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Abstract The effects of different nitrogen application rate on the physical characteristics and anatomic structure of rice stems were investigated with rice cultivars Guangliangyou 1128 （with high resistance） and Zhunliangyou 527 （with low resistance） as materials. The results showed that， firstly， plant height， gravity center height and basal internode length of the 2 rice cultivars increased with the increase of nitrogen application rate， while wall thickness and internode filling degree decreased. The breaking??resistance strength per stem and thrust??resistance strength of plants declined with the increase of nitrogen application rate. Secondly， as nitrogen input increased， the number of vascular bundles and the area of vascular bundles increased between the 2 rice cultivars. Thirdly， with the increase of nitrogen application rate， the stem section area of Guangliangyou 1128， which had strong lodging resistance， gradually increased， while that of Zhunliangyou 527 increased firstly and then decreased. The maximum application amount of nitrogen was 240 kg/hm2. Nitrogen fertilizer mainly affected the relative gravity center height， stem wall thickness and internode filling degree of the 2 cultivars， thereby reducing the basal stem breaking resistance and plant thrust??resistance strength.
Key words Nitrogen application rate； Thrust??resistance strength of plants； Stem physical characteristics； Stem anatomic structure
High yield and lodging are a pair of contradictions in rice production， especially with the large??scale promotion of super rice varieties and the increase in fertilizer use， the problem of lodging is becoming more and more serious. Rice lodging not only causes a decrease in yield， but also leads to a decline in quality and an increase in harvesting difficulty. In addition to the natural factors， the management of lodging is also an important influencing factor[1]. Fertilization， especially nitrogen fertilizer， is one of the most rapid and effective measures to increase crop yield， but excessive nitrogen fertilizer causes rice lodging[2]. Therefore， studying the effects of different nitrogen application rates on rice lodging resistance and stem physical characteristics is of great significance for the control of high yield and lodging resistance of rice.
Rice production is a group production process， and its lodging resistance is the integrated result of its own characteristics， cultivation measures and natural environment. However， the existing research mainly relies on the single stem lodging resistance effect， that is， the lodging index， as the index to evaluate lodging resistance， but the index cannot accurately measure the difference in lodging resistance between different nodes of the same plant[3]. Thrust??resistance strength is measured under the normal growth of plants in the field， which reflects the external force required for the lodging of the plant in the field， and also considers the lodging resistance of the variety itself and the external environmental factors and their relationship[4]. Therefore， in this study， the thrust??resistance strength was used to measure the lodging??resistance strength of rice plants. In addition， there are few studies on the mechanism of lodging resistance of rice with different lodging capacities under different nitrogen application rates. In this study， Guangliangyou 1128， a super hybrid rice variety with strong lodging resistance， and Zhunliangyou 527， a super hybrid rice variety with poor lodging resistance， were used as the materials to investigate the physical characteristics and anatomic structure of rice stems under different nitrogen application rate， as well as the relationship with the thrust??resistance strength， so as to find out the leading factors affecting the lodging??resistance performance of rice， thereby providing a theoretical basis for the high??yield and lodging??resistance breeding and cultivation regulation of rice. Materials and Methods
Test materials
The test was conducted at the practice base of Yangtze University. The soil had a pH value of 6.24， available nitrogen content of 90.64 mg/kg2， available phosphorus content of 55.82 mg/kg2， and available potassium of 43.59 mg/ kg2. The tested varieties were Guangliangyou 1128 with strong lodging resistance and Zhunyou 527 with weak lodging resistance. The split block design was adopted with 3 repetitions. The plot area was 18 m2， and the transplanting specification was 30 cm??15 cm. The rice was sown on April 22， transplanted on May 21， and the seedlings were 30 d old. There were 4 nitrogen levels set in the test， namely， 180 （N1）， 240 （N2）， 300 （N3） and 360 （N4） kg/ km2. The base fertilizer was applied with 60% of nitrogen， and at 5-7 d after transplanting， 20% of nitrogen was applied. After re??watering of the dry field， 15% of nitrogen was applied， and at 3-5 d after full heading， 5% of nitrogen was applied with ordinary urea. The application amount of potassium （K2O） was 180 kg/hm2 and phosphorus （P2O5） was 90 kg/hm2. The potassium fertilizer was applied according to 40% of base fertilizer， 30% applied after re??watering of dry field and 30% applied at 3-5 d after full heading， while phosphate fertilizer was applied once as base fertilizer using calcium superphosphate. The other field management was associated with that of local high??yield fields. At the heading stage， 20 plants and single stems with the same heading date in the middle of the plot were marked of each plot.
