METHOD FOR PRECISE CALCULATION OF ROOT STRESS OF SPIRAL BEVEL GEARS

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Proteome analysis of alfalfa roots in response to water deficit stress

To evaluate the response of alfalfa to water deficit(WD) stress, WD-induced candidates were investigated through a proteomic approach. Alfalfa seedlings were exposed to WD stress for 12 and 15 days respectively, followed by 3 days re-watering. Water deficit increased H2O2 content, lipid peroxidation, DPPH(1,1-diphenyl-2-picrylhydrazyl)-radical scavenging activity, and the free proline level in alfalfa roots. Root proteins were extracted and separated by two-dimentional polyacrylamide gel electrophoresis(2-DE). A total of 49 WD-responsive proteins were identified in alfalfa roots; 25 proteins were reproducibly found to be up-regulated and 24 were down-regulated. Two proteins, namely cytosolic ascorbate peroxidase(APx2) and putative F-box protein were newly detected on 2-DE maps of WD-treated plants. We identified several proteins including agamous-like 65, albumin b-32, inward rectifying potassium channel, and auxin-independent growth promoter. The identified proteins are involved in a variety of cellular functions including calcium signaling, abacisic acid(ABA) biosynthesis, reactive oxygen species(ROS) regulation, transcription/ translation, antioxidant/detoxification/stress defense, energy metabolism, signal transduction, and storage. These results indicate the potential candidates were responsible for adaptive response in alfalfa roots.

Physiological and transcriptomic analyses of roots from Malus sieversii under drought stress

Water deficit is one of the main limiting factors for apple growth and production.Root architecture plays an important role in drought tolerance of plants.However,little is known about the molecular basis of root system in apple trees under drought.In this study,we compared root morphology of two widely used rootstocks of apple(R3 and Malus sieversii)under drought.Our results suggested that M.sieversii is more tolerant to drought than R3,since M.sieversii had a higher ratio of root to shoot as well as root hydraulic conductivity under long-term drought conditions.We then performed whole-genome transcriptomic analysis to figure out the molecular basis of root responses in M.sieversii under drought.It was found that genes involved in transcription regulation,signaling or biosynthesis of hormones,and oxidative stress were differentially expressed under drought.Consistent with the gene expression profile,roots of M.sieversii had higher activities of peroxidase(POD)and superoxide dismutase(SOD)under drought,as well as higher content of abscisic acid(ABA)and lower content of auxin.Taken together,our results revealed the physiological and transcriptomic analyses of M.sieversii roots in response to drought.

Does the root to shoot ratio show a hormetic response to stress? An ecological and environmental perspective

Root/shoot(R/S)ratio is an important index for assessing plant health,and has received increased attention in the last decades as a sensitive indicator of plant stress induced by chemical or physical agents.The R/S ratio has been discussed in the context of ecological theory and its potential importance in ecological succession,where species follow different strategies for above-ground growth for light or below-ground competition for water and nutrients.We present evidence showing the R/S ratio follows a biphasic dose–response relationship under stress,typical of hormesis.The R/S ratio in response to stress has been widely compared among species and ecological succession classes.It is constrained by a variety of factors such as ontogeny.Furthermore,the current literature lacks dose-response studies incorporating the full dose–response continuum,hence limiting scientific understanding and possible valuable application.The data presented provide an important perspective for new-generation studies that can advance current ecological understanding and improve carbon storage estimates by R/S ratio considerations.Hormetic response of the R/S ratio can have an important role in forestry for producing seedlings with desired characteristics to achieve maximum health/productivity and resilience under plantation conditions.

