Exogenous Brassinolide Application: A Promising Strategy to Enhance Sorghum Yield and Photosynthetic Performance Under Nitrogen Reduction Conditions

Reducing nitrogen (N) fertilization is essential for sustainable agriculture, but it frequently suppresses photosynthetic capacity and diminishes grain yield in sorghum. To determine whether exogenous brassinolide (BL) can offset these negative effects, a two-year field experiment was conducted using foliar BL application (0.1 mg L−1) under three N levels (0, 75, and 150 kg N ha−1), with assessments of grain yield, photosynthetic parameters, dry matter accumulation, and nitrogen use efficiency (NUE). Results showed that BL significantly increased grain yield under zero N (by 15.47%) and moderately under 50% N reduction (by 4.32%), primarily by increasing grains per panicle. Under N-reduced conditions, BL enhanced net photosynthetic rate (Pn), chlorophyll content, Rubisco/PEPC activities, and dry matter partitioning to panicles, with these traits positively correlated with yield. Under 50% N reduction, BL improved N recovery efficiency (RE) and agronomic efficiency (AE) while leaf N content correlated positively with SPAD, Pn, and yield. No significant BL effects occurred under normal N. Thus, exogenous BL application partially compensates for N reduction-induced yield loss by enhancing photosynthesis, source–sink partitioning, and NUE, providing a promising, environmentally sustainable strategy for sorghum production under reduced N input.