Determination content and method
Determination of plant height and thrust??resistance strength At 20 d before maturity， 10 plants each were selected from plots with different nitrogen fertilizer application rate according to the average number of stems. The measured plant height in the field was the length from the field surface to the top of the panicle. Then， the plant stem strength meter DIK??7401 was used to determine the stem strength at 20 cm from the ground by pressing the rice stem forward to a dip angle of 45 ?? （measured by a protractor）. At that point， the recorded value from the plant stem strength meter was the maximum pushing resistance tolerated by an individual plant， that is， the thrust??resistant strength of a plant[4].
Gravity center height At 15 d before maturity period， 10 plants with effective panicles close to the average effective panicle were selected in each plot. The single plant （cut off the underground part） was placed on a supporting point， which was adjusted by moving from left to right to keep the plant at the horizontal level， and then the distance from the supporting point to the stem base was measured. And then， the relative gravity center height was calculated. Determination of lodging resistance index of single stem Among the selected plants in "Gravity center height"， 5 plants were selected from each plot， and were cut at the stem nodes， which divided a tem into several internodes. The first elongated internode from the panicle was the first internode， expressed as S1， and successively downward came the second internode （S2）， the third internode （S3）， the fourth internode （S4）， the fifth internode （S5）， the sixth internode （S6）. The internode length， internode thickness and internode wall thickness were measured according to the following： the length of each internode was measured using a ruler； digital vernier caliper was used to measure the outer diameter of the long axis and the outer diameter of the short axis to calculate the cross??sectional area using the formula of Cross??sectional area = （1/4） ?? ??outer diameter of long axis ?? outer diameter of short axis， and the cross??sectional area was the internode thickness； the thicknesses at the 4 intersections of long axis and short axis with stem wall were measured， and the average value was the stem wall thickness.
At 15 d before maturity， 5 listed plants were selected from each plot according to the average number of stems， and 5 single stems with consistent heights were selected to measure the internode length， breaking??resistance strength and internode filling degree of the fourth internode （S4） and the fifth internode （S5） below the panicle. The breaking??resistance strength was measured using the stem strength meter DIK??7401. The distance between the 2 supporting points to fix the internode was 5 cm， and the internode was placed on the 2 supporting points， and then the middle of the internode was pressed to make it break. The force was the breaking??resistance strength of the internode. After deactivated at 105 ??， the sample single stems were dried at 60 ??， and the dry weight between the 2 nodes were determined. The dry weight per unit length was the internode filling degree.
Determination of stem anatomical structure At 20 d before maturity， 10 listed single stems were collected from the treatments with different nitrogen application rates. The stem samples were collected from the middle of the fourth internode （S4） and the fifth internode （S5） below the panicle to make s paraffin sections. After microscopic observation， representative sections were taken to get records using OLMPUS??2 microscopy， and the area of vascular bundles was determined by MAP??GIS software. Data statistics and analysis
Microsoft Excel 2003 was used to process the test data， and JMP.8 was used for other statistical analysis like analysis of variance and correlation analysis.
Results and Analysis
Stem morphological traits under different nitrogen application rates
As shown in Table 1， with the increase nitrogen application rate， the plant height and relative gravity center height of the 2 rice varieties increased gradually， and there was no significant difference in plant height in treatments N2， N3 and N4， but they were significantly higher than that of treatment N1. With the increase of nitrogen application rate， the relative height of the center of gravity showed an upward trend， and the relative gravity center heights of both varieties of treatment N1 were significantly lower than those of treatments N2， N3 and N4.
