The Effects of pH on the Growth of
Snow Peas, Pisum sativum
Ryanne Karnes
Biology 402
April 19, 2008
1
Table of Contents
Introduction
 Pollution
 Acid rain
 Agriculture
 Simulated Acid Rain and Cabbage Seeds
 Effect of pH on Marine Phytoplankton
 Water Hyacinths as Pollution Reducers
 Simulated Acid Fog and Green Peppers
 Simulated Acid Rain on Epicuticular Wax
 Topic Hypothesis
3
3
3
4
5
6
7
8
10
Research
 Data Collection
 Results
1. Figure 1
2. Figure 2
 Statistical Analysis
 Discussion
 Acknowledgements
10
13
14
14
15
15
18
Literature Cited
19
2
Abstract
This study was done to determine the effects of pH on the common snow pea, Pisum
sativum. The effects of pH on peas can give an insight into what acid rain could do to crops
across the nation if the amount of pollution continues to rise. The peas were divided into nine
groups. Seven groups were watered with solutions of altered pH levels and two groups were
watered with the controls, deionized and tap water. The plants watered with pH levels below 7
had the most growth in their height, while the plants watered with pH levels above 7 had poor
height growth. A pH of exactly 7.0 produced similar results to the deionized and tap waters,
which had vertical growth in between the low and high pH levels. There was no statistical
difference among the vertical growth of the plants. However, the however had differences. The
peas watered with tap water had more diameter growth than those watered with a pH of 11.0.
There also was more growth in the peas that were watered with deionized water when compared
to those watered with pH levels of 11.0 and 8.0.
Introduction
Air pollution has become problem in the last few decades and it is caused by a number of
different factors including air conditioners, aerosols, the burning of fossil fuels and even natural
environmental changes such as volcanic activites. The amount of air pollution is a problem
because many of these pollutants can cause health problems such as asthma, lung disease,
cancer, eye irritations and even death (Gifford, 2006). Two common air pollutants can lead to
another kind of pollution that many people put into a category of its own, acid rain.
When sulfur dioxide and nitrogen oxide are released into the atmosphere, these molecules
mix with the moisture in the air to form nitric acid and sulfuric acid and return to earth in the
form of acid rain, fog, snow or dust. This is called acid deposition and it is more common in the
3
eastern United States than it is in the western states (Washington Environment 2010, 1989). It is
more common in the eastern states because the weather patterns carry pollution from the west to
the east. It is also because of the numerous industrial factories adding to the pollution on that side
of the country. There is no way to control where and when acid rain will fall since once it forms
in the air, it becomes part of the weather pattern. When acid precipitation falls it can affect
forests, fields, gardens, and aquatic plants. It also affects paint on buildings, erodes limestone
structures and sculptures, kills or dwarfs trees, and reduces food crop yields (Kidd and Kidd,
2006).
In order to live a life where fear for the environment does not rule the decisions we make,
it is important to know the effects that acid rain can have on human life. Although the direct
impact on humans is limited, acid rain can affect agriculture in Washington, the United States
and worldwide. In 2002, the agricultural census showed the amount of land devoted to farming
in the United States was at 15.3 million acres with about half being used for crop growth. The
census also showed that in Washington alone agriculture was a 5.3 billion dollar industry
(WSDA, 2004). As the atmospheric pollutions increase on the west coast, the weather patterns
change and the possibility for acid rain rises, it becomes apparent that it could affect the state of
Washington’s economy. Acid rain can cause damage to plants by eating away at the outer
protective layer of the leaves, but the most damage is seen when the soil that the plants are
grown in stays acidic instead of returning to neutral (Petheram, 2002).
