Biological Responses Of Different Soybean Varieties To Low Potassium Stress And Mechanism Of Tolerance

Soybean[Glycine max(L.)Merr.]is the main oilseed crop in north of China,which yield and quality were extremely affected by potassium fertilizer.In recent years,the decline of K content in soil need input mineral fertilizer,which not only increase cost of agricultural production,but also decrease fertilizer use efficiency.Low fertilizer utilization increased the potential hazards of environmental pollution.All these could limit soybean production. Various studies on crops showed that planting tolerant varieties was an economic and effective way of alleviating potassium resource shortage and prompting sustainable development of agriculture in China.In former experiments,potassium tolerant and non-tolerant genotypes had been screened in field.The present study is an attempt to identify the physiological mechanisms implicated in tolerance to low potassium by testing difference of biological responses,dry matter accumulation of plants,K kinetic parameters and K efficiency of two typical genotypes that differ in tolerance to low K stress.The main conclusion of study were as followed:1.Responses of biomass and economic yield of different tolerance varieties to low potassium stress was different,and difference between varieties was significant.The changes of weight per 100 seeds were slight to low potassium stress for two types of varieties,but for non-tolerant variety,greatly changes has been determined on pods per plant and harvest index under low potassium stress.2.There were distinct differences of stress symptom when tolerant and non-tolerant varieties transplanted in solution culture.Compared with field plant,tank farming was more controllable and repeatable.In general,plant dry matter and root/shoot ratio decreased consistently with the decline of K levels in the growth medium.The two types were hardly reduced dry matter weight of plant under heavy low K stress,but this difference became progressively smaller with the increase of K levels in solution.During culture period,the first noticeable influence of K deficiency on the growth parameters of plant organs appeared at 14^thdate and difference in total biomass between treatments was more acute at 21^stdate for non-tolerant varieties.There was,however,little change in tolerant varieties.These results suggested that 21^stdate could potentially be used as the crucial time for investigating differences of biological responses.3.Morphological and physiological characteristics of root are important for absorbing K efficiently from culture solution.Effect of low K stress on root morphological characteristics was tiny at early stage of growth.By 21^stdate,low K stress produced significant effect on root morphological characteristics for non-tolerant variety.Total root length,average root radius,root surface area,and root volume were all declined due to K deficiency,while those parameters of tolerant variety were slightly reduced by low K stress.Plants had evolved some mechanisms to maximize the efficiency of potassium acquisition.Changes in root architecture are one such mechanism.When soybean seedlings were grown under conditions of low potassium availability,the ply of cells in the cortex of root tip was increased,but the stele was thin,compared with seedlings grown under conditions of high potassium availability.4.The change of leaf areas was tiny for tolerant variety under low potassium stress.And positive effects of low K stress on special leaf weight were slight at the early growing stage for non-tolerant variety,and marked increase was observed at later stage,but there was no difference between two K levels for tolerant variety.Collenchyma and vascular bundle in stem was weaker than that of adequate K plants for non-tolerant than tolerant variety.Those changes were same in comparison with pith in stem.When plants were subjected to low K stress,main veins were slenderer and parenchyma cell were smaller than that of K-adequate plants.Indeed we found that phloem,cambium,and xylem were all affected by the applied low K stress.And greater decreases in vascular bundle and collenchyma of leaf vein occurred for non-tolerant than for tolerant variety.Anatomical traits of leaf cross-sections performed that palisade tissue layer arrayed densely near the veins,but loosely away from veins. However,basic difference between tolerant and non-tolerant genotypes did not exist.5.When the roots of plants grown in K deficient solution are exposed to ample K concentration solution,the rate of K uptake previously in short supply is much increased compared with control plant maintained with an adequate K provision.Representative depletion curves showed the marked contrast between unstressed controls and plants under low K stress.In our uptake experiments,the decline of K concentration was initially rapid and near linear with time,but became more gradual as the concentration approached zero.The non-tolerant variety was superior over the tolerant variety in uptake both speed and time. Potassium kinetic study indicated that K deficiency in plants result in a decrease in K_m value and increase in I_maxand C_minvalue.However,change extent of each parameter was harsh different between tolerant and non-tolerant varieties.Furthermore,root bleeding sap and cation exchange capacity of non-tolerant variety were all declined under low K stress,while that of tolerant variety changed slightly affected by K deficiency.The plasma membrane H⁺-ATPase activities were dramatically greater than those at adequate K supply,especially to tolerant variety.6.Cellulose content of stem of non-tolerant variety at deficient K supply was much lower than that at adequate K supply.Soluble sugar content in leaf significantly declined due to low K stress,while starch content in leaf varied slightly for tolerant variety,but the content of soluble sugar and starch changed on the other way round for non-tolerant variety.Besides, low K stress produced significantly negative effect on Ca²⁺-ATPase and Mg²⁺-ATPase activities in leaf for non-tolerant variety,while those of tolerant variety were slightly reduced by low K stress.7.For non-tolerant variety it was combined with a decrease of chlorophyll content under low K stress,especially decline of Chl a.There was a slightly change in chlorophyll content of tolerant variety,but the ratio of Chl a/b remarkably increased due to K deficiency,which was benefit to capturing more photo electron.Photosynthetic rate,stomatal conductance,and intercellular CO₂ concentration of non-tolerant variety were all significantly reduced in K-deficient compared to K-sufficient leaves at 21^stdate.By contrast,photosynthetic rate of tolerant variety kept a relatively high level under low K stress in spite of heavy decline of stomatal conductance and intercellular CO₂ concentration.Decrease of intercellular CO₂ concentration indicated that in tolerant and non-tolerant variety the stomata were more closed, which was reflected by a high value of stomatal limitation.There was also a decrease of WUE at the same time,especially for non-tolerant variety,which decline was noticeable.In the experiment all CF parameters were affected by low K stress:also F₀ was affected and varied for the non-tolerant but the tolerant.Except for that,the ratio of energy dissipated by reaction center increased in low K stress for non-tolerant variety,and the ratio of transfer energy decreased at the same time,while those change of tolerant variety were tiny.8.With plant growth,potassium accumulation and distribution in plant varied significantly between deficient and adequate K supply.Low K stress reduced the accumulation of K in root followed by leaf and stem,which lead to a change in K partition between organs.Marked differences existed in the non-tolerant variety.K accumulation in plant was the function of K content and dry matter weight,and was positively correlated with dry matter weight.So the potassium contents of tolerant and non-tolerant genotypes were close in the same K level. There were no difference between change of K uptake efficiency for tolerant and non-tolerant varieties,but change of K utilization efficiency between two genotypes in low and normal K. level was dramatically distinct.