Melatonin has been regarded as a promising substance that enhances the abiotic stress tolerance of plants. However, few studies have devoted attention to the role of melatonin in improving salt tolerance in sugar beets. Here, the effects of different application methods (foliar application (100 μM), root application (100 μM), and combined foliar and root application) of melatonin on the morphological and physiological traits of sugar beets exposed to salt stress were investigated. The results showed that melatonin improved the growth of sugar beet seedlings, root yield and sugar content, synthesis of chlorophyll, photosystem II (PS II) activity, and gas exchange parameters under salt stress conditions. Moreover, melatonin enhanced the capacity of osmotic adjustment by increasing the accumulation of osmolytes (betaine, proline, and soluble sugar). At the same time, melatonin increased the H⁺-pump activities in the roots, thus promoting Na⁺ efflux and K⁺ influx, which maintained K⁺/Na⁺ homeostasis and mitigated Na⁺ toxicity. In addition, melatonin strengthened the antioxidant defense system by enhancing the activities of antioxidant enzymes, modulating the ASA-GSH cycle, and mediating the phenylalanine pathway, which removed superoxide anions (O₂^•−) and hydrogen peroxide (H₂O₂) and maintained cell membrane integrity. These positive effects were more pronounced when melatonin was applied by combined foliar and root application. To summarize, this study clarifies the potential roles of melatonin in mitigating salt stress in sugar beets by improving photosynthesis, water status, ion homeostasis, and the antioxidant defense system.

The Songnen Plain, whose climatic conditions are perfectly suited to sugar beet growth, is located in northeastern China. Unfortunately, this region has a lot of saline-alkaline land, which is the most important factor limiting sugar beet production. This study was undertaken to determine whether biochar-based organic fertilizer could alleviate the negative effect of saline-alkaline soil on sugar beet yield and whether such an effect correlated with changes in nitrogen assimilation, antioxidant system, root activity, and photosynthesis. Three treatments were established: Chemical fertilizers were applied to neutral soil (CK), chemical fertilizers were applied to saline-alkaline soil (SA), and biochar-based organic fertilizer was applied to saline-alkaline soil (SA + B). Our results showed that saline-alkaline stress significantly inhibited the nitrogen assimilation and antioxidant enzymes activities in root, root activity, and photosynthesis, thus significantly reducing the yield and sugar content of sugar beet. Under saline-alkaline conditions, the application of biochar-based organic fertilizer improved the activities of nitrogen assimilation enzymes in the root; at the same time, the antioxidant enzymes activities of the root were significantly increased for improving root activity in this treatment. Moreover, the application of biochar-based organic fertilizer could improve the synthesis of photosynthetic pigments, PSII (Photosystem II) activity, stomatal opening, and photosynthesis of sugar beet under saline-alkaline conditions. Hence, the growth and yield of sugar beet were improved by applying biochar-based organic fertilizer to saline-alkaline soil. These results proved the significance of biochar-based organic fertilizer in alleviating the negative effect of saline-alkaline stress on sugar beet. The results obtained in the pot experiment may not be viable in field conditions. Therefore, in the future, we will verify whether biochar-based organic fertilizer could alleviate the adverse effects of saline-alkaline stress on sugar beets yield under field conditions.

The addition of biochar-based organic fertilizer (BOF) can improve sugar beet yield, but its effects on the growth of sugar beet and on soil quality at different densities remain unclear. Six treatments, comprising two densities D1 and D2 (80,000 and 90,000 plant ha⁻¹) and three application rates B1, B2, and B3 (2.75, 3.25, and 3.75 t ha⁻¹) of BOFs + mineral fertilizer, respectively, are investigated in this research. The mineral fertilizers are typically used to supplement the total N, P₂O₅, and K₂O deficiencies. The BOFs were used in the soil before sowing, and the mineral fertilizer was added to the soil after the first pair of true leaves was grown. At 160 days after sowing (DAS), the root-to-shoot ratio under the D2B2 treatment was significantly higher than that under the other treatments. The effect of density on the photosynthesis rate of sugar beet was not significant. The BOF application amount and density exerted interaction effects on soil physicochemical properties and the activities of different soil enzymes affecting each other. Both the D2B2 and D2B3 treatments reduced the content of NO₃^–-N in the 40–60 cm soil layer. Combined 90,000 plants ha^–1 with 3.25 t ha⁻¹ BOFs can increase the soil nutrient content of the 0–60 cm soil layer, improve the rhizosphere soil environment, promote the uniform distribution of dry matter, and increase sugar production.

Maize (Zea mays L.) is the largest grain crop in Heilongjiang Province. Carbon-based fertilizer is a mixed fertilizer produced by adding a certain proportion of chemical fertilizer with biochar as the loading substrate. In this study, the effects of carbon-based fertilizer on the rhizosphere soil microenvironment and maize root system were discussed. Two maize varieties, Xianyu 335 and Jingke 968, were selected and six treatments were set as follows, including no fertilization (CK1), conventional fertilizer (CK2) and the amount of carbon-based fertilizer, which were 3 t/hm², 3.75 t/hm², 4.5 t/hm² and 5.25 t/hm², respectively. The results showed that carbon-based fertilizer increased the total root length, root volume, root area and root tip number of maize, and the root length, root volume, root area and root tip number of 4.5 t treatment performed better at all stages, which was significantly higher than that of chemical fertilizer. On 16 August (early filling stage), most of the root color changed from milky white to dark brown, the root clarity decreased, the number of roots decreased, the root volume significantly decreased and the root began to age, while the number and volume of roots treated with the carbon-based fertilizer remained stable, the root color was milky white, the morphological structure was clear and there was basically no aging. The carbon-based fertilizer treatment significantly increased the root biomass of 0–15 cm above the plant, 15–30 cm and 30–45 cm between the plants and 0–15 cm between the ridges, forming a wide and deep high-yield root system. The carbon-based fertilizer significantly increased the bleeding rate. On 8 July (jointing stage), Xianyu 335 and Jingke 968 reached the maximum value at the 3 t and 3.75 t treatments, respectively. The carbon-based fertilizer treatment had no significant effect on the amino acid content, but significantly increased the amino acid transport rate on 8 July (jointing stage) and 16 August (early filling stage). The transport rate of inorganic phosphorus gradually decreased with the advancement of the growth process. On 8 July (jointing stage), the ammonium nitrogen content and transport rate of the two varieties reached the maximum value at the treatment of 4.5 t and 3.75 t, which was significantly higher than the treatment of chemical fertilizer and no fertilizer, and showed a gradual downward trend with the advancement of the growth process. The soluble sugar content was relatively low in the early stage and increased rapidly on 4 September (waxy ripening stage). Both varieties reached the maximum value at 4.5 t treatment, and the transport rate reached the maximum value at 3.75 t treatment, which was significantly higher than that of the chemical fertilizer treatment. In conclusion, the carbon-based fertilizer significantly increased the yield of maize, and the yield of maize under the 4.5 t treatment reached the maximum, which was 15.02% and 18.24% higher than that of the chemical fertilizer treatment, respectively.