Relationships between upland rice canopy characteristics and weed competitiveness

Relationships between upland rice canopy characteristics

and weed competitiveness

M. Dingkuhn*, D.E. Johnson, A. Sow, A.Y. Audebert

West Africa Rice Development Association, B.P. 01, Bouake 2551, Ivory Coast

Received 5 March 1998; accepted 11 September 1998

Abstract

Weed-competitive upland rices with an acceptable yield potential are needed for labor-limited systems in Africa, particularly

where shortened fallow periods have increased weed pressure. Crosses between weed-competitive but low-yielding African

rice, Oryza glaberrima, and improved Oryza sativa tropical-japonica rices, might reduce tradeoffs between competitiveness

and yield potential. Parallel ®eld studies under moist upland conditions were conducted during the 1996 and 1997 wet seasons

at Mbe in Cote d'Ivoire to (1) characterize canopy properties, growth vigor and yield for O. glaberrima, O. sativa and

interspeci®c progenies under monoculture, and (2) determine their competitiveness when grown in single rows in competition

with natural weed growth, maize (removed 50 days after sowing [DAS]), the highly weed-competitive O. glaberrima IG10,

and with themselves. In the monoculture study with 21 lines, dry matter, leaf area index (LAI), PAR extinction coef®cient

(Kdf), mean tip elevation angle (MTA) and speci®c leaf area (SLA) were measured 31, 46 and 64 DAS. Across lines, LAI was

positively correlated with SLA, dry matter partitioning to leaves (31 and 46 DAS) and Kdf (46 DAS); and negatively with MTA

(46 and 64 DAS). Plant height was negatively correlated with tiller number. In the competition study with 16 lines, the O.

glaberrima landraces had superior relative yield (yield under interspeci®c competition/yield under intraspeci®c competition).

Some breeding lines were competitive with speci®c competitors. Correlations between canopy characteristics under

monoculture and competition indicated that LAI, SLA and tillering ability were predictive of competitiveness regardless of the

competing species, whereas partitioning, Kdf and MTA were correlated with competitiveness only for speci®c growth stages

and/or competitors. Competitiveness was negatively but weakly correlated with yield potential, and positively, with crop

duration. The authors conclude that SLA and tillering ability, which are major determinants of vegetative vigor, and crop

duration, which affects the ability to recover from early competition, are useful traits in the selection of weed-competitive

rices, particularly in breeding programs that use O. glaberrima. The traits are compatible with high yield potential if cultivars

have large SLA during early developmental stages and small SLA during advanced stages. Major knowledge gaps remain

on weed competitiveness under drought- and ¯ood-prone conditions, which are frequently associated with weed problems.

# 1999 Elsevier Science B.V. All rights reserved.

Keywords: Oryza sativa L.; Oryza glaberrima Steud.; Weed competition; Vigor; Leaf area index; Speci®c leaf area; Mean tip angle;

Extinction coef®cient

Field Crops Research 61 (1999) 79±95

*Corresponding author. E-mail: [email protected]

0378-4290/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

P I I : S 0 3 7 8 - 4 2 9 0 ( 9 8 ) 0 0 1 5 2 - X

1. Introduction

Weed competition is a major constraint to rainfed

rice production in West Africa. Yield-gap studies on

farmers' ®elds in various agro-climatic zones in Cote

d'Ivoire revealed that weed competition is the most

important yield-reducing factor, contributing about

50% to the yield gaps, followed in importance by

nitrogen nutrition and pests and diseases (WARDA,

1996; Becker and Johnson, 1997). Current trends

towards shorter fallow periods and more intensive

use of uplands further aggravate weed problems. Due

to the unavailability and limited effectiveness of her-

bicides, and despite the limited availability of labor

(Johnson, 1995; Dingkuhn and Randolph, 1997),

farmers rely largely on hand weeding in combination

with the use of comparatively weed-competitive but

low-yielding traditional O. sativa tropical-japonica

rices. Indigenous O. glaberrima upland rices, which

are highly weed-competitive but low-yielding due to

small panicles, grain shattering and lodging (Kof®,

1980; Dingkuhn et al., 1998; Johnson et al., 1998), have

been largely displaced from the production systems.

Efforts to utilize O. glaberrima genes to improve

Asian rice cultivars have recently intensi®ed after a

breakthrough in interspeci®c hybridization yielded

genetically stable and fully fertile progenies (Jones

et al., 1997a). The sterility barrier, which commonly

prevents interspeci®c breeding beyond the F1 genera-

tion, was partly avoided by selecting parental materi-

als with maximal genetic compatibility, and partly

overcome by backcrossing, anther culture and double-

haploidization (Jones et al., 1997a). On this basis, a

breeding program at the West Africa Rice Develop-

ment Association (WARDA) in Cote d'Ivoire aims at

developing low-management rice plant types for

labor-limited, weed- and drought-prone production

systems. The program seeks to combine the superior

weed competitiveness (Fofana et al., 1995) and resis-

tance to other stresses of O. glaberrima with the

higher yield potential of improved O. sativa tropi-

cal-japonica upland rices (WARDA, 1996; Dingkuhn

et al., 1997a, b).

The present study was conducted to identify mor-

phological properties of rice lines in monoculture that

would be predictive of interspeci®c competitiveness.

The work was based on descriptive studies in mono-

culture and a simultaneously conducted competition

experiment. A wide spectrum of genetic materials was

used to include O. sativa japonica and indica, O.

glaberrima and O. glaberrima � O. sativa interspe-

ci®c progenies.

2. Materials and methods

2.1. Site characteristics and general setup

Three ®eld experiments were conducted on adjacent

plots on an upland rice ®eld at WARDA's research

station at Mbe in Cote d'Ivoire (78520N, 5860W, 300 m

asl). The soil is an al®sol, characterized by loamy sand

texture (22% clay, 31% silt [12% ®ne, 19% coarse],

and 46% sand [23% ®ne, 23% coarse]) and frequent

iron concretions. Plots received supplemental over-

head sprinkler irrigation as necessary to keep the soil

at ®eld capacity.

Experiment 1 was conducted during the 1997 wet

season and served to characterize canopy properties,

growth dynamics and grain yield of 21 rice lines

grown in mono-culture. Experiment 2 was conducted

simultaneously to study the competitiveness with

weeds of a subset of 16 lines. It was part of a larger

trial to screen 100 lines for weed competitiveness.

Experiment 3, which was conducted during the 1996

wet season and repeated during the 1997 wet season,

was similar to experiment 2, but used a further subset

of eight lines. This setup served to determine whether

morphological and agronomic differences among cul-

tivars observed in monoculture (experiment 1) can be

correlated with weed competitiveness (experiments 2

and 3). All sowing dates were uniformly between 17

and 18 June. Weather data are presented in Table 2.

2.2. Experiment 1

Twenty-one diverse rice cultivars and breeding lines

were grown in clean-weeded plots, based on a rando-

mized complete-block design with three replications.

Plants were dry-seeded in moist, harrowed soil at three

to four seeds per hill at 0.25 m spacing and, after

emergence, thinned to two plants per hill.

