Comprehensive Evaluation on Drought Tolerance of Widely-used Rice Varieties with Subordinate Function Method

Sanxiong LIU,Yongchao LI,Jun WU,Jun MIN,Shuoqi CHANG,Licheng LIU,Xiangyang LU,Qiyun DENG,*

1.Hunan Rice Research Institute,Changsha 410125,China;2.Hunan Agricultural University,Changsha 410128,China;3.Key Laboratory of Indica Rice Genetics and Breeding in the Middle and Lower Reaches of Yangtze River,Ministry of Agriculture,Changsha 410125,China;4.Hunan Hybrid Rice Research Center,Changsha 410125,China

Rice is the main food crop in China,but also the crop with largest water consumption[1].In recent years,global climate has turned warmer and warmer and extreme weathers also occur more and more frequently,leading to severer and severe damage to rice production.The evaluation and breeding of drought-tolerant rice varieties not only are beneficial to yield stability but also can save water[2].

There have been many studies on mechanism and identification indexes of rice drought tolerance at domestic and abroad[3].Liet al.determined total 14 growth indexes of 90 rice varieties (lines)at the seedling stage.They screened out 4 indexes,such as leaf age and length of the first leaf under the heart leaf,as the indexes for evaluating the drought tolerance of rice at the seedling stage,and they also evaluated comprehensively the drought tolerance of rice using subor dinate function method[4].Wanget al.studied the morphological and physiological indexes of rice at the seedling and flowering stages under flood and drought conditions.Their study results showed that the 4 indexes,such as leaf age and root length,could be treated as morphological indexes for evaluating the drought tolerance of rice at the seedling stage,and the 4 indexes,such as leaf proline content and relative water content,can be used for identifying the drought tolerance of rice at the flowering stage[5-6].Deivanaiet al.considered that the chlorophyll stability and proline content could be used as the indexes for evaluating the drought tolerance of rice at the early stage[7].Liuet al.studied the chlorophyll contents (SPAD values)and yield traits of 18 rice varieties at the tillering and booting stage under different water conditions.They found that the rice varieties,which showed small differences in SPAD value between two different water conditions at the booting stage,all showed higher seed setting rate,1 000-grain weight and yield under drought condition,so they considered that the booting stage was the critical period for the sensitivity of rice to water[8].

The studies above mostly confine to the early growth stage(seedling stage)of rice or drought tolerance evaluation of germplasm materials and a few varieties.There are rare reports on the drought tolerance evaluation of rice varieties widely used in production at the water sensitive period.In this study,the drought tolerances of 26 rice varieties widely used in production and 2 control rice varieties were identified,providing a reference for the drought tolerance evaluation of rice varieties and production layout of drought-tolerant rice varieties.

Materials and Methods

Materials

The tested materials included 9 two-line hybrid combinations,10 threeline hybrid combinations and 7 inbred conventional high-quality rice varieties.The drought-tolerant rice variety IAPAR9 (upland rice from Brazil)and drought-sensitive rice variety IR64 were used as the control varieties(Table 1).The seeds of control rice varieties were provided by the germplasm resources bank of Hunan Rice Research Institute.

Methods

Test designThe test was conducted in the arid poolofmultifunctional greenhouse of Hunan Rice Research Institute.The application amounts of nitrogen(N),phosphorus(P2O5)and potassium(K2O)were 165,75 and 75 kg/hm2respectively.According to the growth periods of various rice varieties,the sowing was carried out on May 20th,25thand 30threspectively to synchronize the initial heading stages of various varieties.The seedlings were all transplanted simultaneously on June 15th.There were total 30 plants in each plot with plant and row spacing of 17 cm×20 cm.There were 3 replicates for each treatment,and all the plots were arranged randomly.Since the transplanting,the irrigation was stopped and the arid pool was dried up alternatively.The soil moisture content was determined with drying method.When the soil moisture content was about 20%,the irrigation was carried out for 24 consecutive hours,and after that,the drought stress was performed until the soil moisture reaching about 20%.This cycle was performed several times,and till the dough stage,the irrigation wascompletelystopped.Therice plants cultivated in another pool with shallow irrigation for the entire growth period were treated as controls.

