The objective of this study was to investigate the regulatory effects of sowing density on stem lodging resistance in main crop and annual grain yield of precision-hill-direct-seeding ratoon rice. The results could provide a scientific basis for ratoon rice production with high yield and lodging resistance to adapt to direct seeding and simplified cultivation.

From 2022 to 2023, a field experiment was conducted using two hybrid indica rice varieties Yuxiangyou8133 (YXY8133) and Taiyouyuehesimiao (TYYHSM) with five sowing densities, namely, D1: 150000 hills/hm², D2: 195000 hills/hm², D3: 240000 hills/hm², D4: 285 000 hills/hm², and D5: 330000 hills/hm². The study focused on analyzing the relationships of stem mechanical parameters, morphological traits, dry matter accumulation and distribution, and culm and leaf sheath chemical composition with stem lodging resistance in the main crop and grain yield in the main and ratoon crops under the precision-hill-direct-seeding ratoon rice system.

With increased sowing density, rice yield in both main and ratoon crop increased first and then decreased, and annual yield was highest under D3, reaching 14.03 and 13.23 t·hm^-2 for YXY8133 and TYYHSM, respectively. Compared with the D1 treatment, D2 and D3 treatments slightly increased stem lodging index of main crop by 3.91%-14.69% and 1.48%-5.77% for YXY8133 and TYYHSM, respectively; D4 and D5 treatments markedly increased the lodging index by 31.67%-40.74% and 10.54%-23.27% for YXY8133 and TYYHSM, respectively. The increased sowing density induced significant reduction of breaking moment of basal internodes and thereby, mainly contributed to higher stem lodging index in main crop. Internode length, panicle length, plant height, and gravity center height did not differ significantly among sowing-density treatments, but LAI increased significantly with higher sowing density. The culm and leaf sheath dry weight per plant, panicle dry weight per plant, basal internode culm and leaf sheath plumpness, stem diameter and wall thickness of D3 treatment decreased slightly when compared with D1, thereby maintaining high stem mechanical strength; while D4 and D5 treatments showed larger reduction, and with differences reaching significance for TYYHSM. Further, D4 and D5 treatments significantly decreased the soluble sugar content, starch content, cellulose and lignin content in culm and leaf sheath per plant of main crop at heading stage and thus, resulted in poor stem quality and a higher lodging index in main crop. Correlation analysis showed that culm and leaf sheath dry matter weight per plant, culm wall thickness, soluble sugar content, starch content, cellulose and lignin content were significantly and positively correlated with breaking moment of basal internodes, but significantly and negatively correlated with lodging index.

The appropriate sowing density can effectively balance the trade-off between population size and individual plant growth in main crops under precision hill-direct-seeding ratoon rice, enhance individual plant robustness by maintaining high stem quality, improve the population structure, and consequently, enhance stem lodging resistance and yield performance; D3 treatment was the suitable sowing density to coordinate the lodging resistance in main crop and the high yield of ratoon rice system.

The objective of current study was to investigate the regulation mechanism of endogenous hormones, culm anatomical and morphological traits and its relationship with lodging resistance of hybrid indica rice as affected by foliar application uniconazole under shading condition. The study results could provide theoretical basis and technical support for stable and abundant hybrid indica rice yield and emergency chemical regulation cultivation under poor light stress in Sichuan Basin.

A split-split plot field experiment was carried out in Yongchuan, Chongqing from 2018 to 2019. Shading was selected as the main plots, namely, normal light treatment (NL) and shading treatment with decreased incident light by 50% (S). The foliar application uniconazole levels were used as splits-plots, and three uniconazole levels were set: 0 (U₀), 40 mg·L^-1 (U₄₀), and 80 mg·L^-1 (U₈₀), and the varieties were used as split-split plots, namely Yuxiang203 (YX203) and Cliangyouhuazhan (CLYHZ), respectively. The study focused on analyzing the effects of foliar application uniconazole on stem lodging resistance of hybrid indica rice plant under shading condition, and further investigating the changes of culm morphological, anatomical traits and endogenous hormones and its relationship with stem mechanical strength.

Shading and uniconazole treatments had significant effects on culm morphological and anatomical traits, changes of endogenous hormones and stem lodging index (LI) and its physical parameters in two hybrid indica rice varieties. Compared with NL, S treatments significantly raised LI owing to significant reduction of breaking resistance (M), even though bending moment by whole plant (WP) also decreased, especially for YX203. Further, S treatments significantly lowered thickness of mechanical tissue, area of small and large bundle vascular in basal culm internode. And, cell width was decreased slightly, but ratio of cell length to cell width was increased to a certain extent, thus reduced culm wall thickness and stem mechanical strength consequently. Under shading stress, foliar application uniconazole significantly decreased stem LI of two hybrid indica rice varieties, which attributed to lower WP and higher M values, especially for YX203. Foliar application uniconazole reduced plant height and gravity center height by shorter length of lower internodes. And further, endogenous hormones including GA₁₊₃ and IAA content during culm formation stage was declined, induced reduction of cell length and ratio of cell length to cell width and thereby, contributed to shorter lower internodes. Besides, endogenous hormones Z+ZR and IP+IPA content were increased during culm early development, the cells size showed smaller and denser, cell layer of mechanical tissue and cell number of bundle vascular sheath were also increased, and thus improved small and larger bundle vascular number and area and stem mechanical strength. The correlation analysis showed that cell length, ratio of cell length to cell width, thickness of mechanical tissue and area of small and large bundle vascular were significantly and negatively correlated to LI, but positively correlated to M. The trend were consistent in both YX203 and CLYHZ.

Foliar application uniconazole declined cell length, ratio of cell length to cell width and length of basal internodes primarily owing to reduction of GA₁₊₃ and IAA content, and increased cell layer of mechanical tissue, cell number of bundle vascular sheath and cell density in parenchyma tissue by increasing Z+ZR and IP+IPA content, and raised thickness of mechanical tissue, area of small and large bundle vascular and thereby, improved stem mechanical strength and lodging resistance of hybrid indica rice under poor light stress.