Physiological Mechanisims Of Growth Inhibitors On Controlling Shoot And Leaf Growth Of Euonymus Japonicus Thunb.

Euonymus japonicus Thunb.is a widely cultivated hedge species and landscape plants in urban areas.As a result of their vigorous growth,Euonymus japonicus always needs to be pruned continually to maintain their landscape.As a more efficient technique,regulating plant growth via using growth inhibitors can reduce mechanical injury to plants and labor cost.Although growth inhibitors have been widely used in agricultural industry,the mechanisms how these growth inhibitors modulate plant growth remains unclear.In this study,we treated Euonymus japonicus with growth inhibitors and analyzed their influence on growth of leaf and new shoots growth,ornamental value,root nutrient uptake,photosynthesis,characteristics of photosynthetic nitrogen distribution,accumulation and metabolism of non-structure carbon.Finally,we dissected the physiological mechanism how growth inhibitors impact growth of shoots and leaves.This study aims to provide a theoretical basis for further development of application technique of growth inhibitors on dwarfing of Euonymus japonicus.The main results are as follows:(1)Internode length and plant height were reduced upon paclobutrazol(PBZ),abscisic acid(ABA),and triiodobenzoic acid(TIBA)treatment to varying degrees,and the inhibitory effect increased with the elevated concentration.Among them,effect of PBZ on plant height is most obvious,as relative growth upon 800mg/L PBZ was reduced by 141.2%compared to control.Application of the three growth inhibitors increased diameter of new shoots and root shoot ratio,and decreased leaf area.Among them,TIBA has strongest inhibitory effect on leaf area,as leaf area was reduced by 100%compared to control when concentration of TIBA was higher than 400mg/L.Plants showed higher net photosynthetic rate,apparent mesophyll conductance,stomatal conductance,and transpiration rate upon PBZ treatment than control while ABA and TIBA repressed them.(2)The dwarfing effect caused by PBZ was enhanced by combination of PBZ with ABA or TIBA.Treatment of ABA,TIBA and PBZ together showed the strongest repressive effect on height growth,as increment of plant heights were reduced by 263.0%compared to control.Upon all of the combined treatments,relative height growth of Euonymus japonicas significantly positively correlated with ratio of internode length of new and old shoots and leaf area,while negatively correlated with branching density and diameter ratio of new and old shoots.These results suggested that there are synergetic effects between height growth and internode length or leaf area.After height growth was inhibited,growth inhibitors promoted cross growth of Euonymus japonicus.(3)Application of growth inhibitors facilitated root growth and nitrogen assimilation of Euonymus japonicus Thunb.japonicus.Root length,mean diameter,volume and superficial area of roots were increased upon PBZ and TIBA treatment.Both of PBZ and TIBA treatment increased root uptake area and active uptake area.Both of these two growth inhibitors increased maximum absorption rate of ions of root(V_max)and the rate at which ions flow into the root system(?),reduced critical concentration(C_min)and the concentration of ions required to reach the maximum absorption rate(K_m),thus increased root nutrient uptake.Compared to PBZ or TIBA individual application,treatment of combination of PBZ and TIBA showed enhanced root growth,root absorption activity and root nutrient uptake.(4)To some extent,ABA and TIBA antagonize the improvement of photosynthesis by PBZ via changing gas exchange.Upon application of combination of PBZ and ABA(PA),PBZ and TIBA(PT),and all three growth inhibitors together(PAT),net photosynthetic rate were reduced by 19.5%,34.0%and 26.2%than that upon PBZ treatment,but showed no significant difference with control.When the growth inhibitors were mixed,ABA and TIBA reduced the promoting effect of PBZ on net photosynthetic rate to a certain extent.Compared to PBZ treatment,combination of growth inhibitors reduced stomatal conductance,mesophyll conductance,V_c,max,light use efficiency,transpiration rate and dark respiration rate,and raised photorespiration rate and water use efficiency.In addition,carbon efficiency was increased upon PBZ treatment and combined growth inhibitors treatment.(5)Combined growth inhibitors treatment reduced photosynthetic nitrogen use efficiency of leaves effectively via changing leaf nitrogen distribution pattern.Application of PBZ increased photosynthetic nitrogen utilization efficiency and proportion of nitrogen allocation to photosynthetic apparatus,while photosynthetic nitrogen utilization efficiency and proportion of nitrogen allocation to photosynthetic apparatus upon three different compound inhibitors treatments(PA,PT and PAT)reduced by22.3%-23.5%and 11.5%-13.3%,respectively,than that upon PBZ treatment.In addition,proportion of nitrogen allocation to non-photosynthetic apparatus increased by 4.0%-4.6%while proportion of nitrogen allocation to carboxylation reduced.To some extent,treatment of PA,PT and PAT counteracted facilitating effect of PBZ on photosynthetic nitrogen utilization efficiency and proportion of nitrogen allocation to photosynthetic apparatus.(6)Combination of growth inhibitors significantly changed non-structural carbon allocation pattern in leaves and roots.Application of PBZ alone and three different compound inhibitors(PA,PT and PAT)increased content of reducing sugar,sucrose,soluble sugar and starch in leaves and roots.In addition,activities of sucrose synthetase,sucrose phosphate synthase,invertase,starch synthetase and adenosine diphosphate glucose pyrophosphorylase were higher upon growth inhibitors treatment than control.There were synergistic effects between changes of non-structural carbon in leaves and roots.In conclusion,growth inhibitors can inhibit the shoot growth and leaf area of Euonymus japonicus,and affect the material distribution pattern of Euonymus japonicus by affecting the physiological characteristics of photosynthesis,root nutrient absorption capacity and non structural carbon distribution,so as to change the morphological characteristics of shoots and leaves of Euonymus japonicas.