Roll A
Biology 1082 Laboratory
Thursday 2:00-5:00pm
3 April 2013
THE EFFECTS OF LIGHT VARIABILITY ON COMPETITION BETWEEN BRASSICA RAPA AND LOLIUM
MULTIFLORUM
ABSTRACT
The plant species Brassica rapa and Lolium multiflorum, the latter also known as annual rye, are fast
growing plants that serve as good subjects to analyze plant competition. Plant competition is a very
relevant field of scientific study today, as new questions are brought up about the interactions between
the biotic and abiotic elements of the environment. In our experiment, we studied the growth of B.
rapa and annual rye in and out of competition using light as a limiting factor. In particular, we limited
the red wavelength of light in our treatment group to analyze how this resource limitation affected the
two species’ biomasses. Taking into account the relationship between leaf surface area and light
absorption, we predicted that the annual rye, which has a leaf with a larger surface area, would
dominate over B. rapa when in competition under limited light conditions. After conducting our
experiment, we noticed that under full light conditions, annual rye dominated over B. rapa when in
competition. We attributed this observation to the annual rye’s larger leaf, which enhances light
absorption. Our experimental results also contradicted our original prediction as nearly all of both
annual rye and B. rapa died when placed under limited light while in competition. A possibility for this
result could be our unintentional discovery of the limits to the environment in which we can place the
two species together. This arena of study gives way to a new idea that upon further experimentation
would enhance our knowledge about which environmental conditions allow for both species to prosper.
INTRODUCTION
Competition ecology is an area of study at the top of inquiry among modern scientists. Questions
regarding what factors influence competition between two species and how to identify the presence of
competition have created much disagreement among ecologists. However, while the approach
scientists take often differ, common consensus is that the study of competition is an important aspect to
the environment. An invasive plant lineage competes against other plant species in a spatially variable
environment by exploiting the resources in that environment quickly, dominating over other species
that may inhabit the same environment (Bolker and Pacala, 1999). Therefore, setting limitations on
necessary resources can potentially reveal the species that dominates over others in a competitive
environment. The two species we will focus on in our experiment are rapid-cycling Brassica rapa and
Lolium multiflorum, commonly known as annual rye. We are interested in analyzing how B. rapa reacts
to limitations in light when in and out of competition with the annual rye. Plants use mainly red and blue
wavelengths of light for photosynthesis. A previous study showed that flowering plants exposed to a
combination of red, blue, and white light showed high levels of sweetness in their fruit - a result of
heightened production of glucose from photosynthesis (Lin, et al., 2013). The environmental factor in
our experiment is red light, which we will restrict in our treatment group. We operate our experiment
under the null hypothesis that the biomass of neither B. rapa nor the annual rye will change significantly
when they are grown separately or together, under full light or limited red light. Previous studies have
shown that fitness and growth experienced by plants in natural vegetation is highly dependent on
interactions with plants resulting from competition, especially in regards to their absorbance of light
needed for photosynthesis (Schmitt and Wulff, 1993). Field observations have shown that at low
densities the competition for light could be symmetric, while at high densities, the photosynthetic
success is proportional to leaf area (Droz and Pekalski, 2011). We predict that the annual rye will show
dominance over B. rapa when in treatment because its leaf spans the entire length of the plant, whereas
the B. rapa has a smaller leaf. Consequently, the greater surface area of the rye leaf will aid in capture
of the available light.
MATERIALS AND METHODS
We started with picking a competitor for the B. rapa. We decided that the annual rye would be a good
competitor since it is a visually different plant and would be easy to differentiate from the B. rapa. The
annual rye and the B. rapa are to be planted both separately and together by a lab technician. There are
two conditions, control and treatment, and 72 pots total. Each condition group will consist of 12 pots
containing B. rapa alone, 12 pots containing annual rye and B. rapa planted together, and 12 pots of
annual rye alone. In each pot where only one species is growing by itself, 16 seeds of that species is
planted. In each pot where both species are growing together, 8 seeds of each species are planted.