Plots measured 3 m � 5 m and were fertilized with

a basal application of 40 kg N as urea, 20 kg P as triple

superphosphate and 20 kg K and KCl per hectare; an

additional topdressing of 20 kg N as urea was made at

80 M. Dingkuhn et al. / Field Crops Research 61 (1999) 79±95

Table 1

Summary of selected genetic materials and results of experiment 1 (grain yield at maturity (MAT) and canopy architecture parameters in monoculture at 31, 46 and 64 days after

sowing), experiments 2 and 3 (relative grain yield under competition with natural weeds (WEE), maize (MAI) and the O. glaberrima landrace IG10). Mbe, Cote d'Ivoire

Cultivar/line Stature Origin Observations under monoculture (experiment 1, 1997) Relative yield under competition

DUR

(day)

GY

(t haÿ1)

TN

(mÿ2)

LAI SLA (m2 kgÿ1) Kdf MTA

(deg)

Lp Experiment 2,

1997

Experiment 3,

1996/97

WEE MAI IG10 WEE MAI IG10

MAT MAT MAT 31 46 64 31 46 64 46 46 46 MAT MAT MAT MAT MAT MAT

O. sativa indica (improved lowland)

Bouake 189 Semidwarf Indonesia 126 3.86 329 0.4 1.2 2.3 41 28.8 22 0.5 62 0.5 0.58 0.82 0.58 0.71 0.82 0.52

Suakoko 8 Med.-tall Liberia 145 2.67 335 0.5 1.2 3.96 43 28.1 33 0.6 56 0.5 1.7 1.05 0.41

O. sativa japonica (improved upland)

IDSA 6 Med.-tall Cote d'Ivoire 115 3.22 197 0.3 1.1 1.95 34 23.4 20 0.6 54 0.53 0.93 1.28 0.42 0.68 0.66 0.66

Moroberekana Tall Cote d'Ivoire 126 2.81 112 0.3 1.1 3.37 32 28.2 24 0.6 49 0.56 0.61 1.38 0.83 1.05 0.93 0.88

OS6 Med.-tall Nigeria 116 3.46 182 0.3 1.5 2.26 33 27.6 23 0.5 60 0.5 0.94 0.57 0.73 1.39 0.99 1.13

WAB56-104 Med.-tall Cote d'Ivoire 100 3.52 141 0.2 0.7 1.7 27 20.6 19 0.6 60 0.52 0.49 0.79 0.31 0.33 0.22 0.61

O. glaberrima

(traditional upland)

CG14 Med.-tall Senegal 106 2.67 359 0.5 1.9 4.9 42 31.3 30 0.5 46 0.6 1.25 1.25 0.98 0.98 1.1 0.84

IG10 Med.-tall Cote d'Ivoire 132 2.48 336 0.6 2.4 3.83 47 33.5 26 0.6 48 0.6 1.39 1.61 0.75 1.28 1.31 1.12

WAB56-104 � CG14 hybrids (upland)

WAB450-I-B-P26-1-1 Med.-tall Cote d'Ivoire 113 2.76 169 0.3 1.5 2.09 35 28 22 0.5 50 0.57 0.62 0.65 0.36

WAB450-24-3-2-P18 Tall Cote d'Ivoire 102 3.51 133 0.4 1.3 3.04 37 25 24 0.5 57 0.51 0.63 0.57 0.58 0.65 0.75 0.97

WAB450-I-B-B122-HB Med.-tall Cote d'Ivoire 105 3.12 125 0.4 1.2 2.55 30 26.2 21 0.6 51 0.51 0.67 0.47 0.46

WAB450-11-1-P60-1-2-1 Med.-tall Cote d'Ivoire 100 2.49 140 0.3 0.8 1.35 33 23.5 20 0.4 61 0.49 0.97 0.72 0.64

WAB450-I-B-P37-3-1 Med.-tall Cote d'Ivoire 105 3.12 143 0.3 0.8 1.37 43 25.8 19 0.5 57 0.52 0.23 0.53 0.27

WAB450-6-2-9-MB-2-1 Med.-tall Cote d'Ivoire 100 3 149 0.4 1.1 2.21 24 24.3 22 0.5 59 0.52 0.39 0.22 0.46

WAB450-9-2-6-1-1 Med.-tall Cote d'Ivoire 115 3.87 158 0.3 0.8 1.66 29 21.6 18 0.5 59 0.54 0.92 0.66 0.59

WAB450-16-3-6-1-1 Med.-tall Cote d'Ivoire 119 2.55 225 0.3 1.3 2.77 33 29.5 24 0.5 55 0.53 0.64 1.38 1.08

Mean 114 3.07 202 0.3 1.2 2.58 35 26.6 23 0.5 55 0.53 0.81 0.87 0.59 0.88 0.85 0.84

CV (%) 1 22 18 28 26 30 15 8 12 19 8 7 71 71

S.E. 1 0.4 20.6 0.1 0.2 0.44 3.1 1.2 1.6 0.1 2.6 0.02 0.28 0.24

Number of replications 3 3 4

Number of entries 21 16 8

aMoroberekan is a traditional type.

DUR, duration to maturity; GY, grain yield at 14% MC; TN, tiller number; LAI, leaf area index; SLA, specific leaf area; Kdf, PAR extinction coefficient; MTA, mean tip elevation

angle; Lp, dry matter partitioning to leaves; MAT, maturity.

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79±95

81

55 days after sowing (DAS). Weeds were controlled

by post-emergence herbicide (oxadiazon, 1 kg haÿ1)

and two cycles of manual weeding, 1 and 2 months

after sowing.

The test lines were selected to cover the morpho-

logical and genetic diversity of rices relevant to West

Africa (Table 1) and included: two upland-adapted O.

glaberrima landraces; three lowland-adapted O. sativa

indica cultivars (among them, two `modern' semi-

dwarfs and one medium-tall traditional type, Suakoko

8); three traditional, upland, O. sativa tropical-japo-

nica types; three improved, upland, O. sativa tropical-

japonica types; and 10 stable and fertile, O. sativa

tropical-japonica (WAB56-104) � O. glaberrima

(CG14) interspeci®c hybrids selected from breeder's

trials for morphological diversity. In addition, the

creeping, leguminous weed Mucuna pruriens var utilis

was included to provide for an extreme architectural

type for canopy structure measurements.

At 31, 46 and 64 DAS, above-ground growth para-

meters and canopy properties were measured on all

plots. Dry weights (dw) were determined destructively

from six hills per plot separately for stems (culm and

sheath) and green leaf blades. Leaf area index (LAI)

was measured destructively for six hills using an LI-

3000 (LI-COR, Lincoln, Nebraska) leaf area meter,

and non-destructively, with the LI-COR LAI2000

canopy analyzer. The combined area of leaf blades

and stems (SLAI) was determined by separate area

measurements for stems using the LI-3000 area meter.

To avoid errors in non-destructive LAI measure-

ments caused by direct solar radiation (LI-COR,

1989), measurements with the LAI2000 were con-

ducted only at dawn or, if the sky was completely

overcast, during the day. Independent measurements

conducted on breeder's plots gave a good agreement

between direct (destructive) and indirect (LAI-2000)

measurements (Y � 0.03 � 1.04X; R2 � 0.83;

N � 34; LAI range 0.5±2.0); and between PAR inter-

ception measurements with the LAI2000 (reciprocal

of the instrument output parameter `difn') and mea-

surements using conventional, ¯at PAR interceptors

(SOLEMS, Palaiseau, France; length 0.6 m, range

400±750 nm) placed diagonally through the rectangle

formed by six hills (Y � ÿ0.08 � 1.09X; R2 � 0.90;

range 0.2±0.75). The LAI2000 equipment was used to

simultaneously measure LAI, PAR transmission and

mean tip elevation angle (MTA).

