Environmental and Experimental Botany 64 (2008) 180–188
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Environmental and Experimental Botany
journal homepage: www.elsevier.com/locate/envexpbot
Shoot and root competition in potato/maize intercropping:
Effects on growth and yield
Gustave Nachigera Mushagalusa, Jean-François Ledent, Xavier Draye ∗
Laboratoire d’écophysiologie et d’amélioration végétale, Faculté d’Ingénierie biologique, agronomique et environnementale,
Université catholique de Louvain, Place croix du Sud 2-11, 1348 Louvain-la-Neuve, Belgium
a r t i c l e
i n f o
Article history:
Received 23 August 2007
Received in revised form 13 May 2008
Accepted 20 May 2008
Keywords:
Maize
Potato
Intercropping
Competition
a b s t r a c t
Interspecific competitive relationships and their effect on yield have been analysed in the association
of potato and maize, two species with contrasting patterns of root and shoot systems establishment.
Greenhouse experiments were carried out under three configurations (NC: no interspecific competition;
FC: shoot and root interspecific competition; SC: shoot-only interspecific competition). Despite large
variations between replicate experiments associated with seasonal effects, the study revealed consistent
patterns of competition for above- and below-ground resources. Light interception in FC and SC was
dominated by potato (60%) during the first 45 days after planting and by maize thereafter (80%). The extra
shade caused by the companion crop increased soil moisture by up to 10% in SC treatments. The yield
of the two species responded in opposite ways to SC, which was consistent with asymmetric patterns
of competition between the two species. In potato, FC reduced tuber yield (number and size) by 4–26%,
while SC increased tuber size (compared to NC) by 3–39%. In maize, FC reduced LAI and plant height
by up to 45%, shoot and root dry mass, nutrient content, yield, the weight of 100 grains and harvest
index by ca. 30–100%, while SC affected all but LAI and plant height. It appears that the contrast between
the progressive installation of the maize root system and the rapid early extension of the potato root
system is amplified by the restriction of maize root development under competition, which leads to close
interdependencies between root and shoot competitive relationships. Although the specific effects of
root competition cannot be uncovered by this set of experiments, competition effects on maize in the
potato/maize intercropping seem to primarily related to light availability in the mixed canopy.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Intercropping, i.e. the simultaneous cultivation of two or more
crops in the same field is practised in many regions of the world. In
several scenarios of species, region and climate, intercropping can
increase total yield per land area compared to the sole crop of the
same crops. This effect is commonly attributed to the complementarity of resource capture patterns by crops (Rodrigo et al., 2001).
Intercropping is also used for soil erosion management, pest control and soil fertility improvement, but is most widely practiced
in countries where arable land is scarce, where it contributes to
biodiversity and food security.
Among a number of species combinations that are found in
tropical areas, the intercropping of maize and legumes is widely
documented, which can be explained by the complementarity of
the two crops in mineral nutrition and by their world-wide impor-
∗ Corresponding author. Tel.: +32 10 472092; fax: +32 10 472021.
E-mail address: [email protected] (X. Draye).
0098-8472/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.envexpbot.2008.05.008
tance. This paper focuses on the potato/maize association, which is
practised in Latin America (Midmore et al., 1988), Asia (Liu and
Midmore, 1990; Vander zaag and Demagante, 1990) and Africa
(Ifekwe et al., 1989; Bouwe et al., 2000; Ebwongu et al., 2001). This
association takes advantage of the complementary food values and
morphologies of the two species.
The optimization of cultivation practices under given environmental conditions requires a thorough understanding of the
patterns of resource capture by individual plants and how these
patterns depend on morphology. These points require special
attention in intercropping systems combining species with inherently different morphologies, where asymmetric distribution
of capture organs is likely to induce dominance relationships
between plants which may affect their performance (OzierLafontaine et al., 1999). It is worth noting that the presence of
weeds in a crop raises very similar problems. Given these close
interspecific interactions, the outcome of intercropping largely
depends on available resources and on every condition influencing the phenology and growth of each species (Casper et al.,
1998).
G.N. Mushagalusa et al. / Environmental and Experimental Botany 64 (2008) 180–188
In the potato/maize association, competition for light might be
an issue as the leaves of potato and maize exploit different strata
within the canopy. Maize, for example, has rather high requirements for light and responds negatively to shade, which reduces
growth and delays the appearance of leaves and roots (Pellerin,
1991). However, because potato height seldom exceeds 1 m in the
field against up to 3 m for maize (depending on variety and environment), the latter is commonly regarded as the dominant plant in the
association when the two species are planted simultaneously. Evidently, the dynamics of development in time and space should also
be considered as it may strongly depend on variety and cultivation
conditions (Midmore et al., 1988; Ebwongu et al., 2001).
There are less data on the importance of competition for soil
resources in the potato/maize association. The two species differ by
their root system development in time and space. Potato root length
density is generally about one third of that produced by cereal crops,
and is generally higher in the top soil (Gregory and Simonds, 1992).
Based on these contrasting soil exploration patterns, it is generally believed that the combination of the two species may improve
the efficiency of soil water and nutrient uptake throughout the soil
profile.
There are, however, a number of reports on the detrimental
effects of interspecific competition for soil resources in other crop
mixtures. Root competition during the early season has been found
to decrease the initial growth and reduce the ability of individual
plants to compete for light (Cahill, 1999; Aerts et al., 1991). Not surprisingly, several studies also indicate that the relative importance
of root and shoot competition between species can change during
the season according to the development of the partners (Belcher
et al., 1995; Cahill, 2002; Casper et al., 1998; Carlen et al., 2002;
Wilson, 1988; Wilson and Tilman, 1995).
