Ting CUI, Xiaomin WANG, Jianquan QIN, Huabing DENG， Qiyuan TANG

College of Agronomy, Hunan Agricultural University, Changsha 410128, China

With the decrease of rural la bor force, the popularization of mechanization and the advance of modern agricultural production, the seed processing technology, which is suitable for precision sowing, can realize full seedlings, and is no need to fill the gaps with seedlings, will play more and more im portant role in production[1]. In recent years, the improving techniques of seed vigor,such as seed selection and classification, and seed priming, have been increasingly paid attention to and used; in which, the yield-increasing effect of seed selection and classification has been verified in the crops like maize[2]and soybean[3], etc. Wang Xuguang et al.[4]conducted the study on wheat and showed that after seed classification,the seeds with high level and good quality were planted, grains per plant,1 000-seed weight and grain yield were all improved. The successful experience of the monoseeding of Xianyu 335 in Northeast China greatly promoted the development of maize seed industry and extensive production in China[5].

Hybrid rice has made great contribution to guaranteeing food security in China, but hybrid rice seed has high cost of production and high price, so how to make precision drilling of hybrid rice seeds and give play to the greatest productive value is a common problem large-scale rice production and seed industry have to face together. Some studies showed that the average germination rate of selected seeds improved 3.38%compared with the control (CK), and after cleaning,the seeds with low maturity (green seeds) had higher germination rate than CK[6]. In the experiment, the seeds of hybrid rice were graded according to different specific gravities,then the germination rate and seed vigor were tested, after that, the seed germination, seedling emergence andseedling quality were investigated,and the grain yield was compared,so as to systematically study the effects of selected seeds of hybrid rice, hoping to provide some reference for the processing and utilization of hybrid rice seeds.

Materials and Methods

Tested materials

The tested materials were the commercial seeds of two-line hybrid early rice combination Luliangyou 996(in 2012)and two-line hybrid mid-season rice combination Liangyoupeijiu(in 2013).

Experiment design

The grading of specific gravitybased seeds Industrial salt was used to allocate saline solution with the specific gravities of 1.0, 1.1 and 1.2, the seeds of Luliangyou 996 and Liangyoupeijiu in the saline solution with different gravities were sorted according to the sequence of gravity from low to high, thereby forming the seeds with the gravities of ＜1.0(T1),1.0-1.09(T2), 1.1-1.19(T3)and ≥1.2(T4), then these seeds were cleaned with clear water and then dried for reserving.

Experiment methods Luliangyou 996 and Liangyoupeijiu were carried out indoor germination test and seed vigor test in the laboratory of Hunan Agricultural University in 2012 and 2013, respectively, and the seeds of the two were sown at Yong’an base of Hunan Agricultural University in Yong’an Town of Liuyang City in Hunan Province for investigating the seedling emergence and seedling quality,and testing the grain yield.

The seeds of Luliangyou 996 were sown at Chang’an base of Hunan Agricultural University on April 3 of 2012, the seedling quality was tested on May 1 and the seedlings were transplanted on May 2; the seeds of Liangyoupeijiu were sown on May 10 of 2013,the seedling quality was tested on May 28 and the seedlings were transplanted on May 29.The randomized block design with three replications was adopted for yield determination test, there were 5 treatments with different levels of seeds (including unsorted commercial seeds as CK), the plot area was 12 m2,the spacing in the rows and between rows of Luliangyou 996 was 17.5 cm×20 cm, and that of Liangyoupeijiu was 16.7 cm×26.7 cm.The field management like fertilization,pest disease and weed prevention and water management was carried out according to high-yield culture technique.

Measuring items and methods

The determination of seed germination rate and vigor index The determination of seed germination rate,germination index and vigor index was carried out according to standard germination test[7].

Germination rate (%)= Normal seedlings/Tested seeds×100.

Germination index (GI)=∑Gt/Dt;in the formula, Gt = The number of shoots during the 1st-7thd;Dt=Germination days.

Vigor index (VI)=GI×S;in the formula, S= The dry weight of seedlings at the 7thday of sprouting(g).