Resistant Evaluation of Kiwifruit Rootstocks to Root Zone Hypoxia Stress

In this thesis, 10 species of kiwifruit rootstocks were treated with hydroponics hypoxia to study their root zone hypoxia tolerance. The results were as follows: growth of all kiwifruit seedlings was inhibited. The max length of new root, plant height, plant biomass, root activity, relative growth rate of leaves, and content of chlorophyll in leaves under root zone hypoxia stress obviously declined comparing with control. MDA content, relative conductance in the leaves and roots all increased in 10 kinds of kiwifruit seedlings. The sensitivities of 10 kinds’ kiwifruit seedlings to hypoxia stress were obviously different. With the method of subordinate function and cluster analysis, the adversity resistance coefficient of 10 kinds’ kiwifruit seedlings, were comprehensively evaluated in order to appraise their hypoxia-tolerance abilities. According to the results, “Hayward”, “Qinmei”, “Jinxiang”, “Kuoye”, “Huayou” kiwifruit seedlings held higher tolerance to root zone hypoxia stress, while “Hongyang” kiwifruit seedlings were sensitive to root zone hypoxia stress. The others, including “Xixuan”, “Maohua”, “Jinhua”, “Shanli” kiwifruit seedlings kept moderate resistant intensity to root zone hypoxia stress. The kiwifruit seedlings’ resistance order from strong to weak was: “Hayward” > “Qinmei” > “Jinxiang” > “Kuoye” > “Huayou” > “Xixuan” > “Maohua” > “Jinhua” > “Shanli” > “Hongyang”.

Effects of Chronic Stress of Cadmium and Lead on Anatomical Structure of Tobacco Roots

Cadmium (Cd2+) is one of the major widespread environmental pollutants, and can cause serious problems to all organisms. Lead (Pb2+) is another wide spread dangerous heavy metal. Tobacco is a popular growing economic crop in China. Most tobacco growing region soils contain excessive Cd2+ and Pb2+. To assess anatomic changes of tobacco roots under Cd2+, Pb2+, and Cd2++Pb2+ chronic stress, a pot experiment was carried out in field. The tobacco seedlings with 6 leaves were transplanted to pots in which soil was placed. The amounts of Cd2+ added to soil were 0, 3, 6, 10, 30, 60, and 100 mg kg-1 dry soil. The amounts of Pb2+ added to soil were 0, 150, 300, 450, 600, 750, and 1 000 mg kg-1 dry soil. The amounts of Cd2++Pb2+ added to soil were 0+0, 3+150, 6+300, 10+450, 30+600, 60+750, and 100+1 000 mg kg-1 dry soil. The contents of Cd2+ and Pb2+ in root systems were determined by inductively coupled plasma, and the anatomical structure was studied by method of paraffin sectioning. The results revealed that the amounts of exchangeable Cd2+ and Pb2+ and carbonate bound Cd2+ and Pb2+ in soil increased with the amounts of Cd2+ and Pb2+ added to soil, and the contents of both Cd2+ and Pb2+ in roots were significantly increased along with stress time and the amounts of Cd2+ and Pb2+ added to soil. The growing of tobacco in Cd2+ and Cd2++Pb2+ polluted soil for 50, 100, and 150 d resulted in some abnormal external morphological and anatomical changes in ripe region of lateral roots. All the abnormal roots had abnormal vascular cylinders, and the ratio of abnormal external morphological and anatomical changes of roots positively correlated with the Cd2+ contents in roots and stress time. While, there were no abnormal external morphological and anatomical changes of roots under Pb2+ stress. It was suggested that Cd2+ stress could cause abnormal anatomic changes of roots, but Pb2+ stress could not.

Characterization and expression analysis of a novel RING-HC gene,ZmRHCP1,involved in brace root development and abiotic stress responses in maize

RING is a really interesting new gene which plays important regulatory roles in many developmental processes as well as in plant-environment interactions. In the present report, the Zm RHCP1 gene encoding a putative RING-HC protein was isolated from maize and characterized. The Zm RHCP1 protein contained 310 amino acid residues with a conserved RINGHC zinc-finger motif and two transmembrane(TM) domains. Zm RHCP1 was expressed ubiquitously in various organs(root, stem, leaf, seedling, immature ear, and tassel), but its transcript levels were higher in vegetative organs than in reproductive organs. Moreover, the expression pattern of Zm RHCP1 in brace roots indicated that Zm RHCP1 functions in brace root initiation. In addition, Zm RHCP1 expression was regulated by abiotic stresses. The expression results suggested that Zm RHCP1 plays important roles in brace root development and abiotic stress responses. The findings of the present study provide important information to help us understand the function of Zm RHCP1 in maize.

