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Rice is a major food crop in the world. Owing to the shortage of rural labor and the development of agricultural mechanization, direct seeding has become the main method of rice cultivation. At present, the main problems faced by direct seeding of rice are low whole seedling rate, serious weeds, and easy lodging of rice in the middle and late stages of growth. Along with the rapid development of functional genomics, the functions of a large number of genes have been confirmed, including seed vigor, low-temperature tolerance germination, low oxygen tolerance growth, early seedling vigor, early root vigor, resistance to lodging, and other functional genes related to the direct seeding of rice. A review of the related functional genes has not yet been reported. In this study, the genes related to direct seeding of rice are summarized to comprehensively understand the genetic basis and mechanism of action in direct seeding of rice and to lay the foundation for further basic theoretical research and breeding application research in direct seeding of rice.

Main conclusions (1) Mesocotyl elongation is responsive to abiotic stresses, such as deep sowing drought, submergence, chilling, and salinity. (2) Humus soil culture with a burial depth of 6 cm and at the temperature of 30 °C could be the optimum method for mesocotyl length phenotyping, The frequently colocalized quantitative trait loci (QTL) controlling mesocotyl elongation were located on chromosome (3) 1 (RM562-RG146), chromosome 2 (RZ288-RM145), and chromosome 3 (RM426-RM520). Dry direct-seeding is becoming a popular rice cultivation technology in many countries, which reduces water use and labor costs enormously. Meanwhile, direct-seeding rice is also facing the problems of low seedling emergence rate, poor seedling establishment, weed infestation, and high crop lodging rate. To take the full advantages of direct-seeding, both agronomic and genetic solutions are needed. Varieties with optimum mesocotyl length are desired for improving rice seedling emergence rate, particularly under deep sowing and submergence, which is adopted to reduce lodging and increase tolerance to abiotic stresses. In this review, we summarized the physiological and genetic mechanisms of mesocotyl elongation in rice. The elongation of mesocotyl is affected by light, temperature, and water, and, as a result, is responsive to sowing depth, water content, and soil salinity. Plant hormones such as abscisic acid (ABA), brassinosteroid (BR), strigolactones (SLs), cytokinin (CTK), ethylene (ETH), jasmonic acid (JA), gibberellin (GA), and indole-3-acetic acid (IAA) play important roles in regulating mesocotyl elongation. A humus soil culture protocol developed by our team was shown to be a better high-throughput method for measuring mesocotyl length in large scale. Sixty-seven QTL controlling mesocotyl length were reported, which are distributed on all the 12 chromosomes. Twelve chromosomal regions were repeatedly found to have QTL using various mapping populations and methods. These regions should be targeted in future studies to isolate genes and develop markers for molecular breeding. Two genes with very different molecular functions have been cloned, highlighting the genetic complexity of mesocotyl elongation.

Adoption of appropriate agronomic practices, such as optimum seeding and nitrogen (N) rates, in synchronization with proper water management practice could help improve yield, water productivity and N use efficiency (NUE) of rice ( Oryza sativa L.). Field experiments were conducted at the research farm of the Asian Institute of Technology during the dry seasons (November–March) of 2017–2018 and 2018–2019 to evaluate the effects of seeding (S) (95 [S 95 ], 125 [S 125 ] and 155 [S 155 ] kg ha −1 ) and N (0 [N 0 ], 60 [N 60 ] and 120 [N 120 ] kg ha −1 ) rates under different water regimes (continuous flooding [CF] and safe alternate wetting and drying [AWD15]) on yield, water productivity and NUE of wet direct-seeded rice. Application of 120 kg N ha −1 (N 120 ) resulted in the highest grain yield and water productivity (by 76% and 78%, respectively, compared with N 0 ), which were statistically similar with N 60 . Less plant population in lower seeding rates (S 95 and S 125 ) provided better results in terms of vegetative and reproductive growth, grain yield and water productivity. Rice grown under AWD15 resulted in statistically similar grain yield with rice maintained under CF, but there was 40–44% more water savings depending on seeding rates and 68% higher water productivity in AWD15-treated plots. Improving sink capacity and dry matter accumulation is key to ensuring better grain yields even with reduced N and seeding rates under AWD15. A stronger relationship between grain yield and spikelet number panicle −1 as well as between shoot dry matter and spikelet number panicle −1 was observed under CF (r = 0.61 and r = 0.57, respectively) compared with AWD15 (r = 0.45 and r = 0.41, respectively). N 60 had significantly higher partial factor productivity (64.6) of applied N than N 120 (36.4). S 95 and S 125 resulted in a significant improvement in NUE compared with S 155 . This result suggests that desirable grain yield and water productivity as well as better NUE could be achieved by decreasing N rate from 120 to 60 kg ha −1 , and thus significant reduction in fertilizer input cost could be possible along with environmental benefits. In addition, decreasing seeding rate coupled with AWD15 is recommended to reduce the input cost and total water input to make the wet direct-seeded rice cultivation system more sustainable and profitable.

