The Study On Root Morphology Architecture And Physiological Response Of Alfalfa Root Under Water Deficit

Drought is the main limiting factor restricting the normal growth of plants,especially in arid and semi-arid areas,where water shortages have severely reduced forage yield.Alfalfa(Medicago sativa L.)is a high-quality pasture with a long history of planting in China.It is one of the main legume pastures grown in grassland,and it is also a basic guarantee for soil and water conservation.By comparing the differences in the plasticity,physiological response and molecular mechanism for the root system of alfalfa to water changes will contribute to it provide a reference for further research on the drought resistance mechanism of alfalfa.By comparing root morphological traits,physiological responses of different alfalfa varieties with Aohan(Medicago sativa L.cv.Aohan),Zhaodong(cv.Zhaodong)and Golden Empress(cv.Golden Empress)were used as test materials under drought stress or/and drought and rehydration in this study.On the other hand,molecular mechanism of alfalfa root was compared from the transcriptomics level and identified key candidate genes in response to drought stress under different degrees and durations by polyethylene glycol 6000(PEG-6000)induced drought stress.1.When soil water content was kept at 60%-65% and 75%-80% water holding capacity,the aboveground biomass accumulation was peaked at 7.4 g/pot and 6.5 g/pot for 5 cm cutting height respectively.When soil water content was kept at 60%-65% water holding capacity,the accumulation of aboveground biomass was peaked at 6.7g/pot,which revealed reasonable cutting management is beneficial to alleviate plant damage caused by drought stress.2.Alfalfa root system tended to exhibit dichotomous branching under the most severe water deficit(30%-35% water holding capacity).The root system tended to "herringbone" branch under suitable soil moisture conditions with smaller root biomass and larger aboveground biomass.The findings improve our understanding of the distribution and pattern(growth and lost)on dry matter between alfalfa root system and aboveground part under different water conditions.3.Root ABA directly regulated root system traits,and indirectly affected spatial distribution of the root system,and ultimately affected the forage root system and aboveground yield through structural equation model analysis,which revealed the morphological structure of alfalfa root system and its change characteristics with the above-ground biomass under the action of root ABA were investigated.4.Our results demonstrated that plants had higher fractal dimensions(1.47),fractal abundances(3.84),and branching ratios(0.39)under severe water deficit and rehydration conditions.Plant root systems tended to exhibit herringbone branching when soil moisture was at 60%-65% water-holding capacity with the soil being then rehydrated.When the soil water content was kept at 30%-35% of the field water holding capacity and gradually rehydrated to 75%-80% of the field water holding capacity,the content of active oxygen in roots shows a downward trend;indicating that rehydration after severe drought can alleviate the effects of active oxygen on plants.5.Under PEG-6000 simulated drought stress,with the extension of the PEG stress time,the contents of MDA,carbohydrates and nitrogenous compounds showed an upward trend.During the process of treatments,during the whole stress process,with the increase of PEG concentration and treatment time,the root ABA content increased first and then decreased.6.RNA sequencing transcriptomics results showed that a total of 6854 differential expressed genes(DEGs)(2616 up and 4238 down-regulated genes)involved in drought stress response were identified for7 days between FC(0 MPa)and FO(-1.2 MPa).A total of 3959 differential expressed genes(2323 up and 1636 down-regulated genes)involved in drought stress response were identified for 14 days between SC(0 MPa)and SO(-1.2 MPa).A total of 2780 differential expressed genes(1587 up and 1193 downregulated genes)involved were identified for 21 days between TC(0 MPa)and TO(-1.2 MPa).Meanwhile,the number of different expressed genes that regulate root ABA biosynthesis and signal transduction gradually decreased with the increasing of duration of drought stress.In the end,seven candidate different expressed genes were found to be the same as the results of transcriptome sequencing.This study explored the relationship between root morphological changes and physiological responses of different alfalfa varieties under water fluctuations,and further understood the effects of root ABA on the changes of alfalfa root morphological distribution and plant biomass under drought stress.The mechanism of candidate genes on root ABA synthesis and signal transformation in improving the drought resistance of alfalfa need to be further investigation.