Increasing Phosphorus Application Level Alleviated Adverse Effects of Low-Temperature Stress on Antioxidant Metabolism and Carbohydrate Metabolism in Tobacco Seedlings

Low temperature, as a major abiotic stress, impacts the formation of high-quality tobacco seedlings. It is urgent to take appropriate measures to improve the low-temperature tolerance of tobacco seedlings. A hydroponics experiment was conducted with a tobacco <i>cv</i>. Y2001 under 25 °C (control temperature) and 10 °C (low-temperature stress). Three phosphorus (P) levels including the traditional P concentration (2 mM PO₄³⁻) and higher P levels (3 mM PO₄³⁻ and 4 mM PO₄³⁻) were applied to investigate their effects on antioxidant metabolism and carbohydrate metabolism in low-temperature-stressed tobacco seedlings. The results showed that the low temperature decreased plant height, stem diameter, and biomass of shoots and roots, while the higher P levels promoted plant height and shoot biomass of low-temperature-stressed tobacco seedlings compared to the traditional P level. The leaf net photosynthetic rate (<i>A</i>_N) was decreased by the low temperature, while the <i>A</i>_N of low-temperature-stressed tobacco leaves was increased by 38.6–61.3% for the higher P levels than the traditional P level. Higher O₂⁻ and H₂O₂ were observed in tobacco leaves exposed to low-temperature stress, damaging the <i>A</i>_N, although the low temperature upregulated the expression of encoding <i>superoxide dismutase</i> (<i>NtSOD</i>), peroxidase (<i>NtPOD</i>), and catalase (<i>NtCAT</i>). However, compared with the traditional P level, the higher P levels further upregulated the expression of <i>NtSOD</i> and <i>NtCAT</i> in low-temperature-stressed tobacco leaves to accelerate O₂⁻ and H₂O₂ removal. Higher leaf sucrose content was detected since the low temperature significantly downregulated the expression of <i>NtSuSy</i>, <i>NtCWINV</i>, and <i>NtNINV</i> encoding sucrose synthase, the cell wall, and alkaline invertases, respectively, inhibiting sucrose hydrolysis. Compared with the traditional P level, higher P levels downregulated the expression of <i>NtCWINV</i> in low-temperature-stressed tobacco leaves, further promoting leaf sucrose content. The low temperature downregulated the expression of <i>NtAGP</i> encoding ADP-glucose pyrophosphorylase, <i>NtSSS</i> encoding soluble starch synthase, and <i>NtGBSS</i> encoding granule-bound starch synthase, thereby restricting starch biosynthesis. Additionally, the low temperature upregulated the expression of <i>α-amylase</i> and <i>β-amylase</i>, accelerating starch hydrolysis. These led to a lower starch content in low-temperature-stressed tobacco leaves. The higher P levels further upregulated the expression of <i>α-amylase</i> in low-temperature-stressed tobacco leaves than the traditional P level, further lowering the starch content. Moreover, the leaf soluble sugar content was higher under the low temperature than the control temperature, which helped the tobacco plants resist low-temperature stress. And higher P levels further promoted the soluble sugar content in low-temperature-stressed tobacco leaves compared with the traditional P level, further improving tobacco seedlings’ low-temperature tolerance. Therefore, these results indicated that increasing the P application level can alleviate the adverse impacts of cold stress on antioxidant metabolism and carbohydrate metabolism in tobacco seedlings.