Changes in Several Physiological and Biochemical Indices of Maize Seedling Roots Caused by Drought Stress

The changes in several physiological and biochemical indices of seedling roots of new maize variety Qingnong 8 were studied under the simulated drought condition with 18% PEG-6000 and water shortage. The results showed that under drought conditions,the contents of soluble protein and malonialdehy( MDA) and the activities of superoxide dismutase( SOD) and peroxidase( POD) in roots of maize seedlings significantly increased,and the increasing amplitude reduced after water shortage for 96 h. The re-watering treatment results after 48 h water shortage showed that the SOD and POD activities and the MDA content could recover to normal level,and the soluble protein content was lower than normal content. This study showed that the maize seedlings of Qingnong 8 suffered drought injury could grow normally after re-watering treatment.

Long Term Salinity Stress Reveals Variety Specific Differences in Root Oxidative Stress Response

Salinity stress induces oxidative stress caused by reactive oxygen species(ROS):superoxide radicals,hydrogen peroxide(H2O2) and hydroxyl radicals.Activities of both enzymatic and non-enzymatic components of the antioxidant system and related growth parameters were studied in the roots of the salt tolerant rice variety FL478 and the sensitive variety IR29 in response to long term stress(12 d) induced by 50 mmol/L NaCl.The comparative study showed that FL478 maintained higher relative growth rate and lower Na+/K+ in the roots than IR29 due to a higher membrane stability index that effectively exclude Na+.Lower TBARS(thiobarbituric acid reactive substance) content in FL478 roots indicated that its membrane was relatively unaffected by ROS despite high H2O2 content recorded under the salinity stress.Relatively higher superoxide dismutase activity along with a parallel increase in transcript level of superoxide dismutase(Os07g46990) in FL478 indicated that this protein might make a vital contribution to salt stress tolerance.Although the content of ascorbic acid remained unchanged in FL478,the activity of ascorbic peroxidases(APOXs) was reduced comparably in the both varieties.Transcriptomic data showed that a larger number of peroxidase genes were upregulated in FL478 compared to IR29 and several of which might provide engineering targets to improve rice salt tolerance.

Relationship between acute stress and sleep disorder in grass-root military personnel:Mediating effect of social support

Background: To explore the relationship between acute stress, social support and sleep disorder in grass-root military personnel, and construct the relational model through structural equation modeling. Methods: A total of 2,411 grass-root military personnel were randomly selected by cluster sampling, and administered the Chinese Military Personnel Sleep Disorder Scale, Military Acute Stress Scale and Social Support Rating Scale.Results: The total score of acute stress scale was positively correlated with the total score and factor scores of sleep disorder scale(r=0.209~0.465, P<0.01); The total score of social support scale was positively correlated with the total score of acute stress scale and the total score and factor scores of sleep disorder scale(r=0.356~0.537, P<0.01). The analysis of mediating effects showed that lack of social support partially mediated between acute stress and the factors of sleep disorder. The analysis of structural equation model showed that acute stress not only had a direct effect on sleep disorder(the path coefficient was 0.29, P=0.000), but also on lack of social support(the path coefficient was 0.39, P=0.000); lack of social support had a direct effect on sleep disorder(the path coefficient was 0.48, P=0.000).Conclusions: Acute stress and lack of social support are two significant factors of sleep disorder in grass-root military personnel. Well-established social support could alleviate sleep disorder induced by acute stress. Lack of social support was a partial mediator between acute stress and sleep disorder.

Effects of salt-alkali stress on active oxygen metabolism in roots of Spiraea × bumalda 'Gold Mound' and Spiraea × bumalda 'Gold Flame'

Under artificially-simulated complex salt-alkali stress, the levels of active oxygen metabolism in roots were studied using three-year-old cutting seedlings of Spiraea × bumalda 'Gold Mound' and Spiraea × bumalda 'Gold Flame'. The present study aimed at exploring the antioxidant capacity in roots of spiraeas and revealing their adaptability to salt-alkali stress. Results indicate that the oxygen free radicals contents, electrolyte leakage rates and MDA contents in roots of Spiraea × bumalda 'Gold Mound' and Spiraea × bumalda 'Gold Flame' show an increasing tendency with the increases of the salinity and pH value, whereas the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) all increased firstly and then decreased. With the increase in intensity of salt-alkali stress, the CAT activity in roots of Spiraea × bumalda 'Gold Flame' is higher and the increasing extents in the oxygen free radicals contents, electrolyte leakage rates as well as MDA contents are lower compared with Spiraea × bumalda 'Gold Mound', indicating that Spiraea × bumalda 'Gold Flame' has a stronger antioxidant capacity.

