Effects of Planting Density on Yield and Sourcesink Characteristics of Sweet Potato[Ipomoea batatas(L.)Lam]

Haiyan ZHANG,Beitao XIE*,Baoqing WANG,Qingmei WANG,Shunxu DONG,Aixian LI,Fuyun HOU,Wenxue DUAN,Liming ZHANG

1.Crop Research Institute of Shandong Academy of Agricultural Sciences,Jinan Shandong 250100,China;2.Shandong Academy of Agricultural Sciences,Jinan Shandong 250100,China

The sweet potato(Ipomoea batatas)is the fifth energy crop worldwide[1],as well as the fourth most-produced cereal in China,playing an important role in maintaining crop safety,releasing energy crisis,and developing functional food[2].It is now cultivated worldwide,as high as 111 countries and regions,and China is the country most planting and producing sweet potato.For example,growing area and gross yield of sweet potato represented 42.8%and 71.4%worldwide in 2013[3].

Density is a key factor influencing crop yield[4]by coordinating balance among three yield factors of sweet potato.Currently,researches available on sweet potato only concentrate on two aspects.The first is research on density effects oriented by high yield.For instance,Xuet al.[5]believed that the number of leaf,the number of branch,the weight of stem and leaf and the weight of tuber per plant of Jishu No.22 kept decreasing upon density,while leaf area index,the number of tuber,and yield of fresh tuber were growing.Secondly,attention is paid to density optimization research oriented by highly-yielding and highlyefficient planting on basis of comprehensive agronomical measures.For instance,Jiaet al.[6]carried out re-search on the interplay between density and fertilizerand the results showed that the quantity of K fertilizer and planting density had complementary effect on accumulation of dry matter.

Recently,with developmentof new sweet potato cultivars and improvement of cultivation technology,yields of both individual and groups are increasing.Therefore,it has become an important way for increasing gross yield by improving group yield.In China,sweet potatoes are most grown in hills and mountainous areas,cultivation density is too sparse of 37 500-40 500 plants/hm2.Consequently,it is hard for sweet potato groups to formulate with abundant lands and heat resources in early stage.In later stage,however,sweet potatoes are prone to premature senility,affecting accumulation of dry matters in roots.Hence,the research on relationship of density with source-sink and yield on basis of new cultivars and new technology is conductive to exploring highly-yielding groups and improving yield per unit area of sweet potato.

The research selected starchoriented Jishu No.21[7]and raw-eating oriented Jishu No.22[8]as test materials and analyzed effects of density on yield formation and source-sink characteristics in order to determine the suitable planting density and rational group structure ofdifferentsweet potato cultivars and to provide references forhighly-yielding cultivation modes of sweet potato in hills.

Materials and Methods

Test cultivars

The testcultivars were Jishu No.21 and Jishu No.22.Specifically,Jishu No.21,officially approved in 2007 in China,was bred based on female parent CHGU1.002 and male parent PC94-1 through sexual hybridization by Crop Research Institute of Shandong Academy of Agricultural Sciences.Jishu No.22,officially approved in 2008 in China,was developed with Beijing No.553 as a female parent and highly-qualified cultivar at home and abroad as a male parent by Crop Research Institute of Shandong Academy of Agricultural Sciences.

Test design

The test was carried out in Linyi andZouchengcitiesinShandong Province in 2010.The test plots are arranged in flat areas where soils are loose and convenient for irrigation and drainage.In Linyi,test soils contain 16.8 g/kg organic matter,90.5 mg/kg alkali-hydrolyzable N,10.6 mg/kg available P,and 63.6 mg/kg available K.In Zoucheng,test soils contain 13.8 g/kg organic matter,87.3 mg/kg alkalihydrolyzable N,11.7 mg/kg available P,and 60.5 mg/kg available K.

The test designed 5 treatments with different densities, including treatment A with 30 000 plants/hm2,treatment B with 45 000 plants/hm2,treatment C with 60 000 plants/hm2,treatment D with 75 000 plants/hm2,andtreatmentEwith90000plants/hm2.According to randomized block design,the test was designed with three replications and 30 test plots.Sweet potato was grown 10 rows in every test plot,with row length of 5 m and line spacing of 78 cm.Every test plot has an area of 39 m2.Besides,seedlings were transplanted on May 9 and harvested on October 29 in Linyi and transplanted on May 12 and harvested on October 31 in Zoucheng.

