Increased soil salinity significantly inhibits crop production worldwide, and biochar may alleviate salt stress. In the present study, the application of biochar significantly increased the biomass of rice under salt stress treatment. The analysis of soil properties demonstrated that biochar application significantly decreased electrical conductivity and soluble Na⁺ and Cl⁻ contents in the soil under salt stress. In addition, biochar application increased the soil cation exchange capacity, soil organic matter, humic acid, total nitrogen, and total phosphorus contents in the soil, suggesting that biochar improved the soil nutrient conditions. The application of biochar further increased the abundance of soil bacteria and changed the bacterial community structure under salt stress. Proteobacteria, Chloroflexi, and Acidobacteria were the top three phyla in bacterial abundance. Biochar increased Proteobacteria abundance and decreased Chloroflexi abundance, which were considered to be eutrophic bacteria and oligotrophic bacteria, respectively. Redundancy analysis showed that soil bacterial communities were mainly affected by soil pH and EC (p < 0.05). In conclusion, the application of biochar alleviated salt stress in rice via modifying soil properties and regulating the bacterial abundance and community structure.

The hormonal imbalances, including abscisic acid (ABA) and brassinosteroid (BR) levels, caused by salinity constitute a key factor in hindering spikelet development in rice and in reducing rice yield. However, the effects of ABA and BRs on spikelet development in plants subjected to salinity stress have been explored to only a limited extent. In this research, the effect of ABA and BRs on rice growth characteristics and the development of spikelets under different salinity levels were investigated. The rice seedlings were subjected to three different salt stress levels: 0.0875 dS m⁻¹ (Control, CK), low salt stress (1.878 dS m⁻¹, LS), and heavy salt stress (4.09 dS m⁻¹, HS). Additionally, independent (ABA or BR) and combined (ABA+BR) exogenous treatments of ABA (at 0 and 25 μM concentration) and BR (at 0 and 5 μM concentration) onto the rice seedlings were performed. The results showed that the exogenous application of ABA, BRs, and ABA+BRs triggered changes in physiological and agronomic characteristics, including photosynthesis rate (Pn), SPAD value, pollen viability, 1000-grain weight (g), and rice grain yield per plant. In addition, spikelet sterility under different salt stress levels (CK, LS, and HS) was decreased significantly through the use of both the single phytohormone and the cocktail, as compared to the controls. The outcome of this study reveals new insights about rice spikelet development in plants subjected to salt stress and the effects on this of ABA and BR. Additionally, it provides information on the use of plant hormones to improve rice yield under salt stress and on the enhancement of effective utilization of salt-affected soils.

Cold stress inhibits rice germination and seedling growth. Brassinolide (BR) plays key roles in plant growth, development, and stress responses. In this study, we explored the underlying mechanisms whereby BR helps alleviate cold stress in rice seedlings. BR application to the growth medium significantly increased seed germination and seedling growth of the early rice cultivar “Zhongzao 39” after three days of cold treatment. Specifically, BR significantly increased soluble protein and soluble sugar contents after three days of cold treatment. Moreover, BR stimulated the activity of superoxide dismutase, catalase, peroxidase, and ascorbate peroxidase; thereby alleviating cold-induced damage and increasing glutathione content and the GSH/GSSG ratio while concomitantly reducing H₂O₂ content. BR upregulated the expression levels of cold-response-related genes, including OsICE1, OsFer1, OsCOLD1, OsLti6a, OsSODB, OsMyb, and OsTERF2, and downregulated that of OsWRKY45, overall alleviating cold stress symptoms. Thus, BR not only upregulated cellular osmotic content and the antioxidant enzyme system to maintain the physiological balance of reactive oxygen species under cold but, additionally, it regulated the expression of cold-response-related genes to alleviate cold stress symptoms. These results provide a theoretical basis for rice breeding for cold resistance using young seedlings.

Excessive nitrogen (N) fertilization, low use efficiency, and heavy pollution are the dominant issues that exist in intensively cultivated double rice cropping systems in China. Two-year field and ¹⁵N microregion experiments were conducted to evaluate the N fate in a soil-rice system under a series of different N rate treatments from 2020 to 2021. The economic N application rate that simultaneously improved rice yield and N use efficiency in the rotation system was also investigated. Results demonstrated that soil residues and mineralized N accounted for more than 58.0% and 53.2% of the total N input in the early and late rice seasons, respectively. Similarly, most of the total N input was absorbed by rice, ranging from 43.7% to 55.6% in early rice and from 36.8% to 54.7% in late rice. Rice N use efficiency significantly decreased with increasing N application, while rice grain yield and its N uptake increased when the N application rate was below 150 kg ha⁻¹ in early rice and 200 kg ha⁻¹ in late rice. Exceeding this point limited rice N uptake and yield formation. The apparent N recovery rate, N residual rate, and N loss rate were 23.5–34.4%, 17.0–47.1%, and 26.0–47.8% for the early rice, and 32.8–37.3%, 74.2–87.0%, and 71.5–92.1% for the late rice. The linear plateau analysis further indicated that the recommended N application rate (118.5–152.8 kg ha⁻¹ for early rice and 169.9–186.2 kg ha⁻¹ for late rice) can not only maintain a relatively higher rice yield and N utilization but also significantly reduce soil N residue. Our results provide theoretical guidance for improving N management in double-cropping rice fields in southern China.