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Bragantia, Campinas, v. 70, n. 1, p.40-45, 201140
Crop Production and Management | Article
Eects of plant density and proportion on
the interaction between wheat with
alexandergrass plants
Leonardo Bianco de Carvalho (1*); Pedro Luis da Costa Aguiar Alves (1); José Valcir Fidelis Martins (1)
(1) Faculdade de Ciências Agrárias e Veterinárias – UNESP/Câmpus de Jaboticabal, Departamento de Biologia Aplicada à
Agropecuária, Rod. Paulo Donato Castellane, s/n, 14884-900 Jaboticabal (SP). E-mail: agrolbcarvalho@gmail.com
(*) Autor correspondente.
Recebido: 28/Sept./2009; Aceito: 6/June/2010
Abstract
Determination of competitive relationships among plant species requires appropriate experimental designs and method of
analysis. The hypothesis of this research was that two species growing in coexistence show dierent growth and develop-
ment due to their relative competitiveness. This research aims to measure the relative competitiveness of wheat crop com-
pared to Alexandergrass by the interpretation of plant density and proportional eects using replacement series experiments.
Monocultures were cultivated in densities of 1, 3, 5, 10 and 15 plants per pot and analyzed by regression of dry mass data.
Mixture experiment was cultivated in wheat:Alexandergrass proportions of 0:6, 1:5, 2:4, 3:3, 4:2, 5:1 and 6:0 plants per pot and
analyzed by graphical interpretation of growth and production characteristics. Both experiments were carried out in random-
ized complete block design with four replicates. Alexandergrass was more sensitive to intraspecic competition than wheat.
Alexandergrass was lightly more competitive than wheat. Number and weight of spikes and number of tillers were the wheat
characteristics more aected by Alexandergrass interference.
Key words: Competition, Brachiaria plantaginea, Triticum aestivum, replacement series experiment.
Efeitos da densidade e proporção de plantas na interação entre plantas de trigo e
capim-marmelada
Resumo
A determinação das relações competitivas entre espécies de plantas requer delineamentos experimentais e métodos de
análise apropriados. A hipótese da pesquisa foi que duas espécies crescendo em convivência têm comportamento de cres-
cimento e desenvolvimento distintos em função da sua competitividade relativa. O objetivo desta pesquisa foi quanticar
a competitividade relativa da cultura do trigo com o capim-marmelada através da medida dos efeitos da densidade e da
proporção de plantas, usando experimentos em série substitutiva. Monoculturas foram cultivadas em densidades de 1, 3,
5, 10 e 15 plantas por caixa e analisadas por regressão dos dados de massa seca, em 2006. Experimento em mistura foi
cultivado nas proporções trigo:capim-marmelada de 0:6, 1:5, 2:4, 3:3, 4:2, 5:1 e 6:0 plantas por caixa e analisado através de
interpretação gráca de características de crescimento e produção, em 2007. Ambos os experimentos foram realizados em
delineamento completamente casualizado com quatro repetições. Capim-marmelada foi mais sensível que trigo à competi-
ção intraespecíca. Capim-marmelada foi levemente mais competitivo que trigo. Número e massa de espigas e número de
alhos foram as características do trigo mais afetadas pela interferência do capim-marmelada.
Palavras-chave: Competição, Brachiaria plantaginea, Triticum aestivum, experimento substitutivo.
Bragantia, Campinas, v. 70, n. 1, p.40-45, 2011 41
Interaction between wheat and alexandergrass
1. INTRODUCTION
Determination of competitive relationships between plant
species requires appropriate experimental designs and
method of analysis (R et al., 1989; C, 1991).
ese competitive relationships are influenced for several
factors. P (1985) reports a diagram model with
these factors, through adaptation of Bleasdale’s model.
us, with respect to factors linked to weeds, plant densi-
ty is one of the most important, so that the higher density,
the higher number of individuals competing for the same
environmental resources, and then the competition with
crops will be more intense (C and V-
F, 1996).
