Read the following passage and
answer the questions below.
A mutualistic relationship between two species
increases the chances of growth or survival for both of them.
Several species of fungi form mutualistic relationships called
mycorrhizae with the roots of plants. The benefits to each
species are shown in the figure below.
| mycorrhiza |
increases the ability of
roots to
absorb water and minerals from the soil |
| fungus |
---------------------->
<---------------------- |
plant |
releases plant-synthesized
sugars
on which fungus feeds |
Some plants species that require or benefit from the presence
of mycorrhizal fungi are noted below.
| Cannot survive without mycorrhizae |
Grow better with mycorrhizae |
All conifers
Some deciduous trees
(e.g. birch, beech)
Orchids |
Citrus trees
Ericaceae(e.g. heath, rhododendrons, azaleas)
Grapes
Soybeans |
Agronomists investigated the effects of mycorrhizae on plant
growth and survival in the following studies.
Study 1
Three 4-acre plots were prepared with soil from a pine forest.
The soil for Plot A was mixed with substantial quantities of
cultured mycorrhizal fungi. The soil for Plot B contained only
naturally occurring mycorrhizal fungi. The soil for Plot C was
sterilized in order to kill any mycorrhizal fungi. Additionally,
Plot C was lined with concrete. After planting, Plot C was
covered with a fabric that filtered out microorganisms while
permitting air and light to penetrate, as shown below. Two
hundred-fifty pine seedlings were planted in each of the three
plots. All plots were treated to the same environmental
conditions. The six-month survival rates were recorded in the
table below.
|
# Seedlings alive after 6 months |
Utilization of available K (average) |
Utilization of available P (average) |
| Plot A |
107 |
18% |
62% |
| Plot B |
34 |
10% |
13% |
| Plot C |
0 |
N/A |
N/A |
Study 2
The roots of surviving seedlings from Plots A and B were
analyzed to determine how efficiently they absorbed potassium (K)
and phosphorus (P) from the soil. The results were added to the
table above.
1. The most likely purpose of the concrete liner was:
A. to block the seedlings from sending out taproots to water
below the spot.
B. to prevent mycorrhizal fungi in the surrounding soil from
colonizing the plot.
C. to absorb potassium and phosphorus from the soil for later
analysis.
D. to provide a firm foundation for mycorrhizal fungi in the
plot.
2. Mycorrhizae are highly susceptible to acid rain. Given the
information from the passage, acid rain is probably most harmful
to:
F. wheat fields.
G. birch forests.
H. orange groves.
J. grape vines.
3. In a third study, pine seedlings were planted in the soil
from a different location. The soil was prepared as in Study 1.
This time, the survival rates for seedlings planted in Plot A and
Plot B were almost identical to each other. Which of the
following theories would NOT help to explain these results?
A. Sterilization killed all the naturally occurring
mycorrhizal fungi in the new soil.
B. The new soil was so mineral deficient that it could not
sustain life.
C. The new soil was naturally more fertile than that used in
Study 1.
D. Large quantities of mycorrhizal fungi occurred naturally in
the new soil.
4. According to the passage, in which of the following ways do
plants benefit from mycorrhizal associations?
I. More efficient sugar production
II. Enhanced ability to survive drought
III. Increased mineral absorption
F. I only
G. III only
H. II and III only
J. I, II, and III
5. Which of the following generalizations is supported by the
results of Study 2?
A. Mycorrhizal fungi are essential for the survival of pine
seedlings.
B. Growth rates for pine seedlings may be improved by adding
mycorrhizal fungi to the soil.
C. Mycorrhizal fungi contain minerals that are not normally found
in pine forest soil.
D. Pine seedlings cannot absorb all the potassium that is present
in the soil.