Among the different nitrogen fertilization treatments， the internode lengths of the 2 varieties increased with the increase of nitrogen application rates， except the lengths of the first and second internodes （S1 and S2） of variety Guangliangyou 1128， which were slightly lower in treatment N3 than in treatment N2. Moreover， there was no significant difference in the lengths of internodes S1 and S3 under different nitrogen fertilizer rates， while the length of internode S3 in treatment N1 was significantly lower than that in treatment N4， but all other treatments showed no significant difference. The length of internode S5 of Guangliangyou 1128 in treatment N4 was significantly higher than that of the other treatments， and for Zhunliangyou 527， treatment N4 was significantly higher than treatment N1. In treatment N1， there were only a few single stems had internode S6， so the length of internode S6 of treatment N1 was not included， while for other treatments， the length of internode S6 in treatments N3 and N4 were higher than that in treatment N2. Therefore， different nitrogen fertilizer rates could cause the differences in plant heights by affecting the basal internode lengths and the number of internodes.
Internode physical characteristics and plant thrust??resistance strength under different nitrogen application rates
As shown in Table 2， with the increase of nitrogen application rates， the thrust??resistance strengths of the 2 rice varieties decreased， and the differences between treatment N1 and the other 3 treatments were significant， while there was no significant difference between treatments N2， N3 and N4 （a large area of lodging occurred in treatment N4 for variety Zhunliangyou 527， so no data was obtained）. There were slight differences in the internode filling degrees of S4 and S5 of the 2 rice varieties. For Guangliangyou 1128， the internode filling degree decreased with the increase of nitrogen application rates， that is， N1>N2>N3>N4， while for Zhunliangyou 527， the filling degree first increased and then gradually decreased with the increase of nitrogen application rates， that is， N2>N1>N3>N4.
The thrust??resistance strengths of internodes S4 and S5 decreased with the increase of nitrogen application rates， and the 2 rice varieties showed the same trend. For the thrust??resistance strength of internode S4， there was no significant between treatments N1 and N2， and treatments N3 and N4 of Guangliangyou 1128， while there were significant differences between treatments N1， N2， N3 and treatment N4 of Zhunliangyou 527. The differences in the thrust??resistance strength of internode S5 of Guangliangyou 1128 were significant among the treatments with different nitrogen application rates， but for Zhunliangyou 527， the difference was not significant between treatment N1 and N2， but the difference reached the significant level for all other treatments. Therefore， the nitrogen application rate had great influences on the thrust??resistance strength of internode S5.
As shown in Table 2， the stem walls of internodes S4 and S5 of the 2 rice varieties became thinner with the increase of nitrogen application rates， while the wall thickness of internode S4 showed no significant difference in different treatments. However， the effects of different nitrogen application rates on stem section area were different for different rice varieties. As for Guangliangyou 1128， the stem section area gradually increased with the increase of nitrogen application rate， and the differences between treatment N1 and N2， treatments N3 and N4 were significant， while for Zhunliangyou 527， the section area first increased and then decreased， which was in the order of N2>N1>N3>N4.
Stem vascular bundles under different nitrogen application rates
As shown in Table 3， with the increase of nitrogen application rate， although the number of vascular bundles and vascular bundle areas of internodes S4 and S5 increased， the nitrogen application rates had no effect on the number of vascular bundles for both Guangliangyou 1128 and Zhunliangyou 527. However， the effect was significant on the number of big vascular bundles， resulting in differences in the vascular bundle areas between different treatments. Moreover， the differences were significant between treatment N1 and treatment N4 in the number of big vascular bundles and vascular bundle areas of internodes S4 and S5. Agricultural Biotechnology 2018Conclusions and Discussion
The reconciliation between high yield and anti??reverse contradiction is the focus of many studies. High??yield cultivation should ensure sufficient biological yield under the premise of ensuring a certain number of populations， especially for large ears， and higher nitrogen fertilizer input is one of the necessary conditions for high yield， but increases the risk of lodging[5-6]. This study shows that different nitrogen application rates have significant effects on the thrust??resistance strengths of rice plants， and with the increase of nitrogen application rates， the thrust??resistance strengths of both varieties decreases， namely， the lodging resistance of the plant decreased， which was consistent with the previous results by using lodging index to measure the resistance to lodging[6-7].