In agriculture areas, crop density is kept constant
whereas weed density varies in accordance to local infes-
tation degree. erefore, variation in plant proportion
of crops and weeds is established. us, in competition
studies, it is important to measure the influence of plant
density on competitive process as well as the variation in
plant proportion (C and Victoria F,
1996).
ere are several methodologies used to study plant
competition (R, 1987; R et al., 1989;
C, 1991). However, most researchers measured
just the interference of weeds on crop growth and produc-
tion without concerning on the competition process.
us, it is important to use appropriate experimental
designs and method of analysis with a view to understand
the competition process not just quantifying crop losses
but in a mechanistic way (C and V-
F, 1996). Among methodologies already used,
replacement series experiments are an alternative way to
understand the plant competition process in especial the
relation to plant density and proportion. is method
allows clearing up competitive relationship among differ-
ent plant species (R et al., 1989).
Replacement series experiments allow the control
of plant density and proportion, where plant density is
kept constant while plant proportion is changed for both
studied species (W, 1960; H, 1977). In these
experiments, it is admitted that the total plant density is
sufficient to satisfy the “final constant production”, where
the biomass production per area is independent of plant
density (C and W, 1994; C-
and V F, 1996).
Data interpretation of replacement series experiments
results in a measurement of species competitiveness based
on relative response to biomass production in function of
plant proportion variation (W and V D B,
1965; M and T, 1974; H,
1977). us, it is possible to establish the competitive
relationship between species by graphical visualization
(S, 1983; J et al., 1984; R
et al., 1997). is methodology has been successfully
used to study crop-weeds interaction by E
J. et al. (2002), H and B (2002), VILÁ
et al. (2004), B et al. (2006), A et al.
(2008), F et al. (2008) and R et al. (2008). e
confidence degree of this method of data interpretation is
equivalent to competition coefficient estimative (R-
et al., 1997), so that it can be used securely instead
of any other method.
Although Brachiaria plantaginea (Link) Hitch (Alex-
andergrass) is not the most important weed in Triticum
aestivum L. (wheat crop), this species is frequently found
in areas cropped with this cereal in the South of Brazil.
R et al. (2000) reported that, although the Alex-
andergrass emergence is more frequent in summer season,
it can also emerge substantially in winter season and then
infest winter crops. Moreover, this species is resistant to
ACCase inhibitor herbicides commonly used in soybean in
a rotation with wheat (C et al., 2001). us,
this weed can increase its population if the control is not
well done, and then compete with wheat for environmen-
tal resources needed to plant growth and development.
Considering the importance of Alexandergrass in
wheat crop and because both species are monocots, the
use of herbicides for Alexandergrass control becomes a
challenge in wheat crop management. So, knowledge of
Alexandergrass and wheat competitive relationship may be
used to help establish management strategies for control-
ling this weed. e research hypothesis was that two
species growing in coexistence have different growth and
development behavior due to their relative competitive-
ness. us, the objective of this research was to measure
the relative competitiveness of wheat crop with Alexan-
dergrass by the interpretation of plant density and propor-
tional effects using replacement series experiments.
2. MATERIAL AND METHODS
e experiments were carried out in July and Septem-
ber 2006 (monocultures) and July and October 2007
(replacement series). Formerly, two experiments were
conducted with wheat and Alexandergrass monocultures.
Pots (63×63×30 cm) were filled with substrate composed
by soil and manure in a proportion 3:1. It was kept the
numbers of 1, 3, 5, 10 and 15 plants per pot. Wheat and
Alexandergrass were randomly planted in the pots. e
experiments were conducted in randomized complete
block design with four replicates.
e replacement series experiment was conducted
subsequently to monoculture experiments. It was also
conducted in pots (63×63×30 cm) filled with substrate
composed by soil and manure in a proportion 3:1. It
was established different plant proportions keeping final
plant density constant. Proportions between wheat: Alex-
andergrass plants were 0:6, 1:5, 2:4, 3:3, 4:2, 5:1 and 6:0.
Bragantia, Campinas, v. 70, n. 1, p.40-45, 201142
L.B. Carvalho et al.
Wheat and Alexandergrass were also randomly planted in
the pots. e experiments were conducted in randomized
complete block design with four replicates.
It was not necessary to apply either fungicides or pesti-
cides as long as water was supplied sufficiently to a good
development of weed and crop, in both experiments.
In monoculture experiments, shoot of both species
were collected on September 6th (50 days after planting).
