Abstract: 【Objective】Enhancing the mechanization level is now one of the most important goals in sorghum production. Important progress has been made in the dwarf sorghum hybrid breeding suitable for mechanization planting. The aim of this experiment is to investigate the photosynthetic characteristics and yield formation regularities of dwarf stalk sorghum hybrid, the results of the study will beneficial for the breeding and popularization of the new type sorghum hybrids. 【Method】 In this experiment, the newly bred dwarf sorghum hybrid Liaoza 35, suitable for mechanized production, was served as the main test material, with three planting densities (90 000 plants/hm², 120 000 plants/hm² and 150 000 plants/hm²), and the middle-high stalk sorghum hybrid Liaoza 19, the regional main cultivar with a similar growth duration, was used as the control (planting density was set as the suitable level of 105 000 plants/hm²), to explore the photosynthetic characteristics and yield formation regularities of the dwarf stalk sorghum hybrid through analysis of plant morphological, photosynthetic and ecological environment indexes and yield components of the sorghum population. 【Result】 At filling stage of the most flourishing population, with the increase of planting density, the dwarf sorghum Liaoza 35 showed significant reductions of light intensity, light transmittance, stomatal conductance and photosynthetic rate in the middle and lower parts of the canopy, and also significantly lower than those of the control hybrid Liaoza 19, but no significant difference in the above each index at the upper canopy between the two hybrid varieties. The electron transport rate (ETR), coefficient of photochemical quenching (qP) and the maximal photochemical efficiency (Fv/Fm) on the second leaf from the top of Liaoza 35 at flowering and filling stages showed a trend of decrease along with the increase of planting densities, while the initial fluorescence (Fo) showed a trend of increase, and all the fluorescence parameters of Liaoza 19 were better than those of Liaoza 35 on the condition of middle-high planting densities. Leaf area index increased with the increase of planting density, showing a trend of increase until anthesis and then gradual decline, with the fastest decline in the treatment of 150 000 plants/hm², however, a considerable leaf area index was also maintained at the maturity stage of Liaoza 35; leaf SPAD values declined significantly at lower parts of the canopy. Under high planting density conditions, the morphological, photosynthetic and ecological environment indexes of Liaoza 35 performed poorly compared to Liaoza 19, which might be associated with its plant type: dwarf in height, but no corresponding reduction of leaf number and no noticeable changing of leaf angle. With the increase of planting density, Liaoza 35 gave higher biological yield and grain yield, with significant reduction of grains per panicle but no significant change in 1000-kernel weight. Liaoza 35 produced the highest grain yield, 9 923.5 kg·hm^-2, at the planting density of 150 000 plants/hm², and over-yielded the control Liaoza 19. 【Conclusion】 Reasonable increase of planting density is a key to promote the grain yield increase of dwarf sorghum. However, high planting density may induce negative impacts on the photosynthetic characteristics and dry matter production in the population canopy, thus it is important to make the negative impact on the individual plants less than the positive effect on the population performance through improving sorghum plant type and the better management of water, fertilizers and planting densities to coordinate the relationship between the sorghum population and individual plants.

Key words: dwarf sorghum, mechanized production, photosynthetic characteristics, yield components