The lodging resistance of rice is the result of the interaction of the physical traits of the basal stems. Any change in traits can affect the lodging resistance of the plant. Since the basal internode length is in significantly positive correlation with the lodging trait， while the basal internode thickness， wall thickness and section area is in significantly negative correlations with the lodging trait， which play an important role in the lodging resistance of rice， stem thickness and internode length have been used as the lodging resistance indices[8-9]. This study shows that with the increase of nitrogen fertilizer application， the stem cross??sectional area of Guangliangyou 1128 also increases gradually， which is significantly negatively correlated with plant thrust resistance （r=-0.834 3**）， while that of Zhunliangyou 527 increases first and then decreases， which is positively correlated with plant thrust resistance （r=0.157 6）. It indicates that the amount of nitrogen fertilizer had different effects on the growth and development characteristics of the 2 rice varieties with different lodging resistances， which leads to different trends in the cross??sectional area， namely the stem thickness， of different varieties， and it also suggests that the 2 rice varieties have different self??generated nitrogen utilization efficiencies. For Guangliangyou 1128， as a variety with high nitrogen utilization efficiency， with the increase of nitrogen application rate， the absorption and transport capacities of nutrients in the plants gradually enhanced， the stems are thickened， the internode vascular bundle area is increased， and the number of vascular bundles is increased， enhancing the toughness of the stem[5，10]. Vascular bundles are the main channel for the transport of photosynthetic products， mineral nutrients and water. The number， size and function of these vascular bundles are closely related to the transport of these materials， especially the transfer of photosynthetic products into grains. Therefore， the application of nitrogen fertilizer can promote the material metabolism in rice stems， reduce stem??sheath filling degree， increase panicle weight， shift upward the relative height of gravity center， resulting in an increase in the load weight of the aboveground stem of the plants， thereby reducing the lodging resistance of the plant[3， 11-12]. On the other hand， Zhunliangyou 527 is a nitrogen??inefficient variety. The genetic characteristics determine that the optimal source??flow pool is coordinated under low nitrogen levels， and the morphological characteristics， physiological characteristics and anatomical structure of the stems achieve the optimal combination. The increase of nitrogen application rate to over 240 kg/hm2 can result in the excessive growth of plant， increase of plant height， weak stems， decrease of stem??sheath filling degree， thereby reducing the carrying capacity of aboveground stems， and weakening the lodging resistance of plants. Therefore， in cultivation and breeding， attention should be paid to the stem wall thickness and internode filling degree， and increasing the carrying capacity of the stems is the basis for improving the lodging resistance of rice. References
[1] LI J， ZHANG HC， GONG JL， et al. Effects of different planting methods on the culm lodging resistance of super rice[J]. Scientia Agricultura Sinica， 2011， 44 （11）： 2234-2243.
[2] YANG HJ， YANG RC， LI YZ， et al. Relationship between culm traits and lodging resistance of rice cultivars[J]. Fujian Journal of Agricultural Sciences， 2000， 15（2）： 1-7.
[3] LI GH， ZHONG XH， TIAN K， et al. Effect of nitrogen application on stem lodging resistance of rice and its morphological and mechanical mechanisms[J]. Scientia Agricultura Sinica， 2013， 46（7）： 1323-1334.
[4] CHEN GH， DENG HB， ZHANG GL， et al. The correlation of stem characters and lodging resistance and combining ability analysis in rice[J]. Scientia Agricultura Sinica， 2016， 49（3）： 407-417.
[5] MA J， MA WB， TIAN YH， et al. The culm lodging resistance of heavy panicle type of rice[J]. Acta Agronomica Sinica， 2004， 30（2） ： 143-148.
