Determining nitrogen topdressing rates in accordance with actual seedling density and crop nitrogen status in direct-seeded rice.

BACKGROUND: Adjusting nitrogen (N) input based on actual seedling density (ASD) and plant N status is a practical approach for improving the yield stability of direct-seeded rice. However, the adjustment of topdressing N rates has been empirical in the past. This study aimed to establish a quantitative approach for determining N topdressing rates during tillering (Ntil ) and panicle development (NPI ) based on ASD and crop N status in direct-seeded rice. Field experiments were conducted involving 12 treatments, consisting of four Ntil and three seeding rates in 2017, and eight treatments combining seeding rate, Ntil , and NPI in 2020. RESULTS: Linear regression analysis revealed that the tiller number at panicle initiation (TILPI ) was predominantly influenced by ASD and Ntil . The determination coefficients (R2 ) of the regression models ranged from 0.887 to 0.936 across the four-season experiments. The results indicated that Ntil could be determined accurately using ASD and the target maximum tiller number. Similarly, grain yield was influenced significantly by the N uptake at panicle initiation (NUPPI ) and NPI , with R2 of 0.814 and 0.783 in the early and late seasons of 2020, respectively. This suggested that NPI could be calculated based on NUPPI and the target grain yield. CONCLUSION: The findings offer a quantitative method for establishing N topdressing rates for tillering and panicle development, relying on the monitoring of actual seedling density and plant N status in direct-seeded rice production. © 2023 Society of Chemical Industry.

Growth, nutrient uptake and transcriptome profiling of rice seedlings in response to mixed provision of ammonium- and nitrate-nitrogen.

Nitrogen (N) is a principal macronutrient and plays a paramount role in mineral nutrition of rice plants. Mixed provision of ammonium- and nitrate-nitrogen (MPAN) at a moderate level could enhance N uptake and translocation and promote growth of rice, but current understanding of their molecular mechanisms is still insufficient. Two rice lines of W6827 and GH751, with contrasting ability of N uptake, were subjected to four levels of MPAN (NH4+/NO3- = 100:0, 75:25, 50:50, 25:75) in hydroponic experiments. In terms of plant height, growth rate and shoot biomass, growth of GH751 tended to increase firstly and then decrease with enhancement in NO3--N ratio. It attained maximal level under 75:25 MPAN, with an 8.3% increase in shoot biomass. In general, W6827 was comparatively less responsive to MPAN. For GH751, the uptake rate of N, phosphor (P) and potassium (K) under 75:25 MPAN was enhanced by 21.1%, 20.8% and 16.1% in comparison with that of control (100:0 MPAN). Meanwhile, the translocation coefficient and content in shoots of N, P and K were all increased significantly. In contrast to transcriptomic profile under control, 288 differentially expressed genes (DEGs) were detected to be up-regulated and 179 DEGs down-regulated in transcription under 75:25 MPAN. Gene Ontology analysis revealed that some DEGs were up-regulated under 75:25 MPAN and they code for proteins mainly located in membrane and integral component of membrane and involved in metal ion binding, oxidoreductase activity and other biological processes. KEGG pathway enrichment analysis indicated that DEGs related to nitrogen metabolism, carbon fixation in photosynthetic organisms, photosynthesis, starch and sucrose metabolism, and zeatin biosynthesis were up- or down-regulated in transcription under 75:25 MPAN, and they are responsible for improved nutrient uptake and translocation and enhanced growth of seedlings.

Improving grain yield and nitrogen use efficiency of direct-seeded rice with simplified and nitrogen-reduced practices under a double-cropping system in South China.

