Identification of Drought Tolerance in Peanut(Arachis hypogaea)Germplasm at Seedling Stage

Yonghui LIU, Yi SHEN, Zhide CHEN

Institute of Economic Crops, Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China

Drought is a common problem all over the world.One-third of the world’s area is arid or semi-arid land. Fifty-three percentage of the arable land is arid or semiarid in China, which is one of the world’s major countries influenced by drought[1]. Peanut (Arachis hypogaea L.), an important economic and oil crop of the family Fabaceae, is native to the tropical and subtropical regions of South America[2]. Drought stress during peanut growth stage can decrease its quality and yield. The influence of drought stress on peanut has been reported a lot, especially on its physiological and biochemical traits,such as plant morphology, root morphology, photosynthetic characteris tics and SOD activity[3-6]. Moreover,molecular mechanism related to drought tolerance of peanut such as the identification of drought QTLs[7],the cloning of drought-tolerant genes[8]and leaf proteomics research under drought stress[9]have been reported previously. In our earlier work, it was found that the plant height, plant fresh weight and plant dry weight of peanut were all decreased significantly under drought stress, while its root length,root fresh weight and root dry weight showed no significant difference[10].Therefore, this study evaluated various peanut varieties or lines using plant height, plant fresh weight and plant dry weight under simulated drought stress in laboratory,with an attempt to screen some excellent germplasm for the breeding of drought-tolerant peanut varieties.

Materials and Methods

Materials

Among the 80 peanut varieties or lines we tested,19 had been approved by related authorities, 17 were local varieties,and 44 high-generation lines.

Methods

The preliminary experiments suggested that 15% PEG6000 solution was suitable for laboratory simulation of drought stress. Water was used as the control (CK). First, 90 full seedswere selected from each variety or line, germinated in an incubator, at a photoperiod of 12 h light: 12 h dark and a temperature of 27-28 ℃. At three-leaf stage, the well-grown and similar-sized seedlings were selected and divided into two groups, one was subjected to drought stress simulated by 15%PEG6000 solution and the other to control treatment with water.Three repetitions were prepared for each treatment. PEG6000 solution and water in the two treatments were refreshed every 2 d during the experimental period. The plant height (from the cotyledon node to the growing point), fresh weight (after removal of cotyledons) and dry weight (after removal of cotyledons) of the seedlings in each treatment were measured 10 d later.

Data analysis

In order to eliminate the intrinsic differences among these peanut varieties or lines, all the data of the three indices we measured were converted to relative values using the formulas as follow: Relative plant height (plant fresh weight or dry weight)=The plant height (plant fresh weight or dry weight) in drought treatment / The plant height (plant fresh weight or dry weight) in control treatment × 100%.Excel and SPSS19.0 software were adopted for data processing and analysis.

Results and Analysis

Variation of plant traits under drought stress

As shown in Table 1, compared with the control, the plant growth of peanut was inhibited after 10 d of treatment with 15% PEG6000. In detail, the average plant height of the 80 peanut varieties or lines was up to 7.61 cm in control treatment, but only 5.41 cm in drought treatment with 15%PEG6000. The difference reached an extremely significant level. In addition,the plant fresh weight and dry weight of the 80 peanut varieties or lines also exhibited extremely significant difference between the control and drought treatments.The plant fresh weight and dry weight under drought stress were only 59.8%and 71.6%of those in control treatment(2.91 and 1.16 g). Every index varied in different range among these varieties or lines, with different coefficients of variation.

Variation of relative plant traits among peanut varieties or lines

For comparing the drought tolerance among these varieties or lines more comprehensively,relative values of the plant traits were calculated for all the varieties or lines. As shown in Table 2 and Fig.1, the relative plant height of these peanut varieties or lines was 71.4% on average, although 66.3%of the varieties or line had a relative plant height ranged from 60% to 80%, and only one variety had a relative plant height more than 90%. The relative plant fresh weight of these peanut varieties or lines ranged from 36.63% to 92.58%, and was 60.45%on average. Among them, 10 varieties or lines had a relative value more than 70%. The relative plant fresh weight had high coefficient of variations among these varieties or lines, up to 18.41%. The relative plant dry weight of all the varieties or lines was above 50%, among which the relative plant dry weight of 62 varieties or lines ranged from 60% to 80%, and that of two varieties or lines was above 90%.Especially,the relative plant dry weight of one variety was up to 94.74%.

