A Comparison between Interspecific vs. Intraspecific Competition of
Peas (Pisum sativum) and ginger (Zingiber officinale)
Steven Pham
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Table of Contents
Sections
Pg
Abstract
Introduction
1
Background information
Plant physiology
Nitrogen Fixation and Nodule Development
1
2
Related studies
Nitrogen Stress effect on plant development
Intensity of intraspecific competition
Response of grassland plants to competition
Effect of density and time of planting
Perennial grass response to defoliation
Effect of nitrogen source on plant growth
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4
5
6
7
9
Hypothesis
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Methods
Results
Discussion
Acknowledgments
Literature Review
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14
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21
22
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Abstract
Plants competition between interspecific versus intraspecific groups was studied for six
weeks in a laboratory setting. Results of these two groups were compared against the control
group. Interspecific groups contained plants of peas and ginger, and intraspecific group
contained of peas and peas. The control group consisted of only 1 pea plant per pot. Pea
seedlings were sown 5.08 cm into the soil. Ginger was introduced to pea plants of the
interspecific group when seedlings reached approximately 5 cm. Each group had a 1:1 ratio of
peas and peas or peas and ginger. Results showed that there was no difference in final plant
height between the interspecific and intraspecific group. Initially, plant heights of all three
groups were different from each other. As measurements were taken throughout the experiment,
the difference in plant height between three groups became smaller compared to each other. The
average stem width of the control group was higher than the other two groups. The analysis test
showed that the stem width of the interspecific group was not different from the intraspecific
group. The leaf surface area was also not different between the interspecific and intraspecific
group.
Introduction
Competition among plants for limited resources has been proposed as an important
factor in a number of ecological and evolutionary issues (Wilson et al., 1987). The root system
in plants functions as an anchor, absorbs water and minerals, and stores carbohydrates. Legumes
are plants such as peas and beans that are able to reduce atmospheric nitrogen into ammonium
within their nodule cells and incorporate it into amino acids before being released into the
phloem, thus providing nitrogen to the whole plant (Henry and Raper, 1986). Nodules are
outgrowths formed on the roots of legumes that contain symbiotic bacteria that fix atmospheric
nitrogen and provide it to the plant in exchange for carbon. The atmosphere contains
approximately 80 percent nitrogen gas, and most organisms can not use that form of nitrogen
(Lindermann and Glover, 2003). Instead, organisms use the ammonia (NH3) form of nitrogen to
manufacture amino acids, proteins, nucleic acid and other nitrogen containing components
necessary for life (Lindermann and Glover, 2003). Legumes, such as peas, grow best after they
have developed nitrogen fixation nodules that convert N2 into a usable form. When two plants
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such as ginger and peas compete for limited resources such as nitrogen, the competitive effects
may show more on the plants that lack this source of nutrition.
According to Lindermann and Glover (2003) nitrogen fixation is a process that changes
inert N2 to the biologically available compound NH3. In nature, this conversion is performed
only by bacteria. Plants benefit from this process when soil bacteria die and releases nitrogen in
the form of ammonia or when the bacteria live in a symbiotic relationship with the plant. The
nitrogen fixation process of a plant occurs within root nodules, and it produces NH3 as a product
for plants to absorb. Nitrogen contribution to the natural ecosystem by legumes can range from
25-75 pounds per acre for one year, and even up to several hundred pounds in a crop of legumes.
The legume nodules are formed by the soil bacterium Rhizobium, which invades the root and
multiplies within the cortex cells of the plant. As the bacteria multiply, they receive their
nutrients and energy from the plant. Small nodules can be seen within the second week of
seedling development. A young nodule is not capable of fixing nitrogen, and this is indicated by
the gray or white color of the nodule. Indication of nitrogen fixation nodules occurs when the
inside of the nodule turns pink or reddish as it grows in size. It requires a large amount of energy
for bacteria in the nodules to fix nitrogen. The bacteria obtain the required energy from the plant
in the form of photosynthate, which is a sugar formed during photosynthesis carried out by the
plant. During the middle of a growing season, the pink or red nodules should dominate,
indicating that nitrogen fixation is occurring (Lindermann and Glover, 2003). The color of pink
or red comes from an oxygen carrying enzyme called leghemoglobin, which is found in the
cytosol of infected cells. Leghemoglobin is produced by the bacteroid and the plant. The N2
fixing enzyme (nitrogenase) in the process of nitrogen fixation would not function in the
presence of O2. Therefore, leghemoglobin buffers the O2 concentration within the nodule and
4
allows respiration to occur without inhibiting nitrogenase activity. However, if little nitrogen
fixation occurs, then gray or white nodules should dominate. This lack of pink or red nodules
illustrates that there was insufficient Rhizobium, lack of plant nutrition, or other plant stresses
that inhibit nitrogen fixation.
