Lodging is an important factor limiting rice yield and quality by bending or breaking stem in japonica rice (Oryza sativa L.) production. The objectives of this study were to determine the mechanism of lodging resistance in japonica rice as affected by carbohydrate components, especially its related arrangement in culm tissue and response to top-dressing nitrogen (N) fertilizer. Field experiments were conducted in Danyang County, Jiangsu Province, China, by using two japonica rice varieties Wuyunjing 23 (lodging-resistance variety) and W3668 (lodging-susceptible variety) with three top-dressing N fertilizer rates (0, 135 and 270 kg N ha^–1) in 2013 and 2014. Lodging related physical parameters, morphological characteristics and stem carbohydrate components were investigated at 30 d after full heading stage. Results showed that with increasing N fertilizer rates, the lodging rate and lodging index increased rapidly primarily due to significant reduction of breaking strength in two japonica rice varieties. Correlation analysis revealed that breaking strength was significantly and positively correlated with bending stress, but negatively correlated with section modulus, except for significant correlation at W3668 in 2014. Higher stem plumpness status and structural carbohydrate contents significantly enhanced stem stiffness, despite of lower non-structural carbohydrate. With higher N fertilizer rate, the culm wall thickness was almost identical, and culm diameter increased slightly. The structural carbohydrates, especially for lignin content in culm, reduced significantly under high N rate. Further histochemical staining analysis revealed that high N treatments decreased the lignin deposition rapidly in the sclerenchyma cells of mechanical tissue, large vascular bundle and small vascular bundle region, which were consistent with reduction of bending stress, especially for W3668 and thus, resulted in poor stem strength and higher lodging index. These results suggested that structural carbohydrate plays a vital role for improving stem strength in japonica rice. N rate decreased lodging resistance primarily due to poor stem stiffness, by reducing structural carbohydrate content and lignin deposition in the secondary cell wall of lower internode culm tissue.

The objective of this study was to determine the morphology mechanism of nitrogen (N) fertilizer rates and ratio on lodging resistance through analying its effects among lodging index (LI), lodging-related morphological traits and physical strength in basal internodes by comparing japonica and indica super rice cultivars. Field experiments, with three nitrogen levels (0, 150 and 300 kg ha-1) and two ratios of basal to topdressing (8:2 and 5:5) with two super rice cultivars (Yliangyou 2 and Wuyunjing 23), were conducted in the Baolin Farm, Danyang Country, Jiangsu Province, China, in 2011 and 2012. Effects of N fertilizer rates and ratios on morphology of whole plant, morphology traits in basal internodes and culm’s physical strength parameters were investigated at 20 d after full heading stage. LI of Yliangyou 2 was significant greater than that of Wuyunjing 23 due to larger bending moment by whole plant (WP) with higher plant height and gravity center height. With higher volume of N fertilizer, LI of two super rice cultivars was increased conspicuously. However, no significant effect was detected with increase of panicle fertilizer ratio. The size of breaking strength (M) in basal internodes was the key factor determining LI among N fertilizer treatments. Correlation analysis revealed that M value was positively related bending stress (BS) of Wuyunjing 23 and section modulus (Z) of Yliangyou 2, respectively. The higher N fertilizer levels induced reduction of BS of Wuyunjing 23 due to weak culm and leaf sheath plumpness status and reduced Z of Yliangyou 2 owning to small diameter and culm wall thickness, consequently, influencing their M indirectly. These results suggested that breaking strength was the key factor influencing LI with increase of N fertilizer levels. However, the lodging-related morphology mechanism was different with genotypes. Culm wall thickness and diameter in basal internodes of indica super rice and culm and leaf sheath plumpness status of japonica super rice influenced breaking strength, as well as lodging index, respectively.