Fei TENG，Defeng ZHU，Huizhe CHEN，Xueqing CAl，Yicheng XU

1.China National Rice Research Institute/State Key Laboratory of Rice Biology，Hangzhou 310006，China；

2.Beijing Agricultural Machinery Testing and Extension Station，Beijing 100079，China

Effects of Seed Rate on Root Twining Power and Seedling Quality of Machine-transplanted Super Rice

Fei TENG1，2，Defeng ZHU1*，Huizhe CHEN1，Xueqing CAl1，Yicheng XU1

1.China National Rice Research Institute/State Key Laboratory of Rice Biology，Hangzhou 310006，China；

2.Beijing Agricultural Machinery Testing and Extension Station，Beijing 100079，China

In order to investigate the effects of seed rate on root twining power and seedling quality ofmachine-transplanted superrice cultivars，Yongyou 12 and Zhongzheyou No.1，total 6 seed rates were arranged，and the growth characteristics，quality and root twining power of machine-transplanted rice seedlings were studied.The results showed that seed rate had great effects on root twining power and quality of rice seedlings；with the increase of seed rate，the root twining power of rice seedlings was increased，but the quality of rice seedlings was significantly reduced；under the condition of high seed rate，rice seedlings were weak with less accumulate dry matter and nonuniform growth，and the proportion of weak seedlings was higher.When the seed rate of both rice cultivars was reduced by 10 g/tray，the average seedling height was increased by 0.5 cm，leaf age was increased by 0.1，stem base diameter was increased by 0.1 cm，and root number per plant was increased by 0.3.When the seed rate was increased by 10 g/tray，the accumulated dry matter per plant was reduced by 5.1 mg.With the increase of seed rate，the seedling quality of Zhongzheyou No.1 was decreased rapidly，and the proportion of weak seedlings was increased rapidly.However，Yongyou 12 has a larger elasticity in seed rate.When the seed rate reached to 50 g/tray，blanket seedlings of Yongyou 12 were nursed with uniform growth and higher quality，but the shoots were sparse with lower leaf area index.However，when the seed rate was up to 50 g/tray，blanket seedlings of Zhongzheyou No.1 were nursed with uniform growth and higher quality，as well as higher-density shoots.From a comprehensive point of view，the optimum seed rates of Yongyou 12 and Zhongzheyou No.1 were 65 and 50 g/tray，respectively.

Rice；Super rice；Mechanical transplanting；Seed rate；Blanket seedling；Seedling quality

W ith social and economic development，rural labor transfer and aging，mechanized production of rice，especially mechanical transplanting technology，is developed more and more rapidly.The comparison of different planting patterns and production characteristics show that mechanical transplanting is development direction of rice industry in China.Mechanical transplanting has been applied in China for several years，and the cultivation area of machine-transplanted rice accounts for 40%of the total cultivation area.There are also some factors constraining the promotion of mechanical transplanting in China.Mechanical transplanting is difficult to be applied in double cropping rice and hybrid rice，so the cultivation area of machine-transplanted double cropping rice and hybrid rice is smaller.The main aspect restricting the promoting of mechanical transplanting in rice is nursing［1-2］.Mechanical transplanting is characterized bylarge seed rate and uniform seedling age，and it has higher requirements for seedling management［3-7］.In the cultivation of hybrid rice，if seed rate is higher，seedling quality will be lower，and the advantage of hybrid rice will not be better played；if seed rate if lower，it is difficult for rice roots to twine together，affecting the effectofmechanical transplanting. The ideal seedlings for mechanical transplanting should have uniform and robust growth and strong root twining power［8-12］.

In recent years，there are great differences in optimum seed rate for mechanical transplanting among different rice-growing regions.It is considered that seed rate has close relationships with seedling quality and root twining power.With the increased seed rate on trays，seedling quality is trended to be reduced.Under the condition oflow seed rate，although seedling quality is high，root twining power is weak，and seedling roots are easy to get loose and damaged［11-17］. The relationships between seed rate of different rice cultivars，especially super rice cultivars，and seedling growth and quality are still unclear，and there are no quantitative indicators for evaluatingroottwining powerofnursed seedlings.In this study，different seed rates were arranged，and the effects of seed rate on seedling growth，quality and root twining power of indica and japonica super hybrid rice were investigated，providing theoretical basis and technical support for mechanical transplanting of hybrid rice.

Materials and Methods

Overview on test site

The test was conducted in the Fuyang experimental base of China Rice Research Institute （30.1°N，120.0°E），which is the main ricegrowing area in the middle and lower reaches of the Yangtze River.

Experiment design

The test materials were indicajaponica hybrid rice cultivars，Yongyou 12 and Zhongzheyou No.1.They are recognized as super rice cultivars by the Ministry of Agriculture，and are also dominant rice cultivars in Zhejiang Province.