Nodules on various legumes have a variety of shapes and life spans. Nodules that
develop on perennial legumes, such as alfalfa or clover, will fix nitrogen for the entire season,
and the nodules usually grow around the central root tap (Lindermann and Glover, 2003). The
nodule shape of perennial legumes is fingerlike, and mature nodules resemble a hand. The entire
nodule of a perennial legume is only approximately 1/2 inch in diameter. Nodules on annual
legumes like pea plants are short-lived and replaced constantly during the growing season
(Lindermann and Glover, 2003). Annual legumes have pod-shaped nodules, which can reach the
size of large peas. When pods develop on pea plants, the nodules on the legumes lose their
ability to fix nitrogen. More nutrients are used for the development of the seeds than for the
nodules (Lindermann and Glover, 2003).
Henry and Raper (1986) studied the effect of nitrogen stress on soybean leaf expansion
and photosynthetic rate in young and old leaves, as well as the degree of recovery of these
activities following the resupply of nitrogen. Cultures of soybean plants were divided into
control and experimental groups. The experimental group was treated with different levels of
nitrogen. Leaf surface areas were measured at 1-2 day intervals for both groups. Plants from
both groups were sampled at intervals of 3-4 days over the 25 day experimental period. Upon
harvesting, leaf areas were determined photometrically with a Hayashi Denko AAM-5 area
meter. Results show that the effect of nitrogen stress on photosynthetic activity and recovery of
photosynthesis after resupplying nitrogen varied with leaf age. In mature leaves, photosynthetic
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rate per unit leaf area grew continuously until day eight, and after day 8, it declined steadily.
This decline of photosynthetic rate reflected the natural process of aging in leaves. When aging
occurred in leaves, nitrogen and other mobile nutrients were relocated to growing regions within
the plant. Senescence refers to the processes of a living organism approaching advance age. The
process of senescence of leaves increases as nitrogen decreases. Nitrogen deficiencies start
protein degradation as well as amino acid export to younger growing regions where nitrogen is in
high demand. Photosynthetic activity in young leaves increased when nitrogen was resupplied,
and it eventually returned to the level of nonstressed plants in the control group. This study is
directly related to my project because it illustrates the importance of photosynthetic recovery
rate. In intraspecific competition, nitrogen is a key source of a nutrient among the pea plants.
When the peas compete for a common source of nutrient such as nitrogen, how well a plant
recovers from environmental stress may determine its growth.
Nicotra and Rodenhouse (1995) tested whether the intensity of intraspecific plant
competition of Chenopodium album varies under different conditions of resource supply. One to
eight seedlings were planted per pot and later were thinned to 6 plants per pot. The potting
medium consisted of two parts sand and 1 part perlite. This potting medium was chosen because
it would not contribute to or accumulate nutrients added in solution. Five days after planting,
five replicate pots of each density were assigned to each of five nitrogen/light treatments. The
five resource treatments were composed of two contrasting gradients. One gradient extended
from the treatment of low light and low nitrogen to high light and high nitrogen (treatments
LL:LN, ML:MN, and HL:HN). The second gradient ran from conditions of low light and high
nitrogen to low nitrogen and high light (treatments LL:HN, ML:MN, and HL:LN). Plants were
watered with 200mL of distilled water every other day. To ensure that water was not limiting,
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soil was kept damp. The results showed that intraspecific competition occurs in every treatment
and noticeably affected plant growth. In treatment of LL:LN, ML:MN, and HL:HN, plant height
and number of leaves were smaller at high density (6 plants per pot) than at low density (grown
alone). LL:LN plants grown alone were 58% shorter and had 87% fewer leaves than HL:HN
plants grown alone. In the second gradient, LL:HN and HL:LN treatments showed less
competition where either light or nitrogen was in short supply than when both light and nitrogen
were available at intermediate level (ML:MN). HL:HN treatment plants showed the greatest
competition and productivity. This study relates to my project because it illustrated the effect of
resource availability on intraspecific competition. Competition is most intense when resource
availability is high. Based on the results of this study, as the peas compete with each other for
nutrients, I predicted that there will be intense competition when resources, such as nutrients in
the soil, amount of light, and water, are highly available.
