A scanning electron microscope study of the morphological changes in the common green bean Phaseolus

1
A scanning electron microscope study of the
morphological changes in the common green bean
(Phaseolus vulgaris) from mercury contamination

• Kimberly A. Wilson

0.364 µg/g leaf
Method Design Seeds of Phaseolus vulgaris were
planted approximately 3 cm deep in Peat Pots
containing 54g organic potting soil. The plants
were grown in an incubator at 25C set to 1410
light dark cycle to mimic the early summer.
Upon germination to a height of approximately 1.0
cm above the soil, the plants were dosed with
water containing mercuric chloride of 0.5, 1.0,
2.0 and 5.0 mg/L. The duration of the growing
period was twenty-five days.
Abstract Plants have the capability of absorbing
toxic metals such as mercury. These plants may be
consumed by humans. Exposure to high
concentrations of mercury usually results in
degenerative neurological effects in humans.
Green beans, Phaseolus vulgaris, have been shown
to be bioaccumulators of heavy metals, including
mercury, through the soil from air deposition of
pollutants. This study was conducted to determine
any morphological changes that occurred within
the plants which were exposed to varying mercury
concentrations in the soil and to identify the
lowest observed concentration which displayed
the negative structural effects. SEM micrographs
display various structural effects such as the
visible absence of fine root hairs on the
secondary root structure and a reduced number of
stomata on the surface of the leaf at 0.364 µg/g
of mercury in the soil.
Control leaf
Introduction The primary source of mercury
contamination in the environment is air
pollution. Mercury content in coal for the
United States ranges between 0.070 33 part per
million (ppm). As the coal is burned, the
mercury volatizes into the atmosphere, where it
can then settle out and be deposited on
vegetation or the soil. The Environmental
Protection Agency (EPA) found the average
concentration of mercury in US soil to be less
than 0.2 µg/g. If the growing conditions of a
plant are altered, such as the increase in
mercury concentration in the soil, the plant may
respond with abnormal growth conditions. At high
enough concentrations, mercury may cause
chlorosis, necrosis, abscess of older leaves,
and overall growth reduction. The roots serve as
the primary site of mercury uptake in the plant.
Root hairs are extended outgrowths of epidermal
cells and provide a large area of surface
contact with the soil, serving as the main source
of absorption of both water and nutrients. The
common green bean, Phaseolus vulgaris, is a
dicotyledonous plant which research has shown to
be sensitive toward mercury. Due to its
sensitivity, it was used for this study.
Previous research has determined the mercury
content within the beans by cold vapor atomic
absorption spectroscopy.
0.364 µg/g root
Green bean plants used in study
Control root
SEM Sample Preparation Five samples were
prepared for each bean plant. They were fixed in
formalin overnight before undergoing a five step
graded ethanol series followed by sublimation of
Peldri. The specimen were mounted on stubs using
carbon double-sided tape. They were sputter
coated in gold-platinum before being analyzed
under the SEM.
Unifoliate Leaf
Unifoliate Leaf
Secondary Root
Secondary Root
Conclusion This research has determined that the
lowest observed effective concentration (LOEC) of
mercury to show morphological differences in the
green bean is the 2.0 mg/L (0.364 µg/g in soil).
Morphological changes include the absence of fine
root hairs on the secondary root structure and a
reduced number of stomata per area on the leaf
surface. The absence of the fine root hairs on
the secondary root system may be due to the lack
of extended epithelial cell formation or due to
sonication in their weakened state. The reduced
number of stomata may be contributed toward the
mercury causing poor growth development in the
plant. While this concentration is above the
EPAs average concentration found in top soils
across the United States, if the rate at which
mercury is released into the atmosphere continues
to increase, then the concentrations may approach
this LOEC value. This may result in global
detrimental agricultural implications.
Control root magnified
0.364 µg/g root magnified
Bean plant showing symptoms of chlorosis, the
yellowing of the leaves.