs (La, Ce, and Tb) had an effect on the growth of alfalfa seedlings

Summary

Introduction

Rare earth elements (REEs) are divided into two parts, light rare earth elements (LREE: cerium group elements) and heavy rare earth elements (HREE: yttrium group elements) (Binnemans et al, 2013), which are important plant growth stimulants in agriculture (Cotruvo, 2019; Kovaríková et al, 2019). It was found that REEs had a “hormesis effect” on plant growth and development (d’Aquino et al, 2009; Wen et al, 2011; Ochi et al, 2014). A suitable concentration of REEs could stimulate plant growth and development (Hong et al, 2017), and improve photosynthetic capacity (García-Jiménez et al, 2017) and crop quality (Ramírez-Olvera et al, 2018). The positive effects of REEs on crop production and plant physiological responses have been reported extensively (Liu et al, 2012). Experiments with REEs on alfalfa have not been reported, and it is important to know how REEs affect the seedling growth of alfalfa

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