Effects of Density and Nitrogen Application Rate on Population Structure and Yield of Early-maturing Late Japonica Tongjing 981

Genyou ZHOU Bo WANG Hua XIA

Abstract The study adopted split block design with different treatment levels of density and fertilizer to investigate the growth period， population tiller dynamics， leaf area， dry matter accumulation， plant traits， panicle-grain structure， yielding ability and stress resistance of Tongjing 981 under different density and nitrogen fertilizer levels， so as to make clear the effects of different densities and nitrogen fertilizer levels on the population development and yield of Tongjing 981. The results showed that a too-low density was not conductive to the formation of sufficient number of panicles， and when the density was too high， it affected the number of grains per panicle and 1 000-grain weight; and when nitrogen fertilizer was at a too-low level， it would restrict the population development， and a too-high nitrogen fertilizer easily led to the extension of the growth period and the aggravation of sheath blight. Reasonable density and fertilization level could make the development of individuals in Tongjing 981 population coordinated， and further help to establish a reasonable population structure， maintain a high leaf area and dry matter accumulation during the filling period， and balance the relationship of population dry matter weight with economic coefficient and panicle， grain and weight， thereby improving the population quality and yield of Tongjing 981.

Key words Tongjing 981; Density; Nitrogen application rate; Yield

In addition to genetic factors， rice yield is also affected by ecological environment and cultivation measures. Among them， planting density and nitrogen nutrition level have the most significant impact on rice yield. Appropriate density is conducive to the establishment of a reasonable population structure， which enables the coordinated development of yield components. Nitrogen plays an important role in promoting rice growth and development， increasing the tillering rate and improving rice quality and yield components. Therefore， determining the appropriate rice planting density and nitrogen application rate is of great significance to ensure the high and stable yield of rice. Tongjing 981 is a new high-yield， high-quality， and multi-resistance early-maturing late japonica rice variety bred by Institute of Agricultural Science of the Region Along the Yangtze River in Jiangsu Province. Its average yield was higher than the control by 7.2% in regional test of new rice varieties in Jiangsu Province， and higher than the control by 10.7% in the production test. It was approved by the Jiangsu Crop Breeds Examination and Approval Committee in 2011. This variety has the characteristics of strong resistance to stripe virus disease and sheath blight， strong lodging resistance， high yield， and excellent rice quality. It was identified as the main rice variety in Jiangsu Province by the Jiangsu Agricultural Commission in 2012. This study aimed to explore the effects of different densities and different fertilization levels on the population development and yield of Tongjing 981， and to obtain suitable nitrogen fertilizer application rate and reasonable planting density for achieving high yield， with a view to providing a theoretical basis for formulating high-yield cultivation techniques and measures and promoting the application of Tongjing 981 in large scale in areas suitable for rice cultivation along the river.

Materials and Methods

Materials and experimental design

The experimental material was Tongjing 981. The experiment adopted split zone design with 2 factors of different levels[1]. Factor A was the density （basic seedlings） treatment， set with four treatment levels， respectively 600 000 plants/hm2 （A1）， 900 000 plants/hm2 （A2）， 1.2 million plants/hm2 （A3）， 1.5 million plants/hm2 （A4）. Factor B was the treatment of pure nitrogen application rate， set with four treatment levels as well： 150 kg /hm2 （B1）， 225 kg/hm2 （B2）， 300 kg/hm2 （B3） and 375 kg/hm2 （B4）. There are a total of 16 treatment combinations （Table 1）. The experiment was repeated three times， and included 48 plots， each having an area of 13.4 m2.

In order to prevent the nitrogen fertilizer from leaking between treatments， ridges were formed between plots， and covered with plastic films， which were buried deeply on both sides of the ridges[2]. Each plot was independently irrigated. There were protection lines around the plots， and the application rates of phosphorus and potassium fertilizers and other cultivation measures were consistent among the treatments.

General situation of the experimental field

This experiment was conducted in 2014 on the experimental field of Institute of Agricultural Science of the Region Along the Yangtze River in Jiangsu Province. The soil was sandy loam， with medium to high fertility， and the previous crop was wheat. The seedling raising method of Tongjing 981 was conventional wet seedling raising. Sowing was performed on May 19 （seeds were soaked for 2 d before sowing， and then pre-germinated until whiten buds exposed）. The cultivated strong seedlings were transplanted on June 21. The fertilizer and density levels of each treatment were strictly in accordance with the design requirements. In terms of fertilizer management， the ratio of the base and tillering fertilizer to panicle-grain fertilizer was 6∶4. The field management was the same as the high-yield fields， and the prevention and control of rice planthoppers should be done well in the later stage.

