Drought Tolerance Identification and Eevluation of Maize during Flowering Stage in Guangxi

Maize Research Institute, Guangxi Academy of Agricultural Sciences, Nanning 530007, China

AbstractTotal 20 maize varieties were subjected to drought stress at flowering stage, and then the relative water content, soluble sugar content, chlorophyll content, malondialdehyde (MDA) content and superoxide dismutase (SOD) activity in their leaves, as well as their yields were determined. The drought tolerance of the physiological and biochemical indexes was scored by five-level scoring method, and the drought tolerance index was calculated by the yield index to comprehensively evaluate the drought tolerance of maize during flowering stage. The results showed that the scores of drought tolerance of the maize varieties ranged from 1.929 3 to 5.659 5. Among them, the scores of Zhengda 619, Guidan 162 and Guidan 0810 were greater than 5.0, followed by Dika 008, Xianyu 30T60, Xianzhengda 901, Qingnong 68, South America No.1 and Wanchuan 1306 of which the scores were in the range of 4.0-5.0. The drought tolerance indexes were in the range of 0.410 4-1.096 3. Among the test maize varieties, the drought tolerance indexes of Guidan 0810, Pacific 99 and Zhengda 619 were greater than 1.0, and those of Xianyu 30T60, Dika 008 and South America No.1 were in the range of 0.9-1.0. The correlation between the two kinds of evaluation results was 0.588 7 and was extremely significant. The five-level scoring method and the drought tolerance index can be used simultaneously for the evaluation of drought tolerance of maize during flowering stage. The two aspects of evaluation results showed that Guidan 0810, Zhengda 619, Xianyu 30T60, Dika 008 and South America No.1 were drought-tolerant varieties, among which Guidan 0810 and Zhengda 619 were extremely highly drought-tolerant varieties.

Key wordsMaize, Drought tolerance identification, Drought tolerance index, Drought stress, Flowering stage

1 Introduction

Maize is the second important food crop in Guangxi after rice. It is of great significance to agricultural production in Guangxi. Guangxi belongs to the subtropical monsoon climate with abundant annual precipitation. But affected by the alternation of winter and summer monsoon, precipitation varies in different seasons, and drought and flood disasters occur frequently[1]. Spring drought affects the sowing and emergence of spring maize, and autumn drought affects the pollination and filling of autumn maize. Both of them have a serious impact on maize production[2]. Drought is the primary factor that restricts the increase of maize yield in Guangxi[3]. The main way to overcome drought is to improve crop growth environment by improving irrigation efficiency and cultivation techniques[4]. However, Guangxi is mainly mountainous, and its geographical environment is complex. There are many difficulties in solving the drought problem through the above measures. Therefore, the research and breeding of maize varieties with strong drought tolerance is the main direction for resisting drought[5-6]. The sensitivity of maize to water is different in different periods. Production practice has shown that maize is most sensitive to water stress at flowering stage. In this stage, water deficiency delays tasseling and reduce pollen viability of maize, leading to serious decrease in maize yield[7-9]. Taking leaf relative water content and survival rate at drought stress of young seedlings as the indexes, Wu Binetal.[10]evaluated the drought tolerance of 53 maize inbred lines at seedling stage, and 13 drought-tolerant lines, 16 moderately drought-tolerant lines and 24 drought-sensitive lines were screened out. Tan Jingetal.[11]analyzed the main yield and drought tolerance-related agronomic and physiological traits of 145 new hybrid maize combinations, and a number of new maize varieties with high yield, strong drought tolerance and other excellent agronomic traits were screened out. Du Caiyanetal.[12]analyzed the changes of thirteen drought tolerance-related phenotypic traits and physiological and biochemical indexes of eight maize varieties, and their drought tolerance at seedling stage was evaluated comprehensively by weighting the standard deviation coefficients. Yu Guihaietal.[13]simulated drought stress with 20% polyethylene glycol solution to evaluate the drought tolerance of 14 maize varieties at germination stage. Du Weilietal.[14]conducted comprehensive evaluation on the drought tolerance of maize varieties using grey correlation analysis and principle component analysis, combined with fuzzy function method, using drought tolerance subordinate degree, yield drought tolerance and drought tolerance index as evaluation parameters. Qin Yongaietal.[15]evaluated drought tolerance of maize inbred lines by using DC, DI and SV together with principle component analysis and cluster analysis, and it was concluded that maize yield, grain number per row, ear length and ear diameter were important indexes for screening drought-tolerant germplasms in production. He Jingdanetal.[16]discussed the response of different maize varieties in Guangxi to drought stress and rewatering after drought at heading stage.

