Research On The Canopy Structure Characteristics Of High Photosynthetic Efficiency And Its Regulation Mechanism In Machine-harvested Cotton(Gossypium Spp.)

Appropriate planting density is one of the most important crop management practices for improving crop yield.Adjusting the plant and row spacing of crops is a crucial method to integrate appropriate planting density and new cultivation techniques.Appropriate planting density contributes to optimize crop canopy structure and improve canopy photosynthetic capacity,and then increase the crop yield and quality.In this paper,we regarded the leaf photosynthetic response to the difference of canopy structure as the core content,and the canopy photosynthetic acclimation to different planting density conditions as the main line of the research.The photosynthetic characteristics of leaf and canopy were determined to study the effect of planting density on dry matter accumulation and spatial distribution of leaves and bolls within the canopy,and its influences on the yield and fiber quality in machine-harvested cotton(Gossypium hirsutum L.).The results indicated that cotton optimized photosynthetic N use efficiency and photosynthetic capacity by adjusting leaf mass per area,which in turn affected leaf N allocation to the photosynthetic apparatus.Moreover,optimal canopy light interception and spatial distributions of light and specific leaf area are the key canopy characteristics for efficient utilization of light and N.This research may help to understand the mechanism of leaf and canopy photosynthetic acclimation to different planting densities.The main results of this research are listed as below:1.Effects of planting density on canopy structure characteristics and canopy photosynthesis were studied in machine-harvested cotton.The changes in canopy structure in response to planting density affected the light distribution within the canopies.As planting density increased,the increase rate of leaf area index and interception of photosynthetic active radiation during earlier growth stage was greater.The fraction of photosynthetic active radiation interception increased in the upper-canopy layer,whereas decreased in the mid-and lower-canopy layers at boll setting stage as planting density increased.The decrease rate of diffuse non-interceptance at the earlier growth stage and increase rate of that at the later growth stage were greater as planting density increased.The changes of canopy apparent photosynthesis rate were similar to the changes of leaf area index during the growth stage under different planting densities.The results indicated that optimal photosynthetic active radiation interception and spatial distributions of light within canopy are the key canopy characteristics for high efficient canopy in cotton.2.Effects of planting density on light-saturated photosynthetic rate and photosynthetic nitrogen use efficiency of leaves were studied in machine-harvested cotton.The results showed that leaf N allocation and partitioning among different components of the photosynthetic apparatus were significantly affected by planting density.As planting density changed,cotton optimized photosynthetic N use efficiency and photosynthetic capacity by adjusting leaf mass per area,which in turn affected leaf N allocation to the photosynthetic apparatus.The medium planting density is the optimum one due to high light utilization efficiency,superior spatial distribution of leaf N allocation to the photosynthetic apparatus and photosynthetic use efficiency of photosynthetic N in leaves within the canopy.The results indicated that high photosynthetic capacity of leaves in the upper-canopy layer and high leaf N allocation to the photosynthetic apparatus and photosynthetic use efficiency of photosynthetic N in the mid-and lower-canopy layers were two key canopy characteristics for efficient utilization of light and N by cotton.3.Effects of planting density on the canopy photosynthetic capacity and the photosynthetic use efficiency of light and N were studied in machine-harvested cotton.Planting density significantly affected both leaf morphological characteristics and canopy photosynthetic characteristics.As planting density changed,cotton optimized canopy apparent photosynthetic light use efficiency and photosynthetic capacity by adjusting specific leaf area,which in turn affected leaf N distribution in the canopy.The specific leaf area in all three canopy layers increased as planting density increased.The increases in specific leaf area increased canopy light interception and canopy apparent photosynthetic N use efficiency in the upper-canopy layer,but reduced canopy apparent photosynthetic light use efficiency.Medium planting density had the highest canopy apparent photosynthesis rate and canopy apparent photosynthetic light use efficiency in mid-and lower-canopy layers,contributing to the optimum whole-canopy photosynthetic capacity and canopy apparent photosynthetic light use efficiency.Overall,the results indicated that optimal canopy light interception and spatial distributions of light and specific leaf area are the key canopy characteristics for efficient utilization of light and N.4.Characters in light-response curves of canopy photosynthetic use efficiency of light and N in responses to plant density were studied in field-grown cotton.The results indicated that the light response curve of canopy apparent photosynthesis rate was similar to that of net leaf photosynthesis rate.The changes in canopy structure and leaf photosynthetic characteristics in response to planting density affected the relationship between canopy apparent photosynthesis rate and light within the canopies.Under low light conditions,canopy apparent photosynthetic light use efficiency and canopy apparent photosynthetic N use efficiency were greater in high planting density than in low planting density.Under high light conditions,maximum canopy apparent photosynthetic light use efficiency and canopy apparent photosynthetic N use efficiency were greater in low planting density than in high planting density.These light response curves support the hypothesis that canopy apparent photosynthetic light use efficiency and canopy apparent photosynthetic N use efficiency were maximized according to the light interception and distribution within its canopy,which is by adjusting canopy structure and leaf photosynthetic characteristics.5.Effects of planting density on dry matter accumulation,yield and spatial distribution of bolls within the canopy were studied in machine-harvested cotton.High dry matter accumulation at the earlier growth stage was the typical characteristic of high-yield canopy in conventional cultivars of cotton.However,it was not limiting factor for the high yield in hybrid cotton.High canopy apparent photosynthetic capacity was the common characteristic for the high efficient canopy in conventional cultivars and hybrid cotton.There was no significant difference in single boll weight among different planting density treatments in conventional cultivars of cotton.The limitation for the high yiled formation was the total boll number in conventional cultivar.Increasing the proportion of bolls in the upper-and mid-canopy layers by optimizing crop cultivation and management measure was the key way to increase the yield of conventional cotton cultivars.The higher total boll number and single boll weight were the main reasons for the high yield formation in hybrid cotton.The lower boll setting percentage and postpone of rapid boll growing period were limiting factors for the increase of yield in hybrid cotton.6.Effects of planting density on leaf defoliation rete,boll opening and fiber quality were studied in machine-harvested cotton.The results showed that planting density significantly affected the leaf defoliation rate within the canopy.However,there was no significant difference in cotton fiber quality among different planting densities within conventional cultivars or hybrid cotton.Leaf defoliation rate was the key factor for trash content of machine-harvested cotton,and it also directly affected the manufacturing procedure of raw cotton and fiber quality.Decreasing the planting density moderately,and breeding cultivars with glossy and long function period leaf were the key way to increase leaf defoliation rate in conventional cotton cultivars.Using suitable thin planting density,and breeding cultivars with fast boll setting and boll opening rate were the most important way to increase the fiber quality in hybrid cotton.