3B soil is used in each pot. The technician will drench the soil with 20cc of 2x fertilizer or until it
runs off, and then scatter the seeds randomly throughout the pot and cover each pot with 1/8 to ¼ inch
of vermiculite. They will wet the vermiculite with distilled water and label the pots, and cover a large
tray with saran wrap and place the smaller trays in it with an inch of distilled water. The technician will
then place the “treatment” half of the plants under a red cellophane umbrella to restrict red light from
the plants. The amount of light waves reaching each plant is the independent variable in this
experiment, and the final biomass is the dependent variable. The plants should be allotted one week of
growth to allow the plants to sprout. The only difference between the controlled tray and experimental
(treated) tray should be the presence of the red cellophane umbrella above the treated tray. This is to
maintain a genuine control group to be used for comparison.
Once the plants have sprouted, the technician will then pluck the plants carefully so the pots
that contain one species have 12 plants in each pot, and the pots that have both species have 6 plants of
each species in each pot. The plants should then be allotted an additional week of growth, and be
watered every other day.
The technician will carefully remove all plants from the soil and then take them to the oven to
dry. The final dry biomass is will be recorded, so the technician must be sure to keep track of which
plants (and how many) come from each pot and from which condition. They will then weigh the dry
mass for each plant from each pot, and average the total biomasses from each pot. This data is then to
be organized in a way that one knows the pot number, the number of plants within that pot (ranging
from 0 if none grew to 12 if all plants grew), the average biomass, and the condition it was grown in (be
it the only species in the pot or grown in competition, and if it were from the controlled tray or the tray
under the treatment umbrella).
Using the ANOVA: Single-Factor and Descriptive Statistics functions in Microsoft Excel,
determine the mean biomass, mean number of plants grown, standard error, f and p-values of each
relative group to determine if there is a statistical difference between two groups. In our experiment,
we compared the average biomass of B. rapa and annual rye when grown alone in the controlled tray
and within the tray under the treatment umbrella, each species when grown in competition with one
another in both controlled and treated environments, and the average biomass of each species when
grown alone versus grown in competition (for both the controlled and treated environments). We also
studied the average number of plants within each pot for each species when in a controlled
environment and treated environment. Remember, if the p-value is less than 0.05, there is a statistical
difference between the two groups.
RESULTS
After the first week, aboveground growth became visible, and we could easily weed out the plants
grown in excess. And because we chose two plants easily differentiated, there was no issue in
determining what was B. rapa and what was annual rye when looking at the pots containing both. We
noticed that all plants (after the first week) seemed to be growing at around the same pace. After the
second week however, one could easily identify the controlled tray from the treated tray by the size of
the plants.
Figure 1 shows the B. rapa in and out of competition without light stress. The ANOVA test gave a
p-value greater than 0.05 which is not significant. Figure 2 shows the Annual Rye in and out of
competition without light stress. The ANOVA test gave a p-value less than 0.05 which shows a significant
difference. Figure 3 shows the B. rapa with and without limited resources; the ANOVA test on the
average biomass gave a p-value greater than 0.05, indicating that there was no difference in the actual
growth of the surviving plants. Figure 4 shows the average number of B. rapa plants in each pot within
the controlled and treated environments. The p-value is 1.7E-14, dramatically less than 0.05, meaning
there is a statistical difference between the two groups. Figure 5 shows the annual rye with and without
limited resources. The ANOVA test gave a p-value less than 0.05, which shows a significant difference
between the bio-mass of each group. Figure 6 shows the average number of annual rye plants in each
pot within the controlled and treated environments. The p-value is 0.246, greater than 0.05, meaning
the two groups are not statistically different. Figure 7 shows B. rapa in and out of competition under
light stress. The ANOVA test gave a p-value less than 0.05 which shows a significant difference. Figure 8
shows Annual Rye in and out of competition under light stress. The ANOVA test gave a p-value less than
0.05 which shows a significant difference. Figure 9 shows the B. rapa in competition with another
species with and without light stress. The ANOVA test gave a value less than 0.05 which shows a
significant difference in the bio-mass. Figure 10 shows the annual rye in competition with a competitor
of another species with and without light stress. The ANOVA test gave a p-value less than 0.05 which
shows a significant difference in the bio mass.
DISCUSSION
In the control group, there was not a significant difference between the biomass of B. rapa
when grown alone and when in competition (Figure 1), but there was a significant increase in the
biomass of the annual rye when in competition (Figure 2). Thus, we concluded that in full light, annual
rye dominates when in competition with B. rapa. We attributed this conclusion to the larger surface
area of the annual rye’s leaf which would aid in its absorption of light needed for photosynthesis. This
wasn’t so surprising because in the study by Schmitt and Wulff in 1993, fitness and growth is highly
dependent on interactions resulting from competition- especially in regards to their absorbance of light.