[6] YANG SM， XIE L， ZHENG SL， et al. Effects of nitrogen rate and transplanting density on physical and chemical characteristics and lodging resistance of culms in hybrid rice[J]. Acta Agronomica Sinica， 2009， 35（1）： 93-103.
[7] ISLAM MS， PENG SB， VISPERAS RM， et al. Lodging related morphological traits of hybrid rice in a tropical irrigated ecosystem[J]. Field Crops Research， 2007， 101（2）： 240-248.
[8] ZHANG ZX， CHEN WF， YANG ZY， et al. Effect of lodging resistance on yield and its relationship with stalk physical characteristics[J]. Journal of Shenyang Agricultural University， 1999， 30（2）： 81-85.
[9] ZHANG YQ， XU FY， WANG XL， et al. Effects of Isolane and Lifengling on lodging resistance and yield of direct sowing rice[J]. Journal of Henan Agricultural Science， 2011， 40 （3）： 43-46.
[10] KASHIWAGI T， ISHIMARU K. Identification and functional analysis of a locus for improvement of lodging resistance in rice[J]. Plant Physiology， 2004， 134： 676-683.
[11] LING QH， CAI JZ， SU ZF. Relationship between the number of vascular bundles and the traits of rice stems and its application[J]. Journal of Jiangsu Agricultural College， 1982， 3（3）： 7-16.
[12] YANG ZY， SUN YJ， XU H， et al. Effects of different cultivation methods on accumulation and transformation of assimilation products and lodging resistance of stem??sheaths of no??tillage rice[J]. Chinese Journal of Rice Science， 2013， 27（5）： 511-519.
Key words Nitrogen application rate； Thrust??resistance strength of plants； Stem physical characteristics； Stem anatomic structure
High yield and lodging are a pair of contradictions in rice production， especially with the large??scale promotion of super rice varieties and the increase in fertilizer use， the problem of lodging is becoming more and more serious. Rice lodging not only causes a decrease in yield， but also leads to a decline in quality and an increase in harvesting difficulty. In addition to the natural factors， the management of lodging is also an important influencing factor[1]. Fertilization， especially nitrogen fertilizer， is one of the most rapid and effective measures to increase crop yield， but excessive nitrogen fertilizer causes rice lodging[2]. Therefore， studying the effects of different nitrogen application rates on rice lodging resistance and stem physical characteristics is of great significance for the control of high yield and lodging resistance of rice.
Rice production is a group production process， and its lodging resistance is the integrated result of its own characteristics， cultivation measures and natural environment. However， the existing research mainly relies on the single stem lodging resistance effect， that is， the lodging index， as the index to evaluate lodging resistance， but the index cannot accurately measure the difference in lodging resistance between different nodes of the same plant[3]. Thrust??resistance strength is measured under the normal growth of plants in the field， which reflects the external force required for the lodging of the plant in the field， and also considers the lodging resistance of the variety itself and the external environmental factors and their relationship[4]. Therefore， in this study， the thrust??resistance strength was used to measure the lodging??resistance strength of rice plants. In addition， there are few studies on the mechanism of lodging resistance of rice with different lodging capacities under different nitrogen application rates. In this study， Guangliangyou 1128， a super hybrid rice variety with strong lodging resistance， and Zhunliangyou 527， a super hybrid rice variety with poor lodging resistance， were used as the materials to investigate the physical characteristics and anatomic structure of rice stems under different nitrogen application rate， as well as the relationship with the thrust??resistance strength， so as to find out the leading factors affecting the lodging??resistance performance of rice， thereby providing a theoretical basis for the high??yield and lodging??resistance breeding and cultivation regulation of rice. Materials and Methods
Test materials
The test was conducted at the practice base of Yangtze University. The soil had a pH value of 6.24， available nitrogen content of 90.64 mg/kg2， available phosphorus content of 55.82 mg/kg2， and available potassium of 43.59 mg/ kg2. The tested varieties were Guangliangyou 1128 with strong lodging resistance and Zhunyou 527 with weak lodging resistance. The split block design was adopted with 3 repetitions. The plot area was 18 m2， and the transplanting specification was 30 cm??15 cm. The rice was sown on April 22， transplanted on May 21， and the seedlings were 30 d old. There were 4 nitrogen levels set in the test， namely， 180 （N1）， 240 （N2）， 300 （N3） and 360 （N4） kg/ km2. The base fertilizer was applied with 60% of nitrogen， and at 5-7 d after transplanting， 20% of nitrogen was applied. After re??watering of the dry field， 15% of nitrogen was applied， and at 3-5 d after full heading， 5% of nitrogen was applied with ordinary urea. The application amount of potassium （K2O） was 180 kg/hm2 and phosphorus （P2O5） was 90 kg/hm2. The potassium fertilizer was applied according to 40% of base fertilizer， 30% applied after re??watering of dry field and 30% applied at 3-5 d after full heading， while phosphate fertilizer was applied once as base fertilizer using calcium superphosphate. The other field management was associated with that of local high??yield fields. At the heading stage， 20 plants and single stems with the same heading date in the middle of the plot were marked of each plot.