BACKGROUND: Enhancing grain yield and nitrogen use efficiency (NUE) of rice is of great importance for sustainable agricultural development. Little effort has been made to increase grain yield and NUE of direct-seeded rice under the double-cropping system in South China. Field trials were conducted during 2018-2020 with four treatments, including nitrogen-free, farmers' fertilization practice (FP), 'three controls' nutrient management (TC), and simplified and nitrogen-reduced practice (SNRP). RESULTS: Grain yield under SNRP averaged 6.46 t ha-1 during the three years and was 23.0% higher than that of FP but comparable to that of TC. Recovery efficiency (REN ), agronomic efficiency (AEN ), and partial factor productivity (PFPN ) of nitrogen under SNRP increased by 12.0-22.7%, 159.3-295.0% and 94.6-112.5% respectively compared with FP. Harvest index and sink capacity increased by 7.3-10.8% and 14.9-21.3% respectively. Percentage of productive tillers (PPT) and biomass after heading increased by 24.0% and 104.5% respectively. Leaf nitrogen concentration at heading and nitrogen accumulation after heading increased by 16.3% and 842.0% respectively. Grain yield was positively correlated with PPT, sink capacity, harvest index, biomass and nitrogen accumulation after heading, REN , AEN , and PFPN . CONCLUSION: Grain yield and NUE under SNRP were superior to those under FP and comparable to those under TC. Increase in sink capacity, higher PPT, more biomass and nitrogen accumulation after heading, and greater harvest index were responsible for high grain yield and NUE in SNRP with reduced nitrogen fertilizer and labor input. SNRP is a feasible approach for direct-seeded rice under a double-cropping system in South China. © 2023 Society of Chemical Industry.

The toxicity of heavy metals and plant signaling facilitated by biochar application: Implications for stress mitigation and crop production.

Heavy metals (HMs) accumulation in soil poses a severe threat worldwide for soil, plants, and humans. The accumulation of HMs in soil and uptake by plants leads to disrupt physiological and biochemical metabolisms. As a potential and sustainable soil amendment, biochar has attained huge attention to reduce HMs toxicity in soil and improve plant growth influenced by HMs stress. Despite an array of research studies, there is a lack of knowledge on how biochar interacts with HMs, moderate plant defence system, induce HMs stress signals pathways and promote plant growth. At first, the review highlights the possible effects of HMs on soil and plant and their consequences on plant signaling network. Secondly, the biochar's impact on soil physiochemical properties and the sorption of HMs on biochar surface through direct and indirect mechanisms are reviewed. Finally, the review shows the key roles of biochar in soil improvement to enhance plant growth and signaling response to HMs by enhancing the activities of antioxidants and reducing chlorophyll injury, reactive oxygen species (ROS) accumulation, and cell membrane degradation under HMs stress. However, future studies are needed to evaluate the role of biochar in diverse climatic conditions as well as the long-term effects of biochar on HMs persistency in soil and crop productivity. This review will provide new avenues for future studies to address and quantify the advancement in biochar's role in alleviating plant's HMs stress on a sustainable basis.

QTLs identification for nitrogen and phosphorus uptake-related traits using ultra-high density SNP linkage.

To understand the genetic basis of nitrogen and phosphorus uptake in the cultivated rice, quantitative trait loci (QTL) analysis for 7 nitrogen and phosphorus uptake-related traits including above-ground biomass (AGB), leaf colour value (SPAD) in heading stage, grain nitrogen concentration (GNC), grain nitrogen content of the plant, total nitrogen content (TNC), grain phosphorus concentration, total phosphorus content (TPC) were conducted using SNP markers in a F2 population derived from a cross between GH128 and W6827. A total of 21 QTLs for nitrogen and phosphorus uptake-related traits distributed in 16 regions along 6 chromosomes were detected using a high density genetic map consisting of 1582 bin markers, with QTLs maximum explaining 8.19% of the phenotypic variation. Nine QTLs (42.9% of total QTLs) were detected on chromosome 2. Among them, two QTL clusters including AGB, TNC, TPC and GNC were also detected in the region bin 140 and bin 146 on the chromosome 2. The distance between the two clusters was only 4.1 cM. The presence of QTL clusters has important significance and could be useful in molecular marker assisted breeding. These genomic regions might be deployed for the simultaneous improving the use efficiency of nitrogen and phosphorus in rice breeding.

Root iron plaque alleviates cadmium toxicity to rice (Oryza sativa) seedlings.

