In the field, close planting inevitably causes mutual shading and depression of leaf photosynthesis. To
clarify the regulative mechanisms of photosynthesis under these conditions, the effects of planting
density on leaf structure, gas exchange and proteomics were carefully studied in field-grown sorghum.
In the absence of mineral deficiency, (1) close planting induced a significant decrease in light intensity
within populations, which further resulted in much lower stomatal density and other anatomical characteristics
associated with shaded leaves; (2) sorghum grown at high planting density had a lower net photosynthetic
rate and stomatal conductance than those grown at low planting density; (3) approximately
62 protein spots changed their expression levels under the high planting density conditions, and 22 proteins
associated with photosynthesis were identified by mass spectrometry. Further analysis revealed the
depression of photosynthesis caused by mutual shading involves the regulation of leaf structure, absorption
and transportation of CO2, photosynthetic electron transport, production of assimilatory power, and
levels of enzymes related to the Calvin cycle. Additionally, heat shock protein and oxygen-evolving
enhancer protein play important roles in photoprotection in field-grown sorghum. A model for the
regulation of photosynthesis under mutual shading was suggested based on our results.