Sampling method

Sweet potatoes were sampled on the 40th,80thand 120thdays after transplanting and during harvesting period.Specifically,five plants were selected in every treatment to survey the length of vine,the number of branches,the length and width of leaf,the number of leaf,fresh weights of leaf,petiole and vine of aboveground parts,fresh weights of root tubers,as well as the ratio and weight of dry matter.

Measurement items and methods

Measurement of yieldThe number of survived plants and the number of tubers per plant were concluded,and weight of stem and leaf of aboveground parts and tubers were weighed to compute the number of tuber per plant,dry weight of tuber per plant and tuber yield after harvesting.

Measurements of fresh weight and dry matter weightFor every treatment,5 plants were selected to weigh leaf,petiole,and vine of group parts and fresh weights of root tubers.After drying,the ratio and weight of dry matter were computed.

Fast determination method of leaf area and measurement of leaf area index(LAI)

Fast determination method of leaf areaSamples were collected from the fields to measure length and width of leaf.Specifically,leaf length was measured along leaf vein and the width was measured based on the widest part.Subsequently,virtual leaf area can be computed on basis of length and width and then practical leaf area was obtained by multiplying compensation coefficient of 0.6[9].

Measurement ofleafareaindex(LAI)Leaf area index (LAI)of sweet potato can be computed according to planting density(LAI=virtual leaf area per plant×0.6 ×density(plants/666.7)/666.7).

Data processing and analysisTest data were processed and maps were drawn with Microsoft Excel 2013,and analysis of variance and significance test of difference were conducted with DPS v8.01.

Results and Analysis

Effects of planting density on yield of sweet potato

Asshown in Table 1,fresh yield and dry yield of sweet potato both reached the highest of 45 000 plants/hm2of Jishu No.21 in Linyi and 60 000 plants/hm2in Zoucheng.As for Jishu No.22,fresh yield of sweet potato reached the highestof75 000 plants/hm2and dry yield of sweet potato reached the highestof60 000 plants/hm2.It can be concluded from surveying results of large and medium-sized ratios of tuberthat Jishu No.21 and Jishu No.22 showed the same trend in Linyi and Zoucheng.For example,the ratio achieved the highest of the two cultivars at 30 000 plants/hm2and the lowest of 90 000 plants/hm2.Besides,density also had effects on the ratio of dry sweet potato.For instance,the ratio of dry tubers of Jishu No.21 showed little differences in Linyi,but the ratios were significantly higher in the treatments with densities of 30 000 and 75 000 plants/hm2in Zoucheng.In contrast,the ratios ofJishu No.22in the treatments with densities of 45 000 and 60 000 plants/hm2proved significantly higher.

Table 1 Effects of planting density on yield of sweet potato

Effects ofplanting density on growth and development of source organs of sweet potato

Effects of planting density on vine length and the number of branchIn the test,40 d after planting sweet potatoes,effects of planting density showed insignificant on vine length and the number of branch,and different treatments showed little differences,also.From the 80th d,however,plant groups differed upon planting density.

As shown in Fig.1,the length of vine kept increasing after planting and stabilizing at the 120 d after planting.For example,the length of vine of Jishu No.21 reached the longest in the treatments with densities of 45 000 and 60 000 plants/hm2and the vine length of Jishu No.22 reached the longest in the treatments with densities of 30 000 and 45 000 plants/hm2.

As shown in Fig.2,branches of sweet potatoes grew until 80 d after planting.Specifically,the number of branches of Jishu No.21 and Jishu No.22 both reached the highest in the treatments with densities of 30 000 and 45 000 plants/hm2.

Effects of planting densities on the number of leaf of individual plant and leaf area indexLeaf area index(LAI)is an important index evaluating sweet potato group structure.If the LAI is too high,crop ventilation and sunning would be affected.If LAI is too low,however,it is hardly for sunshine fully made use of.As shown in Fig.3-Fig.4,the number of leaf per plant and LAI both reached peak at the 80th d after planting for Jishu No.21 and Jishu No.22.Specifically,80 d after planting,the number of leaf per plant of Jishu No.21 and Jishu No.22 both reached the highest in the treatment with density of 30 000 plants/hm2,the lowest in the treatment with density of 90 000 plants/hm2,and the moderate in the treatment with density of 60 000 plants/hm2.Leaf area index achieved the highest in the treatments with densities of 75 000 and 90 000 plants/hm2,the lowest in the treatments with densities of 30 000 and 45 000 plants/hm2and the moderate in the treatments with densities of 60 000 plants/hm2.