Trait determination

The tillering dynamics were observed at a fixed point， and 10 holes were continuously surveyed from the third hole in the third longitudinal row at the same position in each plot. The total number of tillers was recorded. The survey was conducted once every 10 d after transplanting， 5 times in total.

The growth process was recorded in different plots， focusing on the dates of sowing，   transplanting， full heading （the date when 80% of the rice panicles are exposed to the leaf sheath）， maturation date （the date when 95% or more of the grains are yellow and ripe）， and the days of the full growth period （the days from the next day of sowing to the maturation date）. The leaf area and dry matter weight were determined at 20 d after planting （July 10）， 40 d after planting （July 30）， the heading stage （August 30）， the filling stage （September 30） and the maturation stage （October 30） by plot. Representative plants were sampled from 20 successive holes in each plot at each time. The leaf area was measured by the length-width method （Leaf area=Leaf length×Leaf width×0.75）. The dry matter weight was determined by the whole plant dry matter weight. During sampling， the root loss should be reduced as much as possible. The samples were treated on the very day of sampling. They were first baked in a 105 ℃ oven for 30 min， and then baked for about 8 h after reducing the temperature to 60-80 ℃. After taking out， the weight was measured after the temperature was reduced to normal temperature.

During the maturation stage， the incidence and disease index of sheath blight in each plot were investigated. Twenty 20 successive holes were sampled for indoor seed testing to determine plant traits and panicle-grain structure. After maturation， harvesting and yield determination were performed by plot.

Data processing

Excel 2007 and SigmaPlot 10.0 were used for data processing， analysis and drawing. The calculation formula of the correlation coefficient between each economic trait and the test factor was r=∑xy/∑x2∑y2. The correlation significance was based on v=n-2=14， r0.05=0.497， r0.01=0.623[3]. In the formula， r is the correlation coefficient; and x and y are the deviations from the average of the variables.

Results and Analysis

Effects of density and nitrogen application rate on growth period of Tongjing 981

It could be seen from Table 2 that under different densities and nitrogen fertilizer treatments， Tongjing 981 had obvious differences in the growth process. Between different fertilization levels of the same density， the maturation period was postponed with the amount of nitrogen applied increasing. Between different densities at the same fertilization level， the maturation period of Tongjing 981 got early with the increase in the number of basic seedlings overall[4]. Under the combined effect of density and fertilizer， the maturation periods of treatments A3B1 and A4B1 were the earliest， and the full growth period was 159 d， which might be that under the low nitrogen fertilizer level of B1， the higher the plant density was， the less nitrogen fertilizer single plant acquired， and the growth period was thus shortened to relieve competition for limited nitrogen fertilizer. The maturation periods of treatments A1B4 and A2B4 were the latest， and the full growth period was 165 d， which might be due to the high nitrogen fertilizer level and the low plant density， which resulted in more nitrogen fertilizer per plant， which further caused the condition that the plants remained green longer and ripened later. It showed that the lower the density and the higher the amount of nitrogen application， the more sufficient the nutrients per plant， the longer the growth period， and the shorter the growth period.

Effects of density and nitrogen application rate on the growth and decline dynamics of tillers in Tongjing 981 population

It could be seen from Fig. 1 to Fig. 6 that different density fertilizer levels had greater effects on tillering， tiller growth and decline and productive tiller percentage of Tongjing 981. Under all the different treatments， the total tiller number of the population during the growth period of the rice showed a trend of first increasing and then decreasing， reached a maximum around 40 d （July 30） after planting， and then decreased following the polarization of tillering. The overall change trend among different treatments was that the tiller density and the number of panicles increased as the number of basic seedlings and the amount of nitrogen applied increased. The correlation analysis results showed that the correlation coefficients of the highest tiller density with the basic seedling number and nitrogen application rate were 0.911 0** （"**" indicates the extremely significant level） and 0.382 7， respectively， and the correlation coefficients of panicle density （panicle number） with the basic seedling number and nitrogen application rate were 0.819 5** and 0.509 9* （"*" means reaching a significant level）， respectively.

