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Field Crops Research 155 (2014) 245–253

Contents lists available at ScienceDirect

Field Crops Research

jou rn al hom epage: www.elsev ier .com/ locate / fc r

rowth of soybean seedlings in relay strip intercropping systems inelation to light quantity and red:far-red ratio

eng Yanga,b, Shan Huanga, Rencai Gaoa, Weiguo Liua, Taiwen Yonga, Xiaochun Wanga,iaoling Wua, Wenyu Yanga,b,∗

College of Agronomy, Sichuan Agricultural University, Chengdu 611130, PR ChinaKey Laboratory of Crop Ecophysiology and Farming System in Southwest, Ministry of Agriculture, Chengdu 611130, PR China

r t i c l e i n f o

rticle history:eceived 8 August 2013eceived in revised form 16 August 2013ccepted 16 August 2013

eywords:ntercroppingightaize

hadingoybean growth

a b s t r a c t

Maize–soybean intercropping is a common system in several countries. However, different spatial pat-terns of maize and soybean can directly affect the light environment of soybean growth under this systemthrough the combined effects of the altered light quality and the reduced light quanta. This work aimedto investigate the differences in the light environment of the soybean canopy in terms of the red:far-red(R/FR) ratio and the photosynthetically active radiation (PAR) as well as the different rates of soybeanseedling growth under maize–soybean relay strip intercropping and soybean sole planting, to analyze therelationship between the morphological characteristics and the light environment, and to assess the rel-ative contributions of the R/FR ratio and PAR transmittance to soybean seedling growth in intercroppingconditions.

Field experiments were performed in 2011–2013. The intercropping patterns involved the wide-narrow row planting of alternating maize and soybean. The light environment of the soybean canopyand the morphology of the soybean seedlings were estimated in the relay strip intercropping systemby changing the distances of the maize and soybean rows as well as the number of maize vs. soybeanrows per strip. These parameters of the intercropping system were compared with those of the soybeanmonocultures. Furthermore, the relationship between the light environment of the soybean canopy andits morphological parameters were analyzed using correlation analysis.

Incident light in maize–soybean relay strip intercropping systems was partly reflected and absorbed bymaize leaves. Thus, the spectral irradiance, R/FR ratio, and PAR of the soybean canopy were decreased withmaize–soybean intercropping as compared to soybean monocropping. Simultaneously, the stem diam-eter, root length, aboveground biomass, total root biomass, and root–shoot ratio of relay intercroppedsoybean were reduced significantly, while its seedling height was increased. The correlation relationshipbetween morphological parameters of soybean and the light environment (R/FR ratio and PAR transmit-tance) in different planting pattern were significant (P < 0.05). Compared to PAR transmittance, the R/FRratio of the relay intercropped soybean canopy was strongly correlated with morphological parametersof soybean seedling (P < 0.01), and the correlation coefficients were higher than 0.88. The response of

soybean seedlings to shading by maize was not solely influenced by the PAR or the R/FR ratio. It may bethe summed effects of both parameters under relay strip intercropping systems. Therefore, the resultsreveal the physiological response mechanisms of soybean seedlings to changes in the quality and amountof light, which may support the building three-dimensional growth model of the responses of plant tolight quantity and quality, and guide the identification of suitable population planting patterns in theintercropping system in the future.

∗ Corresponding author at: College of Agronomy, Sichuan Agricultural University,uimin Road 211, Wenjiang District, Chengdu 611130, PR China.el.: +86 28 86290960; fax: +86 28 86290870.

E-mail addresses: wenyu.yang@263.net, yangfeng1130@yahoo.com.cnW. Yang).

378-4290/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.fcr.2013.08.011

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The amount of cultivable land is gradually decreasing becauseof the rapid urbanization and industrialization caused by the global

population explosion (Awal et al., 2006). The demand for foodis ever-increasing with the increasing population. Consequently,increasing the multiple crop index of land is particularly impor-tant for the development of grain production (Yan et al., 2010).

2 Resea

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46 F. Yang et al. / Field Crops

elay cropping and intercropping are important in subsistencend food production worldwide (Ofori and Stern, 1987; Rodriguez-avarro et al., 2011). These planting patterns often outyield their

ole crop components because of the more efficient use of resourcess well as the reduced incidence of weeds, insect pests, and diseasesCaviglia et al., 2004; Echarte et al., 2011).