Evans (1960) studied the interrelation of responses of three nonlegumous species of an
annual grassland community relative to nitrogen treatments and plant competition. The three
species were Bromus mollis, Erodium botrys, and Gestuca megalura. Each type of grass was
grown in a monospecific, bispecific, and trispecific environment. Monospecific is associated
with only one type of grass, and bispecific and trispecific are associated with two and three type
of grasses grown together. Plants were grown for 16 weeks in a greenhouse to control
temperature and humidity. Amounts of nitrogen applied varied from 100, 200, and 400 lb. After
16 weeks, plants were harvested and weighed. Results showed that applied nitrogen increased
the yield of 3 species in similar ways and magnitude when they were grown monospecifically.
Bi- and trispecific competition among these species affected yields when nitrogen was applied.
Monospecific yields of species were used as a base in all comparisons of yield differences
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resulting from interspecific competion. In the species E. botrys, adjusted yield increases resulted
from bispecific competition with B. mollis and F. megalura at 400 lb of nitrogen applied. In
trispecific competition where E. botrys were grown together with B. mollis and F. megalura at
400 lb of nitrogen, adjusted yield also increased. For the species B. mollis, adjusted yield
decreased due to bispecific competition with E. botrys at 400 lb of applied nitrogen. In
bispecific competition of B. mollis with F. megalura, adjusted yield increased at 100, 200, and
400 lb of applied nitrogen. For the species F. megalura, no increases in adjusted yield resulted
from bi- or trispecific competition. Under high levels of applied nitrogen, E. botrys was most
competitive, B. mollis was intermediate, and F. megalura was least competitive. Interspecific
competition was not effective in altering yields below 200 lb of applied nitrogen in bispecific
relation. Characteristics of E. botrys which might influence its competitive ability are rapid
emergence and early growth, vigorous and large total growth, and wide leaves, which tend to
shade associated plants. This experiment is directly related to my study because it shows how
interspecific competition in a bi- or trispecific environment affects the yield rate. In my
research, I studied the interspecific competition of ginger and peas, and also the intraspecific
competition between the pea plants themselves. In intraspecific competition among the pea
plants, the pea plant that can outgrow the other pea plants will have a higher competitive ability.
Competition level for a nutrient can be determined by growing different species with each other
as this study illustrated. The least competitive plant will have the lowest yield rate, because it is
not able to compete effectively due to its growing rate.
Auken and Bush (1990) measured the effect of grass density and time of planting on
Prosopis glandulosa (honey mesquite) seeding growth, and the results were compared to the
control group of just P. glandulosa. The type of grass chosen for this study was Cynodon
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dactylon (Bermudagrass). The P. glandulosa seedlings were planted with various densities of C.
dactylon. The results of this experiment showed that the P. glandulosa growth increased when
the density of C. dactylon was low. When one seedling of P. glandulosa was grown with 2 or
more C. dactylon, the total dry weight of P. glandulosa decreased. The time of planting between
P. glandulosa in relation to C. dactylon reduced the dry weight of P. glandulosa. When the two
plants were grown together at the same time, C. dactylon caused the dry weight of P. glandulosa
to decrease 86%. When the time difference was 2 months, the total dry weight of P. glandulosa
was 4.5% lower than the total dry weight of the P. glandulosa planted alone in the same time
interval. The response of P. glandulosa to various densities indicates that it is a poor competitor.
Some species can out-compete other species even when they are introduced late to the
environment. Therefore, the time of introduction is an important factor when one species can
inhibits the growth of another. This research is directly related to my study because it illustrated
the importance of density when plants are grown together. This study showed how to avoid
overcrowding when growing two types of plants together. Overcrowding of one plant can cause
the other plants to decrease their production and die. Therefore, the plant density of ginger and
peas in my study will be a 1:1 ratio to give both plants an opportunity for growth and
development.
