Stress Screening Of Potassium-efficient Germplasm Resources Of Proso Millet (Panicum Miliaceum L.) And Study On Physiological Mechanism

Potassium is a key element in the plant growth process and development.Due to the uneven distribution of potassium in farmland and the unbalanced supply and demand of potassium fertilizer,the element tends to become a key factor restricting crop yield and quality in production.Screen the potassium-efficient germplasms is a way to reduce potassium fertilizer application and improve crop yield and quality.The proso millet is rich in genetic diversity and with great stress resistance.The mining of high-quality potassium-efficient germplasm resources can establish a foundation for the resistance breeding of proso millet and the exploration of the mechanism of crop potassium use efficiency.In this study,a series of hydroponic experiments were conducted with the planting of 280 proso millet germplasms.To determine the index for efficient use of potassium proso millet,primary screening and re-screening were carried out under two potassium concentration treatments corresponding to normal potassium supply level and potassium stress,respectively.Based on this,low-potassium-level-tolerant,high-potassium-use-efficiency,low-potassium-level-sensitive,and low-potassium-use-efficiency varieties were screened out,and comprehensively morphological,physiological,and potassium utilize elevation for the selected varieties were carried out.Furthermore,the response mechanism of proso millet with different potassium use efficiencies to low potassium stress was explored,which provides fundamental knowledge for exploring the mechanism of efficient potassium utilization of crops.The main findings are as follows:（1）Within a certain concentration of low potassium stress,with the decrease of potassium concentration in the nutrient solution,the growth,and development of proso millet were inhibited.The 0.01 mmol/L was the optimum potassium concentration to identify the ability of proso millet under seedling to tolerate low potassium.Under the stress treatment of this concentration,the variation coefficient of the low potassium tolerance coefficient of the eight traits including plant height,root length,stem diameter,fresh stem weight,fresh leaf weight,root dry weight,stem dry weight,and root surface area was the largest,which could maximize to show the difference of low potassium tolerance and potassium efficiency of proso millet with different genotypes.Besides,plant and leaf morphology also showed obvious symptoms of potassium deficiency under this concentration,that is,the stem is slender and weak,the leaf support is weak and easy to bend,and the leaf tips and leaf margins appear scorched and speckled chlorosis.（2）The 280 proso millet germplasms had extensive variation in 11 indexes represented by plant height under low potassium and normal potassium conditions,and there were significant differences with all of them.Based on the results of coefficient of variation,principal component analysis,and correlation analysis,plant height,aboveground dry weight,total root length,root surface area,root volume,root tips number,aboveground potassium accumulation,and total plant potassium accumulation were determined as the indicators for low potassium tolerance and high potassium use efficiency in the seedling stage of proso millet.The clustered heatmap and scatter plot were generated from the low potassium tolerance coefficient and the comprehensive elevation index of potassium use efficiency,and Var189 and Var87 were screened out as low-potassium-level-tolerant and high-potassium-use-efficiency varieties,whereas Var116 was defined as low-potassium-level-sensitive,and low-potassium-use-efficiency variety.（3）The morphological responses of low-potassium-level-tolerant and high-potassium-use-efficiency varieties,Var189 and Var87,and low-potassium-level-sensitive,and low-potassium-use-efficiency variety,Var116,were quite different.The growth of shoots of high-potassium-use-efficiency varieties was inhibited by low potassium stress,and the plant height,stem diameter,and aboveground biomass were decreased significantly,while their roots were developed and dense,which may adapt to low potassium stress by increasing root surface area,root volume,and total root length.The shoots and roots of the low-potassium-use-efficiency variety were severely inhibited,and the plant height,stem diameter,aboveground biomass,and root traits values were significantly decreased with the exception of average root diameter,which indicated that the growth of small lateral roots was inhibited.Low potassium stress seriously affected the biosynthesis of photosynthetic low-potassium-use-efficiency proso millet,and the contents of all photosynthetic pigments were significantly lower than those of normal potassium supply.The content of chlorophyll a and carotenoid in high-potassium-use-efficiency varieties was significantly increased under low potassium concentrations,while the content of chlorophyll b and total chlorophyll decreased,and the contents of all photosynthetic pigments were significantly higher than those in low-potassium-use-efficiency variety.During photosynthesis under low potassium stress,the two types of proso millet varieties have different response mechanisms.The stomatal limit value of high-potassium-use-efficiency varieties increased under low potassium stress,and both stomatal conductance and intercellular CO₂ decreased,but the net photosynthetic rate had an insignificant change.Although the stomatal conductance and intercellular CO₂ of the low-potassium-use-efficiency variety increased,the net photosynthetic rate decreased significantly,indicating that its photosynthetic rate was mainly limited by non-stomatal factors.The root tips of these two types of proso millets showed different response mechanisms to low potassium stress.The number of cortical cell layers in the high-potassium-use-efficiency varieties increased,and the change in the stele was not significant.The diameter of the stele in the low-potassium-use-efficiency variety significantly declined,indicating that the stele diameter of the root tip was the main factor affecting millet potassium use efficiency.The thick horn tissue in the stem of the low-potassium-use-efficiency variety was significantly thinner,the vascular bundle of the leaf was weakly developed,and the thickness of the rosette tissue cells outside the vascular bundle was also significantly thinner.（4）The content and accumulation of potassium in proso millets were closely related to potassium supply levels.Under the low potassium stress,the content and accumulation of potassium in each part of the millet plant were significantly lower than those under the normal potassium supply,and the potassium use efficiency was significantly higher than that under the normal potassium supply.The potassium uptake efficiency of millet with different potassium efficiencies was evaluated by potassium accumulation.The potassium accumulation of low-potassium-level-tolerant and high-potassium-use-efficiency varieties,Var189 and Var87,was significantly more than that of the low-potassium-level-sensitive,and low-potassium-use-efficiency variety,Var116,while there was no significant difference in potassium use efficiency among them,indicating the difference in potassium use efficiency of the two types of screened proso millet was mainly reflected in the potassium uptake efficiency.