Effects of free-air CO2enrichment on adventitious root development of rice under low and normal soil nitrogen levels

*,Jinguo Zhu

aKey Laboratory of Crop Genetics and Physiology of Jiangsu Province,College of Agriculture,Yangzhou University,Yangzhou 225009,China

bInstitute of Soil Science,Chinese Academy of Sciences,Nanjing 210008,China

Effects of free-air CO2enrichment on adventitious root development of rice under low and normal soil nitrogen levels

Chengming Suna,Lijian Wanga,Tao Liua,Doudou Guoa,Yingying Chena,Wei Wua, Yulong Wanga,*,Jianguo Zhub

aKey Laboratory of Crop Genetics and Physiology of Jiangsu Province,College of Agriculture,Yangzhou University,Yangzhou 225009,China

bInstitute of Soil Science,Chinese Academy of Sciences,Nanjing 210008,China

A R T I C L E I N F O

Article history:

Received 17 December 2013

Received in revised form

14 April2014

Accepted 12 May 2014

Available online 20 May 2014

Rice

Free air CO2enrichment(FACE)

Root number

Root length

Model

Free air CO2enrichment(FACE)and nitrogen(N)have marked effects on rice root growth, and numerical simulation can explain these effects.To further define the effects of FACE on root growth of rice,an experiment was performed,using the hybrid indica cultivar Xianyou 63.The effects of increasing atmospheric CO2concentration[CO2],200μmol mol-1higher than ambient,on the growth of rice adventitious roots were evaluated,with two levels of N:low(LN,125 kg ha-1)and normal(NN,250 kg ha-1).The results showed a significant increase in both adventitious root number(ARN)and adventitious root length(ARL)under FACE treatment.The application of nitrogen also increased ARN and ARL,but these increases were smaller thanthat under FACEtreatment.On the basis ofthe FACEexperiment,numerical models for rice adventitious rootnumber and length were constructed with time as the driving factor.The models illustrated the dynamic development of rice adventitious root number and length after transplanting,regulated either by atmospheric[CO2]or by N application. The simulation result was supported by statistical tests comparing experimental data from differentyears,and the modelyields realistic predictions ofrootgrowth.These results suggest thatthe models have strong predictive potentialunder conditions ofatmospheric[CO2]rises in the future.

©2014 Crop Science Society of China and Institute of Crop Science,CAAS.Production and hosting by Elsevier B.V.Allrights reserved.

1.Introduction

The rice root system is a vital organ for water and nutrient acquisition,and root number and activity affect the growth of aerial parts and economic yield[1].Rice roots are relatively short,and most are distributed in the plow horizon[2,3]. Differences in root distribution among different rice varieties have been found[4].The architecture of the root system isalso well known to be a major determinant of root function in the acquisition ofsoilresources,and the increase ofthe volume of the soils explored by the roots,as a result of continuous branching,may reflect the plant’s adaptive ability to make best use of unevenly distributed water and nutrients[5].In recent years,many studies of the effects of different water and fertilization levels on rice root growth have been performed. The growth process and distribution ofrice roots and the effects of various cultivation conditions on root system are described by the results of these studies.Under treatment with high nitrogen(N),the dry weightof roots was higher than that under low N fertilization,and moderate water favored the increase of root dry weight[2,5-10].

Free air CO2concentration is one of the important factors affecting root development[11-15].As it is difficult to alter CO2concentration under field conditions,most information on the responses of rice yield to elevated CO2has been obtained from studies in well-controlled non-field conditions, such as in greenhouses[16],soil-plant-atmosphere research units[17],temperature-gradient chambers[18,19]and open-top chambers[20].However,these experimentalconditions,which are different from natural growing environments(field conditions)in combination with the border effects associated with smallplots,have been shown to modify the responses of plants to increasing[CO2][21,22].FACEexperiments,conducted in fully open-air field conditions without altering microclimatic and biotic variables,represent our best simulations of the future high CO2environment.Over the last decade,only two largescale(12 m×12 m plots)replicated rice FACE experiments have been conducted worldwide(1997-2006).Both experiments used a similar FACE technology and employed the same target [CO2],570μmol mol-1[23-25].