Potato/maize intercropping has been mostly discussed under
conditions of high temperatures in Latin America and Asia
(Midmore, 1990; Vander zaag and Demagante, 1990; Liu and
Midmore, 1990), with a focus on the improvement of the potato
yield by shade cast by maize. In the present study, the effect of
mutual shade between potato and maize in the presence or absence
of root competition is analyzed. The objectives of this study were
to investigate the relative importance of shoot and root competition between maize and potato in mixture, estimated through their
effect on yield and morphological parameters (height, leaf area, root
and leaf dry mass).
181
1.2 m × 0.15 m × 0.60 m) standing above ground on a pit (same
dimensions) sheathed with a plastic film to insure isolation from
the surrounding soil (Fig. 1). Each box/pit element (L × W × H:
1.2 m × 0.15 m × 1.20 m) contained 108 L of a 2:3 volumetric mixture of universal compost (peat and bark, fertilized and limed):
Rhine sand (0–2 mm) which facilitates the separation of roots
from the soil at the time of excavation. The chemical analysis
of the compost (before sowing) gave the following results: total
nitrogen (0.35%, Kjeldhal), P2 O5 (0.06%, ICP-AES), K2 O (1.0%), MgO
(0.26%), CaO (0.63%), Fe2 O3 (0.62%), Na2 O (0.30%), Al2 O3 (2.14%), Zn
(13.5 mg/kg). This provided a total amount of 15.8 g N, 1.2 g P and
37.2 g K per box, containing up to 5 plants in the FC treatments. A
large drainage pipe laid down horizontally at the bottom of the pit
secured the hydraulic isolation of each box-pit unit from the subsoil, allowing a finer control of water availability in the accessible
soil. All root observations were performed in the box volume. Border plants were grown in pots (L × W × H: 0.4 m × 0.4 m × 0.3 m).
They were lifted up or down during the experiment to maintain
similar plant heights for box and border plants.
The arrangement of plants in the three treatments is illustrated
in Fig. 2. In each maize or potato treatment, eight plants were
arranged as two planting rows, with 50 cm between plants and
between rows. Measurements were performed on the four central
plants, grown in two boxes. This arrangement should insure that
all treatments shared the same level of intraspecific competition
(four plants in two boxes). Interspecific competition (SC and FC)
was created by adding six plants of the competing species around
the four central plants. Such additive design corresponds to the way
intercropping is implemented in tropical areas. A traditional design
would have counted on two additional rows of border plants but
could not be applied here for practical reasons. However, due to
the consistent arrangement of sampled and border plants, border
effects were systematic and should not have introduced additional
variability.
Solar radiation was supplemented all day long with artificial Hg
vapour lamps yielding 150 ␮mol/m2 /s PAR at the top of the canopy.
Every 2 days from sowing to harvest, water was applied early in the
morning to restore field capacity. This interval was short enough
to preclude the occurrence of severe water deficit and long enough
to allow the reliable quantification by TRD probes of the decrease
of soil water content (see below). Protection against aphids and
whitefly was insured with sticky yellow traps placed above each
2. Material and methods
2.1. Description of experiments
Three replicated greenhouse experiments (2002, 2003a,b) were
carried out in Louvain-la-Neuve, Belgium. The varieties Désirée
and Tripoli were used, respectively, for potato and maize. Each
experiment followed a complete random blocks design with two
(2003a, 2003b) or three (2002) blocks and three treatments. The
three treatments comprised sole crops (no interspecific competition, labelled MNC and PNC for maize and potato samplings,
respectively), full interspecific competition treatments (MFC and
PFC) and shoot interspecific competition treatments (MSC and PSC).
Intraspecific competition was involved in all treatments and was
considered as part of the common environment. Possible interactions between intra- and interspecific competitions were thus
confounded with the effect of treatments, but we assumed that
such interaction effects were small compared to the effect of treatments.
Plants assigned for sampling were grown in rectangular plywood frames (open on top and bottom, Length × Width × Height:
Fig. 1. Schematic representation of the substrate containers. (A) Plywood box (open
at the top and the bottom). (B) Plastic-sheathed pit. (C) Drainage pipe. (D) Ground
surface of the greenhouse.
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G.N. Mushagalusa et al. / Environmental and Experimental Botany 64 (2008) 180–188
Table 1
General information on the three experiments
Date of planting
Date of harvest
Average day temperature (◦ C)
Average night temperature (◦ C)
PAR at the top of the canopy (␮mol m−2 s−1 )a
Fertilization
Relative humidity (%)
2002
2003a
2003b
11 February
18 May
20–30
9–18
400/500
09 January
09 April
20–32
15–18
350/450
–
25–45
02 May
04 August
25–40
18–20
500/880
–
35–60
b
25–50
a
Minimum and maximum PAR values (including artificial lighting) during the experiment between 11:30 and 12:30.
In 2002 only, each box was supplied twice, before and during tubers initiation, with 2 l of a solution containing (in g l−1 ) 4.72 Ca(NO3 )2 ·4H2 O, 0.87 K2 SO4 , 1.46 FeEDTA,
0.91 NaH2 PO4 ·2H2 O and (in mg l−1 ) 248 H3 BO3 , 11.7 ZnSO4 ·7H2 O, 79 MnCl2 ·4H2 O and 9.98 CuSO4 ·5H2 O.
b
box. Other general information about these experiments is listed in
Table 1.
2.2. Measurements during the experiment
Plant height was monitored weekly between emergence and
harvest on the four plants dedicated for sampling (Fig. 2). The
phenological development of maize was estimated as described
by Ledent and Mouraux (1990). The leaf lamina was considered
fully expanded when the ligule was visible. The total leaf area of
maize was calculated by summing individual leaf areas (ILA) estimated from non-destructive measurements of leaf length (LL) and
maximum leaf width (LW) and using the equation ILA = K × LL × LW
(K = 0.75) (Girardin, 2000). The same formula was applied to potato
leaf area, using a value of K = 0.55 calibrated from 10 leaves of different ages in proportions estimated on eight plants. Plant height was
taken at the ligule of the last leaf lamina fully unfolded for maize
and at the top of the canopy for potato.