Caporn and Hutchinson (1987) studied the effects of temperature, water, and nutrient
conditions on cabbage seedlings. Their main focus was how these three conditions affected the
seedlings when they were exposed to a single treatment of simulated acid rain. The acid rain
used had a pH of 3.0. They put the seedlings in different environments and exposed them to a
4
spray of acid rain in a greenhouse environment and an outdoor environment. After being sprayed
with the simulated acid rain, the plants outside were protected from natural rainfall by being
placed under glass sheets. This was done so the pH of the artificial acid rain spray was not
altered by the pH of the natural rain. It took those seeds that were outdoors five days longer to
germinate than those that were indoors. Of those plants that were sprayed with a pH 3.0
simulated acid rain, the ones indoors had visible damage to the outside of the cotyledons, the first
leaf of the embryonic seed, while those that were outdoors had none. It was also noted that there
was more damage to the seedlings kept outdoors. Caporn and Hutchinson (1987) also tested the
effects of simulated acid rain at different temperatures: 10° and 20° Celsius. This research
showed that the plants at higher temperatures were not as affected when compared to those at
lower temperatures. In addition to environment and temperature, the authors tested the combined
effects of simulated acid rain with a water shortage, as well as simulated acid rain with
insufficient nutrients. In both cases, these tests revealed that the simulated acid rain caused less
injury to those seedlings that were not given enough nutrients or water than to those that were
given adequate amounts of either. To find out if the data that had been collected was statistically
significant, the authors did an analysis of variance. This paper helped developed my hypothesis
and my research proposal because the pH of the acid rain affected the cabbage seedlings, a
vegetable, negatively. Although I am not using cabbage seedlings, I am, however, using a
vegetable, snow peas and the effects may as well be negative.
Chen and Durbin (1994) studied the effects of pH on two species of marine
phytoplankton. The pH range was 7.0 to 9.4. They adjusted the pH of the tanks using two
different methods. The first method involved bubbling different types of compressed gases.
These gases were CO2, compressed air, and a mixture of nitrogen and oxygen. Before the
5
bubbling occurred, the pH was adjusted to a level of 8.9 by the addition of diluted NaOH. Then,
it was split up into four different 1 L containers; three of them were bubbled with the compressed
gases and the fourth was not. The three that were bubbled with CO2, compressed air, and a
mixture of nitrogen and oxygen had resulting pH levels of 7, 8, and 9 respectively. The second
method for adjusting the pH involved changing the pH by adding the necessary acid or base to
obtain the desired levels of ranges 7.03 to 10.09. The authors do not specify which acids and
bases were used to change the pH in the second method. The amount of photosynthesis that
occurred was measured using in vivo fluorescence. This technique is a type of staining that can
be used to see where and how many chemical reactions are taking place within a cell. Chen and
Durbin (1994) found that the phytoplankton performed less photosynthesis at pH levels higher
than 8.8. From this, a correlation was found between the high pH levels and the amount of
carbon available. This research shows that there may be link between pH level and the amounts
of photosynthesis a photosynthetic species can perform. The amount of photosynthesis may be
altered due to the pH.
Bewtra et al. (1982) conducted an investigation to test reports that water hyacinths reduce
the amount of pollution in lakes and rivers by decreasing the amounts of heavy metals. It was
believed that if water hyacinths helped in rivers and lakes, then they could help reduce the
pollution of the local landfills as well. Bewtra et al. (1982) studied how pH levels from landfill
leachates affected the growth of water hyacinths. A landfill leachate is a toxic liquid that comes
from water bubbling through a solid waste disposal site (Bewtra et al.,1982). This experiment
was done in a greenhouse, and samples were taken from a local landfill in Ontario, Canada. The
samples were high in chlorine and sodium, so they were diluted using tap water to concentrations
that would be safe for the plants. This dilution was made using 20% leachate and 80% tap water.
6
Ammonium nitrate and dipotassium hydrogen phosphate were also added to the dilution to serve
as nutrients. Bewtra et al. (1982) added either sulfuric acid or potassium hydroxide to the
leachate to obtain pH levels of 3.2, 3.4, 5.5, 6.5, 7.5, 9.0, 9.9, and 10.8. Two containers of water
hyacinth were maintained at each pH level, as well as two for the pure diluted leachate, and two
for tap water. They found that a pH below 4.0 and above 8.0 hindered the growth of the water
hyacinth. They also found that the water hyacinth grew best between pH levels of 5.8-6.0. These
results were helpful in the formulation of my hypothesis and methods section because it offered a
different technique to change pH than was used in previous papers. It also showed that there may
be some scientific use to knowing different pH level tolerances for different plants such as
reducing pollution.