The Response of Winter Wheat Root to the Period and the After-Effect of Soil Water Stress

To reveal the period and after-effect of soil water stress on winter wheat, the article employs the experiment results carried out in the greenhouse. The results showed that the root-restricted weights varied with stress degrees and stress times during and after water stressing. In the course of stress, the chief reason resticting the weight of root was the stress intensity at this time, and that of severe stress treatment was larger than that of mild stress treatment. After water stress was relieved, the results of the after-effect of soil water stress on root growth were that, the stress intensity of short-time and mild stress was larger than that of long-time and severe stress. Comparing two-stage stress intensities, root-restricted weight resulted from after-effect intensity of stress under all of the short-time treatment, and the mild and the long-time stress treatments, while that resulted from the period stress intensity under the severe and the long-time stress treatments. In general, the effects of water stress on root were attributed to the three factors, a formed basis in the previous stage, the after-effect of water condition before this stage and influence of water in this stage, which lead to the characters of root in the whole growth stage.

Effect of Selenium on Root Oxidizing Ability and Yield of Rice under Ferrous Stress

To study the effects of selenium on root oxidizing ability and yield of rice under ferrous stress, a pot culture experiment was conducted, the activity of glutathione peroxidase (GSH-Px) and the concentration of malonaldelyde (MDA) were determined. The root oxidizing ability and yield characters of rice were examined. Results showed that appropriate amount of Se enhanced the activity of glutathione peroxidase and the oxidizing ability of rice roots significantly, reduced the concentration of MDA, increased 1000-grain weight of rice, F = 26.96**, decreased empty and blighted grain rate, increased the rice yield, F = 11.53**, and enhanced the rice resistance under ferrous stress.

Water Deficit Stress Effects on Corn (<i>Zea mays</i>, L.) Root:Shoot Ratio

A study was conducted at Akron, CO, USA, on a Weld silt loam in 2004 to quantify the effects of water deficit stress on corn (Zea mays, L.) root and shoot biomass. Corn plants were grown under a range of soil bulk density and water conditions caused by previous tillage, crop rotation, and irrigation management. Water deficit stress (Dstress) was quantified by the number of days when the water content in the surface 0.3 m deviated from the water content range determined by the Least Limiting Water Range (LLWR). Root and shoot samples were collected at the V6, V12, and R1 growth stages. There was no significant correlation between Dstress and shoot or root biomass at the V6 growth stage. At the V12 and R1 growth stages, there were negative, linear correlations among Dstress and both root biomass and shoot biomass. The proportional decrease of shoot biomass was greater than the proportional decrease in root biomass, leading to an increase in the root:shoot ratio as water deficit stress increased at all growth stages. Determining restrictive soil conditions using the LLWR may be useful for evaluating improvement or degradation of the soil physical environment caused by soil management.

Loss of Responsiveness to Osmotic Stress in Maize Root:the Effect of Water Channel Blocker HgCl_2

In order to investigate the effect of water channel blocker HgCl2 on the hydraulic resistance in roots of maize seedlings,a xylem pressure probe was used to monitor the changes in root xylem pressure in response to NaCl-or mannitol-induced osmotic stresses before and after the application of HgCl2.When the maize roots were subjected to 500 μmol L-1 HgCl2 in root bathing solution,not only a considerable decline in xylem pressure(increase in xylem tension) was observed,but the loss of responsiveness of the plant to both salt-and mannitol-induced osmotic stresses in terms of xylem pressure change was seen as well when the transpiration rate of the plant was not significantly changed.The results are similar but different from the reversed osmosis by the Fenton reaction in the internodes of Chara coralline,showing that the mechanisms of water transport across cell membrane in plant roots are far more complicated than expected.

