Cadmium contamination impairs alkaline tolerance in sugar beet by inhibiting carbon fixation and tryptophan metabolism.

The contamination of cadmium (Cd) may overlap with alkali stress, exerting a synergistic effect on crop growth. This study examined the morphological and physiological responses of sugar beet seedlings irrigated with Cd solution (50 mM CdCl2) to alkali stress (400 mM NaHCO3). Under alkali stress, the presence of Cd exacerbated the reduction in plant growth, the maximum quantum yield of photosystem II (Fv/Fm), chlorophyll content, and osmoregulatory substances (proline, soluble sugars). Cd augmented the concentration of MDA, O2·-, and H2O2, while reducing the expression of BvPOX. Concurrently, glutathione content, ascorbate peroxidase (APX) activity, and the glutathione transferase gene (BvGST) expression were also inhibited by Cd. Cd constrained the ribulose-5P content under alkali stress and the expression of the glyceraldehyde-3-phosphate dehydrogenase gene (BvGAPDH), thereby inhibiting the Calvin cycle. Additionally, the expression of the alcohol dehydrogenase gene (BvADH) was reduced, leading to decreased sucrose and starch content. Cd also accelerated pectin metabolism. Furthermore, Cd inhibited the expression of key genes involved in melatonin synthesis (aromatic-L-amino-acid/L-tryptophan decarboxylase gene, BvDDC/BvTDC and acetylserotonin O-methyltransferase gene, BvASMT), resulting in restricted tryptophan metabolism. This study provides insights into the inhibition of carbon fixation and tryptophan metabolism induced by Cd, its effects on the growth of beets under alkaline stress conditions.