Heterogeneous population distribution enhances resistance to wheat lodging by optimizing the light environment

At present, lodging is still the key factor that limits continuous increases in wheat yields because the mechanical strength of culms is decreased due to low light stress in populations under high-yield cultivation. The mechanical properties of the culm are mainly determined by lignin, which is affected by the light environment. However, little is known about whether the light environment can be improved by changing the population distribution to inhibit culm lodging. Therefore, in this study, we used the wheat cultivar "Xinong 979" to establish a low-density homogeneous distribution treatment (LD), high-density homogeneous distribution treatment (HD), and high-density heterogeneous distribution treatment (HD-h) to study the regulatory effect and mechanism responsible for differences in the lodging resistance by wheat culms under different population distributions. Compared with LD, HD significantly reduced the light transmittance in the middle and basal layers of the canopy, the net photosynthetic rate in the middle and lower leaves of plants, the accumulation of lignin in the culm, and the breaking resistance of the culm, and thus the lodging index increased significantly, with lodging rates of 67.5% in 2020–2021 and 59.3% in 2021–2022. Under HD-h, the light transmittance and other indicators in the middle and basal canopy layers were significantly higher than those under HD, and the lodging index decreased, with no lodging. During the critical period for culm formation, compared with LD, the activities of phenylalanine ammonia-Lyase (PAL), 4-Coumarate: coenzyme A Ligase (4CL), Catechol-O-methyltransferase (COMT), and Cinnamyl-alcohol dehydrogenase (CAD) in the lignin synthesis pathway were significantly reduced in the culms under HD, and the relative expression levels of TaPAL, Ta4CL, TaCOMT, and TaCAD were significantly downregulated. However, compared with HD, the activities of lignin synthesis-related enzymes and their gene expression levels were significantly increased under HD-h. Partial least squares path modeling analysis found significant positive effects between the canopy light environment, photosynthetic capacity of the middle and lower leaves of plants, lignin synthesis and accumulation, and lodging resistance by culms. Thus, under conventional high-density planting, the risk of wheat lodging increased significantly. Accordingly, the canopy light environment can be optimized by changing the heterogeneity of the population distribution to improve the photosynthetic capacity of the middle and lower leaves of plants, promote lignin accumulation in the culm, and enhance lodging resistance in wheat. These findings provide the basis for understanding the mechanism responsible the lower mechanical strength of the culm under high-yield wheat cultivation, and a theoretical basis and for developing technical measures to enhance lodging resistance.