Transcriptome analysis reveals the effects of alkali stress on root system architecture and endogenous hormones in apple rootstocks

Soil alkalinity is a major factor that restricts the growth of apple roots.To analyze the response of apple roots to alkali stress, the root structure and endogenous hormones of two apple rootstocks, Malus prunifolia (alkali-tolerant) and Malus hupehensis (alkali-sensitive), were compared. To understand alkali tolerance of M. prunifolia at the molecular level, transcriptome analysis was performed. When plants were cultured in alkaline conditions for 15 d, the root growth of M. hupehensis with weak alkali tolerance decreased significantly. Analysis of endogenous hormone levels showed that the concentrations of indole-3-acetic acid (IAA) and zeatin riboside (ZR) in M. hupehensis under alkali stress were lower than those in the control. However, the trend for IAA and ZR in M. prunifolia was the opposite. The concentration of abscisic acid (ABA) in the roots of the two apple rootstocks under alkali stress increased, but the concentration of ABA in the roots of M. prunifolia was higher than that in M. hupehensis. The expression of IAA-related genes ARF5, GH3.6, SAUR36, and SAUR32 and the Cytokinin (CTK)-related gene IPT5 in M. prunifolia was higher than those in the control, but the expression of these genes in M. hupehensis was lower than those in the control. The expression of ABA-related genes CIPK1 and AHK1 increased in the two apple rootstocks under alkali stress, but the expression of CIPK1 and AHK1 in M. prunifolia was higher than in M. hupehensis. These results demonstrated that under alkali stress, the increase of IAA, ZR, and ABA in roots and the increase of the expression of related genes promoted the growth of roots and improved the alkali tolerance of apple rootstocks.

Effects of Grafting on Root Exudates of Cucumber and Rhizosphere Environment under Copper Stress

Abstract[Objective]This study to aimed to investigate the effects of Cu stress on root exudates and microbial activities in rhizosphere of grafted and ungrafted cucumber seedlings,and therefore to elucidate the microbial mechanism of grafting for increasing cucumber plants tolerance to Cu stress[Method]Four treatments:(1)ungrafted seedlings+test soil(U0);(2)ungrafted seedlings+test soil+CuSO4·5H2O(U1);(3)grafted seedlings+test soil(G0);(4)grafted seedlings+test soil+CuSO4·5H2O(G1)were set in the pot culture experiment.The contents of free amino acids,organic acids,phenolic acid and sugars,microbial population and enzyme activity in the four treatment were measured,respectively.[Result]The secretion of amino acids and organic acids were increased under Cu stress.The amino acids secretions of grafted seedlings roots were obviously higher than ungrafted seedlings except for Phe and Val.At the same time,the secretion of oxalic acid,malic acid,acetic acid,citric acid,cinnamic acid,ρ-hydroxybenzoic acid and benzoic acid of grafted seedlings were significantly higher than ungrafted seedlings as well.Therefore,more Cu2+were restricted in soil by chelating,complexing and precipitation with root exudates,and its toxicity was decreased.The soil microbial biomass C and N in grafted cucumber rhizosphere were significantly higher than those in ungrafted cucumber rhizosphere,whereas basal respiration and metabolic quotient were significantly lower.Under Cu stress,the numbers of actinomyces and nitrogen fixing bacteria decreased and the number of fungi increased significantly,whereas there was no significant difference in amounts of bacteria.The numbers of bacteria,actinomyces,and nitrogen fixing bacteria in grafted cucumber rhizosphere were significantly higher than those in ungrafted cucumber rhizosphere,but the number of fungi was opposite.The activities of soil urease,phosphatase,sucrase and catalase in grafted cucumber rhizosphere were significantly higher than those in ungrafted cucumber rhizosphere.[Conclusion]These indicated that the soil microbial environment and soil enzymes activities were improved by grafting under Cu stress,and as a result,the adaptability of cucumber to Cu stress was improved.

Mitigative Effect of <i>Bacillus subtilis</i>QM3 on Root Morphology and Resistance Enzyme Activity of Wheat Root under Lead Stress