Takemoto et al. (1988) studied the effects of acidic fog and ozone on green pepper crops
in California. To start off the experiment, the plants were cultured in a greenhouse for 4 weeks
before they were transferred to a location that was adjacent to the experimental site. The plants
were allowed a 3 week adjustment period to get acclimated to the new conditions before they
were randomly assigned to one of eight testing groups. These groups were then placed in opentop field chambers. Inside the chambers the plants were buried 0.22 m below the surface for root
insulation. Fertilizer was applied weekly or water was added as needed. The open-top chambers
were for fog chemistry/ air quality testing and the air quality was regulated using blowers that
dispensed either charcoal-filtered or non-filtered air. The blowers were only run during the day
and shut off at night to allow dew to form on the plant. Fog simulants were applied bi-weekly for
eleven weeks using a fog application system. To make the fog solutions that were at pH levels of
2.6 and 1.6, the authors added HNO3 and H2SO4 in a 3:1 molar ratio. This ratio supplemented the
amounts of NO3- and SO42- that were already in the simulants. The pH levels of the fog actually
7
produced by the application system had pH levels of 1.68, 2.69, and 7.24 with standard
deviations of 0.16, 0.17 and 0.27 respectively. They evaluated the visible injury to the plants
after eleven fog treatments using a numerical scale of 0 to 4. Plants were assigned a score of zero
if they had 0 to 5% injury, 1 if they had 5-25% injury, 2 if they had 26-50% injury, 3 if they had
51-75% injury and 4 if they had 76-100% injury. After the twenty second fog simulation the
plants were harvested and separated into leaf and stem portions and then air dried for 5-10 days
at a maximum temperature of 45°C. They used 2-way ANOVA tests to determine if the amount
of injury to the different pH levels was significant or not. What they found was that while the
plants in the fog pH levels of 2.69 and 7.24 were only 4-9% injured, those that were in the 1.68
fog pH group were injured 32 % with a standard deviation of 11. They also did tests on the
amount of growth and yield that resulted from the different levels of pH and ambient O3 and
found that a pH level of 1.68 hindered the growth and yield the most with those plants giving 3.8
fruits per plant on average compared to other numbers of 6.2, and 6.1. They found no significant
interactions between fog pH and air quality (ambient O3 levels). These results show that as
mentioned earlier, there more than one type of acid deposition and this research shows the that it
can be damaging to crop plants.
The effects of acid rain on epicuticular wax were investigated by Perce and Baker (1987).
It has been noted that the amount of this wax is affected by the plants environmental
surroundings such as temperature, wind, irradiance, and humidity. It has also been seen that
erosion has occurred when the plants were subjected to a simulated acid rain (Percy and Baker,
1987). The authors of this research chose to look at four different species of plants; dwarf bean,
field bean, pea and rape. All the seedlings were thirteen days old and grown in 10 cm pots with
No. 1 compost. The plants were then placed in controlled environment chambers that provided
8
the plant with a temperature of 20 – 15°C, a humidity of 75 – 85% and a photoperiod of 16
hours. There were twelve plants per pH group and the simulated acid rain was applied on
alternate days over a 17 day period. The simulated rain treatments began as soon as the second
initiated leaf pair emerged and they ended after the leaves were fully expanded. The pH of the
treatments was changed using 1/1 M sulphuric/nitric acid to obtain pH levels of 4.6, 4.2, 3.8, 3.4,
3.0 and 2.6. The amounts of epicuticular wax were measured by washing the leaves of the plants
with redistilled chloroform and then dried with anhydrous sodium sulfate. The solution was then
filtered and evaporated and dried in pre-weighed vials. The wax was the only thing left in the
vial and the weight was determined. The amount of surface area on a leave was also measured
using an optomax series 1. Lesions were removed using fine tweezers and the surface areas were
measured before and after their removal to determine the percentage of injury to the leaves.
While leaf area of the dwarf bean was unaffected by the different pH levels, it was increased in
the pea plant and decreased in both the rape and field bean. The four different plants were
separated into two different categories of cuticle wax type; the first was crystalline wax, which
appeared on the pea and rape plants, and the second was an amorphous wax, which was on the
dwarf and field beans. The amorphous cuticle wax appeared to protect the leaves better than the
crystalline. The dwarf and field beans only had lesions on the leaves of plants that were
submitted to pH ≤ 3.0 and the pea and rape plants had lesions at pH ≤ 3.4. The plant with the
most surface area injury was the rape with 4.4% of its surface area being injured and the least
injured to the two bean plants with less than 1% being injured. That left the pea in the middle
with 1.3% of its surface area injured. This is another study that shows that pH can affect plants.
Specifically, it shows that pH can damage some important parts of a plant’s anatomy that help
protect the plant and its structures.
