Foliar uniconazole application increases rice lodging resistance by altering stem morphological and anatomical traits

Lodging is a limiting factor for rice production in the Sichuan Basin, China. However, the mechanisms of stem lodging resistance, especially its regulation by plant growth regulators are still unclear. A two-year field study, by using the three foliar application rates of uniconazole with two rice varieties, Yuxiang203 (YX203) and C-Liangyouhuazhan (CLYHZ), was conducted to determine stem lodging resistance and its morphological and anatomical mechanisms in rice plants. The results revealed that, compared with that in 2019, the grain yield in 2020 significantly decreased, whereas the lodging index (LI) significantly increased. Uniconazole treatment increased the rice yield by 4.6%−11.2% and 2.1%−7.0%, and decreased LI by 21.1%−33.9% and 11.4%−29.6% in YX203 and CLYHZ, respectively. Uniconazole treatment shortened the length of the basal internodes by 19.5%−33.0% (YX203) and 24.7%−40.7% (CLYHZ), resulting in a significant reduction in plant height. Uniconazole treatment increased the mechanical tissue thickness, areas of small and larger vascular bundles, and culm diameter and further increased the breaking strength of the two varieties. Cell wall components, including cellulose and lignin, were increased by foliar application of uniconazole, thereby creating denser sclerenchyma cells and increasing the thickness of the mechanical tissue and area of the vascular bundle. These results suggest that the application of uniconazole enhances stem mechanical strength via increased mechanical tissue thickness and larger areas of small and large vascular bundles, thereby improving the lodging resistance of rice plants.