Measured and determined indexes and methodsThe soil moisture content was determined with drying method.

The chlorophyll contents(SPAD values)in the middle of fag leaves were determined with chlorophyll meter(SPAD502,Japan Minolta Co.)at the booting stage (VI-VIII stages of young panicle differentiation).For the determination of chlorophyll content,3 fag leaves were sampled randomly for each plant,and total 5 plants were sample randomly for each plot.

The leaf relative water content(RWC)was determined according to the method described by Guoet al[2].At the booting stage(VI-VIII stages of young panicle differentiation),total 4-6 top leaves were sampled from the middle of the line which was in the middle of each plot,and the middle parts,in length of 5-10 cm,of the sampled leaves were used for the determinationofRWC.Thesampleleaf pieces were preserved in a preweighed sealed glass bottle,and the sample fresh weight(W)was measured.The bottle was then added with 1 cm-depth water.After 4-h soaking,the leaf pieces were fully expanded,and they were removed.The drops on the surface of leaf pieces were dried gently with paper,and then they were weighed to obtain the full-expansion weight(TW).After then,the samples were dried at 80℃for 24 h to obtain their dry weight(DW).The relative water content was calculated according to the following formula:

After the maturity,total 5 plants in the middle of the row that was in the middle of each plot were harvested for seed test.The effective panicles per plant (EPP),spikelets per panicle(SP),kilo-grain weight(TGW),seed setting rate (SSR),plant height(PH)and panicle length (PL)were measured,and their average values were calculated.

Table 1 Names and serial numbers of the tested rice varieties

Table 2 Relative damage rates of morphological and yield traits of different rice varieties under drought stress %

Grain yield per plant(GYP)(g)=(Effective panicles per plant×Grains per panicle×Seed setting rate×Kilograin weight)/1 000.

Data statistics and analysisThe significance tests for average values among different treatments were performed using Excel.

Before the data statistics and analysis,the original data were all transformed into relative values(K).

Relative value (K) =Measure value in the drought treatment plot/Measured value in the control plot.

Relative damage rate (a) = (Measured value in the control plot - Measure value in the drought treatment plot)/Measured value in the control plot× 100%.

Wherein,Xijrefers to the value ofjtrait oficultivar;XjmaxandXjminrefer to the maximum and minimum ofjtrait,respectively.The average subordinate function value of each index was calculated for each of the tested rice varieties.The larger the average value is,the stronger the drought tolerance of rice varieties is.

Results and Analysis

Effects of drought stress on morphological and yield traits of different rice varieties

As shown in Table 2,drought stress showed effects on all the 7 morphological and yield traits in varying degrees.The effect of drought stress on GYP was greatest,and the average relative damage rate was 27.7%with variation range of 15.9%-57.0%.The average damage rate of EPP ranked second(0.82%),and the relative damage rate of EPP ranged from 5.8%to 19.4%.The SSR differed significantly among different rice varieties under drought stress,and it ranged from 1.6%to 36.0%.The effects of drought stress on PL and SP were moderate,while the effect of drought stress on TGW was smallest,which might be due to mild drought stress designed in this study.

The variance analysisshowed that the GYP,EPP,SSR and PL all differed significantly between treatment and control groups(P＜0.05),but none significant differences were observed in SP,PH or TGW.Therefore,during the comprehensive evaluation on drought tolerance of the tested 26 rice varieties with subordinate function method,the indexes of SP,PH and TGW were excluded.The data on GYP,EPP,SSR and PL were transformed into relative values (K),and then their subordinate functions values were calculated (Table 3).In this study,the drought stress was performed at the tillering stage,and the water shortage resulted in insufficient tillering rate and reduced SSR,ultimately affecting the GYP.