The combination of partner crops in intercropping systemsepends mainly on the crop geometry, but the growth habit, lifepan, and management practices of the crops likewise influence thisombination (Connolly et al., 2001; Awal et al., 2006). Cereal andegume intercropping is recognized as a common cropping systemn several countries (Ofori and Stern, 1987). In China, half of the totalrain yield is produced with multiple cropping, maize–soybeanelay strip intercropping systems in particular (Fig. 1) are one ofajor planting patterns in the southwestern regions (Yan et al.,

010; Zhang et al., 2011).The maize in maize–soybean relay strip intercropping systems

s usually sown according to the narrow-wide row planting patternt the end of March or the beginning of April and harvested at thend of July or the beginning of August. Soybean is sown in the wideows between maize at the beginning of June and harvested at thend of October. The seedling phase of soybean and the reproduc-ive phase of maize overlap over a period of approximately eighteeks between the sowing of soybean and the harvesting of maize.

hus, the two different crop species can be grown during one sea-on in production areas where the growing season is too short forouble cropping. This system increases the productivity as it takesdvantage of the biological N fixation by soybean, thereby reducinghe demand for N-containing fertilizers (Stern, 1993). Aside fromutrient acquisition, the additional components such as the borderow effects contribute to the overyielding of intercropped maizeKnörzer et al., 2009).

The planting patterns of maize-soybean intercropping systemsan induce changes in the microclimate environment within therop canopy, particularly in the light intensity and the spec-ral properties of the soybean canopy with its lower layer (Awalt al., 2006). Soybean is highly sensitive to shading (Wolff andoltman, 1989). Thus, when grown with maize shading underhis system, soybean seedlings have increased height and thin-er stems that are easier to lodge. Therefore, an analysis of theelationships between the light environment and the morpholog-cal characteristics of relay intercropped soybean is important touild three-dimensional growth model and determine the suitableopulation pattern and parameters in the future.

The light quality, which is measured by the red:far-red (R/FR)atio, and the light quanta, which is represented as the photosyn-hetically active radiation (PAR), are two main light properties inhaded environments that can modify plant growth and develop-ent (Smith, 2000). Light that has passed through a canopy is rich in

R light but poor in R light (Evers et al., 2006). Chlorophyll depletes light, whereas FR light is predominantly reflected and transmit-ed by the pigment (Vandenbussche et al., 2005). Plants that canetect a low R/FR ratio will initiate a series of physiological changesnd consequently express shade avoidance characteristics such asncreased stem elongation, reduced stem diameter, and decreasedoot biomass (Page et al., 2010; Afifi and Swanton, 2011). Simi-arly, the reduction of crop plant productivity can be induced byhese changes in light quality (Ruberti et al., 2012). Kasperbauernd Karlen (1986) reported that wheat seedlings exposed to FR hadigher shoot/root ratios than unshaded plants. Pechácková (1999)

ound that a reduced R/FR ratio decreased the root biomass of ahizomatous grass species.

Most of the literature has confirmed the trend of changesn the plant height, stem diameter, root biomass, and above-round biomass under low R/FR ratios or shade stress conditionsKasperbauer and Karlen, 1986; Pechácková, 1999; Page et al., 2010;

rch 155 (2014) 245–253

Afifi and Swanton, 2011; Ruberti et al., 2012). However, thesestudies only explained the morphological changes under shadingconditions or low R/FR ratios. The effect of the light environmenton crop growth under intercropping systems with different spa-tial patterns was not considered. The relationship between themorphological parameters and the light environment (PAR or R/FRratio) in intercropping systems was not addressed in these studies.Likewise, the parameters that were sensitive to the shading envi-ronment were not identified. Relatively little research has focusedon effects of maize shading on soybean seedling growth (Zhanget al., 2011).