The rice seedlings were nursed on trays（580 mm（L）×280 mm（W）× 28 mm （H））with seedling substrate（Jinhai，Hangzhou）.Total 6 seed rates were arranged，including 35，50，65，80，95 and 110 g dry seeds/tray.The rice seeds were soaked on May 27，and sowed on May 30.There were 6 replicates for each seed rate treatment.

lndicator measurement and methods

Growth and dry matter accumulated amount In each treatment，the average leaf age was surveyed 18 d after the sowing.According to the average leaf ages，total 20 plants were sampled for each treatment with 3 replicates.The plant height，root length，root number，yellow leaf number and seedling base diameter were measured.The seedling plants were dried at 105℃and then at 80℃to constant weight，and then weighed.

Uniformity of plant height After a certain time （19 d），three seedling blocks in size of 6 cm ×6 cm were sampled from each treatment.In each seedling block，the heights ofall seedlings were measured.The average seedling height，along with coefficient of variation，was calculated.According to variation coefficientof seedling heightand proportion of seedlings lower than half of the average height，the uniformity of seedling height was observed.

Variation coefficient of seedling height= （Standard deviation of seedling height/Average seedling height）×100%.

Leaf area and SPAD value After a certain time （19 d），three seedling blocks in size of 6 cm ×6 cm were sampled from each treatment.In each seedling block，the fully expanded leaf green degrees of all seedlings were measured using chlorophyllmeter（SPAD-502，Japan）.Theeffectof seed rate on seedling chlorophyll content was analyzed.Subsequently，the seedling leaves with length higher than 2 cm in each seedling block were cut off，and their leaf areas were measured using leaf area meter（LI-3000A，LI-COR Inc.）.

Leaf area index（LAI）=Total leaf area per tray/Tray area.

Root twining power At the 3.5-leaf stage，the entire seedling block was removed from each tray.One end of each seedling block was fixed，and the other end was snapped with a large splint.Subsequently，the seedling block was pulled horizontally by an electronic dynamometer till the seedling block was loosened and broken.The maximum tensile strength was also the root twining power.There were 3 replicates for each treatment.

Data processing and analysis

The data were processed and analyzed using Excel and SAS.The variance analysis was conducted using LSD multiple comparisons.

Results and Analysis

Seedling growth

The seed rate of rice on trays showed significant effect on growth of rice seedlings （Table 1）.With the increase of seed rate，the seedling heights，base diameters，leaf ages and root numbers of both rice cultivars were all trended to be decreased. When the seed rate was reduced by 10 g/tray，the average seedling height was increased by 0.5 cm，leaf age was increased by 0.1，seedling base diameter was increased by 0.1 cm，and root number per plant was increased by 0.3.Under the condition of low seed rate （35-50 g/tray），seedling growth was better，base diameter was larger and roots were more developed.However，under the condition of high seed rate （95-110 g/tray），seedling quality，as well as plant height，leaf age，base diameter and root number，were all reduced greatly.When the seed rate was up to 65 g/tray，yellow leaves appeared in the lower part of seedlings. When the seed rate exceeded 80 g/tray，more withered leaves appeared in the lower part of seedlings，and seedling quality was also reduced rapidly.

Dry matter accumulated amount

With the increased seed rate，dry matter accumulated amount in shoots ofseedlingswasreduced （Fig.1）. When the seed rate was increased by 10 g/tray，the dry weight of single seedling was reduced by 5.1 mg.The critical seed rate for reduced seedling dry matter accumulated amount was different between the two rice cultivars.When the seed rate of Yongyou 12 was increased to 80 g/tray，the drymatter accumulated amount in seedling shoot was significantly reduced；but when the seed rate of Zhongzheyou No.1 was increased to 65 g/tray，the dry matter accumulation amount in seedling shoot was greatly reduced.It indicates that indica super hybrid rice Zhongzheyou No.1 is more sensitive to seed rate，but indicajaponica super hybrid rice Yongyou 12 has larger elasticity in seed rate.

Table 1 Effects of different seed rates on morphological characteristics of rice seedlings

Seedling growth uniformity

According to variation coefficient of seedling height and proportion of seedlings lower than half of the average height，the uniformity of seedling growth was evaluated（Table 2）.Under the condition of low seed rate（30-65 g/tray），the seedling height variation coefficient and proportion of seedlings lower than half of the average height of Yongyou 12 were 11.59% and 0.0%，respectively.However，when the seed rate was increased to 80-110 g/tray，the seedling height variation coefficient and proportion of seedlings lower than half of the average height were 18.6%and 01.8%，respectively. At the low seed rate （30-65 g/tray），the seedling height variation coefficient and proportion of seedlings lower than half of the average height were 19.7% and 0.7%，respectively.But when the seed rate was up to 80-110 g/tray，the seedling height variation coefficient and proportion of seedlings lower than half of the average height were 22.3% and 2.8% ，respectively.When the seed rate was lower than 65 g/tray，there were basically no dwarf seedlings in Yongyou 12.However，when the seed rate was only up to 50 g/tray，dwarf seedlings appeared in Zhongzheyou No.1.The seedling height variation coefficient of Zhongzheyou No.1 was larger than that of Yongyou 12，so the growth uniformityofZhongzheyouNo.1was worse than that of Yongyou 12.Too high seed rate will affect growth uniformity of rice seedlings，resulting in higher proportions of dwarf seedlings and weak seedlings.

