Continuous Cropping Impedes Tartary Buckwheat ( <i>Fagopyrum tataricum</i> ) Growth by Disrupting the Ascorbate–Glutathione Cycle and Nitrogen Metabolism

The yield of Tartary buckwheat is significantly affected by continuous cropping. This study aimed to clarify the physiological mechanisms of continuous cropping on the growth and yield of Tartary buckwheat. A field experiment was conducted on the high-yield Tartary buckwheat cultivar Daku 1 with different continuous cropping years. With increased continuous cropping years, the following parameters decreased continuously: available nutrients in rhizosphere soil; the 3-indoleacetic acid, chlorophyll, and non-structural carbohydrates contents; and the activities of nitrogen metabolism-related enzymes, agronomic traits, and yield. Meanwhile, these features initially increased and then decreased: root morphology and activity; the antioxidant enzymes, ascorbate peroxidase, and glutathione reductase activities; and reduced glutathione content in leaves. The contents of malondialdehyde, superoxide anion free radical, and abscisic acid increased continuously. The contents of ascorbic acid and oxidized glutathione initially decreased and then increased. The yield of continuous cropping for 1, 2, 3, and 4 years was decreased by 5.60%, 21.25%, 39.25%, and 51.93%, respectively. In summary, continuous cropping inhibited the growth and development of the root system and above-ground parts, and caused an imbalance in endogenous hormones, the ascorbate-glutathione cycle, and nitrogen metabolism, thereby reducing the yield of Tartary buckwheat.