Effects of drought stress on physiological indexes of different rice varieties

The variance analysis showed thatthere were significantly differences in leaf RWC and SPAD between treatment and control groups.The data on leaf RWC and SPAD were transformed into relative values(K),and then their subordinate function values were calculated.As shown in Table 3,the subordinate function values of RWC and SPAD of most the rice varieties were basically the same,but there were still some varieties of which the subordinate function values of RWC and SPAD differed greatly.For example,the subordinate function value of RWC of Weiyou 46(RD13)was relatively high (0.74),but its subordinate function of SPAD was 0;the subordination function value of SPAD of Liangyou Peijiu (RD02)was relatively high (0.85),but its subordinate function value of RWC was relativelylow (0.35).It suggests that although the indexes of RWC and SPAD can reflect the strength of drought resistance of rice varieties to some extent,there are differences among different varieties.The difference in average subordinate function value was 0.54 between the two control rice varieties,indicating that there were significant differences in the two physiological indexes between the drought-tolerant and drought-sensitive varieties.

Table 3 Relative values(K)and subordinate function values(X′ij)of phenotypic traits of different rice varieties

Comprehensive evaluation of drought tolerance of different rice varieties

Using the subordinate function method,the drought tolerances of the 26 tested rice varieties were evaluated comprehensively by 2 drought-sensitive physiological indexes and 4 morphological and yield traits.Table 3 showed that there were great differences in average subordinate function values between the drought-tolerant and drought-sensitive control varieties.The average subordinate function values of CK I and CK II were 0.27 and 0.90,respectively.There were total 7 varieties of which the average subordinate function values were higher than 0.85,including Y Liangyou 1(RD01,0.90),Yangliangyou 6(RD03,0.88),Y Liangyou 7(RD04,0.91),Longliangyou 981(RD06,0.87),C Liangyou 343(RD09, 0.88), Fengyuanyou 299(RD18,0.92)andChuangxiang5(RD22,0.86).It was indicated that the drought tolerances of the 7 rice varieties above were relatively strong.There were 3 rice varieties of which the average subordinate function values were lower than 0.5,including Liangyou Peijiu(RD02,0.27),Weiyou 644(RD19,0.40)and Xiangwanxian 13(RD21,0.45).It suggested that the drought tolerances of the 3 rice varieties above were relatively weak.The average subordinate function values of the other rice varieties ranged from 0.50 and 0.85,indicating that their drought tolerances were moderate.A-mong the tested rice varieties,the male parents of Liangyou Peijiu,Y Liangyou 1,Yangliangyou 6 and Fengliangyou 1 were all Yangdao 6,but the subordinate functions values of their drought tolerances ranged from 0.27 to 0.90,indicating that the female patents of hybrid combinations may have greater effects on their drought tolerances.

Discussion

Droughttolerance is generally considered to be a complex quantitative trait that is affected by a variety of factors.The drought tolerance based on a specific index may be different among different varieties.So a single index cannot comprehensively and accurately reflect the drought tolerance of varieties.The subordinate function method eliminates the one-sidedness caused by a single index.Moreover,due to the subordinate function values ranging within a closed interval([0,1]),the drought tolerances of tested materials were comparable.The subordinate function method is a more reliable evaluation method[10].

The drought tolerance of rice is affected by multiple factors,and its mechanism is complex.Phenotypic traits are the reflection of changes in internal physiological structure,and the agronomic and yield traits are the most direct and objective evaluation basis.In most of the previous studies,only a few indexes were used to evaluate the drought tolerances of varieties[11].However,in different growth stages ofrice,the mechanism of drought tolerance may be different.Therefore,the evaluation on drought tolerance by multiple indexes is more reliable and accurate compared with the evaluation by single indexes.In this study, various morphological,physiologicaland yield traits were used to construct the comprehensive evaluation system.Moreover,subordinate function method was adopted,thus the accuracy of evaluation was improved.In addition,the alternative drought stress was performed in this study.Although the differences were not as significant as those by extreme drought treatment, the alternative drought treatment was much similar to the seasonal drought stress in production,and its representativeness was broad.This study will provide a reference for the selection of rice varieties in drought-prone regions.

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