The PAR extinction coef®cient for diffuse light

(Kdf), which is thought to be between 0.4 and 0.6

for rice (Kropff et al., 1994), was calculated from PAR

transmission and LAI using the following equation,

under the assumption that re¯ectance of the canopy

was 0.06 (Kropff and van Laar, 1993; Wopereis et al.,

1996):

Kdf � ÿln�0:94 PAR transmission�LAIÿ1

Kdf was calculated for individual datasets using

destructively measured LAI and the `difn' parameter

of the LA2000 canopy analyzer (Table 1), and across

lines, dates and replications, by regressing ÿln(PAR

transmission) vs. LAI (Fig. 1). Fig. 1(c) and (f) pre-

sents the results of Kdf calculations using six different

combinations of input parameters for the Kdf calcula-

tions (destructive or non-destructive LAI, PAR trans-

mission measured with interceptor bars or with

LAI2000, area values including stems [SLAI] or

excluding stems [LAI]). Differences between trans-

mission measurements using bars or LAI2000 were

small, but the exclusion of stem area led to a marked

increase in Kdf. Calculation of Kdf using exclusively

parameters measured by LAI2000 gave even larger

values. The Kdf based on destructive SLAI and the

transmission measured with LAI2000 (`difn' para-

meter) are henceforth used because it gave the best

agreement with published data for rice (Kropff and

van Laar, 1993; Casanova et al., 1998), and because it

differentiated well between Mucuna and rice. It

remains unclear, however, which combination of para-

meters provides the most accurate results.

Light penetration pro®les through the canopy were

conducted with PAR interceptor bars in 10 cm hight

movements for each plot and sampling date, and

strictly under overcast sky conditions. Speci®c leaf

area (SLA) was calculated on the basis of LAI and leaf

dw. Results obtained with destructive and non-

destructive LAI measurements were similar, and only

the former are reported. Plant height and tiller number

were measured and averaged for six hills per plot.

At maturity, yield and yield components were

determined from 5 m2 per plot following methods

described by Dingkuhn et al. (1998). Graphic repre-

sentations and calculations of con®dence intervals for

regressions were conducted with SIGMA-PLOT 1.03

(Jandel Scienti®c, Erkrath, Germany, 1994). ANOVA

analyses were conducted with MSTATC software.


2.3. Experiment 2

In the 1997 screening experiment for weed compe-

titiveness, 100 rice lines, comprising O. glaberrima,

O. sativa and interspeci®c hybrids, were sown in

single rows 1.5 m long. Only the results for 16 lines

(Table 1) that were also studied in experiment 1 are

presented. The rows were bordered on either side with

a spacing of 0.25 m by rows of the same lines, natural

weed growth, O. glaberrima (IG10) or the local maize

cultivar Bouake 9449SR. The experiment was a split-

plot design with competitors as main plots and test

cultivars as sub-plots and there were three replicates.

Weed growth was controlled by an application 0.75 kg

Fig. 1. Determination of the PAR extinction coefficient (Kdf) by regressing ln of diffuse PAR transmission, with an estimated 6% reflection

subtracted, against leaf area index (LAI); for each of four genetic rice groups (Fig. 1(a)±(d); details in Table 1); and across all 21 test lines

(Fig. 1(e) and (f)), using data from experiment 1. Kdf is -(slope) of the relationships. Fig. 1(e) and (f) compares different methodologies for Kdf

determination, using the LAI2000 fisheye system (output parameter `difn') or interceptor bars (LTR) to measure relative PAR transmission;

and indirectly (LAI2000) or destructively measured LAI or stem-and-leaf area index (SLAI). Lines indicate linear regressions, in Fig. 1(e) and

(f) with 0.05-confidence intervals. DAS, days after sowing. Mbe, Cote d'Ivoire, 1997 wet season.

M. Dingkuhn et al. / Field Crops Research 61 (1999) 79±95 83

oxadiazon per hectare after sowing, except for 0.25 m

strips with intended weed growth that were shielded

during the herbicide application. Maize caused exces-

sive shading and was removed at 50 DAS. Prior to

sowing, a basal fertilizer dose of 20 kg P haÿ1 as triple

superphosphate was applied, and 46 kg N haÿ1 as urea

was applied at 28 and 56 DAS. Yields were obtained

from 1 m of the central row, and were expressed as

yields of rice grown with competitors relative to the

yield in the monoculture treatments.

2.4. Experiment 3

In a second screening experiment conducted during

the 1996 wet season and repeated in 1997 (Table 2),

14 rice cultivars comprising O. glaberrima, O. sativa

and interspeci®c hybrids, were sown in single rows

2.5 m long. The rows were bordered on either side at a

spacing of 0.25 m by rows of the same cultivar, natural

weed growth, O. glaberrima (IG10), or maize. The

design and management were the same as for experi-

ment 2, except that there were four replicates each

year. The results for eight lines, which were also

studied in experiments 1 and 2, are presented here

(Table 1). Data for the 2 years were averaged.

3. Results and discussion

3.1. Canopy properties and agronomic performance

in monoculture (experiment 1)

The 21 lines matured between 100 and 145 DAS

and produced between 96 and 364 surviving tillers

mÿ2 and between 2.5 and 3.9 t grain per hectare at

maturity (Table 1). The highest tiller numbers were

observed for lowland-adapted O. sativa indica rices

(329±364) and O. glaberrima upland rices (336±359).

Upland-adapted O. sativa japonica cultivars produced

between 112 (traditional-tall Moroberekan) and 197

tillers mÿ2 (improved IDSA 6), and interspeci®c O.

sativa � O. glaberrima hybrids produced between 96

and 225 tillers mÿ2. The highest yields (�3.5 t haÿ1)

were observed in locally cultivated cultivars, namely

an O. sativa indica (Bouake 189) and two O. sativa

japonica cultivars (WAB56-104 and Iguape Cateto).

Two interspeci®c hybrids also yielded over 3.5 t haÿ1,

whereas the two O. glaberrima cultivars yielded only

2.5 and 2.7 t haÿ1.

The greatest LAI was observed at 64 DAS for the

two O. glaberrima cultivars CG14 (4.9) and IG10

(3.8) and the long-duration indica type Suakoko 8

(4.0). IG10 has been previously described as being

highly competitive with weeds, while IDSA6 and

WAB56-104 have been described as relatively poor

competitors (Johnson et al., 1997). WAB56-104 and

IDSA6, both improved O. sativa japonica types,

developed LAIs of only 1.5 and 2.0, respectively.

SLA decreased gradually in most lines between 31

and 64 DAS and was comparatively uniform among

the O. sativa japonica and O. glaberrima upland

groups, but varied widely among the interspeci®c

hybrids. SLA, which has previously been found to

be strongly related to vegetative growth vigor and LAI

(Dingkuhn et al., 1998), was generally greatest in the

O. glaberrima and O. sativa indica groups. The two

parents of the interspeci®c crosses, WAB56-104 and

CG14, had the smallest and greatest mean SLA among

the test lines (hybrids excluded), which probably

explains the heterogeneity of SLA among the hybrids.

