Runhua LIU

The No.1 Middle School of Wendeng,Shandong Province,Wendeng 266440,China

Peanut belongs to taproot system[1],and its roots are a organ for supporting and fixing peanut plant,absorbing and transporting moisture and nutrients and synthesiz ing certain amino acidsand hormones[2].The root system of peanut is composed of taproot and various-order lateral roots.In the 1930s,Weaver proposed that to scientifically understand crop production,it is necessary to fully understand the crop root growth,root mass distribution and root vigor for adsorbing water and nutrients at various growth stages,as well as changes in crop root system under different circumstances[3].The soil with high clay content has poor permeability,and it would shorten root and reduce root dry weight[4].This will lead to decreased total leaf area per plant,affecting the accumulation of photosynthetic products[5],and further resulting in reduced crop yield.Previous studies showed that the water use efficiencies ofBeta vulgaris,Glycine max,Lycopersicon esculentum,Zea mays,Helianthus annus,Solanum tuberosumin sandy soil were 22% -25% lower than those in loamy soil[6].However,different crops have different requirements forsoiltexture.The growth and senescence ofNicotianatabacumroots in sandy soil were better than those in clay[7].The root senescence ofZea mayswas slowed down with the increased clay content in soil[8].The root vigor ofZea mayswas closely related to post-heading dry matter accumulation,total dry matter accumulation and grain yield[9].

Wendeng is located in the hilly areas,and its soil depth differs greatly.The suitable soil depth for peanut growth is about 30 cm,which is suitable for cultivation of a variety of peanut cultivars.In this study,the erect type peanut was selected.The deep soil body was prepared by hand so as to weaken the effects of soil structure on root growth of peanut,thus the roots of peanut could be fully extended.The three-dimensional distribution of root system of erect type peanut was studied to explore the growth characteristics of peanut root system and to grasp the rules in root growth of peanut.This study is of great significance for regulation of peanut root growth and development of scientific cultivation techniques.

Material and Methods

Test site and material

The test was conducted in the test field in Xiaoluo Village,Houjia Town,Wendeng City,Shandong Province in 2010.The test peanut cultivar was erect type Qixuan No.1.The kernels,with the weight of 1 g,in the back chambers of two-kernel peanut were selected as the seeds.

Construction of soil system

The deep soil body was constructed by hand.It was the friable soil layer at the depth of more than 50 cm.The friable layer was brown,and was composed of soil and sand.The soil was infertile,and had poor water-holding capacity.The texture of friable layer was soft after reclamation,but was hard before reclamation.The friable soil was all filtered through a sieve,and per unit volume (1 m3)of friable soil was mixed with certain amounts of N (24 g),P2O5(8 g),K2O (16 g)and miscellaneous manure(4.5 kg).At the depth of less than 50 cm,it was the mellow soil layer.Mellow soil refers to the reclaimed soil.In this study,the used mellow soil referred to the soil after seasons of crop cultivation.The mellow soil was all filtered through a sieve,and per unit volume (1 m3)of mellow soil was mixed with certain amounts of N(60 g),P2O5(20 g),K2O(40 g)and miscellaneous manure(50 kg).During the soil-filling process,each 50 cm of soil was watered once for compaction.The soil depth was arranged as 100,200 and 300 cm,respectively.

Cultivation pattern

The peanut seeds were coated with specific agent.They were sowed on May 8.Two seeds were placed in each hole,and the spacing between each two seeds was 10 cm.From the two seedlings in each hole,one seedling wasremoved 7 d after seedling emergence.The seeds collected 20(seedling stage)and 45(late seedling stage)d after the sowing were sowed on soil body with depth of 100 cm;the seeds collected 65 d(late pod-pin stage)after the sowing were sowed in soil body with depth of 200 cm;the seeds collected 95 (late podsetting stage)and 120(mature stage)d after the sowing were sowed in soil body with depth of 300 cm.At various growth stages,the available peanut plants were all much more than sampled peanut plants.The spacing between two adjacent rows was different(100 cm-133 cm)at different sampling period.However,at certain sampling period,the spacing between two adjacent rows was equal to avoid interaction between two adjacent plants.The field management was the same with that in general fields.

