Effects of exogenous melatonin on the uptake and utilization of nutrients in Malus hupehensis seedlings under replant stress

Abstract: 【Objective】Apple replant disease (ARD) is a common soil-borne disease that decreases plant growth, increases damage by predatory insects, and decreases apple yield and quality. Current research on preventing and treating ARD mainly focuses on changes in the physical and chemical properties, metabolites, and microbial community structure of the rhizospheric soil. In contrast, few studies have focused on plant mineral nutrient absorption. In the present study, we investigated the effects of exogenous melatonin on the absorption, utilization, and accumulation of mineral elements in Malus hupehensis Rehd. under replant stress in order to provide a theoretical basis and technical support for the prevention and treatment of ARD.【Methods】In a pot experiment, 400 healthy M. hupehensis seedlings were planted in June, 2021 at the innovation experimental base of Hebei Agricultural University (38° 230 N, 115° 280 E). The seedlings were divided into four groups: plants grown in‘healthy’soil (CK); melatonin control, plants grown in‘healthy’soil containing 200 μmol·L^-1 melatonin (MCK), plants grown in ‘replant’soil (ST), and plants grown in‘replant’soil containing 200 μmol·L^-1 melatonin (MST). Melatonin (200 μmol·L-1 ) was supplemented every 7 days from June to October, 2021. The growth and mineral element contents of the seedlings were analyzed after 120 days of experienment. The indices included dry weight and mineral element concentration, absorption, transfer, accumulation, and distribution in roots, stems, and leaves. Finally, the Mantel test was used to analyze the correlation between the miner-al element content and the dry weight in each part of the M. hupehensis seedlings.【Results】The exogenous melatonin significantly increased the dry weight of the roots, stems and leaves of M. hupehensis seedlings under replant stress compared with the ST plants, by 44.19%, 67.63% and 81.67%, respectively. Additionally, the concentration of mineral elements differed significantly among treatments. Compared with the ST plants, the MST plants showed significantly increased the concentrations of N, P and Mn in the roots and stems, and N, P, Ca, Mg and Mn in the leaves, as well as significantly decreased the concentrations of Fe, Zn, Cu and B in the roots, K, Fe and B in the stems, and Fe, Zn and Cu in the leaves. The replant stress significantly inhibited the absorption of the macroelements N, P, K, Ca, and Mg by 74.04%, 56.46%, 61.17%, 57.96%, and 56.50%, respectively, as well as that of the microelements Fe, Zn, Cu, Mn, and B by 49.49%, 54.28 %, 37.57%, 46.05% and 58.91%, respectively, compared with the control plants. However, the application of the exogenous melatonin significantly reduced this inhibition and promoted the plant nutrient absorption. In terms of the nutrient transfer and accumulation, compared with the ST plants, the application of the exogenous melatonin under replant stress significantly increased the accumulation of N, P, and Mn in the roots by 34.52% , 20.03% and 23.73% , respectively, while it significantly decreased the accumulation of Fe, Zn and B by 25.34% , 22.72% and 61.62% , respectively. Additionally, the exogenous application of melatonin affected the transfer rate of these elements to the leaves and stems. The transfer rate of all the elements (except B and Zn) to the leaves and stems increased significantly. Regarding nutrient distribution, the application of the exogenous melatonin under replant stress increased the distribution ratio of almost all the elements in the roots, stems, and leaves. Compared with the ST plants, the contents of N, P, K, Ca, Mg, Zn, Cu and Mn in the roots of the MST plants increased by 76.13%, 59.62%, 34.30%, 43.43%, 44.21%, 3.17%, 13.11% and 63.63%, respectively, while the contents of N, P, K, Ca, Mg, Fe, Zn, Cu, Mn and B increased by 73.78%, 69.29%, 48.74 %, 57.21%, 69.46 %, 30.86 %, 36.49 %, 45.81%, 121.08% and 41.42% in the stems, and by 128.15% , 119.16 % , 79.73% , 95.63% , 113.28% , 12.81 % , 26.70% , 47.85%, 103.23% and 62.08% in the leaves, respectively. The Mantel test and Pearson correlation analysis showed that after 120 days of treatment, the mineral elements in M. hupehensis plants were positively correlated with the dry weight of each part, and also showed positive correlation with each other. The N content was significantly positively correlated with K, Ca, Mg, Zn and B contents. The P content was significantly and positively correlated with the Ca and Mg contents. The K content was positively correlated with Ca, Mg, Zn and B contents. Moreover, there were significant positive correlations between the contents of Ca and Mg, Mg and Cu, Fe and Zn, Zn and B, and Cu and Mn. In addition, the dry weight of each plant part exhibited significant positive correlations with the N, P, K, Ca, Mg, Zn, and B contents, but not with the Fe, Cu or Mn contents.【Conclusion】The exogenous application of 200 μmol·L^-1 melatonin under replant stress would regulate the absorption and accumulation of mineral elements, and promote plant growth. These findings would provide a theoretical framework for understanding how melatonin regulates the nutrient absorption of M. hupehensis seedlings under replant stress, as well as a theoretical basis and technical information for preventing and treating ARD.