Stage-dependent response of maize grain formation to shading, waterlogging, and their combination: Insights from floret differentiation and floret setting

Climate change has been demonstrated to increase the frequency of complex abiotic stresses in the future, of which the combination of shading and waterlogging caused by persistent rainfall is an important limiting factor affecting maize grain formation and yield. We investigate grain loss in summer maize induced by shading, waterlogging, and their combined stresses at different stages, and how these stresses affect floret differentiation and floret setting. Maize hybrid Denghai 605 (DH605) was chosen as the test material, with three stress treatments: shading (S), waterlogging (W), and their combined stress (SW) for 6 days at the sixth leaf stage (V6) and tasseling stage (VT). Net photosynthetic rate, plant growth rate, floret differentiation, fertilization characteristics, and ovary development were investigated. Results showed that shading, waterlogging, and their combined stress during the V6 stage significantly reduced photosynthetic capacity and leaf growth, delayed the ear elongation and led to a decrease in total floret number. Additionally, the decreased floret setting rate during the reproductive growth stage further exacerbated grain loss. Pollen viability and silking rate were more sensitive to shading, waterlogging and the combined stresses imposed at VT than at V6, reducing floret fertilization rate. Additionally, the reduction in starch content in the ovary increased the risk of post-fertilization abortion by an average of 40.6%-67.1%, representing the highest proportion among the other two causes of kernel loss (loss from silking and fertilization) analyzed. Waterlogging at V6 stage was the dominant factor affecting summer maize yield (whereas at VT was shading), and combined stresses showed greater losses compared to single stresses. The yield of V6-SW and VT-SW was decreased by 11.3% and 2.9%, respectively, compared to the S treatment in the same period; and by 2.0% and 12.6%, respectively, compared to the W treatment in the same period. These findings provide new insights into the responses of maize grain formation to shading, waterlogging, and their combination at different growth stages, and highlight that reducing post-fertilization ovary abortion is an important research objective for mitigating yield losses in maize under persistent rainy conditions.