The survey of seedling emergence rate and seedling quality (1) The determination of seedling emergence rate. 3 plots were selected in the rice seedling bed and each plot was 30 cm×30 cm. When the first complete leaf appeared,the emergence number and ungerminated seeds were surveyed according to the reference[8],then the seedling emergence rate was calculated:

Seedling emergence rate (%)=The emergence number/(The number of seeds in the observation points ×Germination rate)×100.

(2) The determination of adult seedling rate.1-2 d before transplanting rice seedlings, 3-5 representative plots were selected and each plot was 15 cm×6 cm, then total seedlings and the number of shrinking peduncle seedlings (the height of seedlings was less than half of the normal seedling)in the points were surveyed, and then the adult seedling rate was calculated:

Adult seedling rate (% ) =(Total seedlings - The number of shrinking peduncle seedlings)/(The number of seeds in the observation points × Germination rate)×100.

(3)The survey of seedling quality.3 plots in the rice seedling bed of each treatment were selected, then 30 rice seedlings from each plot were separately got and cleaned, and then the height of seedlings, the height of leaf sheath and base stem width were measured according to the reference[8].

The determination of grain yield and the survey of yield components Effective panicles of 30 hills of each plot was counted at the mature period,then the average effective panicles were figured out;according to average panicles, 5 hills were selected to measure total grains per ear, setting percentage and 1 000-grain weight, then the theoretical yield was calculated;the actual output of the rice in the area of 5 m2(2 m×2.5 m)of each plot was measured.

Results and Analyses

Effects of specific gravity-based seed grading on rice seed germination and vigor

As shown in Table 1, in the commercial seeds of two hybrid rice varieties,the seeds with the specific gravities of ＜1.0 (T1),1.0-1.09 (T2),1.1-1.19 (T3) and ≥1.2 (T4) respectively accounted for 13.4%-15.4%, 22.1%-23.1%,38.5%-40.4%and 23%-25.1%,the variation ranges of 1 000-grain weight of Luliangyou 996 and Liangyoupeijiu were 19.6-28.0 and 16.8 -23.5 g,respectively,indicating that the specific gravity and grain weight of different grains of commercial seeds were very nonuniform,thus it was necessary to carry out grain cleaning and grading.

From the Table 1, it was shown that seed cleaning and grading had significant effect on the germination rate,germination index and vigor index of rice. The germination rate, germination index and vigor index of the seeds of Luliangyou 996 variety under the treatment of T1 were all extremely significantly lower than those of CK(unassorted seeds), while those under the treatments of T2, T3 and T4 improved to varying degrees compared with CK, showing that after the cleaning and grading of the seeds of Luliangyou 996, the seed vigor was improved significantly.

The germination rate,germination index and vigor index of the seeds of Liangyoupeijiu variety under the treatment of T1 were all extremely significantly lower than those of CK, while those under the treatments of T2 andT3 showed an increasing trend compared with CK, but the differences were not significant, and those under the treatment of T4 were significantly higher than those of CK,indicating that the level of the cleaning and grading of the seeds of Liangyoupeijiu variety was higher, but it was necessary to further choose from the view of vigor level.

Table 1 Effects of seed cleaning and grading on rice seed germination and vigor

Table 2 Effects of seed cleaning and grading on rice seedling quality

Effects of specific gravity-based seed grading on rice seedling emergence and seedling quality

As shown in Table 2, seedling emergence rate and adult seedling rate of the commercial seeds of Luliangyou 996 in rice field under the treatment of T1 were both extremely significantly lower than those of CK,those under the treatments of T2 and T3 had not significant differences with those of CK, while those under the treatment of T4 were extremely significantly higher than those of CK.Seedling emergence rate and adult seedling rate of the commercial seeds of Liangyoupeijiu under the treatment of T1 were both extremely significantly lower than those of CK; those under the treatments of T3 and T4 were extremely significantly higher than those of CK; although seedling emergence rate under the treatment of T2 was higher than that of CK, the adult seedling rate was lower than that of CK.