This paper reports the different responses of soybean seedlinggrowth to the changing light environment (represented by vari-ations in the PAR and R/FR ratio) that is caused by maize in therelay strip intercropping system. The objectives of this study were:(i) to compare the properties of the light environment in the soy-bean canopy for soybean sole cropping and maize–soybean relaystrip intercropping systems; (ii) to investigate the different mor-phological parameters of soybean under two planting systems;and finally, (iii) to analyze the relationships between the morpho-logical characteristics and the light environment for assessing therelative contributions of light quality (R/FR ratio) and light quanta(PAR transmittance) to soybean seedling growth in intercroppingsystem.

2. Materials and methods

2.1. Study site and experimental design

Experiments were conducted from 2011 to 2013, fields wereassigned to different treatments in a randomized block design withthree replications in the same field at the farm of the SichuanAgricultural University in Ya’an, Sichuan Province, China (29◦59′

N, 103◦00′ E). The field climate was subtropical humid, with amean annual temperature of 16.2 ◦C and a mean annual rainfallof 1200 mm.

2.1.1. Experiment 1This experiment was conducted in 2011, with two treatments

each for the maize–soybean relay intercropping and the soy-bean monoculture. The soybean (Glycine max L. Merr.) cultivarsGongxuan1 and Nandou12, and the maize (Zea mays L.) cultivarChuandan418 are major southwestern cultivars. The intercroppingpatterns used wide-narrow row planting, with alternating stripsof maize and soybean. The number of maize vs. soybean rows perstrip in the relay strip intercropping systems was 2:2. The distancebetween the maize and soybean strips was 60 cm, and both rowspaces were 40 cm under strip intercropping. The row space of thesoybean monoculture was 70 cm (Fig. 2). The experimental unit was6 m long and 12 rows wide for intercrops and 6 m long and 10 rowswide for sole crops. Maize was sown on 25 March 2011, whereassoybean was sown on 15 June 2011 when the maize plants wereat the V12 stage. The plant densities of relay intercropped maize,monocultured soybean, and relay intercropped soybean were 6,10, and 10 plants m−2, respectively. All plots were treated withbasal fertilizers. Before sowing, basal N at 44 kg ha−1 as urea, P at40 kg ha−1 as calcium superphosphate, and K at 10 kg ha−1 as potas-sium sulfate were applied to all plots. At the V6 stage of maize, N at165 kg ha−1 as urea was applied for the relay intercropped maize.

2.1.2. Experiment 2According to the interesting results of light environment change

in experiment 1, the experiment 2 was conducted and was com-prised of seven cropping systems with maize (Z. mays L.) andsoybean (G. max L. Merr.) in 2012–2013 for estimating the relation-ship of morphological characteristics with PAR transmittance and

F. Yang et al. / Field Crops Research 155 (2014) 245–253 247

Fig. 1. 2:2 maize–soybean relay strip intercropping system in August, shortly after maize harvest.

Table 1Planting patterns of the maize-soybean relay intercropping.

Code Cropping system Width of singlestrip (cm)

Distance of maizewide row (cm)

Distance of maizenarrow row (cm)

Distance between maizerow and soybean row (cm)

Distance betweensoybean rows (cm)

P1 2:2 200 180 20 70 40P2 2:2 200 170 30 65 40P3 2:2 200 160 40 60 40P4 2:2 200 150 50 55 40

60

80

Rwpr(n(nt2

F(

P5 2:2 200 140

P6 2:2 200 120

P7 1:1 50

/FR ratio. The maize cultivar in the experiment was Chuandan418,hereas the soybean cultivar was Nandou12. Table 1 lists the maizelanting patterns that were adopted: (P1) “180 + 20” wide-narrowow planting, i.e., wide row of 180 cm and narrow row of 20 cm;P2) “170 + 30” wide-narrow row planting; (P3) “160 + 40” wide-arrow row planting; (P4) “150 + 50” wide-narrow row planting;P5) “140 + 60” wide-narrow row planting; (P6) “120 + 80” wide-

arrow row planting; and (P7) “100 + 100” equal row planting. Inhe P1 to P6 treatments, the ratio of maize to soybean rows was:2. Soybean was planted in the wide rows before the reproductive

ig. 2. Layout of soybean monoculture (A) and maize–soybean relay intercroppingB) systems. S and M stand for soybean row and maize row, respectively.

