Effects And Mechanisms Of Root Size On Water And Nitrogen Uptake In Dryland Crops

Dryland crop production plays a key role in food security.However,dryland crops have been facing the double dilemma of water scarcity and low nitrogen utilization,which has brought certain pressures and challenges to food production.As the main absorption organ of water and nutrients,root system plays an important role in crop production in dry land.But due to the lack of direct and effective soil root research methods,previous studies on yield improvement have been based on the selection of above-ground related traits,and root-related traits have been largely ignored by breeders.Efficient uptake of water and nitrogen by roots is essential for agricultural production.However,the relative relationship of root size and root efficiency to yield,water use efficiency(WUE),and nitrogen use efficiency(NUE)remain unknown.Whether root size is a candidate trait for improving the efficiency of water and nitrogen uptake needs to be further studied.In this study,the effects and mechanisms of root size on water-nitrogen absorption and yield were explored by combining pot and hydroponics experiments,and the mechanisms of water absorption in crop roots by artificially reducing root size(root pruning)were studied.The main results are as follows:1.In both wheat and maize,the root hydraulic conductivity of small roots was significantly higher than that of large roots,and the leaf water potential was no less than that of large root plants,indicating that small roots could improve the water absorption of the whole plant by increasing the root hydraulic conductivity,so as to meet the water demand of the above-ground.Moreover,small roots have a low transpiration rate under drought stress,so small roots can maintain the water balance by increasing root water conductivity and reducing transpiration.The yield of plants with small roots was not lower than that of plants with large roots under both water conditions,indicating that their higher water absorption capacity could meet the demand of yield.Thus,this study reveals that it is possible to increase water absorption and yield,by reducing R/S in modern maize varieties,which may be important for the future breeding of new cultivars suitable for arid regions.2.The leaf water potential,leaf osmosis potential and root xylem osmosis potential of small root plants were not lower than those of large root plants,indicating that the small root system could provide enough water to the shoot.The results were the same as pot experiment.The higher root hydraulic conductivity of plants with small roots compensates for the decrease of roots,and the water absorption capacity is improved,and can maintain the dynamic balance of water absorption and leaf water loss.This may be due to the regulation of aquaporins and root activity.3.Root pruning increased the residual root hydraulic conductivity by 26.2% at 48 h after root pruning under well-watered conditions.,and the leaf water potential of pruned plants was not lower than that of unpruned plants,indicating that plants re-established a new water balance.PEG stress increased the sensitivity of root pruning to osmotic stress and reduced the root hydraulic conductivity of remaining roots in a short time.Root pruning improved the water conduction capacity of residual roots by up-regulating the expression levels of Zm PIP1;1,Zm PIP1;5,Zm PIP2;2 and Zm PIP2;5 and regulating the signals of abscisic acid and jasmonic acid.Compensatory root growth and lateral root growth were stimulated by the increase of auxin and salicylic acid contents after root pruning.4.Under drought stress,the yield of small-root variety Changhan 58(CH)was18.04%,20.77% and 15.72% higher than that of large-root variety Changwu 134(CW)under N0(nitrogen deficiency level),N1(medium nitrogen level)and N2(high nitrogen level),and nitrogen use efficiency was 9.12%,13.93% and 8.32% higher than that of CW,respectively.indicating that CH had a better ability to maintain nitrogen uptake under water stress.The nitrogen uptake per unit root system of CH was significantly higher than that of CW,which improved its own nitrogen uptake capacity and compensated for the decrease of its root system.5.The results of hydroponic experiments showed that the nitrogen uptake per unit root of CH was 47.15%,43.23% and 36.87% higher than that of CW under N1(low nitrogen level),N2(medium nitrogen level)and N3(high nitrogen level),respectively.The increase of CH compensated for the shortage of nitrogen capture caused by the decrease of root,and thus improved the nitrogen uptake capacity of the whole plant,indicating that CH had better nitrogen absorption plasticity than CW under the condition of low nitrogen.The higher nitrogen uptake per unit root of CH was due to the regulation of NR and GS activities in root and leaf and by up-regulating the expression of the nitrogen transporter(Ta NRT1;1,Ta NRT1;2,Ta NRT2;1 and Ta AMT2;1),so that it has high nitrogen affinity and nitrogen absorption potential.This study has important reference value for screening root traits of efficient nitrogen uptake.