During experiments 2003a and 2003b, one box in each block
was equipped with six time domain reflectometry (TDR) probes,
20 cm long, placed horizontally at 20, 40 and 60 cm depth under
each of the two sampled plants. Soil water content was estimated
using a Tektronix impulse generator and a conversion formula calibrated with high organic content substrate (Souza and Matsura,
2003). Measurements of soil water content were taken early in
the morning before each watering (every 2 days—all boxes being
supplied to field capacity) and 6 h after watering, thereby reflecting the water uptake activity of the plants. Soil temperature was
measured at 10–11 and 15–16 h with thermocouple sensors (Hanna
instruments) located at 15 cm depth under each plant in the boxes.
During experiments 2003a and 2003b, the photosynthetically
active radiation (PAR) was measured at several heights in the
canopy using a sunfleck ceptometer (Delta-T Devices Ltd). In the
treatments involving interspecific shoot competition (FC and SC),
the PAR was measured (1) at the top of the dominating species in the
canopy, (2) at the top of the dominated species in the canopy and (3)
at the soil surface. A light extinction coefficient was calculated from
measures (1) and (2) for the dominating species and from measures
(2) and (3) within the mixed canopy. Fractional light interception
by each species was derived according to Wallace (1995). In the NC
treatment, two measurements were carried out, at the top of the
canopy and at the soil surface. All measurements were made during clear days between 11:30 and 12:30. Measurements were taken
every 10 days from 25 to 85 days after sowing.
2.3. Measurements at harvest
Fig. 2. Schematic representation of the experimental design (one block). Potato
plants are represented by open symbols and maize plants by closed symbols. The
square symbols indicate the plants dedicated for sampling. The two planting rows
are represented as dotted lines (oriented N–S) and the boxes as grey rectangles.
Plants dedicated to sampling were harvested 90 days after planting. The boxes were opened and the substrate was washed away
with a gentle flow of water. For potato, shoot and tuber fresh
weights were measured, along with the total number of tubers
and the diameter of each tuber. Root, tuber, leaf and stem dry
weight were measured, including dead material, after desiccation
at 80 ◦ C for 72 h. Tuber dry mass content (TDMC) was measured
on 250 g samples of tubers which were sliced and oven dried for
72 h, and tuber dry weight (TDW) was calculated as total tuber
fresh weight × TDMC. Finally, the harvest index was calculated as
HI (%) = (TDW/total dry biomass) × 100.
G.N. Mushagalusa et al. / Environmental and Experimental Botany 64 (2008) 180–188
For maize, aerial parts and the total root system were oven-dried
at 80 ◦ C for 72 h and weighed. Ears of every plant were oven-dried
at 80 ◦ C for 72 h, shelled and weighed to obtain the grain yield per
plant. The weight of 100 grains and harvest index (HI (%) = (grain
yield/total dry biomass) × 100) were also determined.
The nutrient concentration in the maize shoot was analyzed by
ICP-AES to estimate the effect of interspecific competition on mineral nutrition of maize. Nitrogen content could not be measured due
to accidental loss of samples during analysis. The nutrients concentration in potato is not reported because the vegetative growth of
potato was not affected by interspecific competition with maize.
Relative reductions in plant growth due to interspecific competition were calculated according to Gibson et al. (1999) assuming
that the monospecific competition was similar in all treatments (see above). The effect of interspecific full competition
(EFC) was estimated by comparing the dry weight (Y) of FC
plants to that of NC plants: EFC = ((YFC − YNC )/YNC ) × 100. The
effect of interspecific shoot competition (ESC) was determined
by comparing the dry weight of SC plants to that of NC plants:
183
ESC = ((YSC − YNC )/YNC ) × 100. The effect of interspecific root competition could not be directly estimated due to the lack of a pure root
competition treatment. However, a combined effect of root competition and of the interaction between root and shoot competition
(ERC) was calculated by subtracting the mean of SC plants from
that of the FC plants: ERC = ((YFC − YSC )/YNC ) × 100. In the absence
of interaction between shoot and root competition, this term would
approximate the root competition effect. While some researchers
acknowledge that interaction between the two forms of competition is likely, they usually assume that its amplitude is limited in
the range of conditions encountered in experiments (Casper and
Jackson, 1997). In addition, greenhouse (Wilson, 1988) and field
data (Cahill, 1999) suggest that this latter assumption may be more
often valid than one would assume.
2.4. Statistical analyses
Statistical inference on treatments at harvest was carried
out using a mixed model ANOVA according to the randomized
Fig. 3. Progression of leaf area index (LAI) during the season. PNC (), MNC (), PSC (), MSC (䊉), PFC (), MFC (); vertical bar: standard error on mean.
184
G.N. Mushagalusa et al. / Environmental and Experimental Botany 64 (2008) 180–188
complete block design (Littell et al., 1996). There was important plant size variability among experiments. In 2003b the
plants were larger, had more leaves and accumulated leaf area
much more so than in 2003a and 2002. As the interaction
between experiment and treatment was statistically significant
for many parameters, the three experiments were analyzed separately.
3. Results
3.1. Light interception
During the first month of the experiment, potato exhibited a rapid increase in LAI (Fig. 3) and plant height (Fig. 4)
resulting in a dense coverage of the soil surface in all treatments. Potato plants were taller than maize and intercepted
60% of the incident radiation reaching the top of the canopy,
to the detriment of maize interception, which was significantly affected in comparison with the NC treatment (P < 0.02).
