Weed Research, 1993, Volume 33, 1-8 THE LIBRARV OF THE

FEB 1 5 1993UNIVERSITY OF ILLINOIS

Effects of light and nitrogen and their interaction on thedynamics of rice-weed competition

K. AMPONG-NYARKO* ANDS. K. DE DATTAInternational Rice Research Institute,PO Box 933, Manila, Philippines

Received 24 April 1990Revised version accepted 9 April 1992

Summary: R^sum^: Zusammenfassung

Light and nitrogen (N) interaction effects on rice(Oryza sativa L., cv. UPLRi-7) were studied inthe growth chamber, and effects on its compe-tition with Rottboellia cochinchinensis (Lxjur.)W.D. Clayton and Echinochloa colonum (L.)Link were studied in the field. In the field, N avail-ability increased the canopy light absorptioncoefficient, and reduced the sunlit leaf area indexof rice. In growth chambers, rice plants grownunder conditions of low photosynthetically activeradiation (PAR) had higher shoot N concentra-tion than when grown at higher PAR. The photo-synthetic rate was strongly correlated with leaf Ncontent per unit leaf area. When no N was appliedthere was no difference in dry matter yieldbetween plants grown at 150 and 400 JJLE m~̂ s~'.At various N levels, dry matter increased with in-creasing light intensity. A reduction in lightintensity did not give a proportionate decrease inplant growth. The results suggest that an increasein shoot N concentration is an adaptive mechan-ism of shade plants. The limited plant response toN under shade and acclimation of rice to reducedirradiance could be significant factors in lightand N interaction on rice-weed competition.

Effets de la lumiire, de I'azote et de leur inter-action sur la dynamique de la compitition rizimauvaises herbes

Les effets de l'interaction de la lumiere et de

•Present address: ICIPE, Crop Pests Research Pro-gramme, Mbita, P,O, Box 30772, Nairobi, Kenya,

I'azote (N) sur le riz (Oryza sativa L.). cvUPLRi-7) ont 6te 6tudi6s en chambreclimatique et les effets sur sa competitionavec Rottboeillia cochinchinensis (Lour) WDClayton et Echinochloa colonum (L.) Link ontete etudi6s au champ. Au champ, la disponibil-ite en azote a augmente le coefficient d'absorp-tion de la lumiere par la canopee et a reduitl'index de la surface foliaire ensoleill6e du riz.En chambre climatique, les plants de riz cultiv6sa une faible radiation photosynthetiquementactive (PAR) avaient une concentration en azotede leur tige plus 61ev6e qu'^ une forte PAR. Letaux photosynthdtique a ete fortement correl^avec la teneur en azote foliaire par unite desurface foliaire. Quand aucune azote a 6t€appliquee, il n'y a pas de difference dans lesrendements en matifere sfeche entre les plantescultivees de 150 a 500 JAE m"^ s~'; A des dosesvariees d'azote, la matidre seche a augmenteavec ['augmentation de I'intensitd lumineuse.Une reduction de l'intensit^ lumineuse n'a pasentraind une reduction proportionnelle de lacroissance de la plante. Les r^sultats donnent kpenser que l'augmentation de la concentrationen azote des tiges est un mdcanisme d'adapta-tion des plantes a l'ombre. La rdponse desplantes k I'azote limit^e sous l'ombre et l'accli-mation du riz a une irradiation r6duite pour-raient etre des facteurs significatifs dansl'interaction lumifere-azote sur la competitionriz-adventices.

