To study the effects of different irrigation volumes on the agronomic traits, lodging resistance and yield formation of rice, and to explore a more suitable water management plan for high-yield and stable-yield green cultivation of rice, and to provide a theoretical basis for sustainable rice production.

Using WYD4 and JND667 as experimental materials, six different irrigation volume treatments were carried out: W1 (12000 m³·hm^-2), W2 (10500 m³·hm^-2), W3 (9000 m³·hm^-2), W4 (7500 m³·hm^-2), W5 (6000 m³·hm^-2) and W6 (natural rainfall). By studying the effects of different irrigation volumes on the plant architecture, resistance to lodging, photosynthetic characteristics, dry matter accumulation and yield of rice, the regulatory effects and mechanisms of different irrigation volumes on these aspects were clarified.

Reducing irrigation properly increased the final tiller number, the percentage of productive tillers, leaf length of the top three leaves, and leaf area index of rice, while reducing the leaf width and leaf base angle of the top three leaves, as well as plant height. The W4 treatment can optimize the plant morphology of rice, promote plant growth, and form a good population structure. With the decrease in irrigation volume, the length of the second internode at the base of rice decreased, and the internode plumpness, breaking resistance, lodging index, and the contents of lignin and cellulose in the stem and sheath first increased and then decreased. Proper irrigation was beneficial for improving the lodging resistance of rice stems. Appropriate irrigation volume not only improves water use efficiency but also enhances the photosynthetic performance of the flag leaves of rice, the accumulation and transportation capacity of dry matter in the stem and sheath, which is beneficial to yield formation. Compared with the W1 treatment, the W4 treatment saved 37.50% of the irrigation water consumption, and the yield significantly increased by 9.07%-12.47%.

Under the W4 (7500 m³·hm^-2) treatment, rice has better plant morphology, lodging resistance, photosynthetic performance, and dry matter accumulation compared with other treatments. Therefore, it has the highest yield while having a high water use efficiency and can be used as a more suitable irrigation volume in this region.

The lodging problem restricts the development of the rice industry. This study screened japonica rice varieties with strong lodging resistance and key indicators for lodging resistance identification in japonica rice, and japonica rice varieties with different lodging resistances were further selected to compare the differences in their physicochemical properties, which could provide high-quality parental resources for the breeding of lodging-resistant japonica rice varieties, and offer theoretical basis for optimizing breeding strategies and analyzing the cultivation regulation mechanism of lodging resistance.

This study took 50 japonica rice germplasm resources as experimental materials, and determined 14 lodging resistance traits. Through comprehensive genetic diversity index analysis and comprehensive evaluation, the 50 japonica rice germplasm resources were rated and classified, and the grey correlation analysis was used to screen the key indicators for identifying lodging resistance. On this basis, japonica rice varieties with different lodging resistances were further selected: Jiudao 325 (lodging-resistant type), Jiyujing (relatively lodging-resistant type), and Wuyoudao 4 (lodging-sensitive type) as experimental materials. Subsequently, the stem characteristics, material metabolism, and gene expression characteristics were systematically analyzed to deeply explore the physiological mechanism of lodging resistance in japonica rice.

The coefficient of variation for lodging resistance traits among the 50 japonica rice varieties ranged from 5.38% to 21.08%, with genetic diversity indices between 1.76 and 2.06. Principal component analysis (PCA) extracted four principal components, and the comprehensive evaluation D-values calculated using the membership function method ranged from 0.12 to 0.72. Cluster analysis classified the 50 germplasms into three groups: Group 1 (30%) as lodging-resistant, Group 2 (32%) as moderately lodging-resistant, and Group 3 (38%) as lodging-sensitive. Grey relational analysis indicated that the length and wall thickness of the second basal internode (N2) exhibited the highest correlation with the D-value. The correlation degree between spike weight, the third basal internode (N3) length, N2 length and N2 intercellular wall thickness and japonica rice yield was the highest. Compared with lodging-sensitive varieties, lodging-resistant and moderately lodging-resistant varieties showed significant reductions in N2 internode length by 37.02% and 9.09%, respectively, accompanied by increases in large vascular bundle area by 20.55% and 6.81%, respectively, and cortical thickness by 15.04% and 12.86%, respectively. Upregulated expression of cellulose synthase and lignin synthase genes led to increased cellulose content by 37.30% and 9.19%, respectively, and lignin content by 49.32% and 17.71%, respectively, with Guaiacyl lignin (G-type) and Syringyl lignin (S-type) lignin monomers accounting for 92.36%-93.90% of total lignin.

This study screened elite germplasm resources with excellent lodging resistance traits, such as Jiudao 325, Qinglin 168, and Hajingdao 1. The internode length and internode wall thickness of N2 could be used as key indicators for evaluating and identifying high-yield lodging resistance. Compared with lodging-sensitive japonica rice varieties, lodging-resistant and moderately lodging-resistant varieties upregulated the expression levels of genes related to cellulose and lignin synthase by promoting the biosynthesis of cellulose, lignin monomers, and lignin. Meanwhile, they accumulated higher contents of starch and soluble sugars by optimizing the physical and anatomical structural characteristics of stems, thereby enhancing the lodging resistance of japonica rice.