At present, there are many reports on drought tolerance of maize at seedling stage[17], but the drought tolerance of maize at flowering stage is rarely mentioned. Agronomic traits and yield are mostly used as the evaluation indexes[18-19], and the method is single. It has been reported that drought tolerance of maize varieties in Guangxi at flowering stage was evaluated by yield index combined with drought tolerance physiological and biochemical indexes. In this study, 20 major maize varieties planted in Guangxi were used as test materials, which were subjected to drought stress simulated by controlling water in rainproof greenhouse at flowering stage, and the drought tolerance coefficient and drought tolerance index were calculated according to the yield to comprehensively evaluate the drought tolerance of maize at flowering stage, so as to provide a reference for the breeding and screening of drought-tolerant maize varieties in Guangxi.

2 Materials and methods

2.1 Test materialsA total of 20 maize varieties were selected, including Tiandan 101, Zhengda 619, Zhengda 808, Guidan 0810, Xianyu 30T60, Guiyu No.1, Chuangui 336, Xianzhengda 901, South America No.1, Wanchuan 1306, Pacific 98, Pacific 99, Zuankababa, Qingnong 68, Luhai 733, Luyu 269, Dika 008, Guidan 162, Yahang 0919 and Nanxiao 969, of which the seeds were purchased from Guangxi Agricultural Science and Technology Market.

2.2 Test designThe test was carried out in the rainproof greenhouse of the Scientific Research Core Demonstration Area of Guangxi Academy of Agricultural Sciences. The soil was red loam with organic matter of 10.39 g/kg, alkali-hydrolyzable nitrogen of 65.8 mg/kg, available potassium of 157.5 mg/kg, available phosphorus of 15.68 mg/kg and pH value of 5.60. Direct seeding was used. Two lines were arranged for each maize variety in each treatment group. The length of each line was 3 m, and the spacing between every two lines was 0.7 m. The spacing between every two adjacent plants was 0.3 m. Randomized block design was adopted. Two treatments were designed, normal water supply (CK) and drought stress (T). The two treatments were conducted in different drought-tolerant ponds separated by 1.5-m-wide cement floor, and three replicates were arranged for each treatment. The seeds of the maize varieties were sown on August 8, 2016. After the sowing, 48 g of Jinzhengda controlled-release fertilizer (N∶P2O5∶K2O=24∶8∶10) was applied as base fertilizer in each line at one time. In the normal water supply treatment group, the soil water content was maintained above 75% of the maximum water holding capacity during the whole growth period of maize. In the drought stress treatment group, the water supply was cut off about 15 d before flowering (September 26). The flowering period of the maize varieties was concentrated between October 11 and October 18. During the period, water supply was controlled to maintain the soil water content at 52.8%-55.2% of the maximum water holding capacity, reaching the moderate drought stress level[20]. Three ear leaves were collected from each maize variety in each treatment on October 19, and the physiological and biochemical indexes were measured after mixing. The soil water content was measured as 50.1% of the maximum water holding capacity 10 d after the end of flowering of all varieties (November 3), and irrigation was resumed then. The maize ears were harvested on December 1, and the yield was converted after threshing and drying.

2.3 Measured indexes and methods

2.3.1Physiological and biochemical indicators and five-level scoring method. Relative water content, chlorophyll content, soluble sugar content and malondialdehyde (MDA) content of maize leaves were determined in accordance with the methods recorded inExperimentGuideofPlantPhysiology[21]. Superoxide dismutase (SOD) activity was determined by the assay provided by Nanjing Jiancheng Institute of Bioengineering.