Kdf, as determined for individual plots and sampling

dates by destructive area determination for SLAI and

independent measurements of diffusive light transmis-

sion (difn) using the LAI2000 canopy analyzer, varied

Table 2

Weather data for the wet season 1997 as monthly means (cumulative for rainfall) at the experimental site at Mbe, Cote d'Ivoire

Period T (max) T (min) Rs Wind ETo Epan Rainfall

8C 8C MJ m sÿ1 mm mm mm

June 31.0 21.4 14.4 0.8 4.2 2.9 134

July 29.7 21.2 12.6 0.8 3.8 2.6 81

August 30.9 21.3 13.3 0.6 4.0 2.8 13

September 32.7 21.5 13.5 0.4 4.1 3.1 39

October 33.7 21.7 16.0 0.4 5.5 4.5 154

T, air temperature at 2 m; Rs, solar radiation; ETo, Penman evaporation; Epan, class-A pan evaporation.


between 0.29 and 0.64 among lines and sampling

dates, with no apparent pattern in relation to genetic

groups. Similar values (0.44 to 0.65; Fig. 1) were

obtained when Kdf was determined across genetic

groups by linear regression (ln[0.94 difn] vs. SLAI),

following the approach described by Casanova et al.

(1998). Measurements of Kdf were highly sensitive to

methodology, however, and depended on the inclusion

of stems in the area determinations, the choice of

destructive or non-destructive (LAI2000 analyzer)

area measurements, and PAR transmission measure-

ments with either the LAI2000 ®sheye system or

conventional interceptor bars. The combination of

destructive measurements of SLAI and the difn para-

meter of the canopy analyzer is used hereafter in this

report.

The mean elevation angle of leaf tips (MTA;

Table 1), varied between 468 (CG14) and 628 (Bouake

189) at 46 DAS; the smallest values were found in the

two O. glaberrima cultivars and the traditional tropi-

cal-japonica upland cultivar, Moroberekan, which are

known for having comparatively lax leaves. The

leaves of the O. glaberrima landraces were bent along

the entire length of the blades, whereas those of

Moroberekan were mainly bent towards the tip.

Two interspeci®c hybrids, WAB450-I-B-P26-1-1

and WAB450-I-B-B122-HB, which have been speci-

®cally selected for droopy leaves, had a similar MTA

(508 and 518, respectively).

Partitioning ratios for dry matter between 31 and 46

DAS (dw increase for leaf blades over the increase of

the total above-ground dw) were greatest for the two

O. glaberrima cultivars (0.60) and the smallest for the

three O. sativa indica lines (0.50). Comparatively high

leaf partitioning ratios have previously been reported

for O. glaberrima cultivars (Dingkuhn et al., 1997b;

Johnson et al., 1998).

Fig. 2 illustrates time courses of SLA, MTA and Kdf

for the four genetic groups investigated. Mucuna was

included to (i) test the measurement techniques on a

profoundly different canopy structure (near-horizontal

leaves) and to (ii) determine these canopy descriptors

for a C3 species that developed leaf area 5±10 times as

rapidly as rice (LAI � 8 at 46 DAS, based on the same

population density; data not presented).

The SLA of Mucuna was twice that of rice, but

decreased over time in a way similar to rice

(Fig. 2(a)), indicating that the species was able to

produce about twice the leaf area of rice with the

same amount of assimilates invested. The MTA of

Mucuna was 138 elevation at 31 DAS and increased

gradually to 348 at 64 DAS as the canopy became

Fig. 2. Time courses of specific leaf area, mean tip elevation angle and the PAR extinction coefficient, averaged for three two genetic groups

of O. sativa (indica, tropical-japonica), O. glaberrima, interspecific O. glaberrima � japonica progenies, and the creeping legume Mucuna

(details in Table 1) for experiment 1. Vertical bars indicate standard error (SE) for group means. Mbe, Cote d'Ivoire, 1997 wet season.


crowded and leaves were physically forced into arbi-

trary positions (visual observations), whereas MTA of

rice was comparatively constant. Among the genetic

groups of rice, the O. sativa japonica and indica

groups, as well as the interspeci®c progenies, had

lower MTA at 31 DAS than thereafter (corresponding

to a visibly laxer foliage for seedlings than for plants at

tillering stage (46 and 64 DAS), whereas no such

change was observed for the two O. glaberrima land-

races.

The Kdf of rice for individual plots and dates was

mostly below 0.4 at 31 DAS (Fig. 2(c)) and increased

to values between 0.5 and 0.6 at 46 DAS, whereas that

of Mucuna was about 0.7 at 31 and 46 DAS, and

increased to nearly 1.0 at 64 DAS, indicating that the

canopy behaved like a single, horizontal and opaque

sheet. The relatively low Kdf observed for seedlings of

all species (31 DAS) was probably not due to leaf

elevation angle (which was rather small at that stage),

but to gaps in the canopy resulting from the 0.25 m

spaces between hills (Kropff and van Laar, 1993).

Regression-based calculations of Kdf did not show a

markedly lower Kdf at 31 DAS (Fig. 1).

In summary, the canopies of genetically and agro-

ecologically diverse rices revealed strong commonal-

ities when compared with a contrasting type such as

Mucuna (Fig. 2), although differences were evident

between the two upland rice groups, O. glaberrima

and O. sativa japonica. The O. glaberrima cultivars

were among the lines with the lowest grain yields,

highest tiller numbers, greatest SLA, lowest MTA and

highest leaf partitioning ratios for dry matter. The

interspeci®c progenies varied widely in all parameters

but were mostly, with a few notable exceptions, closer

to the O. sativa japonica parental group than to O.

glaberrima, probably because the main selection cri-

teria in the breeding program had been yield potential

at various input levels, and not weed competitiveness.

3.2. Profiles of PAR penetration and time courses of

shading (experiment 1)

The ability of a rice crop to deprive a competitor of

light resources depends on interception of PAR at the

highest stratum attained by the foliage of the compe-

titor. Taller growth may in some cases be the most

effective strategy, but more assimilates are required to

achieve a given light interception at a greater height.

Four examples of PAR penetration pro®les through

the canopy, as well as the optical density distribution

derived from these, are presented in Fig. 3. At 31

DAS, the density of all canopies was greatest near the

ground and tailed off to the top. That pattern was

associated with very erect top (young) leaves and lax

lower leaves. This density distribution was observed

also at 46 and 64 DAS for the three O. sativa indica

lines (exempli®ed by Bouake 189), allowing deep

light penetration into an erect foliage. By contrast,

in the traditional upland rice Moroberekan, a sharp

density maximum was located at mid-height of the

canopy, resulting from the broad, long and lax leaves.

The O. glaberrima CG14 had a near-normal (Gaus-

sian) distribution of canopy density on the vertical

pro®le (Dingkuhn et al., 1997a) owing to a large

number of tillers inserting at different angles and

lax leaves. Different canopy architectures were found

among the interspeci®c progenies. Some had an archi-

tecture resembling that of maize with a broad plateau-

like maximum of canopy density at mid-height

(Fig. 3, bottom).