Determination content and method
Determination of plant height and thrust??resistance strength At 20 d before maturity， 10 plants each were selected from plots with different nitrogen fertilizer application rate according to the average number of stems. The measured plant height in the field was the length from the field surface to the top of the panicle. Then， the plant stem strength meter DIK??7401 was used to determine the stem strength at 20 cm from the ground by pressing the rice stem forward to a dip angle of 45 ?? （measured by a protractor）. At that point， the recorded value from the plant stem strength meter was the maximum pushing resistance tolerated by an individual plant， that is， the thrust??resistant strength of a plant[4].
Gravity center height At 15 d before maturity period， 10 plants with effective panicles close to the average effective panicle were selected in each plot. The single plant （cut off the underground part） was placed on a supporting point， which was adjusted by moving from left to right to keep the plant at the horizontal level， and then the distance from the supporting point to the stem base was measured. And then， the relative gravity center height was calculated. Determination of lodging resistance index of single stem Among the selected plants in "Gravity center height"， 5 plants were selected from each plot， and were cut at the stem nodes， which divided a tem into several internodes. The first elongated internode from the panicle was the first internode， expressed as S1， and successively downward came the second internode （S2）， the third internode （S3）， the fourth internode （S4）， the fifth internode （S5）， the sixth internode （S6）. The internode length， internode thickness and internode wall thickness were measured according to the following： the length of each internode was measured using a ruler； digital vernier caliper was used to measure the outer diameter of the long axis and the outer diameter of the short axis to calculate the cross??sectional area using the formula of Cross??sectional area = （1/4） ?? ??outer diameter of long axis ?? outer diameter of short axis， and the cross??sectional area was the internode thickness； the thicknesses at the 4 intersections of long axis and short axis with stem wall were measured， and the average value was the stem wall thickness.
At 15 d before maturity， 5 listed plants were selected from each plot according to the average number of stems， and 5 single stems with consistent heights were selected to measure the internode length， breaking??resistance strength and internode filling degree of the fourth internode （S4） and the fifth internode （S5） below the panicle. The breaking??resistance strength was measured using the stem strength meter DIK??7401. The distance between the 2 supporting points to fix the internode was 5 cm， and the internode was placed on the 2 supporting points， and then the middle of the internode was pressed to make it break. The force was the breaking??resistance strength of the internode. After deactivated at 105 ??， the sample single stems were dried at 60 ??， and the dry weight between the 2 nodes were determined. The dry weight per unit length was the internode filling degree.
Determination of stem anatomical structure At 20 d before maturity， 10 listed single stems were collected from the treatments with different nitrogen application rates. The stem samples were collected from the middle of the fourth internode （S4） and the fifth internode （S5） below the panicle to make s paraffin sections. After microscopic observation， representative sections were taken to get records using OLMPUS??2 microscopy， and the area of vascular bundles was determined by MAP??GIS software. Data statistics and analysis
Microsoft Excel 2003 was used to process the test data， and JMP.8 was used for other statistical analysis like analysis of variance and correlation analysis.
