Improved photosynthetic performance under unilateral weak light conditions in a wide-narrow-row intercropping system is associated with altered sugar transport.

Intercropping improves resource utilization. Under wide-narrow-row maize intercropping, maize plants are subjected to weak unilateral illumination and exhibit high photosynthetic performance. However, the mechanism regulating photosynthesis under unilateral weak light remains unknown. We investigated the relationship between photosynthesis and sugar metabolism under unilateral weak light. Our results showed that the net photosynthetic rate (Pn) of unshaded leaves increased as the level of shade on the other side increased. On the contrary, the concentration of sucrose and starch and the number of starch granules in the unshaded leaves decreased with increased shading due to the transfer of abundant 13C into the grains. However, sink loss with ear removal reduced the Pn of unshaded leaves. Intense unilateral shade (40% to 20% normal light) reduced grain yield (37.6% to 54.4%) but not mild unilateral shade (60% normal light). We further found that in unshaded leaves, Agpsl, Bmy, and Mexl-like significantly influenced sucrose and starch metabolism, while Sweet13a and Sut1 were crucial for sugar export. In shaded leaves, Sps1, Agpsl, and Sweet13c were crucial for sugar metabolism and export. This study confirmed that unshaded leaves transported photosynthates to the ear, leading to a decrease in sugar concentration. The improvement of photosynthetic performance was associated with altered sugar transport. We propose a narrow-row spacing of 40 cm that provides appropriate unilateral shade and limits yield reduction.