The five-level scoring method referred to Wu Wenrong’s method[22]. The formulas for calculating drought tolerance coefficients of relative water content, chlorophyll content and SOD activity are as follows:

Drought tolerance coefficient of relative water content=Measured value of relative water content under drought stress/Measured value of relative water content under normal water supply

Drought tolerance coefficient of chlorophyll content=Measured value of chlorophyll content under drought stress/Measured value of chlorophyll content under normal water supply

Drought tolerance coefficient of SOD activity=Measured value of SOD activity under drought stress/Measured value of SOD activity under normal water supply

The drought tolerance coefficient of soluble sugar content was calculated according to the following formula:

Drought tolerance coefficient of soluble sugar content=(Measured value of soluble sugar content under drought stress-Measured value of soluble sugar content under normal water supply)/Measured value of soluble sugar content under normal water supply

The drought tolerance coefficient of MDA content was calculated according to the following formula:

Drought tolerance coefficient of MDA content=(Measured value of MDA content under drought stress-Measured value of MDA content under normal water supply)/Measured value of MDA content under normal water supply

The range between the maximum and minimum of each index was set at five points. The minimum value was assumed as one point, and the maximum value was assumed as six points.

The formulas for cultivating the score of arbitrary value of each index are as follows:

D=(Hh-Hl)/5

F=(H-Hl)/D+1

whereDis the range of 1 point,Hhis the maximum value of each index;Hlis the minimum value of each index;Fis the score; andHis the arbitrary value of each index.

According to the coefficient of variation of each index, the weight coefficient matrix of each index participating in the comprehensive evaluation was determined according to the following formula:

Weight coefficient of an index=Coefficient of variation of the index/The sum of coefficient of variation of each index

On the basis of Wu Wenrong’s leveling drought tolerance of maize varieties[22], one level of extremely high tolerance was added. The variety with comprehensive score greater than and equal to 5.0 was identified as extremely highly drought-tolerant variety; the variety with comprehensive score in the range of 4.0-5.0 was identified as highly drought-tolerant variety; the variety with comprehensive score in the range of 3.0-4.0 was identified as moderately drought-tolerant variety; and the variety with comprehensive score smaller than 3.0 was identified as drought-sensitive variety.

2.3.2Yield, drought tolerance coefficient and drought tolerance index. Total eight consecutive plants were sampled randomly from each line. The collected plants of each treatment were threshed, and the grains were dried, weighed and converted to yield per hectare with 14% water content in grains. The following formulas are used to calculate drought tolerance coefficient (DC) and drought tolerance index (DI).

DC=Grain yield of a maize variety under drought stress/Grain yield of the maize variety under normal water supply

DI=DC×Grain yield of a maize variety under drought stress/Average grain yield of all maize varieties under drought stress

Referring to the classification of drought-tolerant crop types of Lan Jushengetal[23], combined with the practice of maize production in Guangxi, the variety withDIvalue greater than and equal to 1.00 was identified as extremely highly drought-tolerant variety; the variety withDIvalue in the range of 0.90-1.00 was identified as highly drought-tolerant variety; the variety withDIvalue in the range of 0.80-0.90 was identified as moderately drought-tolerant variety; and the variety withDIvalue smaller than 0.80 was identified as drought-sensitive variety.

2.4 Statistical analysisData processing was performed using Excel 2007, and significance test and correlation analysis were carried out using DPS 16.05.

3 Results and analysis

3.1 Effect of drought stress on relative water content of maize leaves at flowering stageRelative water content is often used as a parameter to indicate the degree of water deficit of plants under drought stress. Table 1 shows that under drought stress, the leaf relative water contents of the maize varieties at flowering stage reduced in varying degrees compared with those of the control group. The smaller the reduction was, the greater the drought tolerance coefficient was, and the stronger the drought tolerance was. Based on the drought tolerance coefficient calculated by relative water content, the drought tolerance coefficients of Guidan 162, Qingnong 68, Guidan 0810, Zhengda 619 and South America No.1 were all greater than 0.9, which belonged to highly drought-tolerant varieties.

Table 1 Effect of drought stress on relative water content of maize leaves at flowering stage