B. rapa does not show a significant difference in biomass when grown alone in full light versus
grown alone in limited red light (Figure 3). However, there is a significant decrease in the number of B.
rapa plants when grown alone in limited red light from when they were in full light (Figure 4). We then
concluded that limiting the red wavelength of light does not affect the growth of B. rapa but does affect
the intraspecific competition between the individual plants, decreasing the number that grow. A
possible explanation for this phenomenon could be that B. rapa absorbs mainly blue wavelengths of
light. This prediction would have to be further analyzed in a different experimental setup.
The annual rye shows a significant increase in biomass when grown alone in limited red light
from when it was grown alone in full light (Figure 5). However, there is no significant difference in the
amount of individual plants of annual rye when grown alone in treatment versus under control (Figure
6). Therefore, we concluded that limiting the red wavelength of light enhances the growth of the annual
rye but does not affect the intraspecific competition between the individual plants. We were confused
by this result, as we did not expect that the limitation of a resource would enhance the growth of the
plant. This result could have occurred by error of data collection or input, or could also be attributed to
the species’ general invasive nature, in which could also be further studied in an additional experiment
There was a significant decrease in the biomass of both B. rapa (Figure 7) and annual rye (Figure
8) when they were grown in competition under treatment from when they were grown alone under
treatment. Also, both B. rapa in competition (Figure 9) and annual rye in competition (Figure 10) show a
significant decrease in biomass when placed in limited red light, from when they were in full light. The
decrease for both species was so drastic that the average biomass of both species under treatment
when in competition was almost zero. Therefore, we concluded that when under limited red light when
in competition, neither B. rapa nor annual rye can survive. This result proved our prediction wrong. The
result observed might have occurred because putting the two species in competition and limiting such
an important resource could have been the final push beyond the sustainable limits to the environment
that the species require to grow, but the only way to know for sure would be to repeat the experiment
and limit a different resource, such as a different colored wavelength of light or nutrients within the soil
or amount of fertilizer or water used.
After analysis of our results, we arrived at four conclusions: (1) in full light, annual rye dominates
when in competition with B. rapa; (2) limiting the red wavelength of light does not affect the growth of
B. rapa but does affect the intraspecific competition between the individual plants, decreasing the
number that grow; (3) limiting the red wavelength of light enhances the growth of the annual rye but
does not affect the intraspecific competition between the individual plants; (4) when under limited red
light and in competition, neither B. rapa nor annual rye can survive.
LITERATURE CITED
Bolker B, Pacala S. 1999. Spatial Moment Equations for Plant Competition: Understanding Spatial
Strategies and the Advantages of Short Dispersal. Journal. 153:575-602
Droz M, Pekalski A. 2011. Role of asymmetry in competition for light in a model of annual plants.
Journal. 391:1243-1254
Lin KH, Huang MY, Huang WD, Hsu MH, Yang ZW, Yang CM. 2013. The effects of red, blue, and white
light-emitting diodes on the growth, development, and edible quality of hydroponically grown lettuce
(Lactuca sativa L. var. capitata). Journal. 150:86-91.
Schmitt J, Wulff R. 1993. Light spectral quality, phytochrome and plant competition. Journal. 8:47-51.
TABLES AND FIGURES
0.04
Average Bio (g)
0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
1
2
Competition
Figure 1: Average Biomass of the Controlled B. rapa – Column 1 shows the average biomass of the
controlled B. rapa when grown alone, and column 2 shows that of the controlled B. rapa when grown in
the same pot as the Annual Rye. The standard error from the ANOVA: Single-Factor test for the B. rapa
alone was ± 0.012861, and for the B. rapa in competition was ± 0.000782. (F(1,22)=0.765, p=0.391265).
0.009
Average Biomass (g)
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0
1
2
Competition
Figure 2: Average Biomass of the Controlled Annual Rye – Column 1 shows the average biomass of the
controlled Annual Rye when grown alone, and column 2 shows that of the controlled Annual when
grown in the same pot as the B. rapa. The standard error from the ANOVA: Single-Factor test for the
Annual Rye alone was ± 0.000355, and for the Annual Rye in competition was ± 0.000653. (F(1,22)=7.01,
p=0.0147128).