Wirkungen und Wechselwirkungen von Lichtund Stickstoffdiingung auf die Konkurrenzzwischen Reis und Unkraut

Der EinfluB des Lichts und der Stickstoffdiin-gung (N) auf Reis (Oryza sativa L. 'UPLRi-7')wurde in der Pflanzenwuchskammer und dieWirkungen auf die Konkurrenz mit Rottboelliacochinchinensis (Lour.) W.D. Clayton und

• • >

2 K. Ampong-Nyarko & S. K. De Datta

Echinochloa colonum (L.) Link im Freilanduntersucht. Die Verfugbarkeit von N erhohteim Feld den Licht-Absorptionskoeffizientenund reduzierte den Blattflachenindex beimReis, Bei der Anzucht in Pflanzenwuchs-kammern hatten Reispflanzen bei niedrigerBeleuchtungsstarke eine hohere N-Konzen-tration als bei starkerer Beleuchtung. DiePhotosyntheserate war deutlich mit dem N-Gehalt, bezogen auf die Blattflache, korreliert,Ohne N-Dungung traten bei 150 und 400 \x.Em~^ s~' keine Trockenmassenunterschiede auf,Bei verscbiedenen N-Dungungsstufen nahm dieBildung von Trockenmasse mit der Lichtin-tensitat zu, Eine Verminderung der Lichtinten-sitat fuhrte nicht zu einer profxsrtionalenAbnahme des Pflanzenwuchses, Aus denErgebnissen wurde geschlossen, daB dieZunahme der N-Konzentration im SproB einadaptiver Mechanismus von Schattenpflanzenist. Die begrenzte Reaktion der Pflanzen aufStickstoff im Schatten und die Akklimation vonReis auf vermindertes Licht konnten signi-fikante Faktoren fiir die Wechselwirkungen vonLicht und Stickstoffdungung auf die Konkur-renz zwischen Reis und Unkraut sein.

Introduction

Tbe relationship between environmental re-sources and plant-plant interaction is dynamicand complex. Competition for light occurs inalmost all cropping situations (Donald, 1%3).Nitrogen (N) is also indispensable for plantgrowth and development. Inadequate N nutri-tion affects almost all aspects of plant growthand metabolism (Smith, 1982), As N is themajor nutrient commonly applied to rice (DeDatta, 1981), light and N competition oftenoccur simultaneously.

Interactions between light and N have beenreported. For instance, low N fertility and lowirradiance stimulated shoot dry matter and in-creased shoot N concentration in several foragecrops (Blackman & Templeman, 1938; Wong &Wilson, 1980; Eriksen & Whitney, 1981;Navarro-Chavira & McKersie, 1983), At high Nlevels, increasing shade decreased plant drymatter of Cynodon dactylon (L,) Pers, almostlinearly (Burton et al., 1959),

However, the causes and effects of competi-tion are not easily separated, and resource

interactions may be so interrelated that it maybe impossible to untangle them (Radosevich &Holt, 1984). Also, the ability of plants to adaptto limited environmental resources furthercomplicates our understanding of ecologicalprocesses (Jeffers, 1982),

Pertinent studies will enable a better under-standing of weed competition, which is essentialfor integrated weed management, and makecrop-weed interaction studies less site-specific.The objective of this study was to assess lightand N effects and their interaction on thedynamics of rice-weed competition.

Materials and methods

Light and N interaction effects on rice-weedcompetition were assessed in two sets of fieldexperiments in 1987 and 1988, and in twogrowth chamber experiments conducted in 1988at the Rice Research Institute, Los Baiios,Philippines (latitude 14° 11' N, longitude 121°15' E; elevation 21 m). The soil used was VerticTropaquept, pH 6-6, with the following proper-ties; organic C, 0-6%; total N, 0-07%; K, 1-34mEq 100 g~' air-dried soil; cation exchangecapacity, 26 mEq 100 g~' air-dried soil; andavailable P (Olsen), 40 ppm.

In the first field experiment conducted in the1987 wet season, R. cochinchinensis and E.colonum were grown in monoculture at 100plants m"^, and in mixtures at the same plantdensity where one weed species was mixedwith another at rates of 25-75, 50-50 and 75-25.Rice variety IR43 was the target species grownat a constant density of 100 kg ha"'. Nitrogenat 100 kg ha"' was split-applied, two-thirdsat 21 days after seeding (das) and one-third atabout 1 week before panicle initiation. Lighttransmittance was measured at 100 cm and50 cm within the canopy with four samples fromthe plots.

