| Foxtail millet(Setaria italica L.)is an important food crop in northwest China,which is rich in nutrients and is gradually becoming a functional food raw material.However,decreased and uneven rainfall limits its growth and production.Therefore,cultivating new drought-resistant and high-yield varieties is an effective measure to cope with climate change.Study on the phenotypic characteristics of millet root is the basis of breeding new varieties.To reveal the root phenotypic and physiological and biochemical basis of millet to adapt to drought,and provide an important theoretical basis for breeding new varieties.In this study,65 varieties of millet were collected from four ecological regions in China and cultivated in water.Eight dominant varieties were selected by identifying root phenotype;secondly,the effects of drought on root phenotype,physiology and biochemistry characteristics were preliminarily determined by using water culture method to simulate drought with PEG-6000;finally,a pot experiment was conducted to determine the effects of drought stress on root phenotype,physiology and biochemistry by selecting drought resistant variety(Yugu1)and drought sensitive variety(An04)during the whole growth period.The main results are as follows:1.There are great differences in root growth and functional traits among different varieties.Root dry weight,maximum root length,root surface area and root density contributed most to the principal components.Root surface area ratio and root volume ratio were positively correlated with root length ratio,while root length density and root weight density were negatively correlated with root length ratio(P<0.05).Among the four ecological zones,An04 in the early maturing region of northwest China had the largest root length,shoot dry mass,root dry mass and root density;the root system of the early maturing region in northeast China was the smallest.The root length and root dry weight of the varieties in the northwest early maturing area were about 2.2 times of those in the northeast early maturing area.2.The response mechanism of millet seedling roots to drought stress was different in different ecological regions.From north to south,the root-shoot ratio,root length,root surface area and root volume of millet increased in turn.The nonstructural carbon content of millet root was the highest in the early maturing region of northwest China,and the water absorption ability of millet varieties in summer millet region was the strongest.Under drought stress,the millet biomass decreased and the ratio of root to shoot increased;root length,root surface area and root volume decreased significantly,especially for summer millet cultivars;more photosynthetic sugar was distributed to the root system to keep the ratio of soluble sugar to starch stable in millet of early maturing region in northwest China.3.The water use efficiency of Yugu1 and An04 increased by 18.4% and 63.2%under drought stress respectively,compared with contrast;root exudation(RE)decreased by 66.7% and 89.0%,photosynthetic rate by 34.3% and 61.8%,and yield by 40.6% and 62.6%,respectively.RNA-Seq data showed that the expression of β-amylase 2 in Yugu1 was lower than that in An04.The expression levels of three SWEET genes and four PIP genes in Yugu1 were higher than those in An04,so more photosynthetic sugar was allocated to roots to prevent the ratio of soluble sugar to starch from increasing significantly.The high expression level of Si PIP also enhanced the ability of root water transport.Under drought stress condition,foxtail millet maintained higher RE,stomatal conductance and net photosynthetic rate.In conclusion,the millet varieties in the early maturing region of northwest China had larger roots and more non-structural carbon in the roots,and more photosynthetic sugar was allocated to the roots to stabilize the ratio of soluble sugar to starch under drought stress,thereby improving the resistance. |
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