50 4040 4050

stage of maize, with a row spacing of 40 cm. Meanwhile, the ratio ofmaize to soybean rows was 1:1 for P7 treatment, and the distancebetween rows was 50 cm. The seven intercropping patterns werecharacterized by the distance between the alternating maize andsoybean rows (Table 1). Each treatment was replicated three times,the plot size was 6 by 7 m. Maize was sown on 1 April and 30 Marchin 2012 and 2013, respectively. The harvesting day was 1 Augustand 28 July in 2012 and 2013, respectively. Soybean was sown on15 June and 16 June in 2012 and 2013, respectively. Hand sowingat high density was performed, and subsequently seedlings werethinned to achieve target density and uniformity of plants spac-ing. Maize and soybean plant density was 6 and 10 plants m−2 forintercrop, respectively.

2.2. Spectral irradiance and photosynthetically active radiation

The measurement of spectral irradiance and photosyntheticallyactive radiation (PAR) in different intercrops were made to charac-terize the light environment at different positions in the soybeancanopy. Sensors were placed on the horizontal arm of observingscaffold, which was at the height of 5 cm above the soybean canopy.Measurements were carried out in the midway between maize andsoybean rows, above the soybean canopy and in the center of thesoybean rows. Moreover, a reading was taken above the maizecanopy. Placement of sensors in the 2:2 and 1:1 system were illus-trated in Fig. 3. The two horizontal arms were moveable and couldbe fixed according to crop height. The scaffold was painted blackto minimize the effects of radiation distribution on the measure-ments. Data were collected on cloudless days at noon, to minimizethe external effects of the atmospheric conditions. Due to weatherconditions, the measurement of spectral irradiance and PAR weremade during the V2 and V3 stage of soybean in 2012 and 2013,respectively.

The spectral irradiance of different wavelengths in the soybeancanopy under relay intercropping system was measured using afiber-optic spectrometer (AvaSpec-2048; Avantes, Netherlands).The sensor’s field of view was 25◦, with a full sky irradiance remote

248 F. Yang et al. / Field Crops Research 155 (2014) 245–253

Fig. 3. Setting of spectral irradiance and PAR sensors in the maize-soybean relaystrip intercropping system for 2 M:2SB (a) and 1 M:1SB (b). 2 M:2SB, two rows ofsoybean alternated with two rows of maize. 1 M:1SB, one row of soybean alternatedw

cwTp

ouwapsafsml

P

Wt

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ffuvssWIvwtr

ping varied significantly on 7 July 2011 when maize and soybean

ith one row of maize. Si means the ith position of sensor.

osine corrector. Spectral irradiance was originally measured atavelengths ranging from 400 nm to 1000 nm at 0.6 nm intervals.

he spectral irradiance was determined as the mean of differentosition in crop canopies.

After each spectral irradiance measurement, the flux densitiesf PAR above the soybean canopy were measured at 10 s intervalssing LI-191SA quantum sensors (LI-COR Inc., Lincoln, NE, USA)ith a LI-1400 data logger. The incoming PAR was measured at

bove the maize canopy in relay planting conditions. Three inde-endent measurements were made at different position of theoybean canopy within each plot between 10:30 h and 14:00 h on

clear day. The mean of these three measurements were obtainedor each plot. All the radiation measurements were made on theame day. The PAR transmittance of the top of soybean canopy inaize-soybean relay intercropping system was calculated as fol-

ows (Serrano et al., 2000):

AR transmittance (%) = IsIm

× 100% (1)

here Is is the PAR at the top of soybean canopy, Im is the PAR athe top of the maize canopy.

.3. Morphological characteristics

After light environment measurement, ten plants were removedrom each plot for morphological analysis. The seedling heightrom the soil surface and the stem diameter were measured beforeprooting. Cores (8 cm diameter and 15 cm height) for root obser-ation of soybean seedling were collected using a core sampler. Theoil cores were immersed in water and kept for 3 h to disperse theoil. A root washer then separated roots from the dispersed soil.

inRHIZO system (WinRHIZO Pro LA2400; Regent Instrumentsnc., Quebec, Canada) was used to estimate root lengths of indi-idual samples. After these measurements, all the roots and shoots

◦

ere gathered together, exposed to 105 C for 0.5 h, and then driedo a constant weight at 80 ◦C to determine the stem-leaf and totaloot biomass.