In the second field experiment, rice varietyUPLRi-7 and the weeds R. cochinchinensis andE. colonum were used in the 1988 wet seasonunder upland rice culture. Seven treatments infactorial combination with 2 N levels (0 and 100kg ha~') were tested in a randomized completeblock design with four replications. The landwas ploughed once and harrowed twice. Plotsmeasured 2x3 m. Rice seeds at 100 kg ha"'were drilled into furrows 25 cm apart. A weed

population of 100 plants m ^ was achieved bybroadcasting weed seeds to the plots already in-fested with these weeds, and thinning 1 weekafter weed germination. Urea was broadcast toN plots at 21 das.

Light transmittance (photon flux density 400-700 nm) within the canopy was measured with adigital illuminance meter (lm-3, Tokyo OpticalCo. Ltd, Japan), starting from the bottom andup through the canopy at 49 das. The canopywas stratified into 25-cm vertical layers. Threelight readings were taken randomly per plot.The leaf area of the species was measured bydestructive sampling from three randomquadrats (0-25x0-25 m) per plot using anautomatic leaf area meter (Model AAM-7,Hayashi Denkoh Co. Ltd, Tokyo, Japan). Leafsamples for the determination of leaf area ofeach canopy stratum were obtained by cutting25-cm portions of the plants starting from thesection after the plants in the quadrats had beencut at ground level. Soil moisture at the 10- and20-cm depths was determined gravimetrically.Soil moisture at 10 cm depth was 24-1 at 0 kgfertilizer N ha"', but at 100 kg fertilizer N ha"'it was 23-1. Soil moisture for rice monoculturewas 22-6, but it was 23-7 for rice-/?.cochinchinensis mixture 56 das. The experimentwas terminated at rice harvest 120 das.

The canopy absorption coefficient (extinctioncoefficient) was calculated using the Beer-Lambert law:

where PIo and Pig are photosynthetic irradiance((jiE m"^ s"') above the canopy and at thedesired level in the canopy station, respectively,k is the canopy absorption coefficient (dimen-sionless), and LAI is the leaf area index(m^m"^). The relative competitive ability of thespecies for light interception was calculated asthe sunlit leaf area index (SLAI) (Walker etal.,1988) for each canopy layer.

Light, nitrogen and rice-weed competition 3

100 g~' air-dried soil; cation exchange capacity,26 mEq 100 g~' air-dried soil; and available P(Olsen), 40 ppm).

Growth-chamber conditions were maintainedat 34/23°C day/night temperature, 70% relativehumidity, and 12-h daylength. Treatmentsconsisted of three light levels (150, 250 and 400jtE m"^ s"') and six N levels equivalent to 0,22,44, 88, 132 and 176 mg N applied to 1 kg of air-dried soil. Rice plants were grown in the glass-house for 10 days before they were transferredto the growth chamber.

Gas exchange measurements were conductedin situ 3 weeks later on two plants from each pot,using the youngest, fully expanded leaf of amain stem or primary tiller of vegetative plants.A 50-mm segment of the mid portion of a leafwas enclosed in a steady-state gas-exchangecuvette and exposed to controlled humidity,temperature, CO2 concentration, and light in-tensity (Walz, Effeltrich, Germany). Thecuvette was maintained at 30°C. Relativehumidity was 70%. The gas flow rate, leaf andcuvette temperature, leaf transpiration and CO2uptake rate, and quantum flux (PAR) weremonitored continuously with a Walz gas-exchange system equipped with two BINOS-1gas analysers (Leybold-Heraeus, Hanau,Germany).

Dry weights of leaves, stem and roots weredetermined after oven-drying at 70°C for 3 days,21 days after transfer to the growth chamber forthe first experiment, and after 42 days for thesecond experiment. At harvest, leaf area wasmeasured using the automatic leaf area meter.Dried material was ground and N concentra-tions were determined after Kjeldahl digestion.

