Effects of exogenous melatonin on potato growth，chromium tolerance，and soil chromium reduction under chromium stress

【Objective】This study aimed to explore the effects of exogenous melatonin application on the heavy metal adsorption capacity and photosynthetic rate of potatoes under the stress of the heavy metal chromium（Cr），providing reference for for increasing yield of potato cultivation and soil ecological restoration in Cr polluted areas in northwestern Yunnan.【Method】The main potato variety Lishu 6 cultivated in northwestern Yunnan was used as the experimental material. The plants were stressed with soil containing heavy metal Cr⁶⁺，and after sprouting，they were treated with 100 μmol/L of exogenous melatonin. Six treatments were set up：CK group（non-Cr stress，clear water spraying），Cr group（Cr stress with clear water spraying），MT1 group（non-Cr stress，melatonin irrigation），MT2 group（non-Cr stress，melatonin spraying），Cr+MT1 group（Cr treatment with melatonin irrigation）and Cr+MT2 group（Cr treatment with melatonin spraying）. The melatonin content，Cr content，and photosynthetic parameters in potato leaves were measured on the 7^th and 21^st d，and the yield traits were determined after harvest and changes of Cr content in soil were detected.【Result】From the 7^th to the 21^st d after the application of exogenous melatonin，under non-Cr stress conditions，the mean value of melatonin content in potato leaves increased by 42.6%，the mean value of Cr content decreased by 7.0%，and the mean value of Cr content in soil decreased by 10.1% in the two groups with melatonin（MT1 and MT2）with melatonin. Under Cr stress，the mean melatonin content in potato leaves of the two groups with melatonin（Cr+MT1 and Cr+MT2）increased by 28.7%，the mean Cr content decreased by 48.5%，and the mean Cr content in soil decreased by 6.4%. Compared with Cr group，on the 7^th and 21^st d after melatonin application，the mean net photosynthetic rate（P_n）of the 4 groups with melatonin（MT1，MT2，Cr+MT1 and Cr+MT2）after melatonin application decreased by 27.2% and increased by 9.3 times respectively，and the mean stomatal conductance（G_s）increased by 7.1% and 35.2% respectively，the mean transpiration rate（T_r）increased by 4.0% and 26.5% respectively，while the mean intercellular CO₂ concentration（C_i）increased by 17.2% and decreased by 7.4% respectively. After the application of melatonin，the leaves of the plants were greener and the growth traits were better，and the mean tuber mass and number of tubers at harvest were 2.4 and 1.2 times higher than those in Cr group respectively. In addition，Cr stress had small effect on the change of Cr content in potato tubers， and potato tubers were not the main sites for Cr adsorption.【Conclusion】The application of exogenous melatonin can sustainably enhance the adsorption of heavy metal Cr by Lishu 6 potatoes under Cr stress，improve Cr tolerance，increase plant photosynthetic rate and biomass synthesis，effectively reduce soil Cr content，and provide new solution to increasing potato yield，soil ecological restoration and carbon sequestration.