There have been reports on the effects of elevated[CO2] and N supply on the growth,nutrient uptake,root development,and yield of inbred japonica cultivars[13,14,25-29],but no simulated prediction for root number and length has been made.Compared with conventional rice cultivars,hybrid rice cultivars exhibit better tillering ability,thus a relatively higher growth rate.The effects of FACE and Non root growth may be different.In the present study,the hybrid rice cultivar Shanyou 63,the mostwidely used hybrid rice variety in China for the past 15 years[30],was used to study the effects of FACE under two N levels on root number and length,and the results were used for model development.The models may provide information for root growth controland high-yield cultivation of rice.

2.Materials and methods

2.1.Experiment site and its weather

The experiment was conducted in Xiaoji,Yangzhou,Jiangsu, China(32°35′5″N,119°42′E)in 2005 and 2006.The farm used in this study had fluvisol soil(local name,Qingni soil)with annualmean precipitation of980 mm,evaporation of 1100 mm, temperature of 14.9°C,total sunshine hours of 2100 h,and frostfree period of 220 d.The physical and chemical properties of the soilwere as follows:soilorganic carbon(SOC)18.4 g kg-1, total N 1.45 g kg-1,total P 0.63 g kg-1,total K 14 g kg-1,available P 10.1 mg kg-1,available K 70.5 mg kg-1,sand(0.02-2.00 mm) 578.4 g kg-1,silt(0.002-0.020)285.1 g kg-1,clay(＜0.002 mm) 136.5 g kg-1,and pH 7.2.

2.2.FACE system

The FACE system comprised six FACE plots located in different fields with similar soil and agronomic histories.Of these plots, three were allocated for FACE experiments(hereafter called E-[FACE])and another three for ambienttreatments(hereinafter called A-[FACE]).To reduce the influence of CO2emission,the distance between E-[FACE]plots and A-[FACE]was more than 90 m.Each E-[FACE]plot was designed as an octagon with a largest diameter of 12.5 m.In the E-[FACE]plots,pure CO2gas was released from peripheral emission tubes and the[CO2]was about 570μmol mol-1.The FACE treatment was controlled by a computer system.A-[FACE]plots had no octagon structures and the[CO2]was about 370μmol mol-1.

2.3.Crop cultivation

Seeds of Shanyou 63 were sown in a nursery on May 20. Seedlings were manually transplanted at a density of one seedling per hill into E-[FACE]and A-[FACE]on 15 June.Hill space was 16.7 cm×25.0 cm(equivalent to 24 hills m-2).Two levels of N were supplied as urea:low(LN,125 kg ha-1)and normal(NN,250 kg ha-1).Half of the E-[FACE]and A-[FACE] plots had the LNregime and the other half NN.Nwas applied as basalfertilizer one day before transplanting,as side-dressing at early tillering on 21 June(60%of the total),and at panicle initiation on 28 July(40%).Phosphorus(P)and potassium(K) were applied as basal fertilizer at equal rates of 70 kg ha-1on June 14.The paddy fields were flooded with water(about 5 cm deep)from June 13 to July 10,drained severaltimes from July 11 to August 4,and then flooded intermittently from August 5 to 10 d before harvest.Disease,pests and weed were controlled according to standard practice.

2.4.Plant sampling and measurements

Fifty hills from different locations(three locations in each subplot)were selected to record the number of tillers at 14,25, 44,56,73,and 90 d after transplanting.At the same time,a soil block around a plant with dimensions 25.0 cm×16.7 cm× 20.0 cm was removed.The number of adventitious roots and total root length in every hill were recorded after washing with pure water.

2.5.Data analysis and tests

The experiment data was analyzed by MATLAB software and Microsoft Excel 2003.The root mean square error(RMSE)and relative root mean square error(RRMSE)between observed value and simulation value were used to describe the precision ofthe model.A 1:1 relation graph ofthe observed and simulated values was drawn based on this model.RMSE and RRMSE were expressed as follows:

where Oidenotes the observed value and Sithe simulated value. Oadenotes the mean of the observed values.n denotes the sample size.