Nitrogen/carbon limitation of photosyntheticperformance was calculated from the y-intercept of the linear regression of photo-synthesis on leaf N (Hirose, 1984). If the y-intercept of the linear regression is negative,photosynthesis is limited by N; if the intercept ispositive, photosynthesis is carbon-limited.

In the growth chamber at the IRRI phyto-tron, four UPLRi-7 rice seedlings were grown in13-cm diameter plastic pots with drainage holesfilled with 3-5 kg of air-dried soil, pH 6-6(organic C, 0-6%; total N, 0-07%; K, 1-34 mEq

Results

Canopy structure

At 100 kg N ha"', the LAI of R. cochinchinensiswas 6-16 and that of rice was 0-13 when seededtogether (Table 1). Similarly, E. colonum had aLAI of 2-13, compared with 0-52 in rice (Table

4 K. Ampong-Nyarko & S. K. De Datta

TaWe 1, Distribution of suntit teaf area index (SLAl) and total leaf area index (LAI) in rice and Rottboelliacochinchinensis canopy at 49 days after seeding (1988)

Treatment

Rice, ON

Rice,100N

Rice+R. cochinchinensisseeded 0 days after rice, 0 N

Rice-I-R. cochinchinensisseeded 0 days after rice, 100 N

Rice + /?. cochinchinensisseeded 14 days after rice, 0 N

Rice-t-/?. cochinchinensisseeded 14 days after rice, 100 N

R. cochinchinensis seeded0 days after rice, 0 N

R. cochinchinensis seeded0 days after rice, 100 N

SE(88df)

Rice

Canopy layer(cm)

SLAl

0-25

0-37

0-53

0-02

0-02

0-18

0-31

0-335

25-50

0-15

1-27

0-05

0-03

0-13

0-93

TotalLAI

0-51

1-79

0-10

0-13

0-43

2-09

0-25

0-06

0-03

0-16

0-36

0-09

0-24

R. cochinchinensis

Canopy layer(cm)

SLAl

25-50

0-62

0-55

0-09

0-55

0-49

0-95

50-150

1-88

3-62

0-36

1-64

2-91

2-61

TotalLAI

2-98

6-16

0-81

3-14

3-41

5-06

Table 2, Distribution of sunlit leaf area index (SLAl) and total leaf area index (LAI) of rice and Echinochloacolonum canopy at 49 days after seeding (1988)

Rice, 0N ,0 days

Rice, 100 N,0 days

Rice-^£. colonum seeded0 days after rice, 0 N

Rice-^ £. colonum seeded0 das after rice, 100 N

E. colonum seeded 0 dasafter rice, 100 N

E. colonum seeded 0 dasafter rice, 100N

SD(46df)

Rice

Canopy layer (cm)

SLAl

0-25

0-33

0-41

0-15

0-06

0-14

25-30

0-39

1-57

0-12

0-16

TotalLAI

1-23

1-91

0-31

0-52

E. colonum

Canopy layer (cm)

SLAl

0-25

0-45

0-15

0-37

0-29

25-50

0-45

1-54

0-62

1-81

TotalLAI

1-01

2-13

1-46

2-75

2), Rice monoculture sunlit LAI at 100 kg ferti-lizer N ha~' shifted from 1-79 to 0-05 in mixturewith R. cochinchinensis; it shifted from 1-91 to0-52 when in mixture with E. colonum. TotalLAI of rice increased from 0-13 to 2-09 and sun-

lit LAI increased from 0-05 to 1-24 when R,cochinchinensis was seeded 14 das rice insteadof 0 das rice (Table 1),

When rice and the weeds were grown to-gether, N significantly (P<0-01) increased LAI

more in the weeds than in rice (Tables 1 and 2).The canopy absorption coefficient, K was 0-53at 100 kg N ha~' and 0 26 when no fertilizer Nwas applied, and R. cochinchinensis was plantedat the same time as rice. The absorptioncoefficient of the R. cochinchinensis canopy at100 kg N ha-' planted at 14 das rice was 0 36,and that of E. colonum at 100 kg N ha"' plantedat 0 das rice was 0-33.

