Original data for for intercropped maize experiments, root barrier experiments in intercropped maize, and unilateral shading experiments

<p><span>Intercropping improves resource utilization and yield; however, shading causes a decline in yield. Under wide-narrow-row maize-soybean 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. Therefore, we conducted an experiment with different planting configurations and two controlled environments to investigate photosynthesis, sugar metabolism, and photosynthate allocation in unshaded and shaded leaves under weak light. All three experiments detected an increase in the net photosynthetic rate (<em>Pn</em>) of unshaded leaves on one side with an increase in the level of shade on the other side leaves. 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 <em>Pn</em> of unshaded leaves, limiting their ability to transport sugar. Detailed analysis showed that 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, <em>Agpsl, Bmy, </em>and<em> Mexl</em>-like significantly influenced sucrose and starch metabolism, while <em>Sweet13a</em> and <em>Sut1</em> were crucial for sugar export. <em>Sps1, Agpsl, </em>and<em> Sweet13c</em> were crucial for sugar metabolism and export in shaded leaves. Thus, our study confirmed that under unilateral illumination, unshaded leaves, which transported photosynthates to the ear, contributed significantly to the grain filling and reduced sugar concentration, resulting in a higher photosynthetic performance, while the shaded leaves synthesized fewer photosynthates, primarily for their growth and development. We, therefore, propose a narrow-row spacing (40 cm) that provides appropriate unilateral shade (60% normal light) and limits yield reduction in a wide-narrow-row intercropping system. </span></p>