Effects of planting density on fresh and dry weight of aboveground parts of sweet potatoAs shown in Fig.5-Fig.6,both of fresh and dry weights of aboveground parts reached the highest 80 d after planting,followed by declining.For both Jishu No.21 and Jishu No.22,fresh and dryweightsofaboveground parts reached the highest in the treatment with 30 000 plants/hm2,the lowest in the treatment with 90 000 plants/hm2,and moderate in the treatment with 60 000 plants/hm2.

Effects of planting density on development of sink organs of sweet potato

Effects of planting density on root tuber developmentAs shown in Fig.7-Fig.8,fresh and dry weights of underground parts kept growing.Specifically,40 d after planting,effects ofdensity showed insignificanton fresh and dry weights of underground parts and treatments.Then,80 d after planting,aboveground parts showed significant differences among treatments,affecting root tuber growth.For example,fresh and dry weights of underground varied dramatically,and the differences reached the peak during harvesting period.

About 80 d after planting,fresh weight of underground parts of Jishu No.21 reached the highest in the treatmentwith densityof30 000 plants/hm2and the lowestin the treatmentwith densityof90 000 plants/hm2,and the fresh weight of Jishu No.22 reached the highest in the treatmentwith densityof45 000 plants/hm2,and the lowestin the treatmentwith densityof75 000 plants/hm2.During harvesting period,however,the fresh weights of Jishu No.21 and Jishu No.22 both reached the highest in the treatment with density of 30 000 plants/hm2and the lowest in the treatment with density of 90 000 plants/hm2.With growth of density,fresh weight of underground part was decreasing.

About 80 d after planting,dry weight of underground parts of Jishu No.21 reached the highest in the treatmentwith densityof30 000 plants/hm2and the lowestin the treatmentwith densityof90 000 plants/hm2,and the dry weight of Jishu No.22 reached the highest in the treatmentwith densityof45 000 plants/hm2,and the lowestin the treatmentwith densityof90 000 plants/hm2.During harvesting period,however,the dry weights of Jishu No.21 and Jishu No.22 both reached the highest in the treatment with density of 30 000 plants/hm2and the lowest in the treatment with density of 90 000 plants/hm2.With growth of density,dry weight of underground part was decreasing.

Effects of planting density on dry matter content of root tubersAs shown in Fig.9,the content of dry matter of tuber showed single-peak curve.Specifically,dry mattercontentof Jishu No.21 reached the peak 120 d after planting,and of Jishu No.22 reached the peak 80 d after palnting.Besides,dry matter of Jishu No.21 showed significant differences among treatments 120 d after planting,which achieved the highest in the treatment with 60 000 plants/hm2and the lowest in the treatment with 75 000 plants/hm2.For Jishu No.22,dry matter showed significant differences among treatments in harvesting period,which achieved the highest in the treatment with 45 000 plants/hm2and lowest in the treatment with 90 000 plants/hm2.

Effects ofplanting density on source-sink of sweet potato

Effects of planting density on T/R value(fresh weight)of sweet potatoAs shown in Fig.10,T/R value(fresh weight)was decreasing with growth of sweet potato and reached the lowest until harvesting period.For Jishu No.21,T/R values (fresh weights)were 45 000＞60 000＞90 000＞75 000＞30 000 plants/hm2at 40 and 80 d after planting;the values were 45 000＞75 000=90 000 ＞60 000 ＞30 000 plants/hm2at 120 d after planting,and 45 000=90 000＞75 000＞30 000＞60 000 plants/hm2until harvesting period.In contrast,T/R values of Jishu No.22 were 30 000＞60 000＞90 000＞45 000＞75 000 plants/hm2at 40 d after plants,and 75 000＞30 000＞90 000＞45 000＞60 000 plants/hm2at 80 d after planting;the values were 45 000＞90 000 ＞60 000 ＞75 000 ＞30 000 plants/hm2at 120 d after planting and 75 000＞60 000＞90 000＞45 000＞30 000 plants/hm2until harvesting period.