Iron plaque (IP) on root surface can enhance the tolerance of plants to environmental stresses. However, it remains unclear the impact of Fe2+ on cadmium (Cd) toxicity to rice (Oryza sativa) seedlings. In this study, the effects of different Fe2+ and Cd2+ concentration combinations on rice growth were examined hydroponically. Results indicated that Fe2+ concentration up to 3.2 mM did not damage rice roots while induced IP formation obviously. Cd2+ of 10 µM repressed rice growth significantly, while the addition of 0.2 mM Fe2+ to 10 µM Cd2+ solution (Cd+Fe) did not damage rice roots, indicating that Fe2+ could ameliorate Cd toxicity to rice seedlings. Microstructure analysis showed Cd+Fe treatment induced the formation of IP with dense and intricate network structure, Cd adsorption on the root surface was reduced significantly. Cd concentration of rice roots and shoots and Cd translocation from roots to shoots with Fe+Cd treatment were reduced by 34.1%, 36.0% and 20.1%, respectively, in comparison to a single Cd treatment. Noteworthy, the removal of IP resulted in a larger loss of root biomass under Cd treatment. In addition, Cd+Fe treatment increased the activities of root superoxide dismutase and catalase by 105.5% and 177.4%, and decreased H2O2 and O2·- accumulation of rice roots by 56.9% and 35.9%, and recovered Cd-triggered electrolyte leakage obviously, when compared with a single Cd treatment. The results from this experiment indicated that the formed dense IP on rice roots decreased Cd absorption and reactive oxygen species accumulation, and Fe2+ supply alleviated Cd toxicity to rice seedlings.

Nitrogen losses and greenhouse gas emissions under different N and water management in a subtropical double-season rice cropping system.

Nitrogen non-point pollution and greenhouse gas (GHG) emission are major challenges in rice production. This study examined options for both economic and environmental sustainability through optimizing water and N management. Field experiments were conducted to examine the crop yields, N use efficiency (NUE), greenhouse gas emissions, N losses under different N and water management. There were four treatments: zero N input with farmer's water management (N0), farmer's N and water management (FP), optimized N management with farmer's water management (OPTN) and optimized N management with alternate wetting and drying irrigation (OPTN+AWD). Grain yields in OPTN and OPTN+AWD treatments increased by 13.0-17.3% compared with FP. Ammonia volatilization (AV) was the primary pathway for N loss for all treatments and accounted for over 50% of the total losses. N losses mainly occurred before mid-tillering. N losses through AV, leaching and surface runoff in OPTN were reduced by 18.9-51.6% compared with FP. OPTN+AWD further reduced N losses from surface runoff and leaching by 39.1% and 6.2% in early rice season, and by 46.7% and 23.5% in late rice season, respectively, compared with OPTN. The CH4 emissions in OPTN+AWD were 20.4-45.4% lower than in OPTN and FP. Total global warming potential of CH4 and N2O was the lowest in OPTN+AWD. On-farm comparison confirmed that N loss through runoff in OPTN+AWD was reduced by over 40% as compared with FP. OPTN and OPTN+AWD significantly increased grain yield by 6.7-13.9%. These results indicated that optimizing water and N management can be a simple and effective approach for enhancing yield with reduced environmental footprints.

Silicate-mediated alleviation of Pb toxicity in banana grown in Pb-contaminated soil.

Silicate (Si) can enhance plant resistance or tolerance to the toxicity of heavy metals. However, it remains unclear whether Si can ameliorate lead (Pb) toxicity in banana (Musa xparadisiaca) roots. In this study, treatment with 800 mg kg(-1) Pb decreased both the shoot and root weight of banana seedlings. The amendment of 800 mg kg(-1) Si (sodium metasilicate, Na(2)SiO(3)·9H(2)O) to the Pb-contaminated soil enhanced banana biomass at two growth stages significantly. The amendment of 800 mg kg(-1) Si significantly increased soil pH and decreased exchangeable Pb, thus reducing soil Pb availability, while Si addition of 100 mg kg(-1) did not influence soil pH. Results from Pb fractionation analysis indicated that more Pb were in the form of carbonate and residual-bound fractions in the Si-amended Pb-contaminated soils. The ratio of Pb-bound carbonate to the total Pb tended to increase with increasing growth stages. Treatment with 100 mg kg(-1) Si had smaller effects on Pb forms in the Si-amended soils than that of 800 mg kg(-1) Si. Pb treatment decreased the xylem sap greatly, but the addition of Si at both levels increased xylem sap and reduced Pb concentration in xylem sap significantly in the Si-amended Pb treatments. The addition of Si increased the activities of POD, SOD, and CAT in banana roots by 14.2% to 72.1% in the Si-amended Pb treatments. The results suggested that Si-enhanced tolerance to Pb toxicity in banana seedlings was associated with Pb immobilization in the soils, the decrease of Pb transport from roots to shoots, and Si-mediated detoxification of Pb in the plants.