3.Results

3.1.Dynamicchanges in numberand totallength ofadventitious roots per hillin FACE treatment

FACE treatment significantly increased the number and total length ofadventitious roots per hill(Fig.1).The increase in root number was 25.1,19.8,and 15.9%,respectively,at tillering, jointing,and heading stages and the root length increases were 25.3,23.8,and 29.2%,respectively.In contrast,N showed much lower effects on both the number and total length of adventitious roots per hill,although NN tended to increase the number and total length of adventitious roots.The increases in root number were 9.3,4.0,and 11.5%,respectively,at tillering,jointing and heading stages under Ntreatment,and the increase ratios of root length were 10.8%,5.5%,and 12.2%respectively.The changes in ARN and ARL per hillshowed an S curve under both FACE and AMB(ambient CO2)treatments under different Nrates (Fig.1).

Fig.1-The number(a)and total length(b)of adventitious roots per hill of FACE-treated rice.LN:low nitrogen;NN: normal nitrogen;FACE:free air CO2enrichment;AMB: ambient CO2.

3.2.Model of ARN

3.2.1.Normalstate modelof ARN

Based on the above results,inwhich the number ofadventitious roots per hillchanged following an S curve under both FACE and AMB conditions,an improved logistic equation may be used to describe the model:

where RNambdenotes the number of adventitious roots (number hill-1)ata certain time t;RNmaxdenotes the maximum number of adventitious roots(number hill-1);a1,b1,and c1are modelcoefficients.

3.2.2.Influence of FACE

FACE treatment markedly increased ARN,and trends among the different treatments were consistent(Fig.1).Accordingly, a generalduty modelmay be applied to describe the influence of CO2[31]:

where FCO2denotes the effect coefficient of CO2,Cxrepresents future atmospheric CO2concentration (μmol mol-1),C0represents the CO2concentration of ambient treatments (370μmol mol-1),and k1is a model coefficient with a value of 0.391(based on 2006 statistics).

3.2.3.Influence of N

Combining the previous studies with the results of this experiment,the effect coefficient of N may be calculated as follows[31]:

where FNdenotes the effect coefficient of N application rate (values between 0 and 1)and NAA denotes the N application rate(g m-2).

From the above,the model(RNface)of ARN may be described as follows:

3.3.Modelof ARL

3.3.1.Normalstate modelof ARL

The change of ARL was similar to that of ARN,and the improved logistic equation was accordingly suitable:

where RLambdenotes the total length of adventitious roots (m hill-1)at time t,RLmaxdenotes the maximum length of adventitious roots per hill,and a1,b1,and c1are model coefficients.

3.3.2.Influence of FACE

The influence on ARL was congruent with the results of ARN:

where FCO2is the effect coefficient of CO2;Cxrepresents the future atmospheric CO2concentration(μmol mol-1);C0represents the CO2concentration of ambient treatments (370μmol mol-1),and k2is a model coefficient with the value 0.618 according to Sun et al.[31].

3.3.3.Influence of N

The equation of the N effect coefficient is consistent with Eq.(3).

From the above,the model(RLface)of ARL is described as follows:

3.4.Parameter estimation

Parameters of the equations were calculated by successive fitting of a nonlinear equation with the contraction-expansion algorithm[32],aiming to reach a degree of optimization by minimizing the sum of squares of deviations(SS)between observed and simulated values.Based on the experimentaldata in 2006,parameters were calculated as follows(Table 1).

3.5.Model test

3.5.1.ARN model test

The data observed in 2005 were used to test the ARN model in this study.The results demonstrated that there was a good correlation between the simulated values from the 2006 experiment and the observed values from the 2005 trial,with R2for both NN and LN treatments under the AMB condition high and significant(0.982 and 0.983,respectively,P＜0.01). The correlation coefficients between simulated and observed values were also significant under FACE conditions(0.981 for NN and 0.977 for LN treatment,P＜0.01).The RMSE values for the NN and LN treatments were 32.168 and 30.134,respectively, under AMB,and 34.118 and 36.316 under FACE.The RRMSE values for NN and LN treatments were 0.056 and 0.053, respectively,under AMB,and 0.051 and 0.055 under FACE. Fig.2 shows that the simulated values estimated from the 2006 trial are in good agreement with the observed values obtained from the 2005 trial. Table 1-Parameter values of models for number and length of adventitious roots(NN).