Effects of planting density on T/R value(dry weight)of sweet potatoAs shown in Fig.11,with growth and development of sweet potato,T/R value(dry weight)was decreasing and reached the lowest until harvesting period.For Jishu No.21,T/R values(dry weights)were both 45 000＞60 000 ＞90 000 ＞75 000 ＞30 000 plants/hm2at 40 and 80 d after planting,and 45 000＞75 000＞90 000＞60 000＞30 000 plants/hm2at 120 d after planting;the values achieved the highest in the treatment with density of 45 000 and 75 000 plants/hm2,and the lowest in the treatments with densities of 30 000,60 000 and 90 000 plants/hm2.In contrast,forJishu No.22,the values were 30 000＞60 000 ＞90 000 ＞75 000 ＞45 000 plants/hm2at 40 d after planting,75 000＞30 000＞90 000＞45 000＞60 000 plants/hm2at 80 d after planting,45 000＞90 000＞60 000＞75 000＞30 000 plants/hm2at 120 d after planting,and 75 000＞60 000=90 000＞45 000＞30 000 plants/hm2until harvesting period.

Conclusion and Discussion

Effects of planting density on yield of sweet potato

Rational density is important for achieving a high yield of plant.As for sweet potato,for instance,if grown too densely,individual plants would be inhibited at growing,roots development become poor,and tubers grow small,so that commercial value would be lower.In contrast,if grown too sparsely,total biomass would decline upon plant groups,affecting sweet potato yield[10].The research has demonstrated that the optimal planting density of Jishu No.21 was 45 000-60 000 plants/hm2and of Jishu No.22 was 60 000-75 000 plants/hm2.In practical production,the density should be determined according to specific plant characteristics,environment,planting stage,and growth stage.

Effects ofplanting density on source-sink of sweet potato

The previous researches conclude that to maintain a rational leaf area in a unit area plays a key role in yield,for rational densely-planting coordinates ground and underground parts of sweet potato,improving yield of dry matter.If sweet potatoes are grown sparsely,the number of plant would reduce per unit area and utilization ratio of water,soil and fertilizer would be lower.If sweet potatoes are grown densely,however,ventilation and transmitting would become difficult for the crops,and roots would hardly grow well[9].

The test proved that 40 d after planting,effects of density showed insignificant on source and sink organs,but after vines grew,however,the plant groups showed differences in fields.For example,the number of branches and the length of vines kept growing.Specifically,branch growth almost stopped 80 d after planting,and vine growth finished 120 d after planting.On the other hand,the number of branches and vine length of an individualplantwere higherifsweet potatoes were grown at low densities of 30 000-60 000 plants/hm2,and were lower if sweet potatoes were grown at high densities of 75 000-90 000 plants/hm2.Leaf area index is an important index evaluating group structure of sweet potato,which increases upon planting density.Provided planting density is fixed,leaf area index showed a single-peak in the whole growth stage.Specifically,LAI grew fast from the 40th-80thd and reached the peak on the 80thd after planting,followed by declining.On the other hand,both fresh weight and dry weight of ground parts reached the peaks on the 80thd after planting.For example,fresh and dry weights of Jishu No.21 and Jishu No.22 both reached the highest in the treatment with density of 30 000 plants/hm2,the lowest in the treatment with density of 90 000 plants/hm2and moderate in the treatmentwith densityof60 000 plants/hm2.

Powerful source and large sink lay foundation for highly yielding of sweet potato,for the effects of sink and source are related to planting density and coordination of the two factors is key for highly yielding of sweet potato.Besides,leaf area index and fresh weight of aboveground parts both reflect strength of source.The research indicated that both of fresh and dry weights of root tubers kept growing and reached the peak,followed by decreasing.The ratio of dry matter of Jishu No.21 showed significant differences among treatments 120 d after planting,which achieved the highest in the treatment with density of 60 000 plants/hm2and the lowest with density of 75 000 plants/hm2.The ratio of Jishu No.22 showed great differences among treatments in harvesting period,which reached the highest in the treatmentwith densityof45 000 plants/hm2and the lowest in the treatment with density of 90 000 plants/hm2.In conclusion,fresh and dry weights of root tubers incorporate the size of sink.The research has concluded thatfresh weightand dry weight of root tubers kept growing and the differences showed the most significant among treatments in harvesting period.The dry weight of Jishu No.21 and fresh weight of Jishu No.22 both decreased upon density.Additionally,the changes of T/R values indicated dynamic changes of relationship between aboveground part growth and tuber expansion of sweet potato.The research has demonstrated that it is aboveground part of sweet potato that grows in the treatments with low densities and underground part grows in the treatments with high densities,such as 60 000 and 75 000 plants/hm2.

Therefore,it can be concluded that rational planting densities would well coordinate sweet potato growth of aboveground parts and underground parts to get a high yield by providing a rational group structure.Considering the optimal planting density differs upon cultivars,it is necessary to take genotype,environment,soilfertility and planting density into consideration in determining planting density.

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