Between 35 and 45 days after planting, the fractional light interception of potato progressively decreased, indicating a change
in dominance relationships as maize plants were growing in
height. After 45 days, maize was taller than potato (Fig. 4) and
intercepted up to 80% of the incident radiation. These light interception patterns were remarkably consistent among experiments
(Fig. 5).
3.2. Soil moisture
In potato as well as maize treatments, shading due to the addition of the companion crop (SC relative to NC) increased soil water
content (Fig. 6). This effect was observed early in the maize SC treatment, when maize was in the shade of potato, and later in the potato
SC treatment, when potato was in the shade of maize. No increase of
the soil water content was detected in the FC treatment, which may
not be surprising as the companion crop was also taking up water
in the same box, but could alternatively be due to some interaction (i.e. non-additive effects) between root and shoot competition
effects.
3.3. Potato development and yield
The response of potato to the different treatments was rather
consistent among experiments despite important differences of
plant growth between experiments (Table 2).
The tuber yield under NC conditions approached high field levels. Compared to NC, FC reduced the tuber yield (significant in
2003a and 2003b) by 4.1–26.8%. This effect, which was also apparent in the dry weight of tubers (significant in 2003a and 2003b),
resulted from a reduction of the number of tubers (significant in
2002 and 2003b) and, in 2002, from a shift of tuber size distribution towards smaller sizes. FC did not significantly affect the shoot
and root dry weight, the root/shoot allocation, the harvest index
(not significant in 2002 and 2003a) and the LAI.
Compared to NC, SC increased tuber yield by 2.4–28.5% (significant in 2002). This increase was the consequence of a shift of
tuber distribution towards large sizes (best pronounced in 2002)
and could not be attributed to a change of TDW. Shoot dry weight
may have benefited from SC (from 7 to 23.4%), but this effect was not
significant and was not paralleled with LAI differences. The effect
of SC on root dry weight was less consistent (−11.9 to +27.4%) and
not significant. Neither the root/shoot ratio nor the harvest index
was consistently affected by SC.
Among experiments however, strong differences were observed
in plant growth and in carbon allocation to shoot, roots and tubers.
Fig. 4. Progression of plant height. PNC (), MNC (), PSC (), MSC (䊉), PFC (),
MFC (); vertical bar: standard error on mean.
While 2002 and 2003b results were close to expectations for potato,
the 2003b experiment was remarkable with tuber yield and TDW
less than one third those of the other experiments. The strong
reduction of tuber size in 2003b was inversely proportional to LAI
(Fig. 3) and root dry weight. High temperatures (35–40 ◦ C) during
the tuber development stage may have contributed to this strong
allocation change.
G.N. Mushagalusa et al. / Environmental and Experimental Botany 64 (2008) 180–188
185
Table 2
Yield and biomass of potato at harvest (90 days after planting)
TFW (g pl−1 )
TDW (g pl−1 )
Number of tubers (pl−1 )
Diameter classes
<29
SDW (g pl−1 )
RDW (g pl−1 )
RSR
HI (%)
Total
29–57
>57
2002
PNC
PFC
PSC
S.E.
CV (%)
EFC (%)
ESC (%)
ERC (%)
922.3a
884.8b
1185.3a
33.7
11.7
−4.1
+28.5
−32.6
199.7a
187.0a
182.6a
8.8
16.1
−6.4
−8.6
+0.02
3.3b
4.4a
2.6c
0.06
6.3
–
–
–
8.1a
6.6ab
4.6b
0.24
12.7
–
–
–
2.6b
1.1b
6.1a
0.08
8.5
–
–
–
14.0a
11.7b
13.2a
1.5
38.9
–
–
–
101.1a
84.2b
108.2a
3.9
14.0
−16.7
7.1
−23.7
1.8ab
1.5b
2.2a
0.001
24.5
−13.1
27.4
−40.6
0.02a
0.02a
0.02a
0.01
6.5
–
–
–
66a
68a
62b
1.11
8.2
–
–
–
2003a
PNC
PFC
PSC
S.E.
CV (%)
EFC (%)
ESC (%)
ERC (%)
720.3ab
680.5c
775.4a
13.7
5.4
−5.52
+10.4
−13.2
150.0a
122.3b
158.5a
3.5
6.8
−18.5
+5.7
−24.1
1.5a
1.5a
1.9a
0.07
11.4
–
–
–
6.0a
6.4a
6.6a
0.01
4.9
–
–
–
5.1a
5.1a
6.0b
0.22
11.3
–
–
–
12.6a
13.3a
14.5a
0.4
9.2
–
–
–
57.7a
66.4a
59.8a
1.99
9.16
+15.2
+3.7
11.5
1.4a
1.5a
1.5a
0.11
22.3
+10.3
+6.6
+3.7
0.03a
0.03a
0.02b
0.001
13.5
–
–
–
71a
65a
69a
1.6
11.0
–
–
–
2003b
PNC
PFC
PSC
S.E.
CV (%)
EFC (%)
ESC (%)
ERC (%)
356.7ab
261.2c
364.6a
9.0
7.8
−26.8
+2.2
−29
56.0a
40.5b
52.4a
1.2
6.6
−27.7
−6.3
−21.4
6.4a
4.4b
7.6a
0.6
9.7
–
–
–
4.4b
2.8c
6.3a
0.07
4.5
–
–
–
1.9a
1.6a
2.3a
0.07
10.5
–
–
–
12.6b
9.0c
16.0a
0.4
9.8
–
–
–
251.2a
215.3a
309.8a
10.6
11.6
−14.3
+23.4
−37.6
8.9a
6.5a
7.8a
0.17
6.1
−26.6
−11.9
−14.6
0.04a
0.03a
0.03a
0.002
13.1
–
–
–
18a
14b
15b
0.24
7.4
–
–
–
TFW: tuber fresh weight; TDW: tuber dry weight; RDW: final root dry weight; SDW: final shoot dry weight; RSR: root shoot ratio; HI: harvest index; PNC: no interspecific
competition; PFC: full interspecific competition; PSC: shoot interspecific competition. Exx: effect of full, shoot or root competition. Within a given experiment, numbers
followed by the same letter (in a column) are not significantly different (P < 0.05).
