Low solar irradiance reaching the canopy due to fog and heavy haze is a significant yield-limiting factor worldwide. However, how plants adapt to shade stress and the mechanisms underlying the reduction in leaf photosynthetic capacity and grain yield remain unclear. In this study (conducted during 2018–2021), we investigated the impact of light deprivation (60%) at the pre-anthesis and post-anthesis stages on leaf carboxylation efficiency, source-to-sink relationships, sucrose metabolism, and grain yield of wheat cultivars with contrasting shade tolerance. Shade stress decreased stomatal conductance, stomatal limitation value, intrinsic water use efficiency, rubisco activity, and carboxylation efficiency of flag leaves during grain-filling, whereas intercellular CO₂ concentration increased. These findings indicate that non-stomatal limitation reduces the net photosynthesis rate in a weak-light environment. Shade-tolerant cultivars (MM-51 and CM-39) adapted to low-light conditions via a higher leaf area of flag leaves, light interception rate, and chlorophyll a and b contents; this increased non-structural carbohydrates and sucrose contents in developing grains, ultimately decreasing yield loss by shade stress. Pre-anthesis shading resulted in a greater yield loss than post-anthesis shading because of decreased plant biomass, grain number per spike and 1,000-kernel weight. This study indicates that Rubisco-mediated non-stomatal limitation reduces P_N and sucrose content in plants exposed to low-light stress, contributing to decreased grain yield. Our study provides information on the mechanism underlying shade stress tolerance, which will help design future strategies for reducing yield loss in the context of global dimming.

Biostimulants sprayed exogenously boost crop yield and quality. However, the effects of the co-application of biostimulants and fertilizers as base fertilizers in soil are still uncertain. The objective of this study was to investigate the overall effects of reducing N levels in conjunction with the application of biostimulants on the yield formation and N utilization of wheat and maize. Therefore, based on the winter wheat–summer maize rotation system in the modern R&D base of Sichuan Agricultural University, soil enzyme activities, soil inorganic nitrogen dynamic content, crop nitrogen accumulation and transportation, crop yields, and composition were determined. To achieve this, a total of nine treatments were established based on the winter wheat–summer maize rotation system. The experiment included the following treatments: no fertilization (CK0); one-time application of common compound fertilizer (CK1, applied at a rate of 225 kg ha⁻¹); common compound fertilizer as base fertilizer + urea as topdressing fertilizer (CK2, applied at a rate of 225 kg ha⁻¹, base/topdressing, 6/4); biostimulant + common compound fertilizer with 20% or 30% N reduction (jf-20%, jf-30%); biostimulant chelated urea-formaldehyde fertilizer reducing N by 20%, 30%, or 40% (jn-20%, jn-30%, or jn-40%); and biostimulant chelated urea-formaldehyde fertilizer reducing N by 40% and combined with organic fertilizer, thereby totally reducing N by 27% (jny-27%). The results demonstrated that the application of a biostimulant increased the activities of urease, nitrate reductase, and nitrite reductase in the soil of wheat and maize during the flowering stage. At the same time, the amount of residual nitrate and ammonium N in the soil at maturity was reduced. Furthermore, when N application was appropriately reduced, wheat and maize plants treated with jf, jn, and jny showed a significant increase in N assimilation after the flowering stage, resulting in higher N accumulation in the grains at maturity and ultimately improving the yield compared to CK1 and CK2. The combined use of biostimulants also had a significant positive impact on N use efficiency (NUE). During the two-year period, the NUE in the wheat season showed an increase ranging from 6.70% to 24.00% compared to CK1 and from 5.30% to 22.60% compared to CK2. Similarly, in the maize season, the NUE increased by a range of 11.60% to 22.57% compared to CK1 and from 11.78% to 22.75% compared to CK2. Overall, biostimulants enhanced N absorption and transportation by matching crop N requirements in the mid-to-late stages and improved NUE and yield under appropriate N reduction. This study contributes to the design of improved measures for N reduction and yield stabilization in order to promote sustainable agricultural development.