The mechanical physicochemical properties of wheat culms are key intrinsic determinants of lodging resistance. This study aimed to determine the relationships between plant growth regulator-induced changes in culm physicochemical traits and improvements in mechanical properties.

Two wheat varieties with different lodging resistance, Yangmai 22 and Ningmai 13, were selected. The experiment included four foliar spray treatments: water (control), uniconazole (20 mg·L^-1), prohexadione- calcium (4.5 g·L^-1) and a combination of uniconazole and prohexadione-calcium, applied at key growth stages. Culm morphological characteristics, mechanical traits, structural carbohydrate content, and yield components were systematically measured to analyze the regulatory effects of different treatments on culm lodging resistance and grain yield.

Plant growth regulator treatments significantly reduced plant height and center of gravity height, shortened basal the first and second internodes, while increasing basal internode culm diameter, culm wall thickness, dry matter accumulation, and culm filling degree, with increased cell wall structural carbohydrate content. In 2022-2023, lignin content in the second basal internode at maturity stage of Yangmai 22 and Ningmai 13 increased by 21.17%-25.52% and 5.87%-8.61%, respectively. This optimization of morphology and matter accumulation further improved culm mechanical properties, as evidenced by significantly increased breaking force, section modulus, and breaking moment of the second basal internode, along with significantly reduced lodging index and markedly improved field lodging conditions. In 2023-2024, field lodging rate at maturity stage decreased by 7.81%, 8.94%, and 11.95%, respectively. Correlation analysis showed that breaking strength and breaking moment were significantly positively correlated with section modulus and culm wall thickness, while significantly negatively correlated with bending stress, center of gravity height, and basal internode length. The lodging index was significantly negatively correlated with breaking strength, breaking moment, section modulus, culm diameter, and wall thickness of the basal internodes, but significantly positively correlated with bending stress, plant height, center of gravity height, and internode length. In terms of yield, foliar application of uniconazole, prohexadione-calcium, and their combination increased grain yield. In 2022-2023, grain yield increased by 4.03%-28.40% in Yangmai 22 and by 9.35%-22.48% in Ningmai 13 compared with the control, respectively. In 2023-2024, spray treatments of Yangmai 22 increased by 15.71%, 14.97%, and 12.33% compared with the control, respectively.

Foliar application of uniconazole, prohexadione-calcium, and their combination significantly enhanced wheat culm lodging resistance by optimizing the morphological structure of basal internodes, including reducing plant height, shortening internodes, and increasing culm diameter; promoting dry matter accumulation, culm filling, and lignin deposition; and improving culm mechanical properties. These treatments also maintained or increased grain yield.

The consumption of biscuits in China is increasing yearly, but soft wheat, the material for making biscuits, has been in shortage for a long time. American soft wheat is of stable quality and excellent processing performance, and is welcomed by Chinese processing enterprises. This study on the interannual dynamic change of grain quality and the relationship between quality indexes of American soft red winter and soft white wheat in multiple years could provide the reference for Chinese weak gluten wheat production.

Based on the quality data of soft wheat published by American Wheat Association from 1999 to 2019, the correlation analysis and cluster analysis were used to analyze the relationship among wheat grain, flour, dough and baking quality, and the fitness of American soft wheat quality to the existing weak gluten wheat standard in China was also analyzed.

For grain quality, the mean value of grain protein content (GPC, %), hardness (H), test weight (TW, g·L^-1) and 1000-grain weight (TKW, g) of soft red winter wheat was lower than that of soft white wheat. The annual variation of quality indexes showed: H>TKW>GPC>TW. For flour quality, the wet gluten content (WG, %) of two kinds of soft wheat were about 22%. Four kinds of solvent retention capacity (SRC, %) of soft red winter wheat were higher than or similar to soft white wheat, while the WG, sedimentation value and four kinds SRC of soft red winter wheat had lower variation coefficients. For dough quality, the development time, stability time (ST, min), alveograph P, L, W value and extensograph parameters of soft red winter wheat were lower than those of soft white wheat, and their water absorption (WA, %) was about 52%. The variation coefficients of farinograph, alveograph and extensograph parameters of soft red winter wheat were lower. According to Chinese weak gluten wheat standard GB 17320-2013, the reaching rate of GPC, WG and ST in soft red winter wheat were 100%, 100% and 57.1%, respectively. The reaching rates of GPC, WG and ST in soft white wheat were 90.5%, 95.2% and 38.1%, respectively. Under GB 17893-1999, GPC, WG and ST of two kinds of soft wheat were as follows: GPC<WG<ST, and the reaching rate was less than 70%. Correlation analysis of soft white wheat showed that there was a significantly negative correlation between TKW and GPC, WG, sucrose SRC, H and WG, while a significant positive correlation was found between GPC and WG, sucrose, lactic acid SRC, sucrose SRC and alveograph W, lactic acid SRC and extensibility. Biscuits diameter was negatively correlated with GPC, W and sucrose SRC, and positively correlated with TW, Biscuits spread ratio was negatively correlated with sucrose SRC. Correlation analysis of soft red winter wheat showed that the protein content of flour was positively correlated with WG and ST, and biscuits diameter was negatively correlated with TW and W.

Soft red winter wheat had softer grain texture, smaller grain weight and weaker dough strength. The TW, flour extraction rate (FER, %) and WA were stable in different years. Soft red winter wheat fit the requirements of Chinese weak gluten wheat standard better, with higher reaching rate. The Chinese weak gluten wheat standard was too strict on GPC, WG and ST. The correlation among quality indexes of soft white wheat was more significant than soft red winter wheat. The GPC, sucrose SRC and alveograph W were significantly correlated with other quality indexes, which could be used to evaluate the quality of weak gluten wheat. The GPC, WG, sedimentation value and alveograph L of soft wheat were similar, which could be classified into same category.