Direct seeding rice (DSR) is a cost-saving and efficient production technology. It is of great significance to identify the characteristics of nitrogen demand of DSR in further improving yield and nitrogen use efficiency. In this study, a field experiment was carried out with inbred rice (HHZ) and hybrid rice (FLY4, YLY6) in Wuhan, China, in 2020 and 2021, respectively. Two fertilization ratios are as follows: basal fertilizer: tillering fertilizer: panicle fertilizer = 5:2:3 (T0); basal-tillering fertilizer: panicle fertilizer = 6:4. Four first fertilization time set as follows: T1, basal-tillering fertilizer was applied at 10 days after seeding (DAS); T2, basal-tillering fertilizer was applied at 15 DAS; T3, basal-tillering fertilizer was applied at 20 DAS; and T4, basal-tillering fertilizer was applied at 25 DAS. The changes of yield and yield components, above-ground biomass accumulation, nitrogen use efficiency, harvest index (HI), and leaf SPAD were investigated. The results showed that the yield of DSR could be significantly increased by properly postponing the first basal-tillering fertilization time. Compared with the T0, the basal-tiller fertilizer application at 20 DAS could significantly increase the yield by 15.1%, mainly because the number of spikelets per unit area increased by 18.9% on average across years and varieties. Delaying the first application time of basal-tillering fertilizer in DSR can significantly increase the biomass accumulation of above-ground leaves and stems and increase the yield, but with the decrease of HI in T3 and T4. In addition, delaying the first application time of basal-tiller fertilizer could also effectively improve the nitrogen use efficiency by 12.1% in DSR. The PFP and AE of T3 were significantly increased by 15.1% and 55.9% compared with T0, respectively. Delayed basal-tiller fertilizer treatment laid the foundation for maintaining higher leaf SPAD value in later developmental stage. To sum up, delaying the first application time of basal-tillering fertilizer was beneficial to build a better population of DSR, with high yield and nitrogen use efficiency, which has important reference significance for further optimization of light and simplified direct seeding nitrogen technology of rice. However, the delay of basal-tillering fertilizer application time leads to the increase of biomass, accompanied by the risk of plant lodging. Therefore, the optimal application amount of basal-tillering fertilizer needs to be further explored.

Dry direct seeding of rice (DDSR) is being increasingly used in water-scarce areas across Asia. When drought occurs, deep sowing may enable germinating seeds to utilize the residual moisture below the surface. Our objective was to examine the effects of different sowing depths on DDSR crop growth. Two yield experiments, one in the Philippines and the other in Japan, were established in rainfed lowlands, but drought did not occur. Across the experiments, sowing at a depth of 6 to 7 cm reduced the percentage of emergence compared with sowing at a depth of 1 to 2 cm (25% vs. 73%), which resulted in lower yield (3.5 vs. 4.4 t ha −1 ). The relative yield (deep-sowing/shallow-sowing) was positively correlated with the percentage emergence under deep sowing. Cultivars with no significant yield reduction under deep sowing maintained high emergence (>30%) under deep sowing. Seedling's ability to emerge after deep sowing greatly differed among cultivars: the percentage emergence ranging from 0% to 18% from a depth of 8 to 10 cm, and from 11% to 44% from a depth of 6 to 7 cm. An ancillary field experiment was conducted under drought during the dry season in the Philippines: deep sowing increased the percentage emergence compared with shallow sowing (41% vs. 5%), when the soil surface was severely dry. Our results suggest that choosing cultivars that can tolerate deep sowing is a prerequisite for using deep sowing in DDSR, and that future breeding of DDSR should focus on this tolerance.