VarietyRelative water content∥%CKTGrowthrateDrought tolerancecoefficientGuidan 16293.6886.86-7.280.927 2 aQingnong 6894.6586.96-8.120.918 8 abGuidan 081092.6384.46-8.820.911 8 abcZhengda 61994.0385.61-8.950.910 5 abcSouth America No.193.1283.87-9.930.900 7 abcdWanchuan 130691.4281.98-10.330.896 7 abcdeXianyu 30T6092.6182.91-10.470.895 3 abcdeDika 00894.7384.73-10.940.890 6 bcdeXianzhengda 90193.0382.71-11.090.889 1 bcdefLuhai 73393.6982.61-11.830.881 7 cdefgZhengda 80894.9782.45-13.180.868 2 defghYahang 091991.4479.08-13.520.864 8 efghiNanxiao 96993.6680.28-14.290.857 1 fghiLuyu 26990.7177.51-14.550.854 5 ghijGuiyu No.191.1077.24-15.210.847 9 hijkPacific 9993.8378.75-16.070.839 3 hijkTiandan 10192.7277.69-16.210.837 9 hijkZuankababa92.1776.83-16.640.833 6 ijkPacific 9893.9577.40-17.620.823 8 jkChuangui 33694.7377.53-18.160.818 4 k

Note: Different lowercase letters in the same column represent significant differences between varieties at the level of 0.05. The same below.

3.2 Effect of drought stress on chlorophyll content of maize leaves at flowering stageThe change of chlorophyll content under drought stress affects the efficiency of plant photosynthesis. Table 2 shows that under drought stress, the leaf chlorophyll contents of the maize varieties at flowering stage reduced in varying degrees compared with those of the control group. The smaller the reduction was, the greater the drought tolerance coefficient was, and the stronger the drought tolerance was. The overall analysis showed that the change of chlorophyll content in maize flowering stage under drought stress was relatively small compared with that of the control group. Among the 20 test maize varieties, the drought tolerance coefficients of nine varieties were greater than 0.9. The variance analysis showed that there were no significant differences among the eleven varieties with the highest drought tolerant coefficients.

3.3 Effect of drought stress on superoxide dismutase (SOD) activity in maize leaves at flowering stageSOD is an important active oxygen scavenging enzyme in plants, and it protects cells from excessive reactive oxygen species stress. As shown

Table 2 Effect of drought stress on chlorophyll content of maize leaves at flowering stage

VarietyChlorophyll content∥mg/gCKTGrowthrate∥%Drought tolerancecoefficientQingnong 682.2022.062-6.380.936 2 aWanchuan 13062.1572.014-6.610.933 9 aGuidan 08102.3172.161-6.740.932 6 aGuidan 1622.8552.657-6.950.930 5 aZhengda 6192.1501.994-7.260.927 4 aSouth America No.12.5172.296-8.780.912 2 abDika 0082.3812.167-9.000.910 0 abXianzhengda 9012.4152.197-9.040.909 6 abXianyu 30T602.6682.420-9.300.907 0 abLuhai 7332.0941.879-10.280.897 2 abcLuyu 2692.3972.137-10.860.891 4 abcZhengda 8081.9171.674-12.650.873 5 bcdPacific 981.8721.631-12.890.871 1 bcdZuankababa2.2891.984-13.340.866 6 bcdNanxiao 9692.0851.785-14.370.856 3 cdeTiandan 1012.1781.837-15.640.843 6 defGuiyu No.12.0471.708-16.580.834 2 defChuangui 3361.9691.635-16.950.830 5 defYahang 09192.5062.045-18.410.815 9 efPacific 992.2081.780-19.390.806 1 f

Table 3 Effect of drought stress on SOD activity of maize leaves at flowering stage

VarietySOD activity∥U/mgCKTGrowthrate∥%Drought tolerancecoefficientZhengda 6191 349.511 150.32-14.760.852 4 aXianzhengda 9011 454.431 216.77-16.340.836 6 a Dika 0081 529.501 274.84-16.650.833 5 a Guidan 1621 516.841 209.68-20.250.797 5 ab Guidan 08101 550.791 200.00-22.620.773 8 bcTiandan 1011 629.251 189.68-26.980.730 2 cdSouth America No.11 647.541 189.03-27.830.721 7 cdeLuhai 7331 588.411 129.68-28.880.711 2 deNanxiao 9691 569.741 096.77-30.130.698 7 defPacific 991 447.721 006.45-30.480.695 2 defgZuankababa1 592.811 099.35-30.980.690 2 defgXianyu 30T601 534.811 058.71-31.020.689 8 defgLuyu 2691 592.451 092.26-31.410.685 9 defgChuangui 3361 656.141 093.55-33.970.660 3 efghQingnong 681 608.421 041.94-35.220.647 8 fghiWanchuan 13061 712.961 085.16-36.650.633 5 ghijYahang 09191 740.291 081.94-37.830.621 7 hijGuiyu No.11 658.52990.97-40.250.597 5 ijkZhengda 8081 689.84974.19-42.350.576 5 jkPacific 981 742.75980.65-43.730.562 7 k