Banyikwa (1987) studied the growth responses of the interspecific and intraspecific
competition of two East African perennial grasses to defoliation, nitrogen fertilizer, and
competition. Interspecific competition occurs between species; while intraspecific is
competition that occurs within a species. The two perennial grasses Digitaria macroblephara
(basionym) and Sporobolus ioclados (pan dropseed) were studied in a greenhouse at Syracuse
University. The experiment was divided into four categories of defoliation, nitrogen level, pure
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or mixed culture, and density. Plants were defoliated weekly to a height of 5 cm above the soil
surface, or they were left alone and served as the control group, throughout the experiment. The
nitrogen concentration applied to the plants in solution ranged from 3 mM to 15 mM. Density
varied from 1, 3 or 5 plants per pot, and plants were grown as pure culture, which consisted of
only S. ioclados or D. macroblephara. The mixed culture consisted of S. ioclados and D.
macroblephara. For the mixed culture, density was kept at one of each species per pot.
Defoliation reduced yield per plant by 81.0%, but the D. macroblephara species decreased yield
per plant more than S. ioclados. Interaction of defoliation with nitrogen increased yield per plant
by 20.4% and 330.9% compared to undefoliated plants of D. macroblephara. For S. ioclados,
the addition of nitrogen did not increase the yield per plant as fast as D. macroblephara. This
indicates that S. ioclados exhibits a slow response to nitrogen. The yield per plant decreased by
55.7% when the density of plants increased from one to five plants per pot. Intraspecific density
dependent competition significantly decreased by 67% compared to interspecific competition,
which decreased by 43%. It was found that addition of nitrogen yielded more biomass to shoots
and roots in D. macroblephare than in S. ioclados. Banyikwa (1987) also found that the growth
of the two species per plant yield less in pure culture than in a mixed culture. This study showed
the important difference between interspecific and intraspecific competition, and it is directly
related to my project because it illustrated that intraspecific competition can decrease the yield
per plant more than interspecific competition. For my research, I studied the interspecific versus
intraspecific competition of peas and ginger. As such, this study gave me a general idea of what
to expect for interspecific competition of the pea and ginger plants and intraspecific competition
of just the pea plants.
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Bedell et al. (1999) measured the effect of nitrogen on the growth and activity of
nitrogen-assimilating enzymes in Douglas fir (Pseudotsuga menziesii Franco) seedlings.
Douglas fir seeds were obtained from the Laboratory of Forest Microbiology and were sterilized
in 30% H2O2 and washed in sterile water. Douglas fir seeds were grown in a controlled chamber
with a pre-set temperature and humidity, then nitrogen was supplied to the seeds in the form of
(NH4)2SO4, KNO3, or NH4NO3 every two days throughout the experiment. After 40 days,
(NH4)2SO4 supplemented seeds yielded the highest growth rate in the category of shoot and root
length. Intermediate results were obtained from plants with NH4NO3 and NO3 as sources of
nitrogen. The numbers of lateral roots were dependent upon the source of nitrogen. From day
48 until 68, the end of the experiment, the number of lateral roots was higher when (NH4)2SO4
was used as a source of nitrogen than for NH4NO3 or NO3. This study directly relates to my
research because it illustrates the importance of different nitrogen sources and their effect on
plant yield. In intraspecific competition, the peas will compete for the same source of nitrogen.
The study shows that nitrogen in the form of (NH4)2SO4 would have the highest yield per plant
compared to NH4NO3 or NO3. For example, if the peas were competing for nitrogen in the form
of (NH4)2SO4, rather than NH4NO3 or NO3, then it would have a higher percent yield.
The purpose of this research was to study the effects of interspecific versus intraspecific
competition. In interspecific competition, the two species were ginger and peas. In intraspecific
competition, it was only peas. As plants compete for nutrition in the soil, physiological and
morphological aspects of the plants resulting from malnutrition would appear. The results of
interspecific and intraspecific competition will be compared to the control group of pea plants
grown alone. Pea plants have nitrogen fixation nodules on their roots to fix nitrogen in the
process of their development. In intraspecific competition, the pea plants can inhibit each other’s
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growth rate by competing for the same source of nitrogen. A shortage of nitrogen can lead to
yellowing of leaves and stoppage of growth. Nitrogen is present as a part of nucleoprotein,
amino acids, amines, amino sugar, polypeptides and other organic compounds in plants (Nagoshi
et al., 2005).