Table 3
Yield and biomass of maize at harvest (90 days after planting)
SDW (g pl−1 )
RDW (g pl−1 )
RSR (g pl−1 )
Yield
W100
HI (%)
2002
MNC
MFC
MSC
S.E.
CV (%)
EFC (%)
ESC (%)
ERC (%)
66.0a
35.2c
53.5b
0.78
7.4
−46.6
−18.8
−27.8
4.3a
2.8c
3.7b
0.11
15.2
−35.3
−12.1
−21.2
0.06b
0.08a
0.07b
0.003
24
–
–
–
45.4a
12.4c
33.2b
0.9
14.5
−72.7
−26.9
−45.8
18.4a
10.5c
14.2b
0.25
8.6
−42.9
−22.8
−20.1
64.6a
32.6b
58.0a
1.3
12.1
–
–
–
2003a
MNC
MFC
MSC
S.E.
CV (%)
EFC (%)
ESC (%)
ERC (%)
47.7a
3.9c
17.2b
1.01
4.4
−91.8
−64
−27.8
3.7c
0.07c
0.27b
0.06
4.5
−98.2
−92.6
−5.6
0.09a
0.07b
0.10a
0.007
7.6
–
–
–
35.6
–
–
–
–
–
–
–
15.2
–
–
–
–
–
–
–
69.2
–
–
–
–
–
–
–
2003b
MNC
MFC
MSC
S.E.
CV (%)
EFC (%)
ESC (%)
ERC (%)
167.1a
121.6b
186.7a
6.0
18.6
−27.3
+11.7
−39
12.8a
7.1b
15.9a
0.8
29.5
−44.7
+23.8
−68.5
0.079a
0.061a
0.087a
0.006
38.4
–
–
–
105.4a
51.7c
87.6b
3.1
18.5
−51
−17
−34
19.8a
13.0c
16.4b
0.85
14.7
−34.6
−17.5
−17.2
60.3a
38.7b
43.1b
2.13
22.0
–
–
–
RDW: final root dry weight; SDW: final shoot dry weight; HI: harvest index; W100: weight of 100 grains; MNc: no interspecific competition; MFc: full interspecific competition;
MSc: shoot interspecific competition; E: effect of full, shoot or root competition. Within a given experiment, numbers followed by the same letter (in a column) are not
significantly different (P < 0.05).
186
G.N. Mushagalusa et al. / Environmental and Experimental Botany 64 (2008) 180–188
Fig. 5. Progression of the fraction of light interception. PNC (), MNC (), PSC (), MSC (䊉), PFC (), MFC (); vertical bar: standard error on mean.
3.4. Maize development and yield
The effects of interspecific competition on maize (Table 3) were
more pronounced than those on potato. It is clear from the NC data
that maize growth was not optimal, which is to be linked with the
relatively low PAR, especially in 2002 and 2003a. Compared to NC,
FC reduced significantly the shoot dry weight (−27 to −91%), root
dry weight (−35 to −98%), yield (−51 to −100%) and weight of 100
grains (−34 to −100%) in the three experiments. This treatment also
reduced significantly LAI and plant height, which was most noticeable in 2003a. The root/shoot ratio under FC was increased in 2002
and decreased in 2003a, while the harvest index was significantly
reduced in 2002 and 2003b and there was no yield at all in 2003a.
Relative to NC, SC also depressed plant biomass and yield
but in significantly smaller proportions than FC. The effect of SC
was significant for SDW and RDW in 2002 and 2003a and for
yield and W100 in the three experiments. Neither LAI nor plant
height seemed to be consistently affected by shoot competition.
These slight effects on plant growth occurred without change in
root/shoot allocation.
As for potato, strong differences were observed between the
three experiments. In 2003a, limiting irradiance at the top of the
canopy negatively affected biomass production in all treatments
while low early temperature (10–14 ◦ C) caused a delay in maize
emergence of about 10 days (data not shown). While the combina-
tion of these effects depressed maize growth and yield in the NC
treatment, ears were empty or contained no marketable grains in
the SC and FC treatments which exacerbated the shortage of light.
Maize plants in these conditions may have been unable to achieve
the minimum rate of root expansion required to allow a satisfactory
level of nutrient uptake, especially for low-mobility nutrients such
as phosphorus for which deficiency symptoms were noticeable.
Such dramatic effects did not occur in 2002, where maize experienced less extreme light conditions, and was less dependent on
root system expansion given a point addition of fertilizers (Table 1)
which supplied readily available amounts of nutrients to the standing root system. This interpretation stresses the importance of
shoot/root interdependency, both above- and below-ground structures acting as a sink for major resources captured by the other.
3.5. Nutrient concentration in the maize shoot
The average nutrient concentration in the maize shoot at harvest
is outlined in Table 4 for some of the elements which exhibited the
largest differences among years and treatments. Compared to NC,
the FC treatment consistently reduced the content of all reported
elements (significant in most cases). The effect of SC appeared to be
intermediate between NC and FC. The nutrient content was globally
higher in 2003b than in 2003a for most elements. This coincided
with a larger root development in 2003b compared to 2003a (about
Fig. 6. Progression of soil moisture (cm3 cm−3 ) (×10−3 ) at 20 cm depth in experiments 2003a and 2003b. PNC (), MNC (), PSC (), MSC (䊉), PFC (), MFC (); vertical bar:
standard error on mean. Because there is no difference between soil moisture at different soil depth (20, 40 and 60 cm) only the soil moisture at 20 cm depth is presented in
this figure.