in Table 3, the leaf SOD activity of the maize varieties at flowering stage under drought stress declined in varying degrees compared with those of the control group. The smaller the decline was, the greater the drought tolerance coefficient was, and the stronger the drought tolerance was. The overall analysis showed that drought stress had a greater impact on SOD activity in maize flowering stage. The maximum reduction even reached 43.73%. Among them, Zhengda 619, Xianzhengda 901 and Dika 008 decreased slightly, and their drought tolerance coefficients were all above 0.8, indicating that they have strong drought tolerance.

3.4 Effect of drought stress on soluble sugar content in maize leaves at flowering stageSoluble sugar is a ubiquitous organic matter with osmatic regulation in plants. Osmotic regulation is an important way for plants to resist drought stress. As shown in Table 4, the leaf soluble sugar contents of the maize varieties at flowering stage under drought stress increased in varying degrees compared with those of the control group. The larger the increase was, the greater the drought tolerance coefficient was, and the stronger the drought tolerance was. The drought tolerance coefficients reflected by the change of soluble sugar content in maize varieties ranked Guidan 162 > Xianyu 30T60 > Guidan 0810 > Zhengda 619 > South America No.1, of which the drought tolerance coefficients were significantly different from those of the other varieties.

Table 4 Effect of drought stress on soluble sugar content of maize leaves at flowering stage

VarietySoluble sugar content∥mg/gCKTGrowthrate∥%Drought tolerancecoefficientGuidan 16221.9729.7035.170.351 7 a Xianyu 30T6021.3628.8435.010.350 1 aGuidan 081022.4830.2034.330.343 3 aZhengda 61919.8126.4933.690.336 9 aSouth America No.119.0425.4333.540.335 4 aXianzhengda 90120.4426.6130.190.301 9 bLuyu 26925.0332.5530.070.300 7 bWanchuan 130623.6930.5028.730.287 3 bDika 00823.4830.2028.620.286 2 bQingnong 6822.5228.5026.560.265 6 cYahang 091925.7032.4326.180.261 8 cZhengda 80821.0226.5126.110.261 1 cLuhai 73319.9825.0225.270.252 7 cdNanxiao 96923.1128.5923.730.237 3 dPacific 9922.4527.7223.440.234 4 dGuiyu No.123.5229.0123.330.233 3 dPacific 9826.3031.4219.460.194 6 eChuangui 33623.6228.1719.260.192 6 eTiandan 10127.2432.2818.470.184 7 eZuankababa24.6628.7016.380.163 8 f

3.5 Effect of drought stress on malondialdehyde (MDA) content in maize leaves at flowering stageMDA is the main product of plasma membrane peroxidation in plants under stress, and its content reflects the degree of plasma membrane peroxidation. As shown in Table 5, under drought stress, the leaf MDA contents of the maize varieties at flowering stage increased in varying degrees compared with those of the control group. The smaller the reduction was, the greater the drought tolerance coefficient was, and the stronger the drought tolerance was. The drought tolerance coefficients of Yahang 0919 and Zhengda 619, reflected by the changes in the MDA contents, indicated that they have strong drought tolerance, followed by Xianyu 30T60, Guidan 162, Dika 008, Guidan 0810, Wanchuan 1306, Zuankababa and Qingnong 68, which showed greater drought tolerance coefficients and strong drought tolerance.

3.6 Comprehensive evaluation of physiological and biochemical drought tolerance of maize at flowering stageThe drought tolerance coefficients calculated from the physiological and biochemical indexes in Table 1 to Table 5 were converted into corresponding scores according to the five-level scoring method, and the coefficient of variation of each index was calculated. The weight coefficient matrix of each index participating in the comprehensive evaluation was determined according to the coefficient of variation. The comprehensive evaluations core of each variety was obtained by compound operation (Table 6). As shown in Table 6, the comprehensive scores of Zhengda 619, Guidan 162 and Guidan 0810 were greater than 5.0, indicating that they are extremely highly drought-tolerant varieties; the comprehensive scores of Dika 008, Xianyu 30T60, Xianzhengda 901, Qingnong 68, South America No.1 and Wanchuan 1306 were in the range of 4.0-5.0, indicating that they are highly drought-tolerant varieties; the comprehensive scores of Luhai 733, Yahang 0919 and Luyu 269 were in the range of 3.0-4.0, indicating that they have moderate drought tolerance; the comprehensive scores of the other varieties were below 3.0, indicating that their drought tolerance was weak.