Ginger is an herbaceous plant that belongs to the family Zingiberaceae (Nagoshi et al.,
2005). Ginger’s uses and effects on people are widely known. It can be used for many purposes
such as a cooking spice, beverages, and medicinal uses. There has been little research on the
effect of an herbaceous plant on leguminous plants. The fresh smell and flavor of ginger is due
to chemicals called gingerols (Nagoshi et al., 2005). Nagoshi and colleagues investigated the
properties of gingerols by testing it against vancomycin-resistant enterococci bacteria. They
found that gingerol has antibacterial effects (Nagoshi et al., 2005). Ginger could therefore
inhibit pea growth rate by secreting the chemical gingerol into the soil, reducing the number of
Rhizobium bacteria available for nitrogen fixation. Therefore, it could reduce the level of
nitrogen fixation and the amount of nitrogen available to the plants.
In most studies in the past, such as Auken and Bush (1990), Evans (1960), and Bedell et
al. (1999), the effect of density, time of planting, competition among species, and different
nutrient source such as nitrogen were observed. The study of interspecific versus intraspecific
competition was developed to test whether it is advantageous to have interspecific or
intraspecific competition when nutrients are temporary or seasonal. The results of Banyikwa
(1987) showed that intraspecific competition decreases yield per plant more than interspecific
competition. Banyikwa (1987) also showed that in a grazed ecosystem, where nutrients are
temporal and seasonal, it may be advantageous to both plants and grazers for a plant to grow in
mix culture since interspecific competition decreases yield per plant less than intraspecific
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competition. In this experiment, the interspecific and intraspecific competition of peas and
ginger was studied for six weeks. The study was divided into three groups: 1) interspecific
(ginger and peas), 2) intraspecific (peas and peas), and 3) control (only 1 pea plant). The
controlled variables in this experiment are the amount of sunlight, temperature, time of planting,
types of fertilizer, density of plants, and level of humidity. Throughout the experiment, I made
careful observations of all three groups. After six weeks of collecting data, plants were
harvested for further analysis to determine the results of interspecific versus intraspecific
competition compared to the control group. Base on previous study of Banyikwa (1987), I
hypothesized that the intraspecific competition between the ginger and peas will yield a greater
mass of shoots and roots than interspecific competition between the peas. I further hypothesized
that the yield per plant of interspecific and intraspecific competition will be lower than the
control group of peas alone.
Methods
There were total of 3 groups in this experiment. Group 1 consisted of ginger and peas, to test
interspecific competition. Group 2 consisted of several pea plants grown together, to test
intraspecific competition. Group 3 consisted of 1 pea plant per pot, the control group. Group 1
had a sample size of 20 pea plants and 20 ginger plants. In interspecific competition, the ginger
and the pea plant were grown together in a 1:1 ratio in a single pot to ensure equal competition.
Group 2 contained 2 pea plants per pot for a total of 20 pots, a sample size of 40 pea plants for
Group 2. In Group 3, there was only 1 pea plant per pot as the control group, for a sample size
of 10 pea plants. The pea seeds (Pisum sativum) were obtained from the Ed Hume Seed
Company in Puyallup. The controlled variables were temperature, humidity, light, seed variety,
and moisture level of the soil (Ellis et al., 1995). Variables checked every day were temperature
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of the soil, amount of water added to the soil, and the humidity of the lab. In order to reduce
plant-to-plant variability, seeds of the pea plants were weighed, and a range of 0.21g-0.25g was
determined (Tricot et al., 1997). Only seeds within this range were used. The temperature of the
soil was controlled from a range of 10°C to 15°C by keeping the soil moisture in a moist
condition to maximize the germination rate (Ellis et al., 1995). The pea seeds were germinated
in a Petri dish. The seeds were placed on a wet paper towel under a photosynthetic lamp for 2-3
days. After germination was confirmed, the seeds in a range of 0.21g-0.25g were sown 5.08 cm
deep into a 10.16 cm x 10.16 cm in pot with regular potting soil, according to the 3 groups. The
soil brand used in this experiment was SunShine Mix, which was obtained from Mc Conky in
Puyallup. The brand SunShine Mix was chosen because it has a certain amount of nutrients such
as nitrogen, phosphorus, and sulfur that plants can compete for compared to other potting soil
such as Miracle-Gro, which contain excess of phosphate, zinc sulfate, and sodium molybdate,
and nitrogen. Soil moisture level of each group was recorded daily using a moisture meter that
measure the relative water content (RWC) in units of ρb (g/cm3) (Auken and Bush, 1990). The
moisture probe was inserted 5.08 cm deep into the soil at the center of each pot. If the moisture
level dropped below 5 ρb (g/cm3), 50mL of water were added to keep the moisture level between
5-7 ρb (Auken and Bush 1990). The moisture level was kept at 5-7 ρb in order to prevent the pea
roots from rotting. If the soil moisture is too wet, then organisms such as Rhizoctonia, Pythium,
Aphanomyces, or Fusarium could cause the roots to rot (Tricot et al., 1997). The amount of light
provided to each group came from an artificial light source, provided by Saint Martin’s
University. Plants received 24 hours of light in order to maximize growth. Observations of
germination and growth rates of the three groups were recorded daily throughout the experiment.