Fig. 4. Spectral irradiance of soybean canopy in sole planting and relay intercrop-ping systems. Shadow regions stand for mainly wavelength of red and far-red light.

2.4. Statistical analysis

A one way-ANOVA was performed to test the effects of lightenvironment on morphological characteristics of soybean in soleplanting and intercropping conditions. Pearson correlation andlinear regression were used for analyzing the relationship of mor-phological characteristics with PAR transmittance and R/FR ratio.All the analyses were run with the SPSS v16.0 and Microsoft Excel2003.

3. Results

3.1. Light quality

The changes in the mean spectral radiance of the soybeancanopy for the sole planting and relay intercropping were shown inFig. 4. The spectral radiance values at different wavelengths underthe relay intercropping system were lower than those under soleplanting. In particular, the spectral radiance from R light to FR lighthad a gradually decreasing trend under sole cropping, whereas theopposite trend was observed in the relay intercropping systems.These results indicated that the quantity of solar radiation thatpassed through the maize canopy under maize-soybean relay stripintercropping systems was rich in FR light but had poor visible light.

The ratio of R light (655 nm to 665 nm) to FR light (725 nm to735 nm) varies significantly during plant growth and development(Pessarakli, 2001). The R/FR ratios of the soybean canopy in the soleplanting and relay intercropping systems were examined, as shownin Fig. 5. Contour plots of the R/FR values were calculated fromthe spectral irradiance data at 655 nm to 665 nm (�1) and 725 nmto 735 nm (�2). In the sole planting system of soybean, the maxi-mum value of R/FR was 1.35, whereas the minimum value was 1.05.For the maize–soybean relay intercropping system, the R/FR valuesranged from 0.55 to 0.85. Therefore, a large difference in the R/FRratio of the soybean canopy was observed for the monocultured andrelay intercropped soybean. This difference may cause changes inthe morphological characteristics of the soybean seedlings underthe two different treatment conditions.

3.2. Light quanta

The PAR of soybean canopy in sole planting and relay intercrop-

were at their R2 and V3 stages, respectively (Fig. 6). The PAR of thesoybean canopy under sole planting was higher than that of relayintercropping from 08:00 h to 18:00 h. The PAR values followed a

F. Yang et al. / Field Crops Research 155 (2014) 245–253 249

Fig. 5. Ratio between the spectral irradiance of R light wavelengths (�1) and FR lightwavelengths (�2) in the soybean canopy of sole cropping (A) and relay intercroppings

“opv

Fi

Fig. 7. Morphological parameters of soybean seedlings under sole cropping andrelay intercropping systems at the V3 stage of soybean development in 2011. Eachvalue presents the mean ± S.E. Means for each treatment that do not have a commonletter are significantly different, as determined by the Duncan’s multiple range testsat P < 0.05 and P < 0.01. S1, R1, S2, and R2 indicate the “Sole Gongxuan1,” “Relay

ystems (B).

low–high–low” trend in the two treatments. PAR transmittancef the top of soybean canopy in maize-soybean relay intercrop-ing system was calculated as Eq. (1), the maximum and minimum

alues were observed at 12:00 and 18:00 h, respectively.

ig. 6. Incident PAR and PAR transmittance of the soybean canopy under sole plant-ng and relay intercropping systems.

Gongxuan1,” “Sole Nandou12,” and “Relay Nandou12” treatments, respectively.

3.3. Seedling growth

The morphological parameters of soybean seedlings in solecropping and relay intercropping were shown in Table 2. Theseedling height was significantly increased in the relay intercrop-ping system as compared to that of the sole cropping using twosoybean cultivars (Gongxuan1 and Nandou12). Meanwhile, oppo-site trends were observed for the stem diameter and root length.The seedling heights of Gongxuan1 and Nandou12 under relayintercropping were increased by approximately 89.8% and 86.9%,respectively, as compared to those under sole cropping. The stemdiameters were decreased by 20.3% and 21.3% in the Gongxuan1and Nandou12 cultivars, respectively. Likewise, the respective rootlengths of the two cultivars were decreased by 23.5% and 30.5%.