Shoot nitrogen concentration

In the 1987 field experiment, shoot N concen-tration (Nc) in rice plants grown in competitionwith R. cochinchinensis was negatively cor-related with light transmittance (Fig. 1).

A similar pattern was observed in the growth-chamber studies, where 176 mg added N kg"'soil resulted in Nc of 3-6% at 150 (j.E m~̂ s~',3-3% at 250 .̂E m"^ s"', and 3 0% at 400 jiEm"^ s"' (Fig. 2). In general, Nc increased athigh N levels.

In the growth chamber, Nc per unit leaf areawas positively correlated with net photosyn-thetic rate (Pn), and was significant at P<0 01(Fig. 3).

2 5

2 0

1-5 Y= 2-24-0000936Xr=-0-85

10 -

0J_ _L J_20 40 60 80

Light transmittance ratio (%)

100

Fig. 1. Effect of light transmittance in the canopy of R.cochinchinensis and £. colonum grown in mixtures at 25-75,50-50,75-25 plants m"^ and in monoculture at 100 plants mon the shoot nitrogen concentration of the associated IR43rice 75 days after seeding (1987).

Light, nitrogen and rice-weed competition 5

3-8

3-6

. 22 44 88 132 176

Nitrogen rate (mg kg"' air-dried soil)

Fig. 2. Shoot nitrogen concentration in rice as affected bynitrogen application and photosynthetically active radiation(o—o, 150; A - - - A, 250; • - - • , 400 nEm"^s~ ' ) , Verticalbars indicate ± SE of means (n = 4),

14

140 2 4 6 8 K)

Leaf nitrogen content (mg dm" )

Fig. 3. Net photosynthetic rate of rice (cv, UPLRi-7) asaffected by leaf N content and photosynthetically active radia-tion (o—o, 150; A - - - ± . 250; • - - a, 400 (tE m - ' s"').

6 K. Ampong-Nyarko & S. K. de Datta

Nitrogen limited photosynthesis at 400 and250 JJLE m"^ s"', as shown by the negative y-intercept in the following equations:

Pn = -7-60 (SE ± 0-70)-H2-629 Nc,(r = 0-97, 400 M,Em"^s"')

Pn = -0-37 (SE ±0-05)-l-0-97 Nc, (r = 0-9,250 jtE m" ' s"'O

Pn = 1-28 SE ± 0-15)-+-0-73 Nc, (r = 0-89,150 m"

Dry matter yield

In the 1988 field experiment, the dry matter ofrice at 49 das in competition with R. cochin-chinensis seeded at the same time was 2-0 g m"^when 100 kg N ha"' was appHed, and 2-4 g m"^when no fertilizer N was applied. Rice respon-ded markedly to N when grown in monoculture.When weed planting was delayed by 2 weeks,the dry matter of rice when N was applied was

5 h A

i

0 Ll-22 44 88 132 172

Nitrogen rate (mg kg"' air-dried soil)

Fig. 4. Dry matter of 52-day-old rice plants at various N levelsafter 42 days' exposure to photosynthetically active radiation(o—o), 150; A - - - A ,250 ;a - -D .400nEm-^s" ' )onr i ceshoot-root ratio. Vertical bars indicate ±SE of means

16-7 g m"^, compared with 5-9 g m ^ when no Nwas applied.

In the second growth-chamber experiment at22 mg N kg"' soil, the dry matter of rice at PAR400 (xE m"^ s"' was 4-66 g, but it was reducedto 1-46 g plant"' at 150 n-E m"^ s"' (Fig. 4). Atall light intensities the dry matter of rice wasmarkedly reduced when no N was applied.

In both experiments, there were significantlight and N interaction effects (P<0-01). Whenno N was applied (0 mg N kg"' air-dried soil),the highest dry matter yield was obtained at 250jjiE m"^ s"' and the lowest at 400 \x.E m"^ s"'(Fig. 4). At high N, dry matter increased withincreasing light intensity. The optimum yieldwas obtained at 22 mg N kg"' air-dried soil for150 and 250 (jiE m"^ s"', and at 44 mg N kg"'air-dried soil for 400 p-E m"^ s"' (Fig. 4).