The different canopy architectures translated into

diverse patterns over time of PAR interception at any

given distance from the ground (Fig. 4). At 31 DAS,

no rice cultivar intercepted more than 20% of PAR at

ground level, whereas Mucuna intercepted about 50%

PAR. At 46 DAS, differences between rice lines

emerged, with the O. glaberrima landraces, CG14

and IG10, intercepting more light than the other lines

at 0 and 20 cm above ground. At 40 cm, however,

these landraces lost some of their advantage due to

their intermediate height. At 64 DAS, the short-sta-

tured and erect-leaved Bouake 189 achieved only 25%

light interception at 40 cm, whereas the other rices

intercepted between 50 and 70%.

The behavior of three interspeci®c progenies

selected by the breeder for their vegetative vigor

was intermediate to that of their parents (Fig. 4,

bottom row), but these lines had no advantage relative

to the tall-traditional, japonica cultivar Moroberekan.

Consequently, breeding for progenies having much

of the lea®ness and vigor of O. glaberrima rices was

partially successful, but the choice of the poorly weed

competitive WAB56-104 as O. sativa parent was

not optimal. In fact, parent selection in this recent

interspeci®c breeding program was initially more

driven by genetic compatibility than potentiality


as donor for weed competitiveness (Jones et al.,

1997a, b).

3.3. Relationships between crop performance and

canopy architecture (experiment 1)

A correlation matrix across lines between canopy

architectural properties and growth parameters

(Table 3) revealed signi®cant parameter relationships,

some of which depended on the developmental stage

of the crop. The most important ones are discussed

below.

3.3.1. Specific leaf area

The most consistent and generally positive correla-

tions (P � 0.05) across development stages were

Fig. 3. Profiles of diffuse PAR transmission (left) and PAR extinction per 10 cm stratum (fraction of incoming PAR; right) for four contrasting

rice cultivars (details in Table 1) at 31, 46 and 64 days after sowing (DAS) in experiment 1. Schematic drawings indicate the visually observed

architecture of the cultivars at 64 DAS. Mbe, Cote d'Ivoire, 1997 wet season.


observed between SLA and LAI at three sampling

dates, tiller number at maturity, and crop duration.

Correlations between SLA and yield were generally

negative, but not signi®cant. Among these correla-

tions, the SLA vs. LAI relationships, which were

signi®cant both within and across sampling dates,

are consistent with previous reports (Dingkuhn et

al., 1997a, b) and can be readily explained on a

physiological basis. A high SLA reduces the amount

of assimilates needed to produce a given leaf area,

resulting in earlier ground cover, and therefore, a

greater light harvest and higher canopy photosynthesis

rates early in the season.

The same principle might have been responsible for

the positive correlations between SLA and tiller num-

ber, as tillering depends on the daily assimilate sur-

plus, and therefore, on the relative growth rate of the

crop (Schnier et al., 1990; Dingkuhn et al., 1991).

Across lines, however, this relationship was certainly

confounded by genetic tillering abilities and SLA. The

present observed correlation between SLA during

vegetative growth and tiller number at maturity was

partly due to the inclusion of diverse genetic groups in

the experiment, such as short-statured and high-tiller-

ing O. sativa indica types (high-SLA), extremely high-

tillering O. glaberrima types (high-SLA) and low-

tillering O. sativa japonica types (frequently, low-

SLA).

The correlations between SLA and crop duration

are likely to have a phenological basis as SLA obser-

vations made at common dates necessarily correspond

to different developmental stages in lines having

Fig. 4. Time courses of diffuse PAR at ground level and 20 cm and 40 cm heights (fraction of ambient PAR), for seven rice cultivars

(including the interspecific progenies WAB450-24-3-2-P18 (1) and WAB450-I-B-B122-HB (2); details in Table 1) and the creeping legume

Mucuna grown in monoculture in experiment 1. Vertical bars indicate standard error (SE) across cultivars. Mbe, Cote d'Ivoire, 1997 wet

season.


Table 3

Correlation matrix across 21 rice lines for descriptors of canopy architecture observed at 31, 46 and 64 days after sowing (DAS) (horizontal) vs. leaf area index (LAI) at 31, 46 and

64 DAS; and tiller number, grain yield and crop duration at maturity (MAT) in experiment 1

Specific leaf area Leaf partitioning ratio Mean tip elevation angle Extinction coefficient (Kdf) Plant height

31 DAS 46 DAS 64 DAS 31 DAS 46 DAS 64 DAS 31 DAS 46 DAS 64 DAS 31 DAS 46 DAS 64 DAS 31 DAS 46 DAS 64 DAS

LAI (31 DAS) 0.51 0.55 0.53 0.52 0.33 0.21 0.28 ÿ0.4 ÿ0.4 ÿ0.41 0.47 ÿ0.16 ÿ0.22 0.07 ÿ0.04

LAI (46 DAS) 0.49 0.81 0.68 0.63 0.63 0.42 0.12 ÿ0.65 ÿ0.44 ÿ0.15 0.26 0 ÿ0.32 ÿ0.21 ÿ0.04

LAI (64 DAS) 0.51 0.62 0.87 0.48 0.58 0.3 0.17 ÿ0.68 ÿ0.51 ÿ0.27 0.44 ÿ0.51 ÿ0.19 ÿ0.09 0.12

Tillers (MAT) 0.61 0.73 0.72 0.2 0.27 0.16 ÿ0.12 ÿ0.24 0.06 0.06 ÿ0.02 ÿ0.29 ÿ0.54 ÿ0.56 ÿ0.59

Yield (MAT) ÿ0.26 ÿ0.41 ÿ0.27 ÿ0.41 ÿ0.27 ÿ0.13 0.25 0.47 0.49 0 ÿ0.01 ÿ0.07 0.22 ÿ0.23 0.15

Duration (MAT) 0.48 0.62 0.63 0.23 0.1 0.44 0.17 ÿ0.18 ÿ0.14 0.01 0.17 ÿ0.32 ÿ0.18 ÿ0.16 ÿ0.3

Critical R (1-tail) for P � 0.05 0.37

Critical R (2-tail) for P � 0.05 0.43

Significant (P < 0.05) correlations are in bold print. Mbe, Cote d'Ivoire, 1997 wet season.

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different durations. SLA of any given cultivar is

known to decrease as a function of developmental

stage (Dingkuhn et al., 1998), and, if measured at a

speci®c number of days after sowing, is likely to be

higher in long- than short-duration cultivars.

The generally negative but non-signi®cant associa-

tions between SLA and grain yield support previous

analyses attributing assimilate source limitations to

high SLA, a result of low leaf-level photosynthetic

rates. According to the present results, this effect

might not be very pronounced in the varietal types

considered.

3.3.2. Assimilate partitioning

Partitioning ratios of dry matter between leaf blades

and stems (leaf sheath plus axis) observed between 0

and 31 DAS and between 31 and 46 DAS were

positively correlated with LAI at 31, 46 and 64

DAS. Partitioning ratios measured between 46 and

64 DAS, however, were not. Consequently, partition-

ing patterns favoring growth of leaf blades increased

LAI, indicating that partitioning and SLA are com-

plementary crop parameters contributing to early

ground cover. In a previous comparisons between

progenies of interspeci®c crosses and their O. sativa

and O. glaberrima parents (Dingkuhn et al., 1997b),

the superior vegetative vigor of the O. glaberrima

parent was also associated with higher SLA and

assimilate partitioning to leaf blades.

