Xuhua Zhong

Optimized nitrogen (N) management for rice production typically involves multiple split applications of N fertilizer to enhance rice yield and nitrogen use efficiency (NUE). However, China's rural labor shortage highlights the need to reduce the frequency of fertilizer applications to minimize labor inputs and improve production efficiency. A simplified N management involved less application frequency at critical stages in rice production, but its effectiveness was still uncertain. Thus, a field experiment was conducted in both early and late seasons of 2018 with 5 treatments: N0 (no N fertilizer), TC (optimized N management involving application at tillering, panicle initiation (PI), and grain filling stages); N1 and N2 (one‐time N top‐dressing at uppermost internode elongation (IE) or PI with the same total N input as TC); and RN1 and RN2 (one‐time N top‐dressing at IE or PI with a reduced total N input compared to TC). The effects of N management on grain yield, yield components, agronomic traits, biomass production, N uptake, and NUE were evaluated. In both seasons, N2 achieved grain yield and NUE comparable to TC, whereas N1 exhibited significantly lower grain yield, agronomic N use efficiency (AE _N ), partial factor productivity (PFP _N ), and physiological N use efficiency (PE _N ). Furthermore, RN1 and RN2 resulted in significant yield reductions in the late season relative to TC. These suggested that tiller and grain N fertilizers might be omitted, and one‐time N top‐dressing at PI stage could effectively support high grain yield and NUE. The comparable performance of N2 and TC was primarily attributed to an optimal number of panicles and filled grains, as well as similar total biomass production and N uptake, resulting from the appropriate timing of panicle fertilizer application. Overall, N2 represented a labor‐saving N fertilizer management strategy that sustained high grain yield and NUE under current labor‐constrained production systems.

Small‐grain‐weight rice varieties are popular in South China. However, to maintain grain yields comparable to medium‐grain‐weight varieties, adjustments in nitrogen (N) and seeding rate management may be necessary. In this study, the small‐grain‐weight variety Mabayinzhan was direct‐seeded in a three‐season field experiment conducted from 2022 to 2023 in Qujiang County, Guangdong Province, South China, using four N rates and three seeding rates. The results showed that grain yield and grain weight ranged from 3.45 to 7.95 t ha ⁻¹ and from 11.4 to 14.1 mg, respectively, across all treatments with N application. The variation in grain yield was primarily attributed to differences in spikelets per m ² , biomass production, and grain weight. Grain yield generally increased with higher N rates, but no significant difference was observed across the range of seeding rates. The grain yield responded more strongly to N rate than to seeding rate. The decrease in grain yield resulting from lower N rates could not be compensated by increasing seeding rate. While dense planting with reduced N application could improve N use efficiency (NUE), the increase in NUE was due to the reduced N rate rather than the seeding rate. Our findings indicate that high seeding rates are not essential for achieving high yields with a small‐grain‐weight rice variety. Moreover, increasing planting density while reducing N application may not be optimal for small‐grain‐weight inbred rice production under direct seeding conditions in South China.

Seed development plays a critical role in determining both crop yield and grain quality in rice. As a key nutrient storage organ, the rice endosperm development not only contributes to grain filling but also plays an essential role during the early stages of seed germination. Amino acid metabolism is active during the process of seed development and seed germination. Asparagine is a primary amino acid responsible for long-distance organic nitrogen transport in plants. Asparagine synthetase catalyzes the synthesis of asparagine from aspartate and glutamine. In this study, CRISPR/Cas9-mediated knockout mutants of the OsASN2 gene of rice were generated. Homozygous mutants exhibited complete failure of seed germination, and heterozygotes could not produce homozygous offspring. Endosperm development of homozygous mutant seeds showed severe defects. Additionally, interacting protein screening combined with pull-down and co-immunoprecipitation (Co-IP) assays confirmed that OsASN2 physically interacted with pyruvate phosphate dikinase OsPPDKB, the mutants of which showed impaired endosperm development. These findings collectively indicate that OsASN2 plays a critical role in seed development and germination in 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 (N_til) and panicle development (N_PI) based on ASD and crop N status in direct‐seeded rice. Field experiments were conducted involving 12 treatments, consisting of four N_til and three seeding rates in 2017, and eight treatments combining seeding rate, N_til, and N_PI in 2020.RESULTS: Linear regression analysis revealed that the tiller number at panicle initiation (TIL_PI) was predominantly influenced by ASD and N_til. The determination coefficients (R²) of the regression models ranged from 0.887 to 0.936 across the four‐season experiments. The results indicated that N_til 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 (NUP_PI) and N_PI, with R² of 0.814 and 0.783 in the early and late seasons of 2020, respectively. This suggested that N_PI could be calculated based on NUP_PI 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.

