Exogenous melatonin enhances the continuous cropping tolerance of Tartary buckwheat (Fagopyrum tataricum) by regulating the antioxidant defense system

The yield of Tartary buckwheat is significantly affected by continuous cropping. Melatonin plays a crucial role in plant defense mechanisms against abiotic stresses. However, the relationship between melatonin and continuous cropping tolerance remains unclear. This study aimed to analyze the physiological mechanism of melatonin in enhancing the continuous cropping tolerance (abiotic stress) of Tartary buckwheat. A field experiment was conducted on Tartary buckwheat cultivar Jinqiao 2 under continuous cropping with five melatonin application rates, 0 (Control), 10, 50, 100, and 200 μmol L−1, applied during the early budding stage. The chlorophyll content, antioxidant enzyme activity, osmolyte and auxin (IAA) contents, root activity, rhizosphere soil nutrient content, and agronomic traits of Tartary buckwheat initially increased and then decreased with an increase in the concentration of exogenous melatonin application, with the best effects observed at 100 μmol L−1. Compared with the Control treatment, the 100 μmol L−1 treatment decreased the contents of malondialdehyde, superoxide anion free radical, and abscisic acid (ABA) by an average of 28.79%, 27.08%, and 31.64%, respectively. Exogenous melatonin treatment significantly increased the yield of Tartary buckwheat under continuous cropping. Plants treated with 10, 50, 100, and 200 μM respectively had 1.88, 2.01, 2.20, and 1.78 times higher yield than those of the Control treatment. In summary, melatonin treatment, particularly 100 μmol L−1, enhanced the continuous cropping tolerance of Tartary buckwheat by increasing antioxidant capacity and osmotica content, coordinating endogenous ABA and IAA content levels, and delaying senescence, ultimately increasing yield.