3.3.3. Mean tip elevation angle

Mean leaf tip elevation angle of rice canopies,

measured with the LAI-2000 canopy analyzer using

the gap fraction approach (Anderson, 1971), was

negatively correlated with LAI (measured indepen-

dently with a destructive method) at 46 and 64 DAS.

i.e., plants with laxer leaves developed a greater LAI,

possibly as a result of greater light interception. MTA

at 64 DAS was positively correlated with crop dura-

tion, which was mainly a result of the relatively long

duration of the three erect O. sativa indica types, and

not an expression of a physiological interdependency

of these parameters.

3.3.4. PAR extinction coefficient

The extinction coef®cient (Kdf) for diffuse PAR was

measured with different methods, of which the com-

bination of direct (destructive) determinations of LAI

and measurements of the transmission ratio using

LAI-2000 gave the most reproducible results.

Although Kdf and MTA are theoretically closely

related, MTA was more strongly correlated with LAI

than was Kdf. The commonsensical hypothesis that

LAI and Kdf should be positively correlated during

early developmental stages of the crop could not be

con®rmed.

3.3.5. Plant height

Plant height was not related across lines with LAI,

yield or crop duration, but was negatively correlated

with tiller number. This observation is not surprising,

considering that in breeding, dwar®ng is generally

associated with greater tillering ability. It may be an

indication of physiological tradeoffs between the two

plant characteristics, based on the fact that tallness and

tillering both require investments of assimilates.

3.4. Relationships between canopy properties and

specific leaf area (experiment 1)

Because SLA is a major determinant of growth

vigor (Table 3, also: Dingkuhn et al., 1998; Johnson

et al., 1998), we investigated its relationship with other

leaf canopy properties. Across the 21 lines and within

speci®c sampling dates, LAI vs. SLA scattergrams

(Fig. 5(a)) indicated that the genetic groups (O. gla-

berrima, O. sativa indica and japonica, and interspe-

ci®c hybrids) had a common linear relationship, from

which none of the lines deviated. In fact, even the

creeping legume Mucuna followed the same pattern

(at 64 DAS: SLA 46 kg mÿ2; LAI 7.1). Consequently,

SLA is a robust predictor of potential LAI under given

resources. Marked deviations may occur, however, if a

different photosynthetic metabolism or major compet-

ing sinks for assimilates are present, or if water stress

occurs.

Given its strong positive effects on LAI, SLA was

negatively correlated with PAR transmission

(Fig. 5(b)). No correlations were observed with Kdf,

but a negative correlation was observed with MTA

(Fig. 5(c)). The MTA vs. SLA relationship was fairly

uniform for O. glaberrima rices, O. sativa japonica

rice and their interspeci®c progenies, but the three O.

sativa indica rices had a higher tip angle relative to

their SLA. These lines were speci®cally bred for an

erect foliage. A hypothesis deserving further study is


that the comparative laxness (smaller MTA) of leaves

with high SLA is partly due to greater mechanical

¯exibility.

3.5. Competitiveness with natural and simulated

weeds (experiments 2 and 3)

In experiment 2, the relative grain yields of 16 lines

(subset from experiment 1) were determined under

interspeci®c competition and expressed as a fraction

of yields under monoculture in the same experiment

(Table 1). The competitors were chosen to create

contrasting competition environments: (1) natural

weed growth of the site dominated by the grass

Digitaria horizontalis during the ®rst month, gradu-

ally replaced by broadleaf species (Synedrella nodi-

¯ora, Ageratum conyzoides and other Compositae

species thereafter); (2) maize rows removed at 50

DAS to simulate intense initial competition for

resources, after which cultivars could demonstrate

their recovery ability; and (3) the O. glaberrima land-

race IG10, which, with its high vigor, tillering rate and

long duration, simulates a continuous pressure from a

grassy weed ¯ora.

In competition with the natural weed ¯ora, the

greatest relative yields were observed for Suakoko 8

(1.70), a highly weed-competitive, traditional, O.

sativa indica cultivar; and in the two O. glaberrima

landraces (IG10, 1.39; CG14, 1.25). Yields of these

cultivars increased when their neighboring rows were

substituted with natural weeds. The same three culti-

vars had the largest LAI in experiment 1 under mono-

culture. By contrast, two interspeci®c progenies and

their O. sativa parent, WAB56-104, had relative yields

of less than 0.5, indicating a >50% reduction. CG14

and IG10 have previously been identi®ed as being

good weed competitors, and WAB56-104 as being

poorly competitive (Fofana et al., 1995; Johnson et al.,

1997).

Under competition with maize sown simulta-

neously, and removed at 50 DAS, the greatest relative

yields were observed for IG10 (1.61), followed by the

traditional, tall upland cultivar Moroberekan and the

interspeci®c progeny WAB450-16-3-6-1-1 (both

1.38). These three cultivars had the longest durations

within their respective genetic groups, indicating that

late maturity provided a decisive advantage under

severe, but temporary competition. All other interspe-

Fig. 5. Relationships between leaf area index (a), PAR transmission at ground level (b) and mean tip elevation angle (c) vs. specific leaf area

at 31, 46 and 64 days after sowing (DAS) in experiment 1, across 21 diverse rice cultivars. Solid lines indicate linear regressions and dotted

lines, 0.05-confidence intervals. Mbe, Cote d'Ivoire, 1997 wet season.


ci®c progenies (relative yields between 0.22 and 0.72)

competed poorly with maize, possibly because of their

early maturity.

Except CG14 and the same interspeci®c progeny

that competed well with maize, the competitiveness of

none of the test lines was equal or better than IG10, the

most weed-competitive O. glaberrima cultivar so far

identi®ed. When competing with itself, the center row

of IG10 lodged, thus explaining the its low relative

yield (0.75).

For comparison, results of an identical competition

trial using a subset of eight cultivars in 1996 and 1997

are also presented in Table 1. Again, IG10 and CG14

were the best competitors across environments, and

WAB56-104 was among the poorest. In summary, the

two O. glaberrima landraces competed consistently

well across the three treatments and 2 years. Other

cultivars performed well in speci®c environments,

such as Suakoko 8 in the neighborhood of natural

weeds.

3.6. Determinants of weed competitiveness

(experiment 1 vs. 2 and 3)

A major objective of the study was to identify crop

parameters under monoculture that would be predic-

tive of competitiveness in mixed stands. A correlation

matrix between crop architectural and agronomic

parameters under monoculture vs. actual competitive-

ness with three species (natural weed ¯ora, maize, O.

glaberrima cv. IG10.) is presented in Table 4. The

parameters most predictive of weed competitiveness

were LAI (positive), SLA (positive), tiller number

(positive) and crop duration (positive). Wherever leaf

partitioning ratios were correlated with competitive-

ness (46 and 64 DAS, with maize as competitor), the

slope was also positive. The mean tip angle was

generally negatively correlated with competitiveness

(signi®cant only at 46 DAS), indicating that lax leaf

canopies compete better with weeds than erect ones.

No signi®cant correlations were observed for Kdf,

except for 31 DAS in experiment 3.