As soon as the pea seedlings reach the soil surface, the height of each pea plant was recorded by
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measuring from the top surface of the soil to the tip of the plant to the nearest 0.01 cm, for every
pot and all groups. The ginger used in this experiment was the generic kind, Zingiber officinale
Roscoe, and it was introduced to the pea plant when the pea plant was approximately 5 cm high.
The ginger was obtained from a local market in the form of rhizomes, and it was cut and
weighed at a range of 9g-13g before it was planted with the pea plants. This size range was
chosen because pieces of ginger being cut within this range contained eye-buds. The ginger was
sown approximately 2.54 cm deep into the soil and 2.54 cm apart from the pea plants in order to
give the ginger and pea roots room to interact. The rhizomes obtained have eye-buds and ginger
seeds were not used because it would have taken too long to germinate. If the ginger was
introduced earlier, it would delay the germination rate of the pea seedlings.
Measurements taken in this experiment were the height of each plant, mass of roots, leaf
surface area, color changes in the leaf, stem of each plant to the nearest 0.01 cm, and weight of
the fruits produced, if any. The leaf surface area of each group was measured by drawing the
leaf on graphing paper and counting the squares to determine the leaf surface area. The color
change, such as yellowing of leaves or shedding of leaves of each plant group, was determined.
The stem of each plant was measured using a caliper to the nearest millimeter. The dry weights
of the fruits were measured with a balance to the nearest 0.01 grams. After six weeks, plants
from each group were harvested. The leaf surface area and the dry weight of the roots were
measured to the nearest 0.01 cm. Dry weight was obtained first by washing the roots of each
plant to make sure that the soil was not being included in the measurement. Then, the roots of
each plant were dried at 90o C for 48 hr in a drying oven (Banyikwa, 1987). In order to
determine the growth of each group, the growth rate of each group was divided into 3 stages.
The first stage was on day 1. The second stage was on day 6, and the third stage was on day 12.
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By dividing the growth period into stages, it could show the trends in growth more clearly. After
all data had been recorded, a program of ANOVA was used to test if there was a significant
difference between groups. If there was a significant difference, then a Tukey test (at 95%
confidence interval) was used to perform pairwise comparisons. If p<0.05, then the HO
hypothesis was rejected, and if the p>0.05, then the HO hypothesis was accepted.
Results
Growth of Seedlings
The moisture level of each group was initially set at 5-7 ρb (g/cm3). The Control (Group 3)
seedlings were first to show signs of germination between the three groups. Several seedlings of
the Peas: Ginger (Group 1) and Peas: Peas (Group 2) germinated soon after the germination of
the Control group. Yellowing of leaves for Group 2 occurred on day 8 of growth. Several pea
plants in the Pea: Ginger group showed sign of wrinkling of leaves on the 9th and 10th day of
growth. Group 1 and 2 required more water in order to keep the moisture level at 5-7 ρb (g/cm3)
than Group 3. During the first 9 days of culture, growth increased steadily between Group 1 and
Group 2 compared to Group 3 (Fig. 1).
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Figure 1. Mean height of pea plants seedlings over a 12-day period for 3 replicates (n = 3).