The mean aboveground and root biomass of the soybeanseedlings were reduced under relay intercropping conditions(Fig. 7). For example, the average aboveground biomass of Gongx-uan1 under the relay intercropping system was 0.39 g per plant ascompared with the 0.71 g per plant under sole cropping. The totalroot biomass was significantly reduced from 0.11 g per plant undersole cropping to 0.05 g per plant under relay intercropping condi-tions (P < 0.01). In addition, changes in the trend of the root–shootratio were similar with the changes in the biomass under both

treatments. The root-shoot ratio of the Gongxuan1 and Nandou12cultivars had decreased by about 25% and 33%, respectively.

250 F. Yang et al. / Field Crops Research 155 (2014) 245–253

Table 2Morphological parameters of soybean seedlings under sole cropping and relay intercropping systems at the V3 stage of soybean development in 2011. Each value presentsthe mean ± S.E. Means for each treatment that do not have a common letter are significantly different, as determined by the Duncan’s multiple range tests at P < 0.05 andP < 0.01.

Parameter Gongxuan1 Nandou12

Sole cropping Relay intercropping Sole cropping Relay intercropping

Seedling height (cm) 21.43 ± 1.69Bb 40.67 ± 2.52Aa 20.33 ± 2.86Bb 38.01 ± 1.71AaStem diameter (cm) 0.25 ± 0.02Aa 0.20 ± 0.01Ab 0.24 ± 0.02Aa 0.19 ± 0.01Bb

± 1.54

3

otr(7ttwwhraRcw

3

tt(Prpwdt

4

4

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Root length (cm) 12.35 ± 1.27Aa 9.45

.4. R/FR ratio and morphological characteristics

To confirm the relationship of the R/FR ratio and the growthf the soybean seedlings under the different planting patterns ofhe maize–soybean relay strip intercropping systems, the R/FRatio was calculated by dividing the total irradiance of red lightR, 655–665 nm) by the total irradiance of far-red light (FR,25–735 nm) (Pecot et al., 2005; Hertel et al., 2012). The rela-ionship between the soybean morphological characteristics andhe R/FR ratio was shown in Fig. 8. The seedling height increasedith the decreased R/FR ratio, whereas the others characteristicsere reduced. A negative relationship between increasing seedlingeight and decreasing R/FR ratio (r = −0.94, P = 0.002 for 2012;

= −0.91, P = 0.004 for 2013) were found. Significant results werelso observed for the relationship of the others parameters and/FR (P < 0.01). The correlation coefficients between morphologi-al characteristics and the R/FR ratio for 2012 and 2013 data setsere all higher than 0.88 (P < 0.01).

.5. PAR transmittance and morphological characteristics

To identify the parameters wherein crop growth was sensi-ive to the PAR transmittance, the relationships between the PARransmittance and the morphological parameters were studiedFig. 9). The relationships of the morphological characteristics withAR transmittance were similar with R/FR (Fig. 8). The significantelationship between PAR transmittance and the morphologicalarameters were found. The correlation coefficient for root lengthas minimal for 2012 (r = 0.81, P = 0.02) and 2013 (r = 0.85, P = 0.01)ata sets as compared with the others parameters to PAR transmit-ance.

. Discussion

.1. Light environment

Light is an abiotic factor that is particularly important for plants.n agricultural production, incident light is partly reflected, partlybsorbed, and partly transmitted by crop leaves (Aphalo et al.,999). Simultaneously, the proportion of light that is reflected,bsorbed, and transmitted depends on its wavelength and thelanting pattern. The spectral irradiances of different wavelengths400–1000 nm) were decreased significantly in the relay inter-ropping systems as compared with sole planting (Fig. 4). Anmportant phenomenon is low ratio of the R light (655–665 nm)o the FR light (725–735 nm) because chlorophyll selectivelybsorbs 655–665 nm light while transmitting 725–735 nm lightPessarakli, 2001). The R/FR ratios in the maize-soybean relayntercropping system were from 0.55 to 0.85, which was sig-ificantly lower than that of sole cropping (Fig. 5). This result

as consistent with the literature, which indicates that the/FR ratio decreases from approximately 1.2 in full sunlight to.05 in closed canopies, with a significant decrease occurringefore canopy closure (Smith, 2000). In addition, the PAR of the

