Biostimulant Response of Foliar Application of Rare Earth Elements on Physiology, Growth, and Yield of Rice

Rare earth elements (REEs) have been intentionally used in Chinese agriculture since the 1980s to improve crop yields. Around the world, REEs are also involuntarily applied to soils through phosphate fertilizers. These elements are known to alleviate damage in plants under abiotic stresses, yet there is no information on how these elements act in the physiology of plants. The REE mode of action falls within the scope of the hormesis effect, with low-dose stimulation and high-dose adverse reactions. This study aimed to verify how REEs affect rice plants' physiology to test the threshold dose at which REEs could act as biostimulants in these plants. In experiment 1, 0.411 kg ha-1 (foliar application) of a mixture of REE (containing 41.38% Ce, 23.95% La, 13.58% Pr, and 4.32% Nd) was applied, as well as two products containing 41.38% Ce and 23.95% La separately. The characteristics of chlorophyll a fluorescence, gas exchanges, SPAD index, and biomass (pot conditions) were evaluated. For experiment 2, increasing rates of the REE mix (0, 0.1, 0.225, 0.5, and 1 kg ha-1) (field conditions) were used to study their effect on rice grain yield and nutrient concentration of rice leaves. Adding REEs to plants increased biomass production (23% with Ce, 31% with La, and 63% with REE Mix application) due to improved photosynthetic rate (8% with Ce, 15% with La, and 27% with REE mix), favored by the higher electronic flow (photosynthetic electron transport chain) (increase of 17%) and by the higher Fv/Fm (increase of 14%) and quantum yield of photosystem II (increase of 20% with Ce and La, and 29% with REE Mix), as well as by increased stomatal conductance (increase of 36%) and SPAD index (increase of 10% with Ce, 12% with La, and 15% with REE mix). Moreover, adding REEs potentiated the photosynthetic process by increasing rice leaves' N, Mg, K, and Mn concentrations (24-46%). The dose for the higher rice grain yield (an increase of 113%) was estimated for the REE mix at 0.72 kg ha-1.

Lanthanum in Water, Sediment, Macrophytes and chironomid larvae following application of Lanthanum modified bentonite to lake Rauwbraken (The Netherlands)

Lanthanum Modified Bentonite (LMB; Phoslock®) is used to mitigate eutrophication by binding phosphate released from sediments. This study investigated the fate of lanthanum (La) from LMB in water, sediment, macrophytes, and chironomid larvae in Lake Rauwbraken (The Netherlands). Before the LMB application, water column filterable La (FLa) was 0.02 µg L^-1, total La (TLa) was 0.22 µg L^-1. In sediment the total La ranged 0.03-1.86 g m^-2. The day after the application the maximum FLa concentration in the water column was 44 µg L^-1, TLa was 528 µg L^-1, exceeding the Dutch Maximum Permissible Concentrations (MPC) of 10.1 µg L^-1 by three to fourfold. TLa declined below the MPC after 15 days, FLa after 75 days. After ten years, FLa was 0.4 µg L^-1 and TLa was 0.7 µg L^-1. Over the post-application years, FLa and TLa showed statistically significant downward trends. While the LMB settled homogeneously on sediment, after 3 years it redistributed to 0.2-5.4 g La m^-2 within shallow zones, and 30.7 g m^-2 to 40.0 g La m^-2 in deeper zones. In the upper 20 cm of sediment, La concentrations were 7-6702 mg kg ^-1 dry weight (DW) compared to 0.5-7.0 mg kg^-1 before application. Pre-application anaerobic sediment release of FLa was 0.006 mg m^-2 day^-1. Three months after the application it was 1.02 mg m^-2 day^-1. Three years later it was 0.063 mg m^-2 day^-1. Before application La in plants was 0.8-5.1 mg La kg^-1 DW, post-application values were up to 2925 mg La kg^-1 DW. In chironomid larvae, La increased from 1.7 µg g^-1 DW before application to 1421 µg g^-1 DW after one month, 3 years later it was 277 µg g^-1 DW. Filtration experiments indicate FLa is not truly dissolved free La³⁺ cations.

