Effect Of Topping At Seedling Stage On Root And Shoot Growth And Grain Yield Of Main-stem Type High-yield Spring Soybean

Soybean branches can produce flowers,pods and set seeds,and increase the yield per plant.New branches grow from the axillary buds of soybean seedlings after the top growth point of the plant is interrupted by mechanical damage,insects,or hail,and the yield of a single plant is often increased rather than decreased,showing a strong yield compensation effect.However,the formation of soybean yield is the result of the overall effect of the group,and the increase in yield of a single plant does not mean that the yield of the group will also increase.In view of this,this paper systematically studies the changes in plant structure,root physiology,photosynthetic material production,pod formation,and yield after seedling topping（topping at different times,topping at different densities,topping of different varieties）of mainstem high-yielding spring soybeans in the field in 2017-2018.The main results are as follows:1.Topping at the first compound leaf stage yields the best results.It increases the number of branches of a single plant by 2,increases the number of nodes of a single plant by 41.87%,delays the reproductive period by one week,however,the grain yield decreases by 2.64%.The decrease in topping density significantly increases the number of branches,nodes,flowers and pods of each plant.The seed yield increased by 5.90%at a density of 21×104 plant-hm-2（70%of the optimum density）;topping with a membrane is better than topping without a membrane,and topping from low-yielding fields is better than topping from high-yielding fields,and the yield increase after topping is different for different varieties.2.The first compound leaf stage topping operation results in the largest increase in root mass and dry matter gain.The root weight increases by 18.66%and the dry matter increases by 3.48%respectively.When the density is reduced,the root weight and dry matter per plant increase significantly.When the density is 21×10⁴plant·hm^-2,the root weight and dry matter accumulation are the largest with the root weight increasing by 10.52%and the dry matter increasing by 11.72%respectively.The topping operations mainly increase root volume at 0-40 cm and decrease root volume at 40-60 cm of the root system and improve root vigor at all layers in the post-reproductive period.3.The number of harvested plants and harvested stems are significantly negatively correlated and extremely significantly negatively correlated with yield.The number of population seeds,the number of pods per node and the 100-seed weight are extremely significantly positively correlated with yield.The number of pods per plant and the number of seeds per plant are significantly positively correlated with yield.Population seed number and 100-seed weight have a direct positive effect on yield,and the direct contribution rates to yield are 0.6720 and 0.3399,respectively.The maximum indirect positive effect of100-seed weight→population number→yield is 0.6368,and the indirect positive effect of population number→100-seed weight→yield is 0.3221.The main reason for the increase in yield is the significant increase in the number of pods per plant and the weight of hundred grains due to the decrease in the density of topping.The compensation effect of topping at the first compound leaf stage on yield is the result of an increase in the number of branches leading to a significant increase in the number of pods and the 100-seed weight of a single plant compared to the control group.The reason for the compensation effect of soybean yield after topping at low density is that topping promotes root growth,branching,leaf area index,dry matter accumulation,and a significant increase in the number of flowers and pods per plant.Therefore,topping at the first compound leaf stage can provide a theoretical basis for soybean high-yield cultivation and branch-type high-yield plant-type variety breeding.