The application of high-quality seeds ensures successful crop establishment, healthy growth, and improved production in both quantity and quality. Recently, biochar-based seed coating has been recognized as a new, effective, and environmentally friendly method to enhance seed quality, seedling uniformity, and nutrient availability. To study the impact of biochar coating on the surface mechanical properties of coated seeds, rice emergence and growth, and related physical and physiological metabolic events, laboratory experiments were performed on two water-saving and drought-resistance rice (WDR) varieties (Huhan1512 and Hanyou73) using biochar formulations with varying contents (20%–60%). The results showed that the appropriate concentration of biochar significantly improved emergence traits and seedling performance of the two rice varieties, compared to the uncoated treatment, and that the optimal percentage of biochar coating was 30% (BC30). On average, across both varieties, BC30 enhanced emergence rate (9.5%), emergence index (42.9%), shoot length (19.5%), root length (23.7%), shoot dry weight (25.1%), and root dry weight (49.8%). The improved germination characteristics and vigorous seedling growth induced by biochar coating were strongly associated with higher water uptake by seeds, increased α-amylase activity and respiration rate, and enhanced accumulation of soluble sugar and soluble protein. Moreover, the evaluation results of mechanical properties related to seed coating quality found that increasing the proportion of biochar in the coating blend decreased the integrity and compressive strength of the coated seeds and reduced the time required for coating disintegration. In conclusion, biochar coating is a cost-effective strategy for enhancing crop seed quality and seedling establishment.

Ammonium and nitrate nitrogen are the two main forms of inorganic nitrogen (N) available to crops. However, it is not clear how mixtures of ammonium and nitrate N affect N uptake and partitioning in major rice cultivars in southern China. This study investigated the effects of different ammonium nitrogen and nitrate nitrogen mixture treatments (100:0, 75:25, 50:50, 25:75, and 0:100) on the growth, photosynthetic characteristics, nitrogen uptake, gene expression, and yield of different rice cultivars (Mei Xiang Zhan NO. 2: MXZ2; Nan Jing Xiang Zhan: NJXZ). Rice root biomass, tiller number, and yield were increased by 69.5%, 42.5%, and 46.8%, respectively, in the 75:25 ammonium-nitrate mixed treatment compared to the 100:0 ammonium-nitrate mixed treatment. The nitrogen content in rice roots, stems, leaves, and grains increased by 69.5%, 64.0%, 65.5%, and 17.5%, respectively. In addition, compared with MXZ2, NJXZ had a greater proportion of N allocated to leaves and grains. Analysis of root enzyme activities revealed that the 75:25 ammonium-nitrate mixed nutrient treatment increased rice root glutamine synthetase activity by an average of 35.0% and glutamate synthetase activity by an average of 52.0%. Transcriptome analysis revealed that the 75:25 mixed ammonium-nitrate nutrient treatment upregulated the expression of genes related to the nitrogen metabolism transporter pathway. Weighted correlation network analysis revealed that some differentially expressed genes (HISX and RPAB5) regulated the activities of nitrogen-metabolizing enzymes in rice and some (SAT2, CYSKP, SYIM, CHI1, and XIP1) modulated amino acid synthesis; greater expression of these genes was detected in the 75:25 ammonium-nitrate mixed nutrient treatment. The expression characteristics of the above genes were further confirmed by RT‒qPCR. Interestingly, the expression levels of the above genes were significantly correlated with the glutamate synthase activity, photosynthetic rate, and root volume. It is noteworthy that increasing the expression of the aforementioned genes coupled with nitrogen uptake was observed in the three main rice cultivars. These results suggest that the 75:25 ammonium-nitrate mixture may have increased nitrogen-metabolizing enzyme activities and promoted nitrogen uptake through the upregulated expression of nitrogen metabolism-related genes, thereby increasing tiller number and improving rice yield.

Lead (Pb) and cadmium (Cd) severely impair rice growth, yield, and grain quality. This study assessed the role of exogenous gamma-aminobutyric acid (GABA) in mitigating Pb and Cd toxicity in aromatic rice ‘Guixiangzhan’. Treatments included the control (no Pb, Cd, or GABA), GABA (1 mM), Pb (800 mg/kg of soil)+GABA, Cd (75 mg/kg of soil)+GABA, Pb+Cd+GABA, Pb, Cd, and Pb+Cd without GABA. GABA improved chlorophyll and carotenoid, protein, proline and GABA contents whilst reducing oxidative stress under Pb/Cd toxicity. GABA application regulated antioxidant enzyme activities, net photosynthesis, and gas exchange, while its effects on nitrate reductase and glutamine synthetase were variable. Compared with Pb+Cd, the grain yields were 34.03%, 31.94%, 15.88%, 24.86%, and 17.32% higher in (Pb, Cd, Pb+Cd)+GABA, Pb, and Cd treatments, respectively. Furthermore, GABA reduced Pb and Cd accumulation in aboveground parts, while Ca, Mg, Fe, Cu, Zn, and Mn levels varied across treatments. Cd translocation was more from root-to-leaves, while Pb translocation was more from leaves-to-grains. Grain Pb and Cd positively correlated with their root, stem, and leaf contents but negatively with mineral nutrients. Overall, exogenous GABA mitigated Pb and Cd toxicity in aromatic rice.

Cerium oxide nanoparticles (CeO₂NPs) can boost crops’ salt tolerance, yet their regulatory mechanisms in rice cultivars with contrasting salt tolerance remain unclear. This study investigated the regulatory differences in poly (acrylic acid)-coated nanoceria (PNC)-primed in salt-sensitive (Huanghuazhan, H) and salt-tolerant (Xiangliangyou900, X) rice. The results showed that PNC priming improved salt tolerance in two cultivars, but the underlying mechanisms differed. In the H cultivar, the enhanced tolerance was primarily attributed to enhanced photosynthesis (net photosynthesis and transpiration rates were 53.27% and 20.52% higher than the X cultivar); increased abscisic acid (ABA) content (up by 18.80% compared to the X cultivar), and activated stress-responsive signaling. Metabolomics further revealed that the differential metabolites were enriched in galactose metabolism, ascorbate, and aldarate metabolism, synergistically maintaining intracellular redox balance. In the X cultivar, PNC boosted reactive oxygen species’ (ROS) scavenging capacity (catalase (CAT) increased 36.07%, H₂O₂ and malondialdehyde (MDA) decreased 27.31% and 48.61% compared to H); elevated endogenous indole-3-acetic acid (IAA) and gibberellic acid3 (GA3) levels by 9.55% and 9.08%; and specifically activated cellular defense response and glutathione metabolism. Transcriptome analysis further revealed that the expression of IAA/GA3 signal-responsive genes (OsARGOS/OsGASR2) and antioxidant genes (OsCatA, OsAPX1) were significantly higher in the X cultivar than the H cultivar (p < 0.05), whereas the H cultivar showed higher expression of GST and ABA-related genes. This study provides a new perspective for the mechanism of PNC-enhanced salt tolerance in rice.

Cadmium (Cd) stress poses significant threats to rice growth and production, worsening food security and safety challenges, necessitating effective mitigat