Heading-stage Mowing Improves Photosynthetic Rate and Biomass Production in High-density Sweet Sorghum

INTRODUCTION: Light intensity is an important environmental factor affecting plant photosynthesis. High-density planting, an important crop cultivation practice for high yields, can reduce canopy light intensity, which decreases yield by reducing the rate of photosynthesis and the number of plant tillers. Given the large size of sweet sorghum plants which can naturally lead to dense canopy, we hypothesized that mowing might be able to increase photosynthetic rate and tillering, and population biomass, by improving the light environment within the canopy in high-density planted sweet sorghum. RATIONALE: Currently high-quality forage shortage is a serious problem in China. Sweet sorghum, a high quality forage crop ideal for silage, has attracted great attention due to its high biomass production and excellent palatability. This study aims to elucidate the patterns and physiological-ecological mechanisms by which mowing increases the population biomass of sweet sorghum, thus providing a theoretical basis for high-yield sweet sorghum cultivation. RESULTS: With increasing planting density (15 000, 75 000, and 135 000 plant∙hm–2), the main stem height of sweet sorghum increases, while stem diameter and leaf area decrease, accompanied by a reduction in the number of tillers. At the same time, the higher the planting density, the lower the canopy light intensity, the leaf photosynthetic performance and the fresh weight per plant, although the population-level fresh forage yield can increase significantly as the planting density increases up to 135 000 plant∙hm–2. We found that heading-stage mowing largely increased the number of tillers (4.7, 3.7, and 2.3 per plant, respectively) in all density treatments and reduced the tiller mortality by 35 days. In addition, heading-stage mowing significantly improved canopy light conditions, leaf photosynthetic efficiency and plant fresh weight in densely planted sweet sorghum, further increasing fresh grass yield. Our analysis showed that the average canopy light intensity and the net photosynthetic rate of functional leaves on the main stem were the main factors that influenced the actual tiller number and the main stem fresh weight, respectively. CONCLUSION: Our results demonstrated that high density planting is an important cultivation strategy for the high yields of sweet sorghum through improving the light environment within the dense canopy, thereby increasing the photosynthetic rate and the number of tillers per plant, and eventually increasing the population biomass. Effects of mowing on light intensity (A), leaf net photosynthetic rate (B), number of tillers per plant (C), and fresh grass yield (D) at different planting densities (15 000, 75 000, and 135 000 plant∙hm–2) in sweet sorghum. Different capital letters indicate significant differences before and after mowing at the same planting density (PP