Sampling and recording methods

The roots of peanut were sampled 20,45,65,95 and 120 d after the sowing by water flushing method.The soil on the surface of peanut roots was washed off by mechanical high-pressure sprayer and generalsmall sprayer.At each growth stage,three peanut plants with intact root systems were sampled.

During the washing process,the lateral root span (farest distance between the longest first-order lateral root and the taproot)and embedded depths of taproot and longest first-order lateral root of each peanut plant were measured.After the washing process,the length of taproot(distance between the lower end of hypocotyl and the tip of taproot),lengths of main first-order lateral roots (distance between insertion point and tip of certain lateral root),order number of lateral roots and piece number of lateral roots at each order were measured.The data were all expressed as the means of the three peanut plants[10-11].

Results and Analysis

Order number of lateral roots of erect type peanut

In the formation of root system of erect type peanut,radicle first breaks out from the seed coat and then elongates vertically downward,forming into taproot.With the rapid growth of the taproot,first-order lateral roots appeared gradually from top to bottom of the taproot.The first generated four rows of first-order lateral roots also grew rapidly,andtheyshoweda cross-shaped arrangement,constituting the three-dimensional framework of peanut root system.With the growth of first-order lateral roots,second-to fifth-order lateral roots appeared in succession.From the taproot and various-order lateral roots,thousands of pieces of roots were generated,forming into the large rootsystem of peanut.The fourth-order lateral roots appeared at the seedling stage;the fifth-order lateral roots appeared at the pod-pin stage;while no more orders of lateral roots appeared at the podding or mature stage.

Piece number of later roots of erect type peanut at various orders

At the mature stage,the pieces of first-to fifth-order lateral roots of erect type peanut were 297,7 834,4 768,316 and 12,respectively.The secondand third-order lateral roots were the most,accounting for 95.27% of the total roots,and they are a major component of root system.However,the fifthorder lateral roots were the least,and only 12 fifth-order lateral roots appeared during the whole growth period of peanut.

The total pieces of roots were 1 712,5 186,8 708,12 340 and 13 227 respectively 20,45,65,95 and 120 d after the sowing.The total piece of roots was increased fastest at the seedling stage with an increment of 5 186.The increment of total roots ranked second at the podding stage(3 632),and ranked third at the podpin stage(3 522).However,the increment of total roots was smallest at the mature stage (887).It indicates that during the whole growth period ofpeanut,the distribution of nutrients is prone to pods,so at the podding stage,the increment of total roots is smaller.No sixth-or more-order lateral roots were shown during the whole growth period of erect type peanut(Table 1).

Table 1 Piece number of lateral roots of erect type peanut at each order

Lengths and embedded depths of taproots and lateral roots of erect type peanut

The lengths of taproot of erect type peanut were 20.1,63.8,132.9,182.7 and 188.1 cm respectively 20,45,65,95 and 120 d after the sowing,while the lengths of longest first-order lateral root were 10.7,74.9,141.1,199.2 and 208.3 cm respectively.The results showed that the lengths of taproot and longest first-order lateral root were increased gradually from the seedling to mature stages.At the seedling stage,the taproot of peanut grew rapidly,and it was the longest root.During the late seedling to mature stages,1-2 out of the four longest first-order lateral roots showed higher growth rate than the taproot,so the longest first-order later root became the longest root of peanut.However,during the whole growth period of erect type peanut,the average length of the four longest first-order lateral roots was still less than the length of the taproot.In addition,among the four crossshaped longest first-order lateral roots,the one exposed to the south was generally the longest.

The embedded depth differences between the taproot and longest firstorder later root were 19.8,22.7,29.2,24.6 and 19.3 cm respectively 20,45,65,95 and 120 d after the sowing.It indicated that during the whole growth period of erect type peanut,the embedded depth of the taproot was still larger than that of the longest first-order lateral root.However,certain firstorder lateral roots of some peanut plants were larger than those of their taproots.Although the lengths of certain first-order lateral roots were not the largest,their embedded lengths were the largest(Table 2).