G.N. Mushagalusa et al. / Environmental and Experimental Botany 64 (2008) 180–188
Table 4
Mean nutrient concentrations in the maize shoot (mg kg−1 )
4.2. Consequences of light interception patterns in intercropping
(shoot competition)
P
K
Mg
Ca
2003a
MNC
MFC
MSC
S.E.
4150a
2798b
2798b
42.02
23,620ab
21,596b
26,137a
115.1
1650.1a
1086c
1341b
105.4
3523a
2134c
2963b
42.8
87ab
82b
96a
4.1
2003b
MNC
MFC
MSC
S.E.
3759a
2641c
2753b
25.6
2061b
2097b
2372a
55.6
9506a
6530b
9503a
65.4
159ab
146b
170a
13.1
23,274b
20,660c
25,616a
122.8
187
Fe
MNC: no interspecific competition; MFC: full interspecific competition; MSC: shoot
interspecific competition. Within a given experiment, numbers followed by the
same letter (in a column) are not significantly different (P < 0.05).
three times in terms of dry biomass) and a significant increase of
lateral root length and density (data not shown), which resulted
in a more than three times increase in root length and root surface. This largely compensated for the reduced root:shoot ratio and
contributed to an improvement of soil exploration and possibly of
mineral uptake.
4. Discussion
The growth of potato and maize in sole crop and in different
intercropping configurations has been recorded from planting until
harvest in order to investigate the relative importance of shoot and
root competition between the two crops and its evolution during
the crop cycle. Despite the relatively small size of the experiments
and the large variation observed between experiments, the study
revealed consistent patterns of light interception by the two species
with a characteristic inversion of dominance relationships near 45
days after planting. The addition of a companion crop resulted in
opposite responses of maize grain yield and potato tuber yield
which were, respectively, decreased and increased under shoot
interspecific competition. The study also reveals consistently that
the competition for soil resources is likely to play a significant role
in potato/maize intercropping.
One of the motivations of intercropping resides in the exploitation of the complementarity in the patterns of resource capture
by the mixed crops (Rodrigo et al., 2001). In terms of solar energy
capture, the potato/maize association relies on much more subtle
relationships than a simple complementarity.
On the one side, the maize plant is clearly affected by low irradiance (Fournier, 2000; Reed and Singletary, 1989; Schoper et al.,
1982; Setter et al., 2001) and, therefore, by the presence of a companion crop competing for light (Kropff et al., 1992; Braconnier,
1998; Cavero et al., 1999; Liedgens et al., 2004). As shown here
in the case of early competition with potato, maize is not able to
recover completely from the limited period of shade experienced
at young stages, and this has irreversible consequences on yield.
In addition, this phenomenon is not limited to early stage of the
crop cycle (Setter et al., 2001). In a number of mixtures, maize is
thus struggling more for light and suffering irreversibly from light
competition than it is sharing light with the companion crop.
The potato plant, however, may also be affected by low irradiance. In a study by O’Brien et al. (1998) artificially shading the potato
crop before or after the period of tuber initiation did not affect
the number of tubers, unless the incident radiation was reduced
by more than 37%. Under continuous shade, TDW was ultimately
reduced by as much as 40% (Sale, 1976). Intercropping conditions
may additionally affect temperature within the canopy and induce
a beneficial microclimate for potato growth, especially when the
ambient temperature in the potato sole crop would be higher than
30 ◦ C (Midmore et al., 1988). Here, the presence of maize above
the potato canopy affected soil temperature (data not shown) and
moisture and was beneficial for tuber fresh weight. In comparison with the artificial shade data, these latter experiments suggest
that the indirect effects induced by maize during the period where
it dominates the canopy can at least compensate for the harmful
effect of low irradiance on potato.
The potato/maize association is therefore a clear example of
asymmetric relationships, where the ultimate outcome of light
interception patterns would depend on a subtle balance between
what can be afforded at the maize level (which strongly depends on
the relative development of the two crops) and the indirect benefit
which can be expected at the potato level.
4.1. Evolution of light interception patterns during the crop cycle
4.3. Root competition patterns in intercropping
Light interception in interspecific and monospecific stands is
probably the most illustrated aspect of competitive relationships
between neighbouring plants. In a number of crop combinations, as in the case of potato/maize (present study), shrub/grass
(Tournebize and Sinoquet, 1995), sorghum/cowpea (Gilbert et al.,
2003), wheat/maize or wheat/soybean (Li et al., 2001), leek/celery
(Baumann et al., 2001), leaf area distribution in the canopy has
determinant effects on light interception. A characteristic of the
potato/maize association is the rather abrupt change which occurs
when new maize leaves arise above the dense potato canopy. While
the young maize plant seems to be able to tolerate an episode of
low incident radiation when it is in the shade of potato (e.g. 2003b),
a slight delay of maize emergence due to unpredictable transient
conditions can have dramatic effect on the final yield (e.g. 2003a).
The range of optimal sowing periods is also likely to be dependent on the environmental conditions and agricultural practices
which may influence the duration of the periods during which one
partner is in the shade of the other and the strength of the shade
(Hall et al., 1992; Rajcan and Swanton, 2001). In this sense, the
association’s yield might be more vulnerable than that of the sole
crops.