From the perspective of productive tiller percentage， when the number of basic seedlings was small， the productive tiller percentage increased with the increase of nitrogen application rate; when the basic seedling number was large， the productive tiller percentage increased first and then decreased with the increase of nitrogen application rate; and when the amount of nitrogen applied was fixed， the productive tiller percentage decreased with the increase in the number of basic seedlings overall （Fig. 6）. The correlation analysis showed that the correlation coefficients of the productive tiller percentage with the basic seedling number and nitrogen application rate were -0.931 2** and -0.014 7， respectively.

Effects of density and nitrogen application rate on leaf area of Tongjing 981 population

It could be seen from Fig. 7 to Fig. 10 that the leaf area of the rice population showed a single-peak curve with the growth period under different treatments， and it showed the highest value at the heading stage （August 30）， and then decreased. Moreover， the leaf area of the population generally showed an upward trend with the increase in the number of basic seedlings and the amount of nitrogen application in different treatments. The correlation analysis showed that the correlation coefficients of population leaf area at the heading stage with the basic seedling number and nitrogen application rate were 0.480 1 and 0.873 6**， respectively. However， under high-density and high-fertilizer conditions， the Tongjing 981 population was too large， and the plants were shaded by each other， resulting in poor ventilation and light transmission conditions， early decay of the lower leaves， a sharp decrease in the leaf area of the population and a shift in the center of gravity of the green leaves[1]. It could be seen that high density and high fertilizer were beneficial to increase the leaf area， but only under suitable dense fertilizer conditions could a large leaf area be maintained during the filling period.

Effects of density and nitrogen application rate on dry matter accumulation of Tongjing 981

It could be seen from Fig. 11 to Fig. 14 that with the progress of the growth process， the accumulation of dry matter in each treatment continued to increase， reaching a maximum value during the maturation period， showing an approximately "S" shaped curve growth. The dry matter accumulation was slower within 40 d after transplanting （July 30）， and it accumulated faster from 40 d after transplanting （July 30） to the maturation stage. In the same growth stage， the total dry matter accumulation of the population showed an upward trend as the number of basic seedlings and the amount of nitrogen application increased in different treatments. The economic coefficient decreased with the increase in the number of basic seedlings and the amount of nitrogen applied （Fig. 15）. Through correlation analysis， the correlation coefficients of dry matter accumulation during the maturation period with the number of basic seedling and nitrogen application rate were 0.388 0 and 0.800 2**， respectively， and the correlation coefficients of economic coefficient with the number of basic seedlings and nitrogen application rate were -0.568 1* and -0.762 0**， respectively. It could be seen that increasing the number of basic seedlings and the amount of nitrogen applied is conducive to increasing the dry matter accumulation of the population， but not conducive to improving the economic coefficient.

From the dry matter accumulation in the filling stage （heading stage to maturation stage） （Fig. 16）， when the number of basic seedlings was small， the accumulation of dry matter increased with the increase of nitrogen application; and when the number of basic seedlings was large， the accumulation of dry matter showed a trend of first increasing and then decreasing with the increase of nitrogen application. When the nitrogen application rate was small， the dry matter accumulation amount increased with the increase of the basic seedling number; and when the nitrogen application rate was large， the dry matter accumulation increased first and then decreased with the increase of the basic seedling number. However， there was a very significant positive correlation between dry matter accumulation during the filling stage （from the heading stage to maturation stage） and yield （r=0.944 2**）. It could be seen that increasing the number of basic seedlings and the amount of nitrogen applied was conducive to increasing the dry matter accumulation of the population， but was not conducive to improving the economic coefficient.

Effects of density and nitrogen application rate on stalk weight， root-shoot ratio and incidence of sheath blight of Tongjing 981

It could be seen from Table 3 that plant height and basal internode length increased with the increase of the number of basic seedlings and the amount of nitrogen applied， while the stalk diameter generally decreased with the increase of the number of basic seedlings. Under different nitrogen application conditions， the stalk diameter of rice increased first and then decreased with the increase in nitrogen application. The stalk diameters of treatments A1， A2 and A3 were the largest at the nitrogen fertilizer level of B3， while the maximum stalk diameter of treatment A4 was at B2 nitrogen fertilizer level. Moreover， the root-shoot ratio decreased with the increase of the number of basic seedlings and nitrogen application rate.