Model parameter

Length of

adventitious roots

Max value 751.291 64.495 a11.728 4.335 b1-0.050 -0.178 c1-0.0009 -0.0003 R20.993 0.994

Number of

adventitious roots

Fig.2-Comparison between the simulated(2006 trial)and observed(2005 trial)number of roots under FACE treatment.

3.5.2.ARL model test

The simulated results for ARL were also significantly correlated with the observed results from the 2005 trial,with R2of 0.952 and 0.959 for the NN and LN treatments,respectively,under AMB and 0.958 and 0.957 under FACE.The RMSE values of the NN and LNtreatments were 5.470 and 4.835,respectively,under AMB and 7.732 and 6.971 under FACE.The RRMSE values of the NN and LN treatments were 0.109 and 0.102,respectively, under AMB and 0.132 and 0.122 under FACE.Fig.3 shows that the simulated values based on the 2006 trial showed great coherence with the observed values from the 2005 trial.

4.Discussion

Fig.3-Comparison between the simulated(2006 trial)and observed(2005 trial)length of roots under FACE treatment.

Various factors affect the growth and development ofrice roots. They include soil type,permeability,type and application rates of fertilizers,fertilization time,irrigation methods,and climate conditions as well as the genetic backgrounds of rice varieties [4,6,33,34].Root number,rootlet number,and dry weight increase with enrichment[CO2][11,35].The FACE system has beenused to investigate the influence ofincreasing atmospheric CO2on rice root growth.Kim[36]showed that FACE treatmentstrongly enhanced the root dry weight of medium maturing japonica rice in Japan.Other researchers showed that ARN and ARL of Wuxianggeng 14(japonica rice)were significantly enhanced under FACE condition atseedling stage,jointing stage and heading date[26,29].Hydroponic experiments gave similar results[12,37].In the present study,results from fully open-air conditions also showed that FACE treatment strongly enhanced the number andtotallength ofadventitious roots of Shanyou 63, consistent with previous results[13].

Previous studies in root growth found that the process of root growth closely followed a sigmoid curve[38-40].Quantitative models for root growth have also been reported[41-44].But previous models for root growth have been constructed under hydroponics or pot cultivation conditions and did not consider effects of[CO2]enrichment.The present study showed that changes in ARN and ARL under both FACE and AMB conditions tended to follow a sigmoid curve.Results under different N rate treatments were also consistent.

Studies of the effects of FACE treatments on rice morphological features are rare.Yang(2008)showed that elevated [CO2]increased adventitious root length and adventitious root number at all developmental stages by 25-71%,a response associated mainly with increased root growth rate during the early growth period and a lower rate of root senescence during the late growth period[45].Chen et al.showed that elevated [CO2]significantly increased root biomass during the whole growth season[12].We studied numericalmodels ofrootvolume and adventitious root dry weight,but simulation models for root number and totallength have not been reported[46].This study used a modified logistic equation to simulate effects on rice ARN and ARL under FACE treatment.The results also showed that there was a good correlation between simulated and observed values.R2values varied from 0.952 to 0.983, reaching significant level.RRMSE ranged from 0.051 to 0.132, indicating that results were reliable.Limited by the conditions of the experiments,two factors were involved in this model: CO2concentration and N rate.Because the results depend mainly on statistical models,the mechanism by which FACE affects rice roots is unclear and awaits further investigation.

Acknowledgements

This work was funded by the National Natural Science Foundation of China(No.30270777),the Key Direction Research of Knowledge Innovation in Chinese Academy of Science (No.KZCX3-SW-440)and the Priority Academic Program Development of Jiangsu Higher Education Institutions.The main instruments and apparatus of the FACE system were supplied by Japan National Institute for Agro-Environmental Sciences(NIAES)and Japan Agricultural Research Center for Tohoku Region(NARCT).

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*Corresponding author.

E-mail address:ylwang@yzu.edu.cn(Y.Wang).

Peer review under responsibility of Crop Science Society of China and Institute of Crop Science,CAAS.