Ab 11.69 ± 1.03Aa 8.13 ± 1.48Bb

soybean canopy in sole planting and relay intercropping variedsignificantly because the incident light reflected and absorbedby maize leaves could reduce the amount of incoming PAR thatwas available for the soybean seedlings in intercropping condi-tions (Fig. 6). Similarly, cotton in the two intercropping systems(the number of rows per strip of wheat and cotton with respec-tive ratios of 3:1 and 3:2) intercepted 73% and 93% as muchPAR as the sole cotton, respectively (Zhang et al., 2008). There-fore, the canopy leaves of maize decreased the captured radiationand altered proportions of the light composition in the soybeancanopy of relay intercropping as compared to the soybean mono-culture.

4.2. Seedling growth

The quality and quantity of irradiance can trigger plant mor-phological responses (Kurepin et al., 2007). Shoot elongation thatis induced by light quality changes may confer high relative fit-ness in shading conditions (Schmitt, 1997) but may lead to reducedcrop productivity (Ruberti et al., 2012). Root growth is likewiseoften changed in the light environment with low R/FR ratios(Ruberti et al., 2012). Many reports indicate that shading con-ditions increase the plant height and reduce the stem diameter(Morelli and Ruberti, 2002; Nagasuga and Kubota, 2008). Signif-icant differences in the seedling height, stem diameter, and rootlength were observed under the maize–soybean relay intercrop-ping system as compared to the soybean sole cropping system(Table 2 and Fig. 7). Similarly, Yan et al. (2010) reported thatthe soybean seedling height was significantly increased whilethe stem diameter was decreased in shading conditions. Theaboveground and total root biomass of soybean seedlings is ofinterest because these were reduced under intercropping. Like-wise, the root-shoot ratio in particular was decreased. Hébert et al.(2001) reported that the maize root/shoot ratio was decreasedby light reduction. These morphological traits are directly cor-related with the lodging resistance and crop yield (Yan et al.,2010). Therefore, our research results suggest that changes inthe quality and amount of light in a maize–soybean relay stripintercropping system both directly affect the morphological char-acteristics of soybean. Thus, further studies on the relationship ofmorphological parameters with light quality and quantity are nec-essary.

4.3. Relationship between light environment and morphologicalparameters

Plants growing in the shade of neighboring taller vegetationare usually receiving altered R/FR ratio and reduced PAR (Kurepinet al., 2012). And plants grown under lower R/FR ratios andlower PAR irradiance conditions normally exhibit increased stem

elongation (Smith, 2000). In this study, similar results were foundthat the decreased PAR transmittance and R/FR ratio in soybeancanopy lead to the increased height of soybean seedlings withdecreasing distance between the maize and soybean rows under

F. Yang et al. / Field Crops Research 155 (2014) 245–253 251

F B), roo( age ofc ) and 2

intoaPcTtp

siw(heorss

ig. 8. Relationships of the R/FR ratio with the seedling height (A), stem diameter (F) under relay intercropping systems. Solid and open symbols indicate V2 and V3 stoefficients (r) and significance (P) are for 2012 data (n = 7; solid line, boldface type

ntercropping conditions (Figs. 8 and 9). There was a significantegative correlation of seedling height with R/FR ratio and PARransmittance (P < 0.05). At the same time, the relationship of thethers morphological with R/FR ratio and PAR transmittance werelso significant. Similar studies reported that the R/FR ratio andAR transmittance significantly decreases as the canopy graduallylose in sole cropping conditions (Smith, 2000; Liu et al., 2012).hese results showed that there were important effects caused byhe PAR transmittance and the R/FR ratio on the morphologicalarameters of the soybean seedlings.

Therefore, soybean seedlings have a sensitive response to maizehading in relay intercropping conditions. Though morpholog-cal characteristics were better correlated with R/FR ratio than

ith PAR transmittance in term of the correlation coefficientsFigs. 8 and 9), neither the R/FR ratio nor the PAR transmittancead influenced the soybean seedling growth. The sum of theffects of PAR and the R/FR ratio under different spatial patterns

f intercropping systems may account for this difference. Theseesults are similar to the notion that the response of the Hopeaeedlings to shade conditions is the sum of the effects of thepectral quality (R/FR) and PAR (Lee et al., 1997).

t length (C), aboveground biomass (D), total root biomass (E), and root-shoot ratio soybean in 2012 and 2013 field data, respectively. Regression lines and correlation013 data (n = 7; dashed line, lightface type).