Average Biomass (g)
0.04
0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
1
2
Condition
Figure 3: Average Biomass of B. rapa When Grown Alone – Column 1 shows the average biomass of the
controlled B. rapa when grown alone, and column 2 shows that of the B. rapa when grown under the
treatment umbrella. The standard error from the ANOVA: Single-Factor test for the controlled pots was
± 0.01286, and for the treated pots was ± 0.00296. (F(1,22)=0.430, p=0.519).
Average number of plants
14
12
10
8
6
4
2
0
1
2
Condition
Figure 4: Average number of B. rapa Plants: Column 1 shows the average number of controlled B. rapa
when grown alone, and column 2 shows that of the treated B. rapa when grown in the same pot as the
annual rye. The standard error from the ANOVA: Single-Factor test for the controlled B. rapa was ± 0,
and for the B. rapa in competition was ± 0.527. (F(1,22)=313.6, p=1.7E-14).
Average Biomass (g)
0.06
0.05
0.04
0.03
0.02
0.01
0
1
2
Condition
Figure 5: Average Biomass of Annual Rye when Grown Alone – Column 1 shows the average biomass of
the controlled Annual Rye when grown alone, and column 2 shows that of the Annual Rye when grown
under the treatment umbrella. The standard error from the ANOVA: Single-Factor test for the controlled
pots was ± 0.000355, and for the treated pots was ± 0.005453. (F(1,22)=63.76, p=6.09E-8).
Average number of plants
12.5
12
11.5
11
10.5
10
9.5
1
2
Condition
Figure 6: Average number of Annual Rye Plants: Column 1 shows the average number of controlled
annual rye when grown alone, and column 2 shows that of the treated annual rye when grown in the
same pot as the B. rapa. The standard error from the ANOVA: Single-Factor test for the controlled
annual rye was ± 0, and for the annual rye in competition was ± 0.641. F(1,22)=1.369, p=0.255).
0.02
Average Biomass (g)
0.018
0.016
0.014
0.012
0.01
0.008
0.006
0.004
0.002
0
1
2
Competition
Figure 7: Average Biomass of the Treated B. rapa – Column 1 shows the average biomass of the B. rapa
when grown alone under the treatment umbrella, and column 2 shows that of the treated B. rapa when
grown in the same pot as the Annual Rye under the treatment umbrella. The standard error from the
ANOVA: Single-Factor test for the B. rapa alone was ± 0.00258, and for the B. rapa in competition was ±
0.000415. (F(1,22)=26.90, p=3.36E-5).
Average Biomass (g)
0.06
0.05
0.04
0.03
0.02
0.01
0
1
2
Competition
Figure 8: Average Biomass of the Treated Annual Rye – Column 1 shows the average biomass of the
Annual Rye under the treatment umbrella when grown alone, and column 2 shows that of the treated
Annual Rye when grown in the same pot as the B. rapa. The standard error from the ANOVA: SingleFactor test for the B. rapa alone was ± 0.005453, and for the Annual Rye in competition was ± 7.43E-5.
(F(1,22)=79.50, p=9.32E-9).
Average Biomass (g)
0.016
0.014
0.012
0.01
0.008
0.006
0.004
0.002
0
1
2
Condition
Figure 9: Average Biomass of B. rapa when Grown in Competition with Annual Rye – Column 1 shows
the average biomass of the controlled B. rapa when grown in the same pot as the Annual Rye, and
column 2 shows that of the B. rapa when grown in the same pot as the Annual Rye under the treatment
umbrella. The standard error from the ANOVA: Single-Factor test for the controlled pots was ±
Average Biomass (g)
0.000782, and for the treated pots was ± 0.000415. (F(1,22)=211.62, p=9.09E-13).
0.009
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0
1
2
Condition
Figure 10: Average Biomass of Annual Rye when in Competition with B. rapa – Column 1 shows the
average biomass of the controlled Annual Rye when grown in the same pot as the B. rapa, and column 2
shows that of the Annual Rye when grown in the same pot as the B. rapa under the treatment umbrella.
The standard error from the ANOVA: Single-Factor test for the controlled pots was ± 0.000653, and for
the treated pots was ± 0.7.43E-5. (F(1,22)=112.17, p=4.205E-10).