9
My research tested the effects of different pH levels on pea plants. Snow peas, Pisum
sativum, were chosen because they are easy to acquire, inexpensive, and can reach full maturity
relatively quickly (Maynard and Hochmuth, 2007). While the peas were still seedlings they were
watered with deionized water every other day until they sprout above the soil. Once the leaves
had formed on all plants, watering with the solutions began. The solutions will be of different pH
levels with 16 plants being watered at each pH. The pH levels I will use are 3.0, 4.5, 6.0, 7.0, 8.0,
9.5, and 11.0. This scale was chosen so that I can compare basic solutions with acidic solutions
and determine which, if either, has a more negative effect on the plants. The pH levels will be
adjusted using sufficient amounts of either sulfuric acid or potassium hydroxide. My control for
this experiment will be unaltered deionized water. I hypothesized that in general the plants
watered with solutions of pH levels below 7.0 would grow better than those that were watered
with levels above 7.0. The plants that were watered with solutions that are neutral or close to
neutral such as tap water, deionized water, and a pH of 7.0 would have growth in-between those
watered with the acidic and basic solutions. Furthermore, I specifically hypothesized that the pea
plants would have the most growth and be the healthiest at a pH range of 5.0 - 6.0. These
hypotheses were based on the previously mentioned research, as well as from the knowledge that
peas grow best in acidic conditions (Maynard and Hochmuth, 2007).
Methods
Data Collection
I purchased approximately 200 snow pea (Pisum sativum) seeds and accelerated the
growing process by germinating the seeds before planting them in the soil. The snow pea was
chosen because it has a short maturation time period, easy to find, and relatively inexpensive
which was essential in finishing my project in the time allotted and within budget. The
10
germination was done by taking wet paper towels and placing them into 100 or 200 mL glass
beakers. The seeds were then placed between the wet paper towels and the side of the beaker.
Fifteen seeds were placed in each 100 mL beaker, while 50 were placed in each 200 mL beaker.
Only 144 seeds were needed, so those that did not germinate or became moldy were discarded.
The 144 seeds that did germinate were planted 5 cm deep in 7.5 x 5 x 7.5 cm pots with
one seed in each pot. The soil used was a Sunshine mix bought from Puyallup, Washington. The
plants were watered with deionized water until they sprouted above the soil. Waiting until the
plants had matured slightly was important because if I started the watering with the altered pH
levels, there was a chance that some would not grow at all. If this had occurred I would not have
been able to get data. After maturing had occurred, the plants were labeled A-I with 16 plants in
each group. The locations of the plants were determined using a random numbers table, to ensure
no patterns existed in assigning plants to treatments. Groups A-G were assigned to one of the
seven pH groups, group H was watered with deionized water and group I was watered with tap
water (Table 1).
Table 1. Assignments of the different pH levels to groups A-I.
Group
pH
A
B
C
D
E
F
G
H
I
Deionized Tap
3.0 4.5 6.0 7.0 8.0 9.5 11.0
water
Water
The pH levels were adjusted using sufficient amounts of either sulfuric acid or potassium
hydroxide similar to the technique used by Bewtra et al. (1982). The potassium hydroxide and
the sulfuric acid were provided by the chemistry lab of Saint Martin’s University. The tap water
was taken from the faucets in the biology lab of Saint Martin’s University. The pH of the
solutions was tested daily using a pH meter and the plants were watered and checked every other
day. The moistness was determined using a meter that was inserted into the soil. Approximately
11
25 mL of the designated solution were added to the pots if the moistness of the soil was below 4
g/cm3. For the duration of the experiment the plants were watered with their designated pH
treatment. My controls were 16 plants that were watered with deionized water and 16 plants that
were watered with the tap water. The controls were kept in the same conditions as groups A
through G. The pH of these solutions was tested daily to make sure that nothing had
contaminated them and changed their alkalinity. The plants were kept in room 402 of Saint
Martins University’s Old Main building. To ensure that all plants had the same amount of light,
they were placed under a light bank. The lights were left on 24 hours a day to shorten the
maturation time. Peas grow best with a photoperiod of at least 16 hours (Percy and Baker, 1987).
Notes were taken daily on how well the plants’ growth and health were reacting to the
pH. Measurements were taken twice a week. The measurements were stem height (distance from
soil to apex) and stem width (diameter of stem). Measurements were taken to the nearest 0.01
mm for the diameter and to the nearest 1.0 mm for the height.
Statistical Analysis
Once the data had been collected over six weeks, it was organized and statistically
analyzed. The first test was a one way ANOVA test, which determined if there was a statistically
significant difference between the data. A p-value of less than 0.05 indicated a statistically
significant difference. In the case that the ANOVA test determined that there was a statistically
significant difference, a Tukey test was used. The Tukey test was set with a 95% confidence
level. This test found where the statistical difference was within the data in a given set. The
difference lied within the data that does not cross zero in the Tukey test. A p-value of less than
12
0.05 indicated that I needed to reject my null hypothesis and a p-value of greater than 0.05
indicated that I should fail to reject my null hypothesis.