Control (◊), Peas and Ginger (■), and Peas: Peas (∆). Measurements of plant heights were
measured to the nearest 0.01 cm. Each data point represents a mean. The mean value was
calculated from all individual plant height from that day. This gives a sample size of 70. Error
bars represent one standard deviation from the mean.
The initial average of Group 3 seedlings height was higher than of Group 1 and 2. After
Day 9, growth between all three groups increased rapidly (Fig. 1). From day 9 to day 12, the
average growth of Group 2 was 7.96 cm, group 3 was 8.00 cm, and group 1 was 6.92 cm. The
initial growth at day 1 showed a significant difference between the control, peas: peas, and peas:
ginger (f = 7.03, df = 2, p = 0.002). A Tukey comparison test (ICL > 98.1%) indicate that the
initial growth was significantly higher for the control compared to the peas: peas treatment. On
day 6, the growth of all three groups showed no significant difference among each treatments (f
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= 2.76, df = 2, p = 0.071). The growth rate among the three groups was not statistically different
on day 12 (f = 1.37, df = 2, p = 0.261).
After the 12th day of growth, the stem width, leaf surface area, below ground biomass,
and above ground biomass were recorded for each group. The average widths was significantly
different (f = 9.09, df = 2, p = 0.001). A Tukey comparison test (ICL > 98.04%) indicate that the
stem width of the control group was statistically higher than the peas: peas.
Figure 2. Average stem width of 3 replicates over 12-day period at Lacey, WA, on February 22,
2008. Stem width were taken 2.54 cm above the soil for all 3 replicates. All measurements were
recorded to the nearest 0.01 cm. Each treatment mean is based on 70 plants. Error bars
represent one standard deviation about the mean.
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The leaf surface area was also taken to determine the growth of each plant from the three
groups. The leaf surface area was not statistically different among three groups (f = 1.26, df = 2,
p = 0.299). The mean leaf surface area was very close to each other within each group (Fig. 3).
Figure 3. Mean leaf surface area of pea plants seedlings over a 12-day period for 3 replicates (n
= 3) were collected at Lacey, WA, on February 22, 2008. Only green leaves were measured.
Each treatment mean was calculated from 30 leaves on different plants. Error bars represent one
standard deviation from the mean.
The average below ground biomass of the control group was lower than the peas: peas
and peas: ginger. However, the average above ground biomass of the control was higher than the
19
other two groups (Table 1). Roots of each group were harvest and dried within the group rather
than individual plant.
Table 1. Mean pea plants seedlings sampled at Lacey, WA, on February 22, 2008. The mean
was determined by dividing the total root weight of each group by the number of plants in that
particular group.
Treatments
Average Below Ground Biomass Average Above Ground
of each group (g)
Biomass of each group(g)
Peas: Ginger
Peas: Peas
Control
3.8
3.90
2.53
0.465
0.471
0.508
Discussion
Based on Figure 1, the initial growth in height at day 1 showed that the control group
grew at a faster rate than the peas: peas group. From day 1 to day 5, the growth rate increased
steadily for the control group. The control group grew best from day 9 to day 10, as indicated by
the steepness of the curve. The growth of the control group began to increase, but at a slower
rate from day 10 to day 12. Plants from each group were not rotated under growth lights during
the six weeks research. The control group received the most amount of light because it was in the
center area of the growth light. Plants that were in the center received the most amount of light
grew at a faster rate (Torrey, 1952). This is because plants absorb light to conduct
photosynthesis, and plants that receive a lesser amount of light would not grow as well as plants
that receive more lights (Tricot et al., 1997).
From day 1 to day 5, the growth in height between groups of interspecific and
intraspecific competition was nearly identical to each other. This trend continued until day 7.
There was a small separation of growth in height between two groups of interspecific and
intraspecific from day 9 to day 12. From day 9 to day 10, the interspecific competition group
grew slightly more than the intraspecific group. Then from day 11 to day 12, the group of
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intraspecific competition outgrew the interspecific group to achieve a higher growth in height.
Prior to the experiment, it was expected that the intraspecific group would grow at a much faster
pace than the interspecific group. Figure 1 indicated that the growth in height between the two
groups of interspecific and intraspecific was not different from each other.
