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

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 (Pn) 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 Pn 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, Agpsl, Bmy, and Mexl-like significantly influenced
sucrose and starch metabolism, while Sweet13a and Sut1 were crucial for
sugar export. Sps1, Agpsl, and Sweet13c 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.