Shoot dry mass was weighted (analytical balance) after
drying at 70ºC in a forced air convection oven during
96 hours. Data were submitted to regression analysis by
Boltzmann sigmoid model:
y = +A2
A1-A2
1+ exp(x-x0)/dx
Boltzmann equation where: y indicates dry mass accumu-
lation; A1 - A2 indicates production loss; x indicates plant
density; x0 indicates plant density achieving 50% of “final
constant production”; and dx indicates tangent in x0.
Dry mass data were also plotted in a graphic and
compared to the standard error. Data interpretation was
in accordance with the previously cited “final constant
production” (C and W, 1994;
C and V F, 1996).
In replacement series experiment, results were evalu-
ated in two dates. Leaf area (Li-Cor 3000A equipment),
number of tillers and dry mass accumulation for both
species, in addition to wheat plant height and number of
spikes, were measured on August 30 (50 days after plant-
ing). Wheat and Alexandergrass leaf area, number of tillers
and dry mass accumulation data were analyzed visually
by interpretation of the graphic of relative production
response in function of plant proportion (W, 1960; W
and V D B, 1965; H, 1977; S,
1983; J et al., 1984). In addition, it was analyzed
the behavior of wheat plant height and number of spikes
in relation to Alexandergrass dry mass accumulation.
Furthermore, on October 9th (90 days after planting),
the behavior of plant height, length, weight and number
of spikes, and number of tillers of wheat crop were also
analyzed in function of Alexandergrass dry mass accu-
mulation. Ninety days after planting, higher agronomic
interesting wheat characteristics more affected by the
coexistence with Alexandergrass were also submitted to
regression analysis.
3. RESULTS AND DISCUSSION
Wheat and Alexandergrass monocultures achieved the
“final constant production” of dry mass accumula-
tion before 15 plants per pot, according to Boltzmann
regression curve (Figure 1). Plant density achieving 50%
of “final constant production” may be used to compare
plant sensitivity to intraspecific competition, so that
lower values indicate more sensitive species (C-
and W, 1994; C and V-
F, 1996). us, comparing the parameter x0 of
Boltzmann equation, Alexandergrass showed x0 equal to
1.57 while wheat showed 2.10, indicating that the weed
was more sensitive to intraspecific competition than the
crop.
Despite the stabilization of wheat and Alexandergrass
theoretical dry mass accumulation occurred after 11 and 8
plants, respectively, wheat dry mass was equal after densi-
ty of 5 plants per pot while Alexandergrass dry mass was
equal after 3 plants per pot, comparing the standard errors
(Figure 1). us, we may consider that the final constant
dry mass production was established over these densities.
Increasing plant density, the intraspecific competition is
established thereby environmental resources are limited
to plants, reducing their development (R et
al., 1997). Due to this fact, as plant density increases, the
individual plant weight gets lower.
According to J et al. (1984), the highest
density achieved in monocultures experiments have to be
used as the maximum number of plants in replacement
series experiments. us, plant density in replacement
Figure 1. Behavior of dry mass accumulation of wheat and Alexandergrass monocultures in response to plant density. e symbol shows
the average value of four replicates.
Wheat
5
10
15
20
25
30
0 3 6 9 12 15
Density (plants per pot)
Dry mass (g per pot)
R2= 0.99 **
y = -89.16 / (1+exp(x+2.10)/1.91) + 21.53
Alexandergrass
50
60
70
80
90
0 3 6 9 12 15
Density (plants per pot)
Dry mass (g per pot)
y = -302.27 / (1+exp(x+1.57)/1.02) + 79.24
R
2
= 0.98 **
Bragantia, Campinas, v. 70, n. 1, p.40-45, 2011 43
Interaction between wheat and alexandergrass
series experiment should be 5 plants per pot. However, we
opted for 6 plants per pot because wheat regression curve
showed better stabilization just over this density.
Wheat and Alexandergrass mixture showed differ-
ent response in function of that it was studied 50 days
after planting. Relative leaf area was increased in both
species once kept in mixture (Figure 2a). However, rela-
tive number of tillers and dry mass accumulation were
slightly increased in Alexandergrass and slightly decreased
in wheat once kept in mixture (Figure 2b, 2c). Accord-
ing to interpretation of R et al. (1997), it is
evident that both species showed higher leaf area once
in mixture, indicating mutual benefices. Alexandergrass
was also beneficial in mixture cultivation while wheat
was denigrated, considering both number of tillers and
dry mass accumulation; furthermore, we may observe
that there was a slight increase of the sum parameter,
indicating that Alexandergrass made more efficiently
use of environmental resources than wheat in spite of
competing for these same resources, according to inter-
pretation of R et al. (1997). Moreover, still 50
days after planting, as Alexandergrass dry mass accumula-
tion was increased, wheat number of spikes gets lower,
mainly after proportion 1:5; but wheat plant height was
strongly reduced just after proportion 4:2 (Figure 3a). It
shows that wheat number of spikes was more sensitive to
Alexandergrass interference than plant height.