MAIN CONCLUSION : Totally, 23 loci were detected, and 383 candidate genes were identified, and four of these candidate genes, Os01g0392100, Os04g0630000, Os01g0904700 and Os07g0615000 , were regarded as promising targets. Direct-seeding cultivation is becoming popular in rice (Oryza sativa L.)-planting countries because it is labor- and time-efficient. However, low seedling establishment and slow seedling emergence have restricted the application and popularity of the technique. Mesocotyl elongation and shoot length are two important traits that can enhance rice seedling emergence. A single nucleotide polymorphism (SNP) is a genome sequence variation caused by a single base within a population, and SNPs evenly distributed throughout the genomes of plant species. In this study, a genome-wide association study (GWAS), based on 4136 SNPs, was performed using a compressed mixed linear model that accounted for population structure and relative kinship to detect novel loci for the two traits. Totally, 23 loci were identified, including five loci located known QTLs region. For the mesocotyl elongation, 17 major loci were identified, explaining ~19.31 % of the phenotypic variation. For the shoot length, six major loci were detected, explaining ~39.79 % of the phenotypic variation. In total, 383 candidate genes were included in a 200-kb genomic region (±100 kb of each locus). Additionally, 32 SNPs were identified in 30 candidate genes. Relative expression level analyses indicated that four candidate genes containing SNP variations, Os01g0392100, Os04g0630000, Os01g0904700 and Os07g0615000, represented promising targets. Finally, eight elite accessions with long mesocotyl and shoot lengths were chosen as breeding donors for further rice direct-seeding variety modifications.

This paper aims to solve the problem of high reseeding rates and mis-seeding rates in the rice multi-grain hole direct seeding process. A multi grain cluster air suction type rice hill direct seed metering device was developed, and its seeding mechanism was analyzed. Based on CFD-DEM coupling simulation and bench tests, this study explored and optimized the performance of the seed metering device, and carried out the seeding adaptability test. The simulation results were as follows: when the negative pressure was −5 kPa, the static pressure, dynamic pressure and velocity of the flow field reached the maximum. When the negative pressure was −4 kPa, the qualification index was 89.62%, the reseeding index was 4.36%, and the mis-seeding index was 6.02%. The results of the orthogonal rotation combination test of three factors and five levels showed that when the rotation speed, negative pressure and the length of stirring brush were 20.70 rpm, −4.0 kPa and 10.50 mm respectively, the seed metering performance was the best, the qualification index was 90.85%, the reseeding index was 4.41% and the mis-seeding index was 4.74%. The seed metering device had strong adaptability to the sowing of different rice varieties, and met the agronomic requirements of direct seeding and seeding in rice holes.

Higher demand and cost of labor and water shortage have forced the farmers to look for an alternate method of cultivation in rice as a substitute to the existing conventional transplanting. Dry direct seeding and water seeding have emerged as better alternatives over transplanting method. These methods not only result in labor saving, but also result in significant water saving in rice. These are important adaptation strategies to the impending climate change. However, the direct seeding method is confronted with severe weed infestation and yield losses if weeds are not managed well. Against this backdrop, a field study was undertaken during kharif seasons of 2019 and 2020 to evaluate the effect of crop establishment methods and weed management practices on rice and its associated weed flora. The results demonstrated that grain yields obtained under water seeding (WS) were statistically at par with transplanting (CT), but significantly superior to dry direct seeding (DDSR). Yield attributes were significantly superior in WS as compared DDSR, but were at par with CT. Weed density followed the order of DDSR > WS > CT. With the advancement in age of the crop, sedges dominated in DDSR, whereas broad-leafweeds (BLW) dominated in WS and CT methods of establishment. All the herbicides reduced the weed density significantly as compared to weedy check. Penoxulam (PE) reduced the weed density and weed dry matter on an average by 91% and 92% at 30 DAS/DAT over weedy check, respectively. PE proved significantly superior in controlling all the sedges and grasses but was less effective against BLW. Maximum reduction in yield due to weeds was observed in weedy check (WC) (58%) and the lowest was observed in PE (3%). Application of PE @ 22.5 g ha−1 under the WS method of crop establishment resulted in highest average weed control efficiency and grain yield.