Leaf area and SPAD value

With the increased seed rate，seedling leaf SPAD values of both rice cultivars were trended to be decreased（Fig.2）.There was a negative correlation between seedling leaf SPAF value and seed rate.

As shown in Fig.3，with the increased seed rate，the leaf area index was increased first and then decreased.Under the condition of low seed rate，increasing seed rate showed significant effect on increasing leaf area index.However，under the condition of high seed rate，increasing seed rate almost showed no effect on leaf area index.When the seed rate was increased to 65 g/tray，the leaf area indexes of both rice cultivars all reached the peaks.However，when the seed rate was further increased，the leaf area indexes were decreased. This might be because that too high seedling density reduced seedling quality，leading to reduced leaf area.

Root twining power

Root twining power is an important indicator evaluating nursed seedling quality.The greater the root twining power is，the more difficult the seedling block’s loosening is.As shown in Fig.4，there was a positive correlation between root twining power and seed rate.The regression analysis showed that root twining power waslogarithmically linearly related to seed rate（r＞0.98，P＜0.05）.It suggests that increased seed rate can inevitably increase root twining power.However，under the condition of high seed rate，when the seed rate was increased further，the increment of root twining power became small.Under the condition of same seed rates，the root twining powers of Yongyou 12 were higher than those of Zhongzheyou No.1 by 9.26% （35 g/tray），24.11%（50 g/tray），32.6%（65 g/tray），33.81%（80 g/tray），34.74%（95 g/tray）and 41.09%（110 g/tray），respectively.Therefore，the root twining power of Yongyou 12 was greater under various seed rates.

Table 2 Seedling uniformity under different seed rates

Discussion

Effect of seed rate on seedling root twining power

Mechanical transplanting and supporting cultivation techniques of rice is one of the development directions of modern，intensive and largescale agriculture，and it has wide application prospect［18］.Tray seedling is one of the most important steps in mechanized cultivation of rice.On trays，rice roots twine together.When they are removed，seedling block is difficult to be loosened.Tray seedling is the basis for mechanical transplanting of rice.Root twining power is closely related to seed rate.Yu et al.［19］found that when the seed rate was low，individual advantages of seedlings were more obvious，but the root twining power was weak；when the seed rate was high，seedling quality was poor，which was not conducive to survival of transplanted seedlings.This study found that there was a logarithmic linear relationship between root twining power and seed rate.When the seed rate was low，increasing seed rate could significantly increase the root twining power；but when the seed rate was further increased，the increment of root twining power became small. The lowest seed rates required by the two rice cultivars to nurse blanket seedlings were all 50 g/tray.However，at the lowest seed rate，the seedling shoots were sparse，increasing missing rate of seedlings during transplanting.When the seed rate was increased to 95 g/tray，seed rate showed smaller effect on root twining power，but the seedling quality was reduced greatly.Even worse，the seed costs were increased.

Effect of seed rate on seedling quality

Different scholars achieve different conclusions about effect of seed rate on seedling quality.Fang et al.［20］found that when the seed rate was 80 g/tray，the seedling quality was best. Zhang et al.［21］considered that substrate seedling could nursed robust seedlings meeting requirements by mechanical transplanting under the condition of low seed rate （300-400 g/m2）.Li et al.［22］and Peng et al.［23］found that the optimum seed rate was 150-180 g/tray for mechanical transplanting of Wuxiangjing 14.In this study，the effect of seed rate on seedling quality of rice was investigated.The results showed that under the condition of low seed rate（＜50 g/tray），the seedlings were robust with uniform growth and small difference in seedling height，but the leaf area index was relatively low.However，under the condition of high seed rate（＞110 g/tray），the seedlings were weak with nonuniform growth，higher proportions of dwarf and weak seedlings and reduced leaf area index.It suggests thatincreased seed rate can increase root twining power.However，the seedling quality is reduced，and the proportion of dwarf and weak seedling is increased，affecting the growth uniformity of rice seedlings.Although the variation trends of various indicators were almost the same between the two rice cultivars，the critical seed rate for reducing seedling quality was different.When the seed rate was increased to 80 g/tray，the seedling growth was restricted， and the seedling quality was reduced.However，the seedling quality of Zhongzheyou No.1 began to be reduced when the seed rate was increased to only 65 g/tray.In conclusion，Zhongzheyou No.1 is more sensitive to seed rate，and its seedling quality is effected more greatly by seed rate；Yongyou 12 has a large elasticity in seed rate. From a comprehensive point of view，the optimum seed rates for Yongyou 12 and Zhongzheyou No.1 are 65 and 50 g/tray，respectively.