Table 5 Effect of drought stress on MDA content of maize leaves at flowering stage

VarietyMDA content∥μmol/gCKTGrowthrate∥%Drought tolerancecoefficientYahang 091931.5936.1114.320.856 8 aZhengda 61934.6840.1115.660.843 4 abXianyu 30T6028.4834.4721.010.789 9 bcGuidan 16228.4934.7421.950.780 5 bcDika 00832.7940.3623.080.769 2 cGuidan 081026.9634.1526.630.733 7 cdWanchuan 130632.3441.0927.040.729 6 cdeZuankababa31.4140.0427.470.725 3 cdeQingnong 6829.0637.1127.710.722 9 cdeLuhai 73328.3437.0630.790.692 1 defXianzhengda 90128.3537.4732.140.678 6 defgPacific 9832.5443.0432.260.677 4 defgZhengda 80828.4038.1334.250.657 5 efghSouth America No.126.8937.0337.710.622 9 fghNanxiao 96930.6242.4438.600.614 0 ghChuangui 33632.1644.9839.860.601 4 hTiandan 10130.1842.5040.800.592 0 hGuiyu No.129.5141.6941.310.586 9 hLuyu 26926.9639.9348.090.519 1 iPacific 9929.8644.3748.590.514 1 i

Table 6 Drought tolerance comprehensive evaluation of physiological and biochemical indexes

VarietyDrought tolerance evaluation score of each indexRelative water contentChlorophyll contentSOD activitySoluble sugar contentMDA contentComprehensivescoreZhengda 6195.232 55.661 86.000 05.606 25.804 55.659 5 Guidan 1626.000 05.780 95.052 56.000 04.886 85.543 7 Guidan 08105.292 35.861 64.643 45.776 54.204 05.150 1 Dika 0084.318 04.993 15.673 84.257 14.721 94.796 6 Xianyu 30T604.534 04.877 83.193 65.957 45.023 94.697 0 Xianzhengda 9014.249 11.977 75.727 34.674 83.400 14.613 7 Qingnong 685.614 06.000 02.468 83.708 94.046 44.358 0 South America No.14.782 25.077 63.744 25.566 32.587 44.348 8 Wanchuan 13064.598 35.911 62.222 04.286 34.144 14.210 4 Luhai 7333.909 04.501 23.563 03.365 63.597 03.784 7 Yahang 09193.132 41.376 62.018 33.607 86.000 03.215 1 Luyu 2692.659 04.278 23.126 34.642 91.073 03.147 3 Zhengda 8083.288 63.590 31.238 23.589 13.092 22.942 6 Nanxiao 9692.778 52.929 33.347 32.955 82.457 52.897 5 Zuankababa1.698 53.325 13.200 61.000 04.081 42.653 6 Tiandan 1011.896 12.441 23.890 91.556 12.136 62.395 4 Guiyu No.12.355 72.079 91.600 62.849 42.062 22.185 1 Pacific 981.248 23.498 11.000 01.819 63.382 62.161 1 Pacific 991.960 51.000 03.286 82.878 71.000 02.041 3 Chuangui 3361.000 01.937 72.684 51.766 42.273 71.929 3 Coefficient of variation (CV)42.270 039.293 442.061 339.710 139.674 1-Weight coefficient0.208 20.193 60.207 20.195 60.195 4-

3.7 Maize yield and drought tolerance evaluationTable 7 shows that the drought tolerance coefficients were not entirely consistent with the drought tolerance index calculated from the yields. Drought tolerance coefficient mainly reflects the stability of crop yield, and drought tolerance index not only reflects the stability of crop yield but also takes into account the high yield. The drought tolerance indexes of Guidan 0810, Pacific 99 and Zhengda 619 were all greater than 1.0, indicating that they are extremely highly drought-tolerant varieties; the drought tolerance indexes of Xianyu 30T60, Dika 008 and South America No.1 ranged from 0.9 to 1.0, indicating that they are highly drought-tolerant varieties; the drought tolerance indexes of Guidan 162, Wanchuan 1306, Xianzhengda 901, Luhai 733, Zhengda 808 and Qingnong 68 were in the range of 0.8-0.9, indicating that they have moderate drought tolerance; and the drought tolerance indexes of the other varieties were all less than 0.8, so they are drought-sensitive varieties.