Lead (Pb) is an environmental pollutant extremely toxic to plants and other living organisms including humans. In order to research the relieve effect of Bacillus subtilis QM3 on wheat roots (Triticum aestivum L.), after wheat seeds germination for two days, wheat root caused, the experimental materials were divided into four large groups and each large group was placed in 6 petri dishes as six small groups, and then four large groups respectively cultivated with sterile water (CK), 108 CFU/ml B. subtilis QM3 (B1), 107 CFU/ml B. subtilis QM3 (B2) and 106 CFU/ml B. subtilis QM3 (B3) for 2 days, after that stressed with lead nitrate, Pb (NO)2, Pb2+ concentration calculation at five concentrations (50, 250, 500, 1000, 2000 mg/L), sterile water and different Pb2+ concentration liquid respectively cultivated the 6 small groups in each large group measuring root morpholog and assaying changes of antioxidant enzyme activity. The results showed that: with the increase of the Pb2+ concentration, root morphology index and the activity of antioxidant enzyme increased first and then decreased. Root morphology index reached the maximum in 50 mg/L Pb2+ concentration. B. subtilis QM3 clearly promoted the growth of the root and the antioxidant enzyme activity (p 0.05). Without Pb stress, B. subtilis QM3 had the best improving effect on root morphology. When Pb2+ concentration was 50 mg/L, superoxide dismutase (SOD) and ascorbate peroxidase (APX) reached the maximum. SOD activity, compared with CK, B1, B2 and B3 respectively, increased by 8.05%, 27.41% and 9.79%. APX activity, compared with CK, B1, B2 and B3 respectively, increased by 52.70%, 111.15% and 14.16%. Catalase (CAT) and peroxidase (POD) reached the maximum at the Pb2+ concentration was 500 mg/L. CAT activity, compared with CK, B1, B2 and B3 respectively, increased by 59.93%, 83.46% and 70.59%. POD activity, compared with CK, B1, B2 and B3 respectively, increased by 2.88%, 10.11% and 7.67%. Result suggested that B. subtilis QM3 could improve root growth and antioxidant enzyme activity of the wheat root under lead stress.

Carbon unloading in roots in relation to root senescence in Cercis chinensis seedlings under drought stress

When Cercis chinensis seedlings suffered from drought treatment,net photosynthetic rates had been significantly reduced at the end of the drought treatment. Compared with the control,the activities of acid invertases in roots had increased 5 and 11 days after drought treatment. Seventeen days after drought treatment,the activities of acid invertases in roots were significantly decreased,while activities of alkaline invertases in roots had also been significantly reduced. As the moisture in culture media decreased,so the activities of sucrose synthases in leaves decreased slightly. In roots,their activities had significantly increased 5 and 11 days after drought treatment. The contents of fructose in roots reduced as the moisture in culture media decreased and 11 and 17 days after drought treatment the reduction was significant. The content of glucose in roots clearly did not change as drought stress occurred further,but was still less than that in the control seedlings. Similarly,the content of sucrose reduced as the moisture in culture media decreased. At the beginning of the drought stress,the content of sucrose was significantly higher than that in the control and afterwards there were no differences between drought-treated seedlings and the control. The gradient of the sucrose content between leaves and roots was 0.0982 mg·g-1 FW 17 days after drought treatment,while the gradient of the seedlings under normal condition was 1.3832 mg·g-1 FW. The sucrose concentration gradient reduced by 92.9%. The reduction in the sucrose content gradient under drought stress decreased the sucrose partitioning in roots. Therefore,our results support the hypothesis of 'shared control'.

An Analysis to the Driving Forces for Water and Salt Absorption in Roots of Maize Seedlings Under Salt Stress

When maize seedlings were subjected to salt stress,a decline in root xylem pressure was observed within seconds,followed by a gradual increase in Na+ deposition in the seedlings.The magnitude of xylem pressure response was positively correlated with,but not proportional to the intensity of the stress.A continuous recording of the xylem pressure profile showed that self-regulation of the xylem pressure existed before and after the imposition of salt stress when the environmental conditions were relatively stable.The salt induced increase in xylem tension dominated the total water potential of the plant when the salt stress was mild,but the osmotic potential became more prominent when the NaCl concentration in the root bathing solution was raised to over 100 mol m-3.The average transpiration rate of the seedlings dropped by 40% when the NaCl concentration in the root ambient was increased to 150 mol m-3.Although salt stress resulted in the decline of both the xylem pressure potential and the osmotic potential in the root xylem,the changes in the total water potential of the root xylem solution were always smaller than the changes in the water (osmotic) potentials of the solution bathing the root.An analysis to the water relations of maize seedlings showed that not only the water potential components,but the radial reflection coefficient of the roots was also dependent on the level of salinity.When the NaCl level in the root bathing solution was raised from 25 to 150 mol m-3,the radial reflection coefficient of the root declined from 0.43 to 0.31.This small change resulted in a remarkable increase in the normalised relative NaCl absorption by 2.4 times,indicating that the radial reflection coefficient of root played a very important role in regulating the absorption of NaCl in maize seedlings under salt stress.