Results
After the plants had grown for 15 days, I began watering with the altered pH
solutions. I measured their stem height and diameter before beginning the watering, as well as
every time I watered. Figure 1 represents the changes in stem height of the nine experimental
groups. It can be seen that all of the plants reacted in the same manner at first. All groups grew
similarly for the first three treatments then separated, showing differences between them. The
plants that were watered with the altered pH level of 11.0 and 9.5 began to flatten out while the
others continued to gain stem height. The groups that were watered with tap water, deionized
water, or ph levels of 8.0, 7.0, 4.5 and 3.0 continued to climb and ended close to each other. The
only plants that showed extreme growth were those that were watered with a pH level of 6.0.
Figure 2 represents the changes in stem diameter of the nine experimental groups. The
overall trends are similar to figure 1 in that the basic solutions had the least amount of growth
while the plants watered with the acidic solutions grew the best. Also, there was no big
difference between the diameter widths until the third treatment. The difference in the overall
trends was that the plants watered with tap water and deionized water had more diameter growth
than those watered with a pH of 7.0.
13
Tap Water
Deionized Water
11
9.5
8
7
6
4.5
3
60
55
50
Height (cm)
45
40
35
30
25
20
15
10
22-Feb
27-Feb
3-Mar
8-Mar
13-Mar
18-Mar
Date
Figure 1. The average stem height (n=15) of P. sativum plants being treated with altered pH levels
(3.0, 4.5, 6.0, 7.0, 8.0, 9.5, 11.0, tap water and deionized water) from February 27, 2008, through
March 22, 2008.
14
23-Mar
Tap Water
Deionized Water
11
9.5
8
7
6
4.5
3
2.6
2.5
2.4
Diameter (mm)
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
22-Feb
27-Feb
3-Mar
8-Mar
13-Mar
18-Mar
23-Mar
Date
Figure 2. The average stem diameter (n=15) of P. sativum plants being treated with altered pH
levels (3.0, 4.5, 6.0, 7.0, 8.0, 9.5, 11.0, tap water and deionized water) from February 27,
2008, through March 22, 2008.
Statistical Analysis
The one-way ANOVA test revealed there was no significant difference between the plant
heights of the different ph groups. This indicated that I failed to reject my null hypothesis. There
was however, a statistically significant difference within the stem diameters. A p-value at this
level indicated that I should reject my null hypothesis. At a 95% confidence level, the Tukey
tests showed that there was a statistically significant difference between the tap water and a pH
level of 11.0 as well as between the deionized water and the pH levels of 11.0 and 8.0. The
growth in stem diameter was significantly higher for the deionized water and tap water than it
was for the pH level of 11.0 and 8.0.
Discussion
In this research, the goal was to determine how pH affects the growth of common snow
peas, Pisum sativum. My hypothesis was that the pea plants that were watered with solutions
15
between 5.0 and 6.0 would be the healthiest and have the most growth due to the fact that peas
grow best in acidic conditions (Maynard and Hochmuth, 2007). Figure 1 showed that the plants
that were watered with the pH levels of 3.0 to 6.0 had more height growth than the other
treatment groups. This means that their height increased the most over the 28 day growth period.
Those watered with a pH of 6.0 had the most vertical growth overall.
The diameter, however, showed a different growth pattern. The plants that had the most
growth in the diameter of their stems were those that were watered with a pH level of 3.0
followed by a pH of 6.0, tap water, deionized water and then a pH level of 4.5. The plants that
were watered with tap water and deionized water grew more than those that were watered with a
pH level of 3.0, which was unexpected. This could have been due to measuring errors. Since the
stems of the Pisum sativum plants were fragile, the amount of pressure I applied to the
micrometer determined the reading. It is possible that I applied too much pressure when
measuring some of the diameters or not enough while measuring others. Although the diameter
results were slightly different then those for the height, they both conveyed the same general
pattern; Pisum sativum grows best in acidic or neutral conditions and grows poorly in basic
conditions. In both sets of data, the hypothesis was supported.