From the point of seedling quality,the height of seedlings, the height of leaf sheath and stem base width of Luliangyou 996 under the treatment of T1 were the lowest in all treatments,and they were extremely lower than those of CK; the height of seedlings under the treatments of T2,T3 and T4 had not significant differences with that of CK,but the height of leaf sheath and stem base width were significantly higher than those of CK.The height of seedlings, the height of leaf sheath and stem base width of Liangyoupeijiu under the treatment of T1 were all extremely significantly lower than those of CK; the height of seedlings under the treatments of T2, T3 and T4 had not significant differences with that of CK, but the height of leaf sheath and stem base width were significantly higher than those of CK,indicating that the seedling quality under the treatments of T3 and T4 whose seed proportion accounted for more than 60%was superior to that of CK.

Effects of specific gravity-based seed grading on rice grain yield and yield components

As shown in Table 3, the grading of commercial seeds of hybrid rice varieties had significant effect on rice grain yield, the grain yield under the treatment of T1 whose specific gravity was the smallest was the same as that of CK,but that under the treatments of T2, T3 and T4 was significantly higher than that of CK, and the yield-increasing range of Luliangyou 996 respec-tively reached 9.1%,7.9%and 10.2%,and that of Liangyoupeijiu respectively reached 6.8%,10.2%and 8.4%.From yield components of view, seed grading treatment had not significant effects on seed setting percentage and 1 000-grain weight, but had significant effects on the effective panicles per unit area and spikelets per panicle;the effective panicles of the grading seeds of Luliangyou 996 and Liangyoupeijiu under the treatments of T2,T3 and T4 were significantly higher than those of CK, and spikelets per panicle were also improved,thereby making the grain yield higher than that of CK.

Table 3 Effects of seed cleaning and grading on rice grain yield and yield components

Conclusions and Discussions

The specific gravity and grain weight of different grains of commercial seeds of two hybrid rice varieties were very nonuniform, with the improvement of selecting degree (specific gravity), rice seed grain weight,germination rate, germination index,vigor index, seedling emergence rate and adult seedling rate were improved gradually, which were beneficial to the improvement of grain yield. Therefore,appropriately doing grading treatment on the seeds of hybrid rice,making the seeds uniform, improving seed vigor,and realizing the full seedlings,uniform seedlings and adult seedlings, is an effective approach to realize the precision sowing of hybrid rice, decrease the use of the seeds and reduce production cost.

Seed vigor is the potential emerging ability of seeds. High seed vigor can cause high seedling emergence rate,fast speed of emergence,uniform seedlings and high utilization efficiency of luminous energy and water as well as fertilizer, thereby giving full play to the yield-increasing potential of the variety and reducing production risk.In recent years, "international seeds"have spread rapidly in China, besides variety factor, the seed vigor is one of the important causes, because it is significantly higher than that of domestic seeds.The monoseeding technology of high vigor seeds simplifies the links of thinning and final singling,saves the manpower and finance of peasant households, and changes the heavy means of rural labor, thus receiving a warm welcome in vast farmers[5].High vigor seeds can reduce the use of seeds, thereby reducing the production area of seeds and the energy consumption of seeds during the transportation and storage.

At present, the seed quality of main crops in China, especially the status of seed vigor, is not optimistic,which has already seriously affected the transformation development of agricultural production. In rice production, low seed vigor will can cause low germination rate, seedling emergence rate and adult seedling rate; when the seeds are planted by the machine, it will cause poor blanket of rice seedlings and line break as well as vacancy plants; when using rice throwing-seedling cultivation, it will cause the missing hills of seedlings and the decrease of seedling-throwing density and basic seedlings; when direct sowing culture is used, it will cause missing seedlings,thereby seriously affecting the grain yield. To make up these defects, the method of increasing the seeding quantity in the production is frequently adopted,thereby greatly making the rise of seed costs.

With rice production being in a transforming period in China, simplified and mechanized cultivation and management method has become a trend in the scale production of rice[9],high vigor seeds will play more and more important role in the production.To obtain high vigor seeds in the production (especially the production of hybrid rice), it shall not only study the technical system of field production of high-yield and high-vigor seeds from varieties, ecological environment and cultivation techniques, but also study the key technologies maintaining and improving seed vigor from precise processing of seeds,so as to effectively improve the level of rice seed vigor,and fully realize precision seeding,thereby providing a basis of excellent seed sources for maintaining the sustainable high yield of rice production and guaranteeing national food security.In the experiment,it was shown that specific gravity-based seed grading can help to get rid of the seeds with low vigor, and sort out the seeds with high vigor.

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