Now, 90 days after planting, as Alexandergrass dry
mass accumulation was increased, wheat number and
weight of spikes and number of tillers got lower; but
wheat plant height and length of spikes were strongly
reduced just after proportion 4:2 (Figure 3b). It shows
that wheat number and weight of spikes and number
of tillers were more sensitive to Alexandergrass interfer-
ence than plant height and length of spikes. So we might
confirm that higher agronomic interesting characteristics
as number and weight of spikes were strongly reduced, so
that at 50% of wheat population the decreasing was over
65% and 63%, respectively, according to the regression
equations (Figure 4a,b).
0
20
40
60
80
100
120
Density (plants per pot)
RDM (%)
Wheat Alexandergrass Sum
0
30
60
90
120
150
180
210
RLA (%)
0
20
40
60
80
100
120
RNT (%)
(a)
(b)
(c)
Wheat
Alexandergrass 0 1 2 3 4 5 6
6 5 4 3 2 1 0
Figure 2. Relative leaf area – RLA (a). Relative number of tillers
– RNT (b). Relative dry mass accumulation – RDM (c) of wheat
and Alexandergrass in response to plant proportion. Each symbol
shows the average value of four replicates. e traced line indicates
the equivalent production.
0
20
40
60
80
100
120
Relative (%)
Plant heigth Number of spikes Alexandergrass dry mass
0
20
40
60
80
100
120
Relative (%)
Plant heigth Number of spikes or tillers
Lenght of spikes Weight of spikes
Alexandergrass dry mass
(a)
(b)
Density (plants per pot)
Density (plants per pot)
0
0
1
1
2
2
3
3
4
4
5
5
6
6
0
0
1
1
2
2
3
3
4
4
5
5
6
6
Alexandergrass
Wheat
Alexandergrass
Wheat
Figure 3. Relation of plant height and number of spikes of wheat
with Alexandergrass dry mass at 50 days after planting (a) and of
plant height, number and length and weight of spikes, and number
of wheat with Alexandergrass dry mass at 90 days after planting (b).
Each symbol shows the average value of four replicates.
Bragantia, Campinas, v. 70, n. 1, p.40-45, 201144
L.B. Carvalho et al.
0
25
50
75
100
Number of spikes (%)
0
25
50
75
100
Wheat population (%)
Weight of spikes (%)
y = 0.46x + 0.01x
2
– 0.15
y = 0.51x + 0.01x
2
– 0.72
R
2
= 0.99**
R
2
= 0.99**
100 75 50 25 0
Figure 4. Behavior of the number of spikes and the weight of
spikes of wheat growing in coexistence with Alexandergrass in
function of the crop population decrease, at 90 days after planting.
Each symbol shows the average value of four replicates.
e fact of lower number of tillers had been observed
when weeds coexisted with wheat crop might be explained
by the highest resources allocation in principal stem caused
by low quality of light reflected to crop, reducing tiller
allocation (A and M, 2001). As a conse-
quence, number, length and weight of spikes were also
reduced; once these production characteristics are deter-
mined by wheat tillering (A et al., 2005). e most
important consequences of light quality cues, often medi-
ated by decreasing in red far-red ratios with respect to the
spectral composition of incident sunlight radiation affect-
ing weed-crop interaction are changes in plant morphology
in anticipation of competition by light quantity, water or
nutrients (M J. et al., 2009). us, as a final conse-
quence of competition for limited environmental resources,
Alexandergrass will cause reduction on wheat crop yield.
4. CONCLUSION
Alexandergrass is more sensitive to intraspecific competition
than wheat and lightly more competitive than wheat. Number
and weight of spikes and number of tillers are the wheat char-
acteristics more affected by Alexandergrass interference.
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