3.8 Correlation between drought tolerance index of yield and drought tolerance coefficients of physiological and biochemical indexesAs shown in Table 8, the drought tolerance index of yield was positively correlated with physiological and biochemical comprehensive evaluation score, drought tolerance coefficient of relative water content, drought tolerance coefficient of chlorophyll content, drought tolerance coefficient of SOD activity an drought tolerance coefficient of soluble sugar content. The correlations with physiological and biochemical comprehensive evaluation score and relative water content drought tolerance coefficient were extremely significant, the correlation with soluble sugar con-

tent drought tolerance coefficient was significant, and the correlations with chlorophyll content drought tolerance coefficient and SOD activity drought tolerance coefficient were insignificant.

Table 7 Effect of drought stress on yield parameters in different maize varieties

VarietyYield∥kg/haCKTDroughttolerancecoefficientDroughttoleranceindexGuidan 08106 0005 4750.912 5 a1.069 3 a Pacific 995 7905 3700.927 5 a1.092 9 a Zhengda 6195 7455 1900.903 4 ab1.028 9 ab Xianyu 30T606 6305 2500.791 9 abcd0.912 3 abc Dika 0085 1604 6200.895 3 abcde0.907 7 abc South America No.15 8654 9200.838 8 abc0.905 7 abc Guidan 1625 5054 6800.850 1 abc0.873 1 abc Wanchuan 13065 4454 6500.854 0 abcd0.871 4 abc Xianzhengda 9016 7655 1000.753 9 abcde0.843 7 abcd Luhai 7335 3104 5000.847 5 abcd0.836 9 abcde Zhengda 8085 0104 3350.865 3 abc0.823 1 abcde Qingnong 685 9554 6950.788 4 abcde0.812 3 abcde Yahang 09195 9404 5600.767 7 abcde0.768 2 abcde Nanxiao 9696 2404 5750.733 2 abcdef0.736 1 abcde Tiandan 1016 2104 5300.729 5 abcdef0.725 1 bcde Zuankababa6 1804 3350.701 5 cdef0.667 3 cdef Guiyu No.15 7303 7500.654 5 def0.538 6 def Luyu 2694 8903 4500.705 5 bcdef0.534 1 ef Chuangui 3366 0303 7800.626 9 ef0.520 0 ef Pacific 986 0903 3750.554 2 f0.410 4 f

Table 8 Correlation between maize yield drought tolerance index and the drought tolerance coefficient of physiological and biochemical indexes

Correlation coefficientDrought toleranceindexComprehensivescoreRelative watercontentChlorophyllcontentSODactivitySoluble sugarcontentMDAcontentDrought tolerance index1Comprehensive score0.588 7??1Relative water content0.685 3??0.795 5??1Chlorophyll content0.406 80.714 3??0.840 4??1SOD activity0.425 30.305 00.512 2?0.459 9?1Soluble sugar content0.538 00.713 3??0.858 1??0.698 8??0.500 9?1MDA content0.274 00.443 90.576 9??0.459 7?0.283 80.412 31

Note:**represents a significant correlation at the 0.05 level, and*represents a significant correlation at the 0.01 level.