Results and Analysis
Stem morphological traits under different nitrogen application rates
As shown in Table 1， with the increase nitrogen application rate， the plant height and relative gravity center height of the 2 rice varieties increased gradually， and there was no significant difference in plant height in treatments N2， N3 and N4， but they were significantly higher than that of treatment N1. With the increase of nitrogen application rate， the relative height of the center of gravity showed an upward trend， and the relative gravity center heights of both varieties of treatment N1 were significantly lower than those of treatments N2， N3 and N4.
Among the different nitrogen fertilization treatments， the internode lengths of the 2 varieties increased with the increase of nitrogen application rates， except the lengths of the first and second internodes （S1 and S2） of variety Guangliangyou 1128， which were slightly lower in treatment N3 than in treatment N2. Moreover， there was no significant difference in the lengths of internodes S1 and S3 under different nitrogen fertilizer rates， while the length of internode S3 in treatment N1 was significantly lower than that in treatment N4， but all other treatments showed no significant difference. The length of internode S5 of Guangliangyou 1128 in treatment N4 was significantly higher than that of the other treatments， and for Zhunliangyou 527， treatment N4 was significantly higher than treatment N1. In treatment N1， there were only a few single stems had internode S6， so the length of internode S6 of treatment N1 was not included， while for other treatments， the length of internode S6 in treatments N3 and N4 were higher than that in treatment N2. Therefore， different nitrogen fertilizer rates could cause the differences in plant heights by affecting the basal internode lengths and the number of internodes.
Internode physical characteristics and plant thrust??resistance strength under different nitrogen application rates
As shown in Table 2， with the increase of nitrogen application rates， the thrust??resistance strengths of the 2 rice varieties decreased， and the differences between treatment N1 and the other 3 treatments were significant， while there was no significant difference between treatments N2， N3 and N4 （a large area of lodging occurred in treatment N4 for variety Zhunliangyou 527， so no data was obtained）. There were slight differences in the internode filling degrees of S4 and S5 of the 2 rice varieties. For Guangliangyou 1128， the internode filling degree decreased with the increase of nitrogen application rates， that is， N1>N2>N3>N4， while for Zhunliangyou 527， the filling degree first increased and then gradually decreased with the increase of nitrogen application rates， that is， N2>N1>N3>N4.
The thrust??resistance strengths of internodes S4 and S5 decreased with the increase of nitrogen application rates， and the 2 rice varieties showed the same trend. For the thrust??resistance strength of internode S4， there was no significant between treatments N1 and N2， and treatments N3 and N4 of Guangliangyou 1128， while there were significant differences between treatments N1， N2， N3 and treatment N4 of Zhunliangyou 527. The differences in the thrust??resistance strength of internode S5 of Guangliangyou 1128 were significant among the treatments with different nitrogen application rates， but for Zhunliangyou 527， the difference was not significant between treatment N1 and N2， but the difference reached the significant level for all other treatments. Therefore， the nitrogen application rate had great influences on the thrust??resistance strength of internode S5.
As shown in Table 2， the stem walls of internodes S4 and S5 of the 2 rice varieties became thinner with the increase of nitrogen application rates， while the wall thickness of internode S4 showed no significant difference in different treatments. However， the effects of different nitrogen application rates on stem section area were different for different rice varieties. As for Guangliangyou 1128， the stem section area gradually increased with the increase of nitrogen application rate， and the differences between treatment N1 and N2， treatments N3 and N4 were significant， while for Zhunliangyou 527， the section area first increased and then decreased， which was in the order of N2>N1>N3>N4.
