Exogenous 6-benzyladenine alleviates the adverse effects of shading on the filling and quality of wheat grains caused by walnut trees in a walnut-wheat intercropping system

Agroforestry intercropping is an effective method for optimizing land use and ensuring food security. However, the physiological mechanisms underlying the inhibition of grain filling and quality formation in disadvantaged crops caused by shade stress from dominant plants in intercropping systems, as well as related mitigation measures, remain to be investigated. We conducted a two-year field experiment in a walnut-winter wheat intercropping system to elucidate the effects of exogenous 6-benzyladenine (6-BA) application on wheat grain weight and quality under shade stress created by walnut trees. The results indicated that grain filling rate and the activity of key starch synthesis enzymes—such as sucrose synthetase [SuS, (EC 2.4.1.13)], starch synthase [SSS, (EC 2.4.1.21)], ADP-glucose pyrophosphorylase [AGPase, (EC 2.7.7.21)] and granule-bound starch synthase [GBSS, (EC 2.4.1.21)] were dramatically declined in the below-canopy area (BCA) than in the far-canopy area (FCA) during the flowering and grain filling stages, which inhibited the amylose and amylopectin content in grains and decreased grain weight by 11.96–12.14%. Grain test weight and flour yield were significantly lower in the BCA, while protein components and flour processing quality parameters were substantially higher than in the FCA. Furthermore, in both BCA and FCA, exogenous 6-BA enhanced grain filling rate, activity of starch synthesis enzymes (i.e., SuS, SSS, AGPase, and GBSS) and total starch content, thereby increasing wheat grain weight by 8.12%-10.68% and 3.76%-1.14%, respectively, when grain protein components (i.e., glutelin, gliadin, and globulin) and flour quality (i.e., sedimentation value and extension resistance) were improved. Therefore, 6-BA alleviated the adverse effects of walnut tree shading on wheat grain filling rate, starch synthesis, and protein composition, thereby reducing wheat grain weight and quality loss.