Effect of Exogenous Melatonin Application on the Grain Yield and Antioxidant Capacity in Aromatic Rice under Combined Lead-Cadmium Stress

Abstract

This study aimed to determine the mechanism of exogenous melatonin application in alleviating the combined Pb and Cd (Pb-Cd) toxicity on aromatic rice (Oryza sativa L.). In this study, a pot experiment was conducted; two aromatic rice varieties, Yuxiangyouzhan and Xiangyaxiangzhan, were selected, and sprays using 50, 100, 200, and 400 μmol L^-1 melatonin (denoted as S50, S100, S200, and S400) and irrigation using 100, 300, and 500 μmol L^-1 melatonin (denoted as R100, R300, and R500) were also selected. The results showed that, under the S50, S100, and S200 treatments, the Pb content of aromatic rice grain decreased, and the grain yield increased significantly. Moreover, the application of exogenous melatonin significantly reduced the accumulation of H₂O₂ in rice leaves at maturity under Cd-Pb stress and reduced the MDA content in Xiangyaxiangzhan leaves. In addition, the microbial community structure changed significantly under S50 and R300 treatments. Some pathways, such as the synthesis of various amino acids and alanine, aspartate, and glutamate metabolism, were regulated by S50 treatment. Overall, melatonin application improved aromatic rice grain yield while reducing heavy metal accumulation by regulating the antioxidant capacity and metabolites in aromatic rice plants and altering the physicochemical properties and microbial community structures of the soil.

Keywords: antioxidants; aromatic rice; cadmium–lead; melatonin; microbial community structures.

Figure 1

The average monthly temperature, humidity, and sunshine hours during the experiment period in 2019.

Figure 2

Effects of exogenous melatonin application on GSH content in Yuxiangyouzhan (a) and Xiangyaxiangzhan (b); ASA content in Yuxiangyouzhan (c) and Xiangyaxiangzhan (d); MTs content in Yuxiangyouzhan (e) and Xiangyaxiangzhan (f) at the heading stage (HS), 15 day after HS, and the maturity stage (MS). Means sharing similar letters indicate no significant difference at p < 0.05 according to the LSD test.

Figure 3

Effects of exogenous melatonin application on SOD activity in Yuxiangyouzhanand (a) and Xiangyaxiangzhan (b); POD activity in Yuxiangyouzhan (c) and Xiangyaxiangzhan (d); CAT activity in Yuxiangyouzhan (e) and Xiangyaxiangzhan (f); MDA content in Yuxiangyouzhan (g) and Xiangyaxiangzhan (h); and H₂O₂ content in Yuxiangyouzhan (i) and Xiangyaxiangzhan (j) at the heading stage (HS), 15 day after HS, and the MS. Means sharing similar letters indicate no significant difference at p < 0.05 according to the LSD test.

Figure 3

Effects of exogenous melatonin application on SOD activity in Yuxiangyouzhanand (a) and Xiangyaxiangzhan (b); POD activity in Yuxiangyouzhan (c) and Xiangyaxiangzhan (d); CAT activity in Yuxiangyouzhan (e) and Xiangyaxiangzhan (f); MDA content in Yuxiangyouzhan (g) and Xiangyaxiangzhan (h); and H₂O₂ content in Yuxiangyouzhan (i) and Xiangyaxiangzhan (j) at the heading stage (HS), 15 day after HS, and the MS. Means sharing similar letters indicate no significant difference at p < 0.05 according to the LSD test.

Figure 4

Rarefaction curve (a); histogram of species distribution (b); PCA plot of fungal communities in 18 soil samples under the XCK, XS50, YCK, YR300 treatments (c); RDA considering the fungal relative abundance at the operational taxonomic unit (OTU) level; SOM, soil total N, P, and K contents, Cd and Pb concentrations, and pH (d). XCK: Xiangyaxiangzhan growing under Cd–Pb stress without melatonin treatment; XR300: Xiangyaxiangzhan growing under Cd–Pb stress and irrigated with 300 μmol L⁻¹ exogenous melatonin; XS50: Xiangyaxiangzhan growing under Cd–Pb stress and sprayed with 50 μmol L⁻¹ exogenous melatonin; YCK: Yuxiangyouzhan growing under Cd–Pb stress without melatonin treatment; YR300: Yuxiangyouzhan growing under Cd–Pb stress and irrigated with 300 μmol L⁻¹ exogenous melatonin; YS50: Yuxiangyouzhan growing under Cd–Pb stress and sprayed with 50 μmol L⁻¹ exogenous melatonin.

Figure 5

Overview of the enriched KEGG pathways. 1: Alanine, aspartate, and glutamate metabolism; 2: Arginine biosynthesis; 3: Aminoacyl-tRNA biosynthesis; 4: Nicotinate and nicotinamide metabolism; 5: Phenylpropanoid biosynthesis; 6: Citrate cycle (TCA cycle); 7: Butanoate metabolism; 8: Arginine and proline metabolism; 9: C5-Branched dibasic acid metabolism; 10: Valine, leucine, and isoleucine biosynthesis; 11: Pantothenate and CoA biosynthesis; 12: Glyoxylate and dicarboxylate metabolism; 13: Carbon fixation in photosynthetic organisms; 14: Pyrimidine metabolism; 15: Isoquinoline alkaloid biosynthesis; 16: Starch and sucrose metabolism; 17: beta-Alanine metabolism; 18: Glycine, serine, and threonine metabolism; 19: Vitamin B6 metabolism; 20: Betalain biosynthesis.

Figure 6

Multivariate pattern recognition analysis of metabolomics. (a) PCA plot of metabolites under XCK, XS50, YCK, and YS50 treatments; (b) PLS-DA plot of metabolites under XCK, XS50, YCK, and YS50 treatments; (c) VIP score of metabolites under XCK, XS50, YCK, and YS50 treatments; (d) PCA plot of metabolites under XCK and XS50 treatments; (e) PLS-DA plot of metabolites under XCK and XS50 treatments; (f) VIP score of metabolites under XCK and XS50 treatments; (g) PCA plot of metabolites under YCK and YS50 treatments; (h) PLS-DA plot of metabolites under YCK and YS50 treatments; and (i) VIP score of metabolites under YCK and YS50 treatments.

Figure 7

(a) Venn diagram showing the numbers of differential metabolites in XCK vs. XS50, XCK vs. YCK, XS50 vs. YS50, and YCK vs. YS50 comparisons. (b) Significant differential metabolites in XCK vs. XS50, XCK vs. YCK, XS50 vs. YS50, and YCK vs. YS50.