Patterns of parameters predictive of competitive-

ness differed between competition treatments (natural

weeds, maize, IG10). MTA (46 DAS) and leaf parti-

tioning ratios (46 and 64 DAS) were signi®cantly

correlated only with competitiveness with maize

whereas LAI and SLA were correlated with competi-

tiveness in all treatments (but not necessarily at all

dates). Tiller number and crop duration in monocul-

ture were correlated with competitiveness with natural

weeds and maize, but not with IG10. This might have

been a result of the high tillering ability and duration

of IG10 (Table 1), a `strategy' that could possibly not

be beaten by rice lines having the same properties. The

advantage of long crop duration and high tiller number

under competition with maize was due to the fact that

maize was removed at 50 DAS, thus giving an advan-

tage to rice lines that were capable of compensatory

tillering and vegetative growth.

Crop parameters that correlated positively with

competitiveness, correlated negatively, although not

signi®cantly, with yield under monoculture, and vice

versa (compare Tables 3 and 4). This ®nding indicates

that there were tradeoffs between weed competitive-

ness and yield potential for all traits measured. To

reduce these tradeoffs, Jones et al. (1997a, b) proposed

selecting cultivars having high SLA to improve com-

petitiveness with weeds during vegetative growth.

Such leaves would be replaced with erect leaves

having low SLA and high chlorophyll content during

the reproductive growth period.

Stepwise, linear, multiple regression analyses were

conducted for canopy architectural properties vs.

competitiveness (means of competition treatments)

(Table 5). SLA ®gured signi®cantly in all models,

whereas plant height was generally rejected. Kdf

and MTA were rejected in all models that were based

on individual sampling dates, but were retained in the

model using averaged canopy observations for the

vegetative growth period (31 and 46 DAS). Dry matter

partitioning to leaves ®gured only in the model cor-

relating monoculture observations at 64 DAS with

competitiveness, but this effect might be partially a

result of a physiological linkage of leaf partitioning

ratios with crop duration during advanced growth

stages. Dynamics of partitioning are commonly seen

as a function of developmental stage and not time, and

lines with longer duration competed well with maize

due to their ability to recover after maize was

removed.

The overall model predicting mean competitiveness

(across competitor species) with averaged canopy

observations during the vegetative growth period

had a probability of 0.0004, and included SLA (posi-

tive), MTA (negative) and Kdf (positive) as predictor


Table 4

Correlation matrix across rice lines for descriptors of canopy structure and grain yield observed in monoculture (experiment 1) vs. competitiveness (yield under competition/

monoculture treatments) in Experiment 2 (16 entries) and Experiment 3 (8 entries)

Experiment Competitor Leaf area index Leaf partitioning ratio Specific leaf area Mean tip elevation angle

31 DAS 46 DAS 64 DAS 31 DAS 46 DAS 64 DAS 31 DAS 46 DAS 64 DAS 31 DAS 46 DAS 64 DAS

2 Natural weeds 0.52 0.51 0.62 0.36 0.27 0.22 0.5 0.41 0.7 ÿ0.08 ÿ0.31 ÿ0.06

2 Maize 0.3 0.52 0.57 0.27 0.58 0.64 0.48 0.57 0.44 ÿ0.19 ÿ0.56 ÿ0.44

2 IG10 0.23 0.5 0.55 0.23 0.4 0.27 0.13 0.57 0.37 ÿ0.28 ÿ0.34 ÿ0.43

2 Mean 0.45 0.63 0.72 0.37 0.52 0.48 0.5 0.63 0.64 ÿ0.21 ÿ0.51 ÿ0.37

3 Natural weeds 0.58 0.72 0.46 0.52 0.4 0.53 0.43 0.76 0.57 0.53 ÿ0.41 ÿ0.36

3 Maize 0.87 0.9 0.73 0.59 0.6 0.53 0.79 0.95 0.79 0.78 ÿ0.61 ÿ0.41

3 IG10 0.52 0.66 0.43 0.57 0.34 0.18 0.3 0.49 0.49 0.2 ÿ0.37 ÿ0.42

3 Mean 0.72 0.83 0.59 0.6 0.49 0.48 0.57 0.82 0.67 0.58 ÿ0.51 ÿ0.42

Experiment Competitor Extinction coefficient (Kdf) Plant

height

Tillers

maturity

Yield

maturity

Duration

maturity

31 DAS 46 DAS 64 DAS 31 DAS 46 DAS 64 DAS

2 Natural weeds ÿ0.38 0.31 ÿ0.22 ÿ0.46 ÿ0.43 ÿ0.29 0.67 ÿ0.38 0.56 Critical R (P � 0.05):

2 Maize 0.06 0.38 ÿ0.1 ÿ0.14 ÿ0.24 0.02 0.54 ÿ0.47 0.53 1-tail 0.43

2 IG10 0.25 0.08 ÿ0.23 0.06 ÿ0.17 0.25 0.34 ÿ0.35 0.17 2-tail 0.49

2 Mean ÿ0.08 0.35 ÿ0.21 ÿ0.27 ÿ0.37 ÿ0.05 0.67 ÿ0.51 0.57 N 16

3 Natural weeds ÿ0.76 0.04 0.07 ÿ0.05 ÿ0.4 0.03 0.29 ÿ0.53 0.6 Critical R (P � 0.05):

3 Maize ÿ0.69 0.08 0.1 ÿ0.26 ÿ0.3 ÿ0.05 0.58 ÿ0.64 0.63 1-tail 0.63

3 IG10 ÿ0.91 0.1 ÿ0.13 0.23 ÿ0.08 0.44 ÿ0.04 ÿ0.5 0.21 2-tail 0.7

3 Mean ÿ0.86 0.08 0 ÿ0.06 ÿ0.31 0.11 0.33 ÿ0.61 0.56 N 8

Significant correlations (P < 0.05) in bold italic print. Mbe, Cote d'Ivoire.

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variables. Consequently, rice lines with thin and lax

leaves and a high extinction coef®cient were the best

competitors.

4. Conclusions

Our study con®rmed that the O. glaberrima land-

races IG10 and CG14 are more weed competitive than

the most important O. sativa cultivars available in

West Africa, and that they owe this advantage largely

to superior SLA (which leads to a high LAI at an early

stage) and tillering ability. Leaf partitioning ratio and

parameters related to foliage architecture (Kdf and

MTA) also contributed to competitiveness, but only

at speci®c developmental stages and/or under compe-

tition with speci®c species. Plant height was not

correlated with competitiveness under the experimen-

tal conditions.

The interspeci®c progenies did not match the com-

petitiveness of their O. glaberrima parent, except for

one case, possibly because an essential trait, high

tillering ability, was not present. It is evident that

several traits need to be combined to achieve competi-

tiveness with weeds. Competitiveness was negatively

but weakly correlated with yield potential, and posi-

tively, with crop duration. Tradeoffs between compe-

titiveness and yield potential were, therefore,

signi®cant, but could possibly be reduced by speci®-

cally expressing traits for competitiveness, such as

high SLA and tillering ability, at an early develop-

mental stage.

Major knowledge gaps remain with respect to weed

competitiveness under drought- and ¯ood-prone con-

ditions, which are frequently associated with weed

problems in West Africa. It is possible that traits for

lea®ness, such as high SLA, affect drought resistance

negatively. The present study, therefore, needs to be

extended to drought- and ¯ood-prone environments

(to which most O. glaberrima types, but few O. sativa

types are well-adapted) in order to develop a screening

system for weed-competitiveness suited to a broad

range of physical environments.