Jasmonic acid (JA) acts as a signal molecule in LaCl(3)-induced baicalin synthesis in Scutellaria baicalensis seedlings

Rare earth elements (REEs) have been widely used to increase accumulation of biomass and secondary metabolites in medicinal plants in China. However, very few studies have investigated how REEs mediate secondary metabolism synthesis in medicinal plants. Lanthanum (La), an important REE, is known to improve the accumulation of secondary metabolites in medicinal plants and is widely distributed in China. However, few studies have evaluated the signal transduction leading to La-induced secondary metabolism in medicinal plants. In this study, LaCl(3) treatment-induced multiple responses in Scutellaria baicalensis seedlings, including the rapid generation of jasmonic acid (JA), sequentially followed by the enhancement of baicalin production. Direct application of JA also promoted the synthesis of baicalin in the absence of LaCl(3). LaCl(3)-induced baicalin synthesis was blocked by two different JA synthesis inhibitors. Our results showed that JA acts as a signal component within the signaling system leading to La-induced baicalin synthesis in S. baicalensis seedlings.

Influences of magnesium deficiency and cerium on antioxidant system of spinach chloroplasts

Magnesium-deficiency conditions applied to spinach cultures caused an oxidative stress status in spinach chloroplast monitored by an increase in reactive oxygen species (ROS) accumulation. The enhancement of lipids peroxide of spinach chloroplast grown in magnesium-deficiency media suggested an oxidative attack that was activated by a reduction of antioxidative defense mechanism measured by analysing the activities of superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, and glutathione reductase, as well as antioxidants such as carotenoids and glutathione content. As the antioxidative response of chloroplast was reduced in spinach grown in magnesium-deficiency media, it caused a significant reduction of spinach plant weight, old leaves turning chlorosis. However, cerium treatment grown in magnesium-deficiency conditions decreased the malondialdehyde and ROS, and increased activities of the antioxidative defense system, and improved spinach growth. Together, the experimental study implied that cerium could partly substitute for magnesium and increase the oxidative stress-resistance of spinach chloroplast grown in magnesium-deficiency conditions, but the mechanisms need further study.

Physico-chemical property of rare earths-effects on the energy regulation of photosystem II in Arabidopsis thaliana

Photosystem II (PSII) from Arabidopsis thaliana treated by lanthanum (La(3+)), cerium (Ce(3+)), and neodymium (Nd(3+)) were isolated to investigate the effects of 4f electron characteristics and alternation valence of rare earth elements (REEs) on PSII function regulation comparatively. Results showed that REE treatment could induce the generous expression of LhcII b in A. thaliana and increase the content of light-harvesting complex II and its trimer on the thylakoid membrane significantly. Meanwhile, the light absorption in the red and blue region and fluorescence quantum yield near 683 nm were obviously increased; oxygen evolution rate was greatly improved too, suggesting that REEs could enhance the efficiency of light absorption, regulate excitation energy distribution from photosystem I (PSI) to PSII, and thus increase the activity of photochemical reaction and oxygen evolution accordingly. The efficiency order of the four treatments was Ce(3+) > Nd(3+) > La(3+) > control.

Absorption and transfer of light and photoreduction activities of spinach chloroplasts under calcium deficiency: promotion by cerium

Chloroplasts were isolated from spinach cultured in calcium-deficient, cerium-chloride-administered calcium-present Hoagland's media or that of calcium-deficient Hoagland's media and demonstrated the effects of cerium on distribution of light energy between photosystems II and I and photochemical activities of spinach chloroplast grown in calcium-deficient media. It was observed that calcium deprivation significantly inhibited light absorption, energy transfer from LHCII to photosystemII, excitation energy distribution from PSI to PSII, and transformation from light energy to electron energy and oxygen evolution of chloroplasts. However, cerium treatment to calcium-deficient chloroplasts could obviously improve light absorption and excitation energy distribution from photosystem I to photosystem II and increase activity of whole chain electron transport, photosystems II and I DCPIP photoreduction, and oxygen evolution of chloroplasts. The results suggested that cerium under calcium deficiency condition could substitute for calcium in chloroplasts, maintain the stability of chloroplast membrane, and improve photosynthesis of spinach chloroplast, but the mechanisms still need further study.