4 Conclusions and discussions

Drought tolerance of crops is caused by many factors. Previous reports only focus on a single physiological index of drought tolerance. Although some progress has been made, the repeatability of many results is poor due to single analysis method and the lack of corresponding statistical methods, so that single index has limitations[24-27]. In this study, several physiological and biochemical indexes related to drought tolerance in maize were analyzed. It was found that the drought tolerance of each variety, reflected by different physiological and biochemical indexes was not consistent, and that one variety was not outstanding in all indicators. Therefore, it is necessary to synthesize multiple indicators for evaluation in order to reduce the one-sidedness of single index evaluation. Many researchers have been aware of the drawbacks of a single index[12, 28]. A set of scientific and reasonable mathematical and statistical methods have been used to evaluate drought tolerance of maize. He Xueyinetal.[29]studied the drought tolerance of 13 maize varieties using drought tolerance coefficient and fuzzy membership function methods and used four drought tolerance physiological indexes to evaluate the drought tolerance of plant comprehensively. Zhang Zhimengetal.[30]studied the drought tolerance of major peanut varieties in northern China. Drought tolerance coefficient method and membership function value method were used to comprehensively evaluate the drought tolerance using thirteen related indexes. Du Caiyanetal.[20]analyzed the changes of thirteen phenotypic traits and physiological and biochemical indexes of eight maize varieties by weighting the standard deviation coefficients to comprehensively evaluate the drought tolerance at seedling stage. In this study, five-level scoring method was used to quantify the measured values of each index by conversion. According to the coefficient of variation (CV) of each index, the weight coefficient matrix of each index participating in the comprehensive evaluation was determined, and through the weight analysis, the drought resistance was evaluated comprehensively. From the maize varieties, extremely highly drought-tolerant varieties of Zhengda 619, Guidan 162 and Guidan 0810 at flowering stage were screened out. This is consistent with the previous research result that Guidan 0810 and Zhengda 619 have strong drought tolerance at seedling stage[31].

In the process of evaluating drought tolerance of crops, yield index is considered to be the most important and direct index for drought tolerance identification of maize varieties[32], and based on yield index, drought tolerance coefficient and drought tolerance index are calculated. Drought tolerance coefficient reflects the stability of varieties, and drought tolerance index takes into account both the stability of varieties and the high yield. Therefore, yield index for crop drought tolerance identification has been substantially improved in the biological sense[23]. Recent studies also show that drought tolerance index has been used to study the difference in crop drought tolerance, and a certain consensus has been reached[33]. The yields of different maize varieties under drought stress at flowering stage were also analyzed, and their drought tolerance coefficients and drought tolerance indexes were calculated. The results showed that Guidan 0810, Pacific 99 and Zhengda 619 had strong drought tolerance. At the same time, their drought tolerance coefficients showed that their yield stability was good. Among them, the results of drought tolerance of Guidan 0810 and Zhengda 619 were similar to those of Chen Kun[18]through yield, and were basically consistent with the results of comprehensive evaluation of physiological and biochemical indexes by five-level scoring method in this study. Considering all the drought tolerance identification results, the drought tolerance calculated based on yield were not entirely consistent with the comprehensive evaluation results of physiological and biochemical indexes. After the correlation analysis between the two aspects of results, it was found that the drought tolerance index was extremely significantly moderately correlated with the comprehensive evaluation result of physiological and biochemical indexes. Considering the correlation between drought tolerance index and each physiological and biochemical index, the drought tolerance index was insignificantly or weakly correlated with all the physiological and biochemical indexes except relative water content. The weak or insignificant correlation might be related to the selection of physiological and biochemical indexes or the duration of drought stress. Therefore, how to evaluate the drought tolerance of maize at flowering stage using single physiological and biochemical index and yield index needs further study.

Although the evaluation results of the two methods were inconsistent, it was found that the drought tolerance index was extremely significantly moderately correlated with the comprehensive evaluation result of multiple physiological and biochemical indexes. Combining the two aspects of evaluation results, it was concluded that Guidan 0810, Zhengda 619, Xianyu 30T60, Dika 008 and South America No.1 were drought-tolerant varieties, and Guidan 162, Xianzhengda 901, Wanchuan 1306 and Qingnong 68 also showed good drought tolerance. Among them, Guidan 0810 and Zhengda 619 showed outstanding drought tolerance, and they were identified as extremely highly drought-tolerant varieties by both the two evaluation methods. Relevant studies[3, 16, 18]and production practice showed that Guidan 0810 had good performance in tolerance to drought and barrenness.

This study shows that both the evaluation results of drought tolerance index and the comprehensive evaluation result of physiological and biochemical indexes by five-level scoring method can be used to evaluate drought tolerance of maize at flowering stage. The two aspects of evaluation results explicate that among the 20 test maize varieties, Guidan 0810, Zhengda 619, Xianyu 30T60, Dika 008 and South America No.1 are drought-tolerant varieties, and among them, Guidan 0810 and Zhengda 619 have extremely strong drought tolerance, and are suitable for cultivation and extension in arid mountainous areas of Guangxi.