Acknowledgements

This research was partly funded by the Department

for International Development (DFID), UK. D.E.

Johnson is employed by the Natural Resources Insti-

tute, University of Greenwich, UK.

References

Anderson, R.S., 1971. Radiation and crop structure. In: Sestak, Z.,

Catsky, J., Jarvis, P.G. (Eds.), Plant Photosynthetic Production.

Manual of Methods. W. Junk, The Hague, Netherlands, pp.

412±466.

Becker, M., Johnson, D.E., 1997. Leguminosen zur nachhaltigen

Verbesserung von Trockenreis-Brache Rotationen in West

Africa. In: Ottow, J.C.G., Sauerborn, J. (Eds.), Leguminosen

zur Verbesserung und nachhaltigen Sicherung der Agrarpro-

duktion. Giessener Beitraege zur Entwicklungsforschung,

series 1, vol. 24. Justus-Liebig-Universitaet, Giessen, Germany

(in press).

Casanova, D., Epema, G.F., Goudriaan, J., 1998. Monitoring rice

reflectance at field level for estimating biomass and LAI. Field

Crops Res. 55, 83±92.

Dingkuhn, M., Randolph, T.F., 1997. The potential role of low-

management rice technologies during the agricultural transition

in West Africa. In: Jones, M.P., Dingkuhn, M., Johnson, D.E.,

Fagade, S.O. (Eds.), Interspecific Hybridization: Progress and

Table 5

Multiple, linear regression analyses across 16 rice lines for descriptors of canopy structure (experiment 1, monoculture) vs. mean

competitiveness with natural weed growth, maize and the O. glaberrima landrace IG10 (experiment 2). Mbe, Cote d'Ivoire, 1997 wet season

Lp SLA MTA Kdf Plant height

Coefficient P Coefficient P Coefficient P Coefficient P Coefficient P Intercept R P

31 DAS Ns Ns 0.212 0.049 Ns Ns Ns Ns Ns Ns 0.0126 0.5 0.049

46 DAS Ns Ns 0.05 0.009 Ns Ns Ns Ns Ns Ns ÿ0.565 0.63 0.009

64 DAS 2.76 0.032 0.041 0.005 Ns Ns Ns Ns Ns Ns ÿ1.38 0.77 0.003

31�46 DAS Ns Ns 0.038 0.005 ÿ0.03 0.028 2.11 0.08 Ns Ns 0.438 0.81 0.0004

DAS, days after seeding; Lp, dry matter partitioning ratio to leaf blades; SLA, specific leaf area (kg mÿ2); MTA, mean tip elevation angle

(deg); Kdf, PAR; extinction coefficient; Ns, not significant (P > 0.05).


Prospects. West Africa Rice Development Association, Cote

d'Ivoire, ISBN 929113113X, pp. 3±20.

Dingkuhn, M., Schnier, H.F., De Datta, S.K., Doerffling, K.,

Javellana, C., 1991. Relationship between ripening phase

productivity and crop duration, canopy photosynthesis and

senescence in transplanted and direct seeded lowland rice. Field

Crops Res. 26, 327±345.

Dingkuhn, M., Jones, M.P., Johnson, D.E., Fofana, B., Sow, A.,

1997a. Oryza sativa and O. glaberrima genepools for high-

yielding, weed-competitive rice plant types. In: Fukai, S.,

Cooper, S., Salisbury, J. (Eds.), Breeding Strategies for Rainfed

Lowland Rice in Drought-Prone Environments. ACIAR

Proceedings No. 77, Australian Centre for International

Agricultural Research, Canberra, Australia, 277 pp.

Dingkuhn, M., Jones, M.P., Fofana, B., Sow, A., 1997b. New high-

yielding, weed competitive rice plant types drawing from O.

sativa and O. glaberrima genepools. In: Kropff, M.J., Teng,

P.S., Aggarwal, P.K., Bouma, J., Bouman, B.A.M., Jones, J.W.,

van Laar, H.H. (Eds.), Application of Systems Approaches at

the Field Level, vol. 2. Kluwer Academic Publishers,

Dordrecht, The Netherlands, pp. 37±52.

Dingkuhn, M., Jones, M.P., Johnson, D.E., Sow, A., 1998. Growth

and yield potential of Oryza sativa and O. glaberrima upland

rice cultivars and their interspecific progenies. Field Crops Res.

57, 57±69.

Fofana, B., Koupeur, T., Jones, M.P., Johnson, D.E., 1995. The

development of rice varieties competitive with weeds. Brighton

Crop Protection Conference, vol. 1. The British Crop Protec-

tion Council, Farnham, Surrey, UK, pp. 187±192.

Johnson, D.E., 1995. Weed management strategies for smallholder

rice production. Brighton Crop Protection Conference, vol. 3.

The British Crop Protection Council, Farnham, Surrey, UK, pp.

1171±1180.

Johnson, D.E., Jones, M.P., Dingkuhn, M., Mahamane, M.C., 1997.

The response of O. glaberrima, O. sativa and interspecific

hybrid rice cultivars to weed competition. Proc. 1997 Brighton

Crop Protection Conference. The British Crop Protection

Council, Farnham, Surrey, UK, pp. 197±202.

Johnson, D.E., Dingkuhn, M., Jones, M.P., Mahamane, M.C., 1998.

The influence of rice plant type on the effect of weed

competition on O. sativa and O. glaberrima. Weed Res. 38,

207±216.

Jones, M.P., Dingkuhn, M., Aluko, G.K., MandeÂ, S., 1997a.

Interspecific Oryza sativa L. � O. glaberrima Steud. progenies

in upland rice improvement. Euphytica 92, 237±246.

Jones, M.P., MandeÂ, S., Aluko, K., 1997b. Diversity and potential

of Oryza glaberrima Steud. in upland rice breeding. Breeding

Sci. 47, 395±398.

Koffi, G., 1980. Collection and conservation of existing rice

species and varieties of Africa. Agronomie Tropicale 34, 228±

237.

Kropff, M.J., van Laar, H.H., 1993. Modelling Crop-Weed

Interactions. CAB International, UK, and IRRI, Los Banos,

Philippines, 274 pp.

Kropff, M.J., van Laar, H.H., Matthews, R.B., 1994. ORYZA_1. A

Basic Model for Irrigated Lowland Rice Production. IRRI,

Manila, Philippines, 110 pp.

LI-COR, 1989. LAI-2000 Plant Canopy Analyzer. Technical

Information. LI-COR, Inc., Lincoln, Nebraska, USA, 24 pp.

Schnier, H.F., Dingkuhn, M., De Datta, S.K., Mengel, K.,

Wijangco, E., Javellana, C., 1990. Nitrogen economy and

canopy CO2 assimilation in tropical lowland rice. Agron. J. 82,

451±459.

WARDA, 1996, Annual Report for 1995. West Africa Rice

Development Association, Bouake, Cote d'Ivoire.

Wopereis, M.C.S., Bouman, B.A.M., Tuong, T.P., ten Berge,

H.F.M., Kropff, M.J., 1996. ORYZA_W: rice growth model for

irrigated and rainfed environments. SARP Research Proceed-

ings. DLO and WAU, Wageningen, The Netherlands, and IRRI,

Los Banos, Philippines, 159 pp.


Relationships between upland rice canopy characteristics and weed competitiveness

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