4.4. Prospects of the consequences of plant response on thecanopy level

The regulation of plant morphogenesis by shading has beenthoroughly studied from germination to flowering (Spaldingand Folta, 2005; Chelle et al., 2007). Plants growing in canopyshade are usually receiving altered light quality (R/FR ratio) andreduced light quantity (PAR) (Smith, 2000). Due to the R/FRratio determines the state of the phytochrome photoequilibriumwhich controls various photomorphogenetic plant responsessuch as accelerated extension growth, retarded leaf, and reducedbranching (Leuchner et al., 2007). Thus, R/FR ratio is usually usedas an important parameter for building functional-structuralplant models, which allow us to analyze the consequences ofchanges in plant architecture for the functioning of individualorgans (Evers et al., 2005; Chelle et al., 2007). Our research resultsalso confirmed that there were strongly correlation relationship

between R/FR ratio and morphological characteristic of soybean inintercropping conditions. Simultaneously, significant results werealso observed for the relationship of the seedling growth and PARtransmittance (Figs. 8 and 9). Therefore, the assessment of R/FR

252 F. Yang et al. / Field Crops Research 155 (2014) 245–253

F eter (Br nd V3c ldface

rglf

5

(

(

ig. 9. Relationships of PAR transmittance with the seedling height (A), stem diamatio (F) under relay intercropping systems. Solid and open symbols indicate V2 aorrelation coefficients (r) and significance (P) are for 2012 data (n = 7; solid line, bo

atio and PAR of relay intercropped soybean canopy at differentrowth stage is crucial for integrating the responses of plant toight quantity and quality in the three-dimensional models in theuture.

. Conclusions

1) The spectral irradiance, R/FR ratio, and PAR of the soybeancanopy are decreased in maize-soybean relay intercroppingconditions as compared to soybean monoculture. Simulta-neously, R/FR ratio and PAR transmittance are decreased withdecreasing distance between the maize and soybean rowsunder intercropping conditions. These results imply that R/FRratio and PAR of soybean canopy are altered with the changesof spatial pattern in soybean–maize relay strip intercroppingsystem.

2) The quality and quantity of irradiance in soybean canopy

can trigger morphological responses. The seedling height ofsoybean seedling is significantly increased in the relay inter-cropping condition as compared to that of monoculture.Opposite trends are found for stem diameter, root length,

), root length (C), aboveground biomass (D), total root biomass (E), and root-shoot stage of soybean in 2012 and 2013 field data, respectively. Regression lines and

type) and 2013 data (n = 7; dashed line, lightface type).

aboveground biomass, total root biomass, and root-shoot ratio.In addition, the regulations of soybean seedling morphogene-sis are response to altered light environment resulted from thechanged distance between maize and soybean rows in relayintercropping system.

(3) There are significant relationship of morphological parame-ters of soybean seedling with R/FR ratio and PAR transmittancein maize–soybean relay intercropping system, especially R/FRratio of soybean canopy. It suggests that the sum of theeffects of PAR and the R/FR ratio affect soybean seedlingsgrowth under different spatial patterns of intercroppingsystems.

(4) This study likewise indicated that the soybean seedlings growthis more sensitive to the R/FR ratio and PAR in intercrop-ping systems. These results would provide further supportfor elucidating the physiological response mechanisms of soy-bean seedlings to shade stress. Eventually, this information

will be helpful for simulating the three-dimensional distribu-tion of R/FR ratio and PAR within soybean canopies and fordetermining the suitable population planting pattern in amaize–soybean intercropping system.

Resea

A

Ro2SS

R

A

A

A

C

C

C

E

E

E

H

H

K

K

K

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F. Yang et al. / Field Crops

cknowledgements

The research was supported by National Program on Key Basicesearch Project (2011CB100402), Public Research Funds Projectsf Agriculture, Ministry of Agriculture of the PR China (201103001;01203096), Program on Industrial Technology System of Nationaloybean (CARS-04-PS19), and Postdoctoral Science Foundation ofichuan Province (04310624).

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