Conclusions

Seed rate has great effects on root twining power and seedling quality of rice.Increasing seed rate can effectively increase root twining power and leaf area index of rice seedlings.However，seedling quality and growth uniformity are worsened，and weak and dwarf seedlings are increased.The indica hybrid rice Zhongzheyou No.1 is more sensitive to seed rate.

Under the condition of low seed rate，increasing seed rate can significantly improve root twining power and leaf area index.However，the seedling quality and SPAD value are greatly reduced.Under the condition of high seed rate，continuously increasing seed rate shows small effects on seedling quality and root twining power，and the change tendencies of various indicators become small.

When the seed rate reaches 50 g/tray，blanket seedlings of Yongyou 12 will be nursed.However，since the seed rate is relatively low，seedling shoots are sparse，and leaf area index is small.The lowest seed rate for Zhongzheyou No.1 to obtain blanket seedlings is also 50 g/tray.On the contrary，its seedling quality and leaf area index are all better.Comprehensively considering root twining power，seedling quality and production costs，the optimum seed rates of Zhongzheyou No.1 and Yongyou 12 are determined as 50 and 65 g/tray.

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Responsible editor：Tingting XU

Responsible proofreader：Xiaoyan WU

超級稻機插播種量對秧苗成毯性與秧苗素質的影響

滕飛1，2，朱德峰1*，陳惠哲1，蔡雪青1，徐一成1
（1.中國水稻研究所/水稻生物學國家重點實驗室，浙江杭州310006；2.北京市農業(yè)機械試驗鑒定推廣站，北京100079）

為探明水稻機插不同播種量條件下秧苗成毯及質量的差異，以超級稻品種甬優(yōu)12和中浙優(yōu)1號為試驗材料，設置6個秧盤播種量，研究秧苗生長特性、秧苗質量及成毯性。研究結果表明，不同播種量對秧苗成毯性及秧苗素質的影響顯著，播種量的增加可顯著提高秧苗根系盤結力，但其秧苗素質明顯下降，秧苗細弱，干物質積累量減少，同時其秧苗整齊度較差，小苗弱苗比例增多。2個品種秧盤播種量每下降10 g，平均秧苗株高提高0.5 cm，葉齡增加0.1個，秧苗基部增粗0.1 cm，單株秧苗根數(shù)增加0.3根；秧盤播種量每提高10 g，平均單株干物質下降5.1 mg。隨著播種量的增加，中浙優(yōu)1號秧苗素質下降較快，小苗比例迅速提高，甬優(yōu)12播種量彈性則相對較大。甬優(yōu)12在播種量達到50 g/盤時可實現(xiàn)秧苗成毯，秧苗生長整齊，秧苗個體素質也較優(yōu)，但其秧盤地上部生長稀疏，葉面積指數(shù)較??；而中浙優(yōu)1號播種量為50 g/盤時，秧苗成毯及秧苗素質各項指標均較優(yōu)，秧苗生長整齊、緊密。綜合各指標研究結果考慮，甬優(yōu)12和中浙優(yōu)1號最適播種量分別為65及50 g/盤。

水稻；超級稻；機械插秧；播種量；秧苗成毯；秧苗素質

國家水稻產業(yè)技術體系項目（CARS-01-04A）；國家科技支撐計劃項目（2012BAD07B02）；科技部科研院所技術開發(fā)研究專項（2013EG134237）；中央級公益性科研院所基本科研業(yè)務費專項基金（2012RG005-3）。

滕飛（1990-），男，江蘇鹽城人，碩士，研究方向：水稻機械化種植技術，E-mail：493861676@163.com。*通訊作者，博士，研究員，博士生導師，從事水稻高產栽培方面的研究，E-mail：cnrice@qq.com。

2015-08-22

Supported by National Rice Industry Technology System（CARS-01-04A）；National Key Technology Research and Development Program （2012BAD07B02）；Ministry of Science and Technology’s Special Fund for Technology Development and Research of Research Institutes （2013EG134237）；National Nonprofit Institute Research Grant of CATAS-TCGRI（2012RG005-3）.

*Corresponding author.E-mail:cnrice@qq.com

Received:August 22，2015 Accepted:November 3，2015

修回日期 2015-11-03