Through correlation analysis， the correlation coefficients of plant height with the basic seedling number and nitrogen application rate were 0.439 6 and 0.884 4**， respectively， and the correlation coefficients of the total internode length of the basal first node and basal second node with the basic seedling number and nitrogen application rate were 0.723 7** and 0.680 5**， respectively， while the correlation coefficients of the stalk diameter with the basic seedling number and nitrogen application rate were -0.713 0** and -0.034 0， respectively， and the correlation coefficients of the root-shoot ratio with the basic seedling number and nitrogen application rate were -0.771 3** and -0.589 6**， respectively.

Under different density and fertilizer conditions， plant traits were closely related to the environmental conditions of the population. When the density and fertilizer level were low， the Tongjing 981 population had an insufficient number; and when the density and fertilizer level were high， the tiller density was large， and the number of population was large， resulting in poor ventilation and light transmission conditions， poor population environment， poor individual development， thinner stalk diameter， longer basal internodes and worse stress resistance[5]. According to observations， the occurrence of field sheath blight in Tongjing 981 gradually increased with the increase in the number of basic seedlings and the amount of nitrogen applied （Table 3）. Under the conditions of this study， the individuals in treatments A3B3， A2B3 and A2B4were relatively coordinated， the tiller density was more suitable， and the population quality was better， so higher yields were obtained.

Effects of density and nitrogen application amount on yield and composition of Tongjing 981

It could be seen from Table 4 that different density and fertilization amount had greater effects on the yield of Tongjing 981. Through analysis of variance， the yield of Tongjing 981 showed differences between different treatments. From the perspective of yield structure， with the increase in the number of basic seedlings and the amount of nitrogen applied， the panicle number of Tongjing 981 showed an overall upward trend （the correlation coefficients were 0.818 1** and 0.514 0*， respectively）， while the number of filled grains per panicle and 1 000-grain weight decreased with the increase of the number of basic seedlings （the correlation coefficients were -0.800 0** and -0.592 8*， respectively）. When the density was low， the number of filled grains per panicle and 1 000-grain weight increased with the increase of nitrogen application rate in A1 and A2， while under high densities， the number of filled grains per panicle and 1 000-grain weight had a trend of increasing first and decreasing then with the increase in the amount of nitrogen in A3 and A4. The results showed that Tongjing 981 had fewer panicles under low-fertilization and low-density conditions， and lower number of grains per panicle and 1 000-grain weight under high-fertilizer and high-density conditions， so high yield could not be obtained. Only when the density and fertilizer level are more appropriate and the panicle， grain， and quality are coordinated， higher yields can be obtained[6-8]. Under the conditions of this study， the density and fertilizer combination of treatment A3B3 was the most reasonable and the yield level was the highest at 12 022.5 kg/hm2.

Conclusions and Discussion

The results of this study showed that reducing the planting density and properly increasing the amount of nitrogen fertilizer were beneficial to the individual development of Tongjing 981 by increasing stalk diameter， increasing the number of panicles per plant， the number of grains per panicle and 1 000-grain weight， improving economic coefficient and enhancing disease resistance， while too-low density would cause too-small population and insufficient number of panicles， thereby affecting yield. Increasing the density and increasing the amount of fertilization were conducive to the increase in population number and the acquisition of sufficient panicles， as well as the increase in leaf area and dry matter accumulation. However， when the density and fertilizer level were too high， it caused the population to be too large， which restricted the individual development of the plant and reduced the number of grains per panicle and 1 000-grain weight， and it easily led to the extension of the growth period and the aggravation of sheath blight， but also was not conducive to the increase in yield. Reasonable density and fertilization level could make the development of individuals in Tongjing 981 population coordinated， and further help to establish a reasonable population structure， maintain a high leaf area and dry matter accumulation during the filling period， balance the relationship of population dry matter weight with economic coefficient and panicle， grain and weight， thereby improving the population quality and yield of Tongjing 981. Under the conditions of this study， when the basic number of seedlings was about 1.2 million plants/hm2 and the nitrogen application rate was about 300 kg/hm2， the density and fertilizer level were more reasonable and the yield level was the highest. Different cultivation management measures should be determined according to different cultivation conditions in production. When transplanting， we should increase the basic seedlings in fields with poor soil fertility， and reduce the basic seedlings in fields with good soil fertility. During field management， nitrogen fertilizers should be appropriately applied to fields with insufficient basic seedlings， and nitrogen fertilizers should be appropriately reduced to fields with more basic seedlings， so as to keep the basic seedlings and the amount of nitrogen fertilizer close to a reasonable level as much as possible and finally achieve the purposes of promoting reasonable development of population individuals and increasing yield.

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