Potato and maize display very contrasting root system architectures both in time and space. Potato may produce 90% of its
nodal roots by the fourth leaf stage thanks to the abundant carbon resources in the planted tuber (Iwana et al., 1979; Iwana, 1998)
while the emergence of nodal roots in maize is progressive and
usually synchronous with that of leaves (Demotes-Mainard and
Pellerin, 1992; Girardin, 2000; Pellerin, 1991). In addition, the roots
of potato are produced at varied angles (from horizontal to vertical)
and are preferentially present in the top 30–60 cm of the soil (data
not shown) while those of maize display clear gravitropic responses
which allow them to reach lower layers. Based on these data, it is
generally assumed that soil exploration by the two species is complementary and that the root system contribution to competition
between maize and potato should be limited.
This study did not isolate the effects of root competition and may
not be relevant to validate this assumption. However, it provides
convincing evidence that the combination of root and shoot competition (compared to shoot competition alone) further reduces the
yield of maize and obviates the beneficial effect of shade on potato.
Soil water content data suggest that the effect on potato might be
188
G.N. Mushagalusa et al. / Environmental and Experimental Botany 64 (2008) 180–188
due to the larger water uptake by the two species mixture, which
would suppress the beneficial cooling effect of the shade cast by
maize, as a dry soil would be more prone to warming than a moist
soil. The case of maize is probably different because the detrimental effect of the early shade cast by potato also affects the growth
of the maize root system and the capacity to rapidly colonize the
soil. The latter effect would be especially important since potato
establishes most of it root system very quickly. The importance
of fast exploration rate in the competition for soil resources has
been reviewed by Robinson (1996). When maize emerges above
the potato leaves, the maize plant has therefore an underdeveloped root system which is likely to hamper its competitive ability to
acquire soil resources, which would contribute to the lower maize
performance in FC compared to SC.
The interdependency between root and shoot was further illustrated by the fact that fertilizer supply on maize in the SC treatment
in 2002 was able to promote maize growth, even though a sufficient
amount of nutrient was present in the soil volume. Most likely,
root growth had been reduced by the competition for light and was
unable to counterbalance the progressive depletion of low mobility nutrients such as phosphorus in the vicinity of the standing root
system.
Although the strict consequence of root competition cannot be
revealed by this set of experiments, it appears that competition
effects on maize are primarily related to light availability in the
mixed canopy, but that they restrict root development and increase
the vulnerability of maize in front of the advanced development of
the potato root system.
Acknowledgements
The authors thank the Evangelisher Entwicklungstudienst-EED
(Germany) for financial support to GM and two anonymous reviewers for constructive comments on the manuscript. XD is a Research
Associate from the Fonds de la Recherche Scientifique (FSR-FNRS,
Belgium).
References
Aerts, R., Boot, R.G.A., van der Aart, P.J.M., 1991. The relation between above- and
below-ground biomass allocation patterns and competitive ability. Oecologia
87, 551–559.
Baumann, D.T., Bastiaans, L., Kropff, M.J., 2001. Competition and crop performance
in a leek–celery intercropping system. Crop Science 41, 764–774.
Belcher, J.W., Keddy, P.A., Twolan-Strutt, L., 1995. Root and shoot competition intensity along a soil depth gradient. Journal of Ecology 83, 673–682.
Bouwe, N.B., Walangululu, M., Kidanemariam, H.M., 2000. Performance de 4 cultivars de pomme de terre en culture associée avec le maı̈s et le haricot. In: African
Association Conference Proceeding, vol. 5, pp. 87–190.
Braconnier, S., 1998. Maize–coconut intercropping: effect of shade and root competition on maize growth and yield. Agronomie 18, 373–382.
Cahill Jr., J.F., 1999. Fertilization effects on interactions between above- and belowground competition in on old field. Ecology 80, 466–480.
Cahill Jr., J.F., 2002. Interactions between root and shoot competition vary among
species. Oikos 99, 101–112.
Carlen, C., Kölliker, R., Reidy, B., Lücher, A., Nösberger, J., 2002. Effect of season and
cutting frequency on root and shoot competition between Festuca pratensis and
Dactylis gomerata. Grass and Forage Science 57, 247–254.
Casper, B.B., Cahill Jr., J.F., Hyatt, L.A., 1998. Aboveground competition does not
alter biomass allocated to roots in Abutilon theophrasti. New Phytologist 140,
231–238.
Cavero, J., Zaragoza, C., Suso, M.L., Pardo, A., 1999. Competition between maize
and Datura stramonium in an irrigated field under semi-arid conditions. Weed
Research 39, 225–240.
Demotes-Mainard, S., Pellerin, S., 1992. Effect of mutual shading on the emergence
of nodal roots and the root/shoot ratio of maize. Plant and Soil 147, 87–93.
Ebwongu, M., Adipala, E., Ssekabembe, C.K., Kyamanywa, S., Bhagsari, A.S., 2001.
Effect of intercropping maize and Solanum potato on yield of the component
crops in Central Uganda. African Crop Science Journal 9, 83–96.
Fournier, C., 2000. Modélisation des interactions entre plantes au sein des peuplements. Application à la simulation des régulations de la morphogenèse aérienne
du maı̈s (Zea mays L.) par la compétition pour la lumière. Ph.D. Thesis. Institut
National Agronomique Paris-Grignon, France.
Gibson, K.D., Foin, T.C., Hill, J.E., 1999. The relative importance of root and shoot
competition between water-seeded rice and Echinochloa phyllopogon. Weed
Research 39, 181–190.
Gilbert, R.A., Heilman, J.L., Juo, A.S.R., 2003. Diurnal and seasonal light transmission
to cowpea in sorghum–cowpea intercrops in Mali. Journal of Agronomy and Crop
Science 189, 21–39.
Girardin, P., 2000. Ecophysiologie du maı̈s. Association Générale des Producteurs de
Maı̈s, France.