Rice–fish co-cultures have been practiced for over 2000 years, and they have tremendous potential in terms of increasing food security and economic benefits. However, little research has been conducted into achieving stable yields and high lodging resistance with regard to rice while simultaneously promoting the harmonious and healthy growth of fish in rice–fish co-culture paddy fields. We conducted a field study aimed at selecting suitable rice varieties for rice–fish co-culture systems (encompassing both ratoon and main crop). This selection process was grounded in an evaluation of lodging resistance and grain yield among 33 rice varieties used throughout the studied region. The results revealed a range of lodging indices of the main crop for the second internode, spanning from 62.43 to 138.75, and the annual grain yield (main crop and ratoon crop) ranged from 7.17 to 13.10 t ha⁻¹ within rice–fish co-culture systems. We found that the use of rice–fish co-culture farming could improve the milling quality, nutrient quality, and appearance quality of rice, though the improvement gained through co-culturing varied across rice varieties. Moreover, the lodging index of the three basal internodes of rice plants was significantly and positively correlated with the plant height and the culm fresh weight, but it was negatively correlated with the bending strength of the rice basal internodes. Additionally, the 33 tested rice varieties were clustered in accordance with their lodging resistance (i.e., high resistance with lodging indices 62.43–75.42; medium resistance with lodging indices 80.57–104.62; and low resistance with lodging indices 113.02–138.75) according to the hierarchical cluster analysis. The 33 rice varieties were also clustered in accordance with the annual (main crop and ratoon crop) grain yield (i.e., high yield with 11.17–13.10 t ha⁻¹; medium yield with 10.15–10.83 t ha⁻¹; and low yield with 7.16–9.88 t ha⁻¹). In all, 11 rice varieties were identified by a comprehensive evaluation as suitable varieties for grain production in the rice–fish co-culture system. These varieties displayed favorable traits, including a high annual rice yield, strong lodging resistance, and good grain quality. This is the first study to systematically evaluate rice varieties based on grain yield, lodging resistance, and grain quality in rice–fish co-culture systems.

The CORIGAP project was implemented in six main rice granaries in South and Southeast Asia. The project introduced several country-specific sustainable best management practices, including nutrient management, pest management, water management, and several postharvest technologies, among other specific practices. This chapter introduces each country and its respective challenges to rice production. It outlines cultivation practices, historical developments, and their impacts on opportunities for the development of the rice sector. This is accompanied by specific case studies that highlight the adoption of specific technologies and practices. Case studies encompass the adoption of various best management practices in Myanmar and Thailand, especially postharvest technologies. Furthermore, the chapter highlights the outreach of “One Must Do, Five Reductions” in Vietnam, the development and implementation of the “Three Controls Technology” and alternate wetting and drying (AWD) practices in China, rodent pest management in Indonesia, and weed and water management in Sri Lanka. These case studies identified positive agronomic, social, and economic changes. The chapter concludes by harnessing the agricultural development strategies in each country with a synthesis of outcomes and impacts.

Increasing evidence shows that improved nitrogen management can enhance lodging resistance and lower internodes play a key role in the lodging resistance of rice. However, little is known about the cellular and molecular mechanisms underlying the enhanced lodging resistance under improved nitrogen management. In the present study, two rice varieties, with contrasting lodging resistance, were grown under optimized N management (OPT) and farmers’ fertilizer practices. Under OPT, the lower internodes of both cultivars were shorter but the upper internodes were longer, while both culm diameter and wall thickness of lower internodes were dramatically increased. Microscopic examination showed that the culm wall of lower internodes under OPT contained more sclerenchyma cells beneath epidermis and vascular bundle sheath. The genome-wide gene expression profiling revealed that transcription of genes encoding cell wall loosening factors was down-regulated while transcription of genes participating in lignin and starch synthesis was up-regulated under OPT, resulting in inhibition of longitudinal growth, promotion in transverse growth of lower internodes and enhancement of lodging resistance. This is the first comprehensive report on the morpho-anatomical, mechanical, and molecular mechanisms of lodging resistance of rice under optimized N management.