Stem vascular bundles under different nitrogen application rates
As shown in Table 3， with the increase of nitrogen application rate， although the number of vascular bundles and vascular bundle areas of internodes S4 and S5 increased， the nitrogen application rates had no effect on the number of vascular bundles for both Guangliangyou 1128 and Zhunliangyou 527. However， the effect was significant on the number of big vascular bundles， resulting in differences in the vascular bundle areas between different treatments. Moreover， the differences were significant between treatment N1 and treatment N4 in the number of big vascular bundles and vascular bundle areas of internodes S4 and S5. Agricultural Biotechnology 2018Conclusions and Discussion
The reconciliation between high yield and anti??reverse contradiction is the focus of many studies. High??yield cultivation should ensure sufficient biological yield under the premise of ensuring a certain number of populations， especially for large ears， and higher nitrogen fertilizer input is one of the necessary conditions for high yield， but increases the risk of lodging[5-6]. This study shows that different nitrogen application rates have significant effects on the thrust??resistance strengths of rice plants， and with the increase of nitrogen application rates， the thrust??resistance strengths of both varieties decreases， namely， the lodging resistance of the plant decreased， which was consistent with the previous results by using lodging index to measure the resistance to lodging[6-7].
The lodging resistance of rice is the result of the interaction of the physical traits of the basal stems. Any change in traits can affect the lodging resistance of the plant. Since the basal internode length is in significantly positive correlation with the lodging trait， while the basal internode thickness， wall thickness and section area is in significantly negative correlations with the lodging trait， which play an important role in the lodging resistance of rice， stem thickness and internode length have been used as the lodging resistance indices[8-9]. This study shows that with the increase of nitrogen fertilizer application， the stem cross??sectional area of Guangliangyou 1128 also increases gradually， which is significantly negatively correlated with plant thrust resistance （r=-0.834 3**）， while that of Zhunliangyou 527 increases first and then decreases， which is positively correlated with plant thrust resistance （r=0.157 6）. It indicates that the amount of nitrogen fertilizer had different effects on the growth and development characteristics of the 2 rice varieties with different lodging resistances， which leads to different trends in the cross??sectional area， namely the stem thickness， of different varieties， and it also suggests that the 2 rice varieties have different self??generated nitrogen utilization efficiencies. For Guangliangyou 1128， as a variety with high nitrogen utilization efficiency， with the increase of nitrogen application rate， the absorption and transport capacities of nutrients in the plants gradually enhanced， the stems are thickened， the internode vascular bundle area is increased， and the number of vascular bundles is increased， enhancing the toughness of the stem[5，10]. Vascular bundles are the main channel for the transport of photosynthetic products， mineral nutrients and water. The number， size and function of these vascular bundles are closely related to the transport of these materials， especially the transfer of photosynthetic products into grains. Therefore， the application of nitrogen fertilizer can promote the material metabolism in rice stems， reduce stem??sheath filling degree， increase panicle weight， shift upward the relative height of gravity center， resulting in an increase in the load weight of the aboveground stem of the plants， thereby reducing the lodging resistance of the plant[3， 11-12]. On the other hand， Zhunliangyou 527 is a nitrogen??inefficient variety. The genetic characteristics determine that the optimal source??flow pool is coordinated under low nitrogen levels， and the morphological characteristics， physiological characteristics and anatomical structure of the stems achieve the optimal combination. The increase of nitrogen application rate to over 240 kg/hm2 can result in the excessive growth of plant， increase of plant height， weak stems， decrease of stem??sheath filling degree， thereby reducing the carrying capacity of aboveground stems， and weakening the lodging resistance of plants. Therefore， in cultivation and breeding， attention should be paid to the stem wall thickness and internode filling degree， and increasing the carrying capacity of the stems is the basis for improving the lodging resistance of rice. References
[1] LI J， ZHANG HC， GONG JL， et al. Effects of different planting methods on the culm lodging resistance of super rice[J]. Scientia Agricultura Sinica， 2011， 44 （11）： 2234-2243.
[2] YANG HJ， YANG RC， LI YZ， et al. Relationship between culm traits and lodging resistance of rice cultivars[J]. Fujian Journal of Agricultural Sciences， 2000， 15（2）： 1-7.
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