Physiological Response of Sugar Beet Seedlings to Ferrous, Hypoxia, and Interactive Ferrous-Hypoxia Stresses

Crops during their growth period can encounter waterlogging problems which often lead to an oxygen-deficient environment in the rhizosphere and rapid increases in the availability of iron in the soil. Therefore, a hydroponic experimental system was designed to evaluate the response mechanism of sugar beet to single ferrous, hypoxia stress, and interactive ferrous-hypoxia stress through physiological and biochemical analyses. In this study, stress conditions were applied in a factorial design: three ferrous treatments (0.06, 0.48, and 1.92 mmol) and three hypoxia treatments (normal, 4-day hypoxia, and 8-day hypoxia) were set against one another, totaling 9 experimental treatments. We measured morphological indexes, biomass production, root vigor, the electrolytic leakage, chlorophyll content, gas exchange, reactive oxygen species concentration, malonaldehyde concentration, antioxidant enzyme activity, proline, soluble sugar concentration, and plant nutrient contents of sugar beet seedlings under different treatments. Under single stress, the growth of seedlings was inhibited, and interactive stress enhanced this inhibition. Compared to the control, 1.92 mmol Fe and 8 days of hypoxia induced a 16% and 43% reduction in the total plant height of seedlings, respectively, while interactive ferrous-hypoxia stress produced a 51% reduction. Single and interactive stress damaged the roots of seedlings, root vigor reached its lowest under interactive stress, and Fe concentrations increased, but N, P, K, Mn, and Zn decreased. Chlorophyll content decreased under ferrous and interactive stress. At the same time, stresses affected the gas exchange of leaves, and the stomatal conductance, transpiration rate, and intrinsic water use efficiency decreased, resulting in insufficient photosynthetic capacity of the seedlings, and biomass accumulation decreased. Furthermore, the antioxidant enzyme activity and osmotic adjustment substance content of seedlings increased with an increase of the degree of stress. Among these, the POD activity of seedlings grown under 1.92 mmol Fe and hypoxia for 8 days was upregulated by 7.5 and 1.5 times compared with the control, while the interactive stress induced a 10.5 times upregulation. Under single stress, seedling growth was inhibited to varying degrees, while under interactive stress, this inhibition was enhanced. Seedlings also improved their antioxidant capacity to resist the damage caused by stress, but on the whole, stress had a profound effect on the plants.