Evaluation of Lodging Resistance and Its Physiological Mechanisms in Japonica Rice Resources

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.