The greater mean stem width of the control group suggested that plants from the control
group were healthier than the peas: peas group. The control group was healthier than the
interspecific and intraspecific groups because there was no competition in the control group.
The mean stem width of the intraspecific group was higher than the interspecific group. This
indicated that competition for the interspecific group was more intense than the intraspecific
group. Stem width in plants can be used as an indicator of plant health because it divides into
nodes that hold buds which grow into leaves, flowers, and other stems (Torrey, 1952). The
greater mean biomass indicated that plants from intraspecific competition were able to grow
more effectively than interspecific competition. The discrepancy for below biomass of the
control compared to the interspecific and intraspecific groups can be explained because some of
the roots could have been left in the soil when harvested. The mean leaf surface area was
another indicator of healthiness for plants in this experiment. The size of the leaf on each plant
can be used as an indicator to tell how healthy plants absorb nutrients from their environment
(Banyikwa, 1987). Plants that are able to absorb more nutrients from their environment will
show the effect on different growth area such as leaf sizes.
Based on the results collected for 6 weeks of research, it suggested that there were not
any differences in plant height between the interspecific and intraspecific group. When nutrients
such as nitrogen, water, phosphorous, and ammonium are temporal, results indicated that plant
competition of interspecific and intraspecific groups were equally competitive among each
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group. Results in this experiment suggest that competition in the wild and on farms are not
different from each other if species are grown together interspecifically or intraspecifically. In
order to obtain the maximum growth from plants for six weeks, results indicated that plants
should have no competition among them.
Further research that could be done in this experiment is to use water as a control
variable. Water is an important source of nutrients for plant growth. Using water as a control
variable would test how effectively plant competition occurs between the interspecific and
intraspecific group. Factors that could be changed for the next experiment are rotating the plants
under growth lights, a smaller range for moisture of the soil, and introduce the ginger earlier.
The amount of light is critical to plant growth, as indicated by Tricot et al. (1997). Plants that
receive more light contribute to the soil moisture. Plants that received the most amount of light
were indicated by growth and dryness of the soil. Moisture level in the soil became dry at a
faster rate for plants that received the most amount of light. The range of moisture levels could
have been smaller to decrease the variance between all three groups. Another factor that
influences plant competition was the time of introduction of the ginger. By introducing it at an
earlier time, this will give the ginger more time to develop, and the difference between all three
groups would have been clearer.
In conclusion, intraspecific competition yielded a greater mass of roots and shoots than
interspecific competition when nutrients of an ecosystem are seasonal. The intraspecific group
yielded a higher roots and shoots than the interspecific group because the presence of ginger
decreased or inhibit the growth of peas when it was introduced. Intraspecific and interspecific
competition both yield a lower mass of roots and shoots compared to the control. Early stages of
growth showed a greater difference between all the groups than late stages. In the early stages of
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growth, each group had to adapt to their environment. The group that adjusts to its environment
the fastest will be most successful because it will utilize nutrients that are available at a faster
rate. The group that takes the most time to adjust to its environment would not grow as
efficiently as the group that takes less time to adjust to its environment. The similarity between
the three groups can be examined if plants had more time to develop. If competition between the
peas and ginger behave as expected when the ginger was introduced at the time of germination of
pea plants, then the H0 hypothesis is accepted. The H0 hypothesis is accepted because this would
give the ginger more time to interact with the pea plant. If interspecific and intraspecific
competition behaves as they did in the present study, then distributing amount of light evenly by
rotating the groups on daily basis would support the H0 hypothesis because amount of light
receive affect plant growth.
Results of this study has major implications on the agriculture system in farms and as
well as in the wild. Most studies have ignored the importance of competitive interactions
between interspecific versus intraspecific. Statistical analysis of results in this study indicated
that competition between the intraspecific group were equally competitive compared to the
interspecific group. Therefore, plants could be grown interspecifically or intraspecifically when
nutrients are seasonal in an ecosystem. In a farm setting, this indicated competitive interaction
do not influence plant yield. In a wild type setting such as the forest, plants have an equal
chance of competition for nutrients in their environment.
Acknowledgments
I would like to thank Dr. Coby, Dr. Hartman, and Dr. Olney for their guidance and helpful
suggestions during this experiment. I would also like to thank Cheryl Guglielmo for the
preparation of equipments that were used in this experiment.
23
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