The slight discrepancy between the results and the hypothesis cannot be explained by
variable differences since they were randomly placed and subjected to equal amounts of all
variables except for those being tested. However, it is possible that the fact that the plants
watered with a pH level of 3.0 had less diameter growth than those watered with Tap or
Deionized water due to a measuring error. The reading that I took from the micrometer depended
on how much pressure was applied. Applying the same amount of pressure to all 144 and plants
became difficult.
16
In both the diameter and height, the plants were unchanged by the treatments until they
had been treated for about a week. This is similar to what was found by Winner (1994). His
research was on how plants respond to pollution and he found that there seemed to be a common
threshold for plants. They were able to handle a certain amount of pollution before changes were
seen. My research and the research done by Winner has brought up new questions about the
relationship between plants and the water they use. It would be interesting to test what it is that
happens with the anatomy of the plant when it is watered with a basic solution versus an acidic
one. Does it fail to take enough water or does it fail to transport it and use it correctly? This
could be tested similarly to what Varney and Canny (1992) did when they tested the water
uptake of maize plants. They added a dye to the solution that they were watering with and later
cut the roots open to determine the rates at which the water was being absorbed.
If done again there may be some things that could be changed in the procedures of this
research. I would start off by finding another way to measure the vertical growth of the plants.
They were very fragile and broke easily while measuring. I would also find another method of
mixing the pH solutions. The technique use by Bewtra et al. (1982) proved more difficult than
first thought. It was difficult to get the pH levels to stabilize, especially those closest to neutral.
Overall my research showed the effects of pH and acid rain on pea plants. If acid rain
becomes a problem in the western United States, the crops of Washington should not be greatly
affected as long as the pH is between 3.0 and 6.0. The effects of a pH lower than 3.0 were not
tested in this research and would be something else to look into at a later date. If for some
reason, a new type of precipitation forms that is basic, our crops may be threatened because the
basic pH levels had less growth in both stem height and width. Since peas grow best in acidic
conditions the effects of acid rain were not drastic. To better understand the effects of pollution
17
and acid rain on other crops in the northwest, it would be a good idea to perform the same
research on those. Any crop that is a leading contributor to the Washington economy would be a
good plant to test. These crops include apples and potatoes. Studying these would provide a
better picture of what acid rain could do to the economy in Washington and the northwest.
Acknowledgements
I would like to extend a thank you to Dr Olney, Hartman, and Coby for all of their
assistance in the proposal and follow through of this research. I would also like to thank Cheryl
Guglielmo and Jeffery Karnes for all of their help in the set up of the study.
18
Literature Cited
Bewtra JK, Biswas N, El-Gendy AS. 1982. Growth of water hyacinth in municipal landfill
leachate with different pH. Environmental Technology. 25: 833-840.
Caporn, S, Hutchinson, T. 1987. The influence of temperature, water and nutrient conditions
during growth on the response of Brassica oleracea L. to a single, short treatment with
simulated acid rain. The New Phytologist. 106: 251-259.
Chen, CY, Durbin EG. 1994. Effects of pH on the growth and carbon uptake of marine
phytoplankton. Marine Ecology Progress Series. 109: 83-94.
Gifford, C. 2006. Planet under pressure: pollution. Heinemann Library, IL, pp. 12-14.
Kidd, JS, Kidd, RA. 2006. Air pollution: problems and solutions. Chelsea House Publishers, NY,
pp. 74-83.
Maynard, DN, Hochmuth, GJ. 2007. Handbook for Vegetable Growers, fifth ed. John Wiley &
Sons, Inc., NJ, pp. 159, 416.
Percy, KE, Baker, EA. 1987. Effects of simulated acid rain on production, morphology and
composition of epicuticular wax and on cuticular membrane development. New
Physiologist. 107: 577-589.
Petheram, L. 2002. Acid Rain. Bridge Stone Books, MN, pp. 20-21.
Takemoto, B, Bytnerowicz, A, Olszyk, D. 1988. Depression of photosynthesis, growth, and yield
in field-grown green pepper (Capsicum annuum L.) exposed to acidic fog and ambient
ozone. Plant Physiology. 88: 477-482.
Varney, GT, Canny, MJ. 1992. Rates of water uptake into the mature root system of maize
plants. New Phytologist. 123: 775-786.
Washington Environment, 2010. 1989. The state of the environment report. pp. 17-18, 31-38.
Winner, WE. 1994. Mechanistic Aanalysis of plant responses to air poullution. Ecological
Applications. 4:651-661.
WSDA [Internet]. Washington State Department of Agriculture [cited 2007 Dec 9]. Available
from http://agr.wa.gov/news/2004/04-47.htm.
19