| One of the most effective ways to deal with the Yangtze River Basin’s low rapeseed yield and low profitability caused by the contradiction of crop rotation is through mechanized transplanting.The current front-runner method for mechanizing transplanting of rapeseed is using blanket seedlings.To satisfy the demands of high-speed and efficient mechanical transplanting,nursery rapeseed seedlings at a high density in a tray are a prerequisite for automated rapeseed transplanting.However,it is clear that good quality and high density are incompatible needs.In our research,Ningza118 and Ningza158 were used as experimental materials to examine the competition effect brought on by increased sowing density on seedling quality,growth recovery after transplanting,yield components,and rapeseed quality of rapeseed blanket seedlings,as well as the mechanism of adverse competition effects regulated by uniconazole,a type of plant growth regulator.Five sowing densities of 200,400,600,800,and 1000 grains per tray were used to duplicate different levels of competitive pressure and examine its effects on seedling quality,growth recovery after transplanting,yield components,and rapeseed quality.A series of 0,250,500,and 750mg L-1 uniconazole was set using the polyvinyl alcohol seed-coating technique.To determine the effects of uniconazole therapy on seedling quality,growth recovery after transplanting,yield components,and rapeseed quality,blanket rapeseed seedlings were assessed under high competition pressure(800 grains per tray)and low competition pressure(400 grains per tray).With the rapeseed blanket seedling transplantation technique,this study aims to provide a solid theoretical basis for selecting the planting density and the proper level of uniconazole seed-coating treatment.The following were the main findings:(1)Competition substantially restricted the growth of rapeseed blanket seedlings,resulting in decreases in the seedlings’ dry weight,leaf area,height,width,stem diameter,chlorophyll content,and non-structural carbohydrate content.The results of the regression analysis revealed that the seedling’s dry weight decreased by 12 mg and its leaf area increased by 2.1 cm2 for every increase in sowing density of 100 grains per tray.At the same time that imbalance growth increased,the seedling’s stem/leaf ratio and root/shoot ratio both significantly increased,and the dry matter allocation direction shifted more in favor of the stem,petiole,and root.On the other hand,the levels of C,N,and P in the seedlings decreased as the level of competition grew.As competition pressure increased,P and S increased in the shoot while P decreased in the root.The equilibrium of the carbon,nitrogen,and phosphorus stoichiometric ratios was tilted by the growing competitive pressure.Additionally,the competition-induced shortage and disorder of nutrients limit seedling growth.However,a small increase in sowing density was good for boosting population biomass and element accumulation in the plate,and the sowing density increase was more advantageous for resource usage.(2)The seedlings’ total antioxidant capacity would be reduced by the increasing competing pressure during the growth period,which would result in a rise in MDA and H2O2 concentration after transplantation.The rising amount of oxidative damage limits the average development rate of seedlings after transplantation.Increased competitive pressure during seedling growing can also lead to decreased biomass accumulation and impaired reproductive growth in the mature stage.The number of effective branches,siliques,and seeds per silique were negatively impacted by the increased competition pressure during seedling nursery.There was a considerable loss in yield as a result of decreasing yield components.The results of the regression analysis revealed that when the seeding density was raised by 100 grains per tray,both the yield per plant and the yield per hectare decreased by 1.83 g and 248 kg,respectively.In terms of rapeseed quality,the increased competition pressure during nursery would lead to a reduction in oil content and an increase in erucic acid and sulfur glycoside concentrations.(3)Uniconazole has the potential to increase the individual dry weight,leaf area,plant height,seedling width,and stem diameter in rapeseed blanket seedlings in addition to reducing the growth inhibition brought on by heightened competition pressure,to reduce the stem to leaf ratio and the excessive stem and petiole growth brought on by imbalanced growth.A treatment with uniconazole promotes plant growth while simultaneously improving the plant’s capacity to absorb nitrogen and assimilate carbon.It also increases the amount of nitrogen and carbon in the seedling and balances the ratio of carbon,nitrogen,and phosphorus between the shoot and root.(4)The amount of soluble sugar,sucrose,and starch increased significantly after treatment with uniconazole.After transplantation,uniconazole treatment increased the activities of α-amylase and β-amylase,increased the amount of soluble sugar in the plants by enhancing their capacity to break down starch.Additionally,by boosting the activities of shoot sucrose phosphate synthase(SPS),sucrose synthase(SuSy),neutral invertase(NI),and acid invertase(AI),the synthesis and decomposition of sucrose were coordinated to maintain the stability of intra-plant sucrose,in order to maintain the carbon balance in the plant.It also reduced the amount of H2O2 and MDA,lessening the degree of oxidative damage.to encourage the rapid transplant recovery of the rapeseed blanket seedlings.(5)The yield decline brought on by increased competition pressure during nursery at mature stage may be successfully reversed with uniconazole.The accumulation of biomass,the number of effective branches,the number of siliques,and the number of seeds per silique all increased along with the uniconazole treatment.The yield increased by over 45%under high competition pressure(800 grains per plate),and by almost 20%at low competition pressure(400 grains per plate).Additionally,uniconazole treatment reduced the yield gap between the two sowing densities from 37%in without the use of uniconazole to 15%after treatment.Uniconazole increased rapeseed oil content while lowering sulfur glycoside and erucic acid levels under the extreme pressure of seedling-raising competition.In conclusion,competition pressure had a detrimental effect on the quality of rapeseed blanket seedlings,recovery growth after transplanting,and yield and seed quality.When used for establishing seedlings,uniconazole has the potential to greatly improve seedling quality,accelerate post-transplant growth recovery,and reduce yield loss brought on by increasing competition pressure.Based on the current requirements for mechanical transplanting and guarantee the yield,according to the results of our study,a recommended sowing density for actual production is between 600 and 800 grains per plate.It was advised to use 5 ml of uniconazole solution per 100 g of seeds for the seed-coating treatment,which ranged from 500 to 750 mg L-1. |
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