Lateral root span and cylindrical three-dimensional structure of erect type peanut

At the seedling stage of erect type peanut,the length of the longest firstorder lateral root was 10.7 cm,and the lateral root span was 10.5 cm.When the first-order lateral roots began to generate,they showed a 90-degree angle with the taproot.After a certain time,the four early generated first-order lateral roots began to grow slightly downward. When their lengths reached 40-43 cm,they began to grow vertically downward.At that time,the vertical distances between the tips of first-order lateral roots and their insertion points ranged from 10 to 13 cm.However,the taproot still grew vertically downward.The spans of the longest first-order later root were 10.5,34.9,40.2,41.8 and 42.6 cm respectively 20,45,65,95 and 120 d after the sowing.The results also showed that at the late pod-pin stage,the lateral root span of erect type peanut reached the peak;in the later period,it showed no significant variation(40 cm-43 cm).

During the late seedling to mature stages,the spans of the four longest first-order lateral roots of erect type peanut began to be remained stable after they reached the peaks(40 cm-43 cm).This was because that the horizontal growth of the four lateral roots had reached the peaks,so they began to grow vertically.However,during that time,the embedded depths of first-order lateral roots were increased rapidly.The shape of each first-order lateral root was similar to that of parabola.Due to the existence of spans of major first-order lateral roots, cylinder-like structure were formed with the taproot as the axis and average span of major first-order lateral roots as the radius.The ratio between the cylinder height and taproot length was increased rapidly from the late seedling (48.7% )to late mature(84.4% )stages of erect type peanut(Table 2).

Three-dimensional distribution of roots of erect type peanut

At the seedling stage,the taproot of erect type peanut grew vertically,and its growth rate was higher than those of lateral roots.The embedded depth of peanut root system is generally defined as the length of taproot.The first-order lateralroots grew slightly downward,and their growth was nearly horizontal.The lengths of first-order lateral roots were reduced in overall from top to bottom of the taproot,so the span of longest first-order later root was also the lateral root span of peanut.During this period,the root system of erect type peanut was handstand cone-typed with taproot as the axis and first-to fourth-order lateral roots,total 5186 pieces,forming the cone body.

During the late seedling stage to mature stage,the root system of erect type peanut was three-dimensional network-typed with taproot as the axis,early-generated major first-and second-order lateral roots as the framework and a huge number of third-order lateral roots as the net meshes.It was an orderly mass system.The total piece of lateral roots reached 13 227.The root system of erect type peanut was no longer handstand cone-typed.The upper part,with height of 10-13 cm,was blunt cone-typed,accounting for only 5.3% -6.9% of the length of taproot.The middle part was cylindershaped with taproot as the axis and lateral root span as the radius.The ratio between cylinder height and taproot length was even up to 84%.The lower part of the root system was irregularhandstand cone-shaped.

Table 2 Observed data about roots of erect type peanut

Conclusions and Discussion

Erect type peanut has a large and structure-complex root system.With the growth oftaproot,nearly300 pieces of lateral roots were generated from the top to bottom.Among them,there were four cross-shaped first-order lateral roots.They grew rapidly and formed into the three-dimensional framework of peanut root system.Till the pod-pin stage,the order number of lateralroots oferecttype peanut reached five.Till the mature stage,thousands of pieces of lateral roots were generated.The second-and third-order lateral roots were a major component of peanut root system,while the fifth-order lateral roots were the least.No more orders of lateral roots were shown at the podding or mature stage.At the mature stage,the increment of total peanut roots was smaller.

During the growth period of erect type peanut,the lengths of taproot and longest first-order lateral root were all increased gradually.At the seedling stage,the taproot was the longest among the peanut roots.Till the late seedling stage,the longest first-order lateral root became the longest.During the pod-pin to late mature stages,the lateral root span of erect type peanut was remained stable.At the late mature stage,taproot was not always the one with largest embedded depth.

At the seedling stage,lateral roots of erect type peanut grew slightly downward,and their growth was nearly horizontal.The three-dimensional structure of peanut root system was a handstand cone.During the late seedling to mature stages,the upper part of the root system was blunt conetyped,the middle part was three dimensional network-shaped,and the lower part was irregular handstand cone-typed.

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