Shoot-root ratio

The interaction effect of light and N on shoot-to-root ratio was important. Shoot growth in-creased proportionally more than root growth atthe 150 and 250 (JLE m"^ s"' compared with the400 (jiE m"^ s"' treatment (Fig. 5). The highestshoot-to-root ratio was observed from 88 to 176

If

1 L-L.22 44 88 132 176

Nitrogen rate (mg kg"' air-dried soil)Fig. 5. Effect of soil N and photosynthetically active radiation(o o, 150; A — ± ,250; D - - n, 400 (lE m"̂ s"') on riceshoot-root ratio. Vertical bars indicate ±SE of means

Light, nitrogen and rice-weed competition 7

mg N kg ' stir-dried soil for 150 and 250 (xE m -s- ' , and 132 to 176 for 400 ji.E m"" s"'.

Discussion

The availability of light and N interactedpositively to intensify weed competition. Pene-tration of radiation into the vegetation dependson differences in height between crops andweeds (Grime, 1979), characteristics of the in-cident radiation, spectral characteristics of thefoliage and canopy structure, including LAI,and the position, distribution, size and shape ofleaves (Norman, 1980), Sunlit LAI of rice wastherefore increased by factors that limited theweed canopy structure, such as delayed weedemergence or low soil N,

Reduced irradiance, as well as high soil N, in-creased leaf N content. Higher N concentrationsin plants grown under low irradiance have beenobserved in both temperate and tropical foragegrasses (Alberda, 1965; Deinum & Dirven,1972; Wong & Wilson, 1980; Navarro-Chavira& McKersie, 1983), Leaves of three maizehybrids were found to have higher nitrate con-centrations when exposed to low light(Knipmeyer et al., 1962),

Leaf N content per unit leaf area was positiv-ely correlated with net photosynthetic rate.Many studies have related the increase in CO2assimilation rate to increase in leaf N (Yoshida& Coronel, 1976; Evans, 1983; Hirose &Kitajima, 1986), Photosynthetic rates have beenobserved to increase proportionally up to N of c,15-18 mg dm~^. As plants under reducedirradiance had increased Nc, they were able tomaximize their photosynthetic rate. However,at low PAR of 150 jiE m~^ s~\ photosynthesiswas limited by the supply of carbon for furtherleaf expansion to capture light, Hirose &Werger (1987) reported that photosynthesis wascarbon limited at photon flux densities of lessthan 125 (xEm'^s" ' .

A reduction in PAR did not lead to a propor-tionate decrease in plant growth because offeedback effects on the plant, such as increasedCO2 assimilation, increased shoot-to-root ratio,and increased specific leaf area, N deficiencyreduced plant growth more than low lightintensity.

Plants grown in soil with no N added at 250jjiE m~^ s~' had a higher dry matter yield than

plants grown at 400 jiE m ^ s ', Similarfindings have been reported for forage grasses(Blackman «fe Templeman, 1938; Wong &Wilson, 1980; Eriksen & Whitney, 1981;Navarro-Chavira & McKersie, 1983), At lowN, plants grown at 150 jxE m""̂ s~' had highershoot Nc concentration, but photosynthesis waslimited by light, while 400 (JLE m"^ s"' leaf Nlimited photosynthetic rate. At 250 jjiE m~^s~\ when no N was added, rice had a higherleaf N than at 4(X) (xE m~^ s~', and also ahigher PAR than at 150 |xE m"^ s~\ andsubsequently produced the highest dry matteryield.

High N does not prevent yield reductions inrice under conditions of weed competitionbecause high N increases canopy absorptioncoefficients, resulting in increased shading. Ashas been shown in the present study, there islittle rice response to N under shade. Thisexplains the observation that oversupply of N,even when it is the limiting factor, cannot sub-stitute for effective weed control. Theagronomic significance of increased shoot Nconcentration under shade needs to bequantified for the understorey crops ofintercrops.

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