As a photophilous plant, rice is susceptible to low-light stress during its growth. The Sichuan Basin is a typical low-light rice-producing area. In this study, eight rice varieties with different shade tolerances were studied from 2021 to 2022. The physiological adaptability and yield formation characteristics of rice were studied with respect to photosynthetic physiological characteristics and dry matter accumulation characteristics, and the response mechanism of rice to low light stress was revealed. The results showed that the shading treatment significantly increased the chlorophyll a, chlorophyll b, and total chlorophyll contents in the leaves of direct-seeded rice after heading, and the total chlorophyll content increased by 1.68–29.70%. Nitrate reductase (NR) activity first increased and then decreased under each treatment, and the shading treatment reduced the NR activity of direct-seeded rice. Compared to the control treatment, the peroxidase (POD) activity of each variety increased from 7 to 24 d after the shading treatment. The transketolase (TK) activity in direct-seeded hybrid rice increased under low light stress. Compared with the control, shading treatment significantly reduced the aboveground dry matter, grain number per panicle, and seed setting rate of direct-seeded rice at the full heading stage and maturity stage, thus reducing the yield of direct-seeded rice by 26.10–34.11%. However, under the shading treatment, Zhenliangyou 2018 and Jingliangyou 534 maintained higher chlorophyll content and related enzyme activities, accumulated more photosynthetic products, and reduced yield. In general, Zhenliangyou 2018 and Jingliangyou 534 still had a yield of 7.06–8.33 t·hm−2 under low light. It indicated that Zhenliangyou 2018 and Jingliangyou 534 had better stability and stronger tolerance to weak light stress and had a higher yield potential in weak light areas such as Sichuan.

Dry direct seeding rice (DSR) is an emerging production system because of increasing labor and water scarcity in rice cultivation. The limited availability of rice cultivars suitable for dry direct seeding hampers the widespread adoption of this cultivation method in Northeast China. This study aimed to investigate grain production and plant characteristics associated with dry direct seeding rice. We conducted a field experiment on 79 japonica rice cultivars in Shenyang City, Liaoning Province, Northeast China, in 2020 and 2021. This study found that the grain yield of the tested rice cultivars ranged from 5.75–11.00 t ha−1, with a growth duration lasting between 144–161 days across the cultivars. These cultivars were then categorized into high yielding (HY), medium yielding (MY), and low yielding (LY) based on daily yield by using Ward’s hierarchical clustering method. The higher grain yield for HY compared to MY and LY was attributed to more spikelets per unit area. The HY alleviated the conflict between higher panicle density and larger panicle size by improving the seedling emergence rate and productive stem rate. It also significantly increased shoot biomass at maturity. The HY reduced the period between seeding and beginning of heading (BBCH 51) and the proportion of dry matter partitioned to the leaf at the heading stage. However, it also increased the accumulation of dry matter in the grain and the proportion of dry matter partitioned to the grain at maturity. Furthermore, the HY markedly increased the harvest index and grain-leaf ratio, which are beneficial to coordinate the source–sink relationship. A quadratic function predicted that 98 days is the optimum growth duration before heading (BBCH 51) for achieving maximum yield. In conclusion, for dry direct seeding rice, it is appropriate to select high-yielding japonica inbred rice cultivars with shorter growth duration before heading (about 93–102 day), higher panicle number (about 450–500 × 104 ha–1), more spikelet number per panicle (about 110–130), higher seedling emergence rate (about 70–75%), higher productive stem rate (about 60–70%), and greater harvest index (about 50–55%).