Table 1 Variation and significance analysis of plant traits between different treatments

Table 2 The relative values of plant traits of the 80 peanut varieties or lines %

Cluster analysis on the varieties(lines) based on their drought resistance

Cluster analysis of the 80 peanut germplasms based on Euclidean distance was performed using their relative plant height, relative plant fresh weight and relative plant dry weight.The results revealed that all the 80 germplasms were divided into four categories A, B, C and D at Euclidean distance of 15.Category A included 51 peanut germplasms, with an average of relative plant height of 70.65%, an average of relative plant fresh weight of 63.44% and an average of relative plant dry weight of 74.03%; The six varieties or lines of category B, which had relative plant height over 85% ,relative plant fresh weight over 70%and relative plant dry weight over 78%,exhibited strong drought resistance.The eight varieties or lines of category C were more sensitive to drought as their relative plant height was only 48%-60%,relative plant fresh weight 44%-55%and relative plant dry weight 57% -75% . The 15 germplasms of category D were characterized by larger relative plant height of 75.73%,smaller relative plant fresh weight of 48.17%and relative plant dry weight of 63.5%on average.

Table 3 Differences of plant traits among approved varieties, high-generation lines and local varieties %

Differences of drought tolerance among different peanut populations

The differences in drought tolerance among the approved varieties,high-generation lines and local varieties tested were illustrated as Table 3. The relative plant height, relative plant fresh weight and relative plant dry weight of the approved peanut varieties were 75.59%, 68.96% and 75.43%, respectively. All the three indices of approved varieties ＞those of high-generation lines ＞those of local varieties. The variance analysis based on relative plant height and relative plant fresh weight revealed that the differences between approved varieties and local varieties reached a significant or extremely significant level,while high-generation lines showed no significant differences with them. The relative plant dry weight had no significant differences among the three populations.

Differences of drought tolerance among different peanut varieties or lines

Cluster analysis showed that the six peanut varieties or lines of category B were drought tolerant, while the eight peanut varieties or lines of category C had poorer drought tolerance.The relative plant height, the relative plant fresh weight and the relative plant dry weight of No.1. No.4 and No.14 of category B were all more than 85%, indicating they had better drought tolerance, while the relative plant height, the relative plant fresh weight and the relative plant dry weight of No.58,No.59 and No.68 in this category were slightly smaller, but still proved that the three varieties were drought tolerant.No.43,No.56 and No.65 of category C, which had smaller relative plant height, fresh weight and dry weight of no more than 65% were sensitive to drought stress.

Discussion and Conclusions

Drought is one of the main factors influencing agricultural development.Due to the global warming and the increasing water scarcity, drought has been more frequent and more persistent in recent years[11-12].Although peanut is tolerant to drought and bar-ren,seasonal drought remains a major factor limiting peanut production in China. At present, about 7 000 germplasm accessions of peanut have been collected in China so far,but their drought tolerance has been rarely reported[13].Therefore,it is important to identify the drought tolerance of existing peanut germplasm and screen drought tolerant varieties, and thus to provide basis for breeding new peanut varieties. Among the various methods and indicators for evaluating peanut drought tolerance,field trial is the most direct, which is able to objectively reflect the drought resistance of every peanut variety. However, the environmental factors in field are changeable and difficult to control. Therefore, in this study, the drought tolerance of peanut seedlings was identified under simulated drought stress using 15%PEG6000 in laboratory, to avoid the influence caused by changeable environment conditions. The results achieved under the simulated drought stress to some extent reflected the drought tolerance of these peanut varieties or lines.In summary, compared with field trials, laboratory tests are time-saving and labor-saving for identifying the drought tolerance of a large amount of peanut samples. Currently,the molecular mechanism related to peanut drought tolerance and the breeding of new peanut varieties have become hot issues in future,especially after the completions of peanut genome sequencing and drought-related transcriptome sequencing[15]. In the present study,three highly drought tolerant, three drought tolerant and three drought sensitive peanut varieties or lines were finally screened out from the 80 germplasm accessions we tested, which can be used as basic materials for molecular research and germplasm improvement in future work.

Table 4 Drought-tolerant and drought-sensitive materials screened in this experiment

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