Gregory, P.J., Simonds, L.P., 1992. Water relations and growth of potatoes. In: Harris,
P. (Ed.), The Potato Crop: The Scientific Basis for Improvement, 2nd ed. Chapman
& Hall, London, UK, pp. 214–246.
Hall, M.R., Swanton, C.J., Anderson, G.W., 1992. The critical period of weed control
in grain corn. Weed Science 28, 441–447.
Ifekwe, O.P., Odurukwe, S.O., Okonkwo, J.C., Nwokocha, H.N., 1989. Effects of maize
and potato populations on tuber and grain yields, net income and land equivalent
ratios in potato/maize intercropping. Tropical Agriculture 66, 329–333.
Iwana, K., 1998. Development of nodal and lateral roots in potato under field conditions. Journal of the Faculty of Agriculture, Hokkaido University 68, 33–44.
Iwana, K., Nakaseko, K., Gotoh, K., Nishibe, S., Umemura, Y., 1979. Varietal differences
in root system and its relationship with shoot development and tuber yield.
Japanese Tropical Crops Science 49, 495–501.
Kropff, M.J., Weaver, S.E., Smits, M.A., 1992. Use of ecophysiological models for
crop-weed interference: relations amongst weed density, relative time of weed
emergence, relative leaf area and yield loss. Weed Science 40, 296–301.
Ledent, J.F., Mouraux, D., 1990. Determination of foliar stage and number of leaves
in maize when lower leaves are missing. Agronomie 2, 147–156.
Li, L., Sun, J., Zhang, F., Li, X., Rengel, Z., Yang, S., 2001. Wheat/soybean strip intercropping. II. Recovery or compensation of maize and soybean after wheat harvesting.
Field Crops Research 71, 173–181.
Liedgens, M., Soldati, A., Stamp, P., 2004. Interactions of maize and Italian ryegrass
in a living mulch system: (1) shoot growth and rooting patterns. Plant and Soil
262, 191–203.
Littell, R.C., Milliken, G.A., Stroup, W.W., Wolfinger, R.D., 1996. SAS© System for Mixed
Models. SAS Institute Inc., USA.
Liu, J., Midmore, D.J., 1990. A review of potato intercropping practices in Western
Hubei, China. Field Crops Research 25, 41–50.
Midmore, D.J., 1990. Scientific basis and scope for further improvement of intercropping with potato in the tropics. Field Crops Research 25, 3–24.
Midmore, D.J., Roca, J., Barios, D., 1988. Potato (Solanum spp.) in the hot tropics. IV.
Intercropping with maize and the influence of shade on microenvironment and
crop growth. Field Crops Research 18, 141–157.
O’Brien, P.J., Firman, D.M., Allen, E.J., 1998. Effects of shading and seed tuber spacing
on initiation and number of tubers in potato crops (Solanum tuberosum). Journal
of Agricultural Science 130, 431–449.
Ozier-Lafontaine, H., Bruckler, L., Lafolie, F., Tournebize, R., Mollier, A., 1999.
Modélisation de la compétition pour l’eau dans une association culturale: influence de la distribution des racines, des propriétés physiques du sol et de la
répartition de la demande climatique. In: Maillard, P., et Bonhomme, R. (Eds.),
Fonctionnement des peuplements végétaux sous contraintes environnementales. Les Colloques INRA, Paris, pp. 459–480.
Pellerin, S., 1991. Effet d’une réduction du rayonnement incident sur l’émission des
racines adventives du maı̈s en début du cycle. Agronomie 11, 9–16.
Rajcan, I., Swanton, C.J., 2001. Understanding maize–weed competition: resource
competition, light quality and the whole plant. Field Crops Research 71,
139–150.
Reed, A.J., Singletary, G.W., 1989. Roles of carbohydrate supply and phytohormones
in maize kernel abortion. Plant Physiology 91, 986–992.
Robinson, D., 1996. Resource capture by localized root proliferation: why do plants
bother? Annals of Botany 77, 179–185.
Rodrigo, V.H.L., Stirling, C.M., Teklehaimanot, Z., Nugawela, A., 2001. Intercropping
with banana to improve fractional interception and radiation-use efficiency of
immature rubber plantations. Field Crops Research 69, 237–249.
Sale, P.J.M., 1976. Effect of shading at different times on the growth and yield of the
potato. Australian Journal of Agricultural Research 27, 557–566.
Schoper, J.B., Johnson, R.R., Lambert, R.J., 1982. Maize yield response to increased
assimilate supply. Crop Science 22, 1184–1189.
Setter, T.L., Flannigan, B.A., Melkonian, J., 2001. Loss of kernel set due to water deficit
and shade in maize: carbohydrate supplies, abscissic acid, and cytokinins. Crop
Science 41, 1530–1540.
Souza, C.F., Matsura, E.E., 2003. Multi-wire time domain reflectometry (TDR) probe
with electrical impedance discontinuities for measuring water content distribution. Agricultural Water Management 59, 205–216.
Tournebize, R., Sinoquet, H., 1995. Light interception and partitioning in a
shrub/grass mixture. Agricultural and Forest Meteorology 72, 277–294.
Vander zaag, P., Demagante, A.L., 1990. Potato (Solanum spp) in an isohyperthermic environment. V. Intercropping with maize. Field Crops Research 25,
157–170.
Wallace, J.S., 1995. Towards a coupled light partitioning and transpiration model for
use in intercrops and agroforestry. In: Sinoquet, H., Cruz, P. (Eds.), Ecophysiology
of Tropical Intercropping. INRA, Paris, pp. 153–162.
Wilson, S.D., 1988. Shoot and root competition. Journal of Applied Ecology 25,
279–296.
Wilson, S.D., Tilman, D., 1995. Competitive responses of eight old-field plant species
in four environments. Ecology 76, 1169–1180.