Download (1975) Photorespiration: a Classroom Demonstration (JNRLSE)

Photorespiration: A classroom demonstration1
D. D. Wolf and E. W. Carson2
ABSTRACT
An easily implemented test to show biochemical
differences between plant species in photorespiration
has proved to be an effective addition to the formal
classroom lecture. The principle of the test is that
leaves in a closed system cause the CO2 concentration
(^CO?) to reach characteristic compensation values.
The test uses a sodium bicarbonate indicator solution
which is blue when in equilibrium with low CO2 atm
(created by non-photorespiring species) and green in
high CO2 atm (created by photorespiring species).
This paper presents several modifications of a
simple experiment to demonstrate differences between photorespiring (high CO2 compensation)
and non-photorespiring (low CO2 compensation)
plants. The method is less costly and less time consuming than that described by Moss (3). The method has also been used in a survey to categorize
species and is modified from that described by
Tregunna et al. (4).
METHODS AND MATERIALS
Additional index words:
Visual aid. Carbon
dioxide. Compensation value. Teaching aid.
Two 50-ml beakers (other containers can be
used) are placed side-by-side on the bottom of a 1
"D LANTS within both the Monocotyledoneae
and Dicotyledoneae classes have distinct anatomical and -biochemical differences, and are subclassified into photorespiring and non-photorespiring groups. Characteristic of the non-photorespiring
group are Kranz type, tropical, and C4-dicarboxyIic
acid plants; plants having low CO2 compensation;
and L syndrome. The identifying term of "nonphotorespiring," however, seems to have gained
wide popular acceptance (2, 4, 5). Characteristic of
the photorespiring group are C3 and Calvin cycle
plants; temperate species; and plants having high
CO2 compensation.
Including the concept of
photorespiration in introductory plant science
courses stimulates student interest in the biochemistry of plant -growth, and can help them to
relate crop yields to efficient photosynthetic and
respiratory mechanisms.
liter (1 qt) wide-mouth glass jar. About 30 ml of
water are placed in one beaker. The leaf material
(preferably with open stomata and a leaf area of at
least 10 cm2) is detached, recut under water, and
placed in the beaker containing water. About 5 ml
of a sodium bicarbonate indicator solution (5 X
10~s M) containing 1% (v/v) Universal Indicator
(Fisher Scientific Co.) is placed in the other beaker.
The indicator solution will be greenish-yellow and
can be stored. A closed system is created by securing the lid on the jar. A cover made of two layers
of polyethylene plastic, such as a plastic bag held in
place by a rubber band, also seals the jar. The plant
material is continuously illuminated by a fluorescent
desk lamp having two 40-W bulbs. A change in
Contribution of the Agronomy Department, Virginia
Polytechnic Institute and State University, Blacksburg, VA
24061.
2
Associate Professor and Professor.
114
JOURNAL OF AGRONOMIC EDUCATION, VOL. 4, AUGUST 1975
Visual demonstrations of biochemical differences
between species help to stimulate interests in plant
characteristics, e.g., the efficiency of yield capabilities. Any plant material that is photosynthetically
active can be used, but the most effective way to
standardize the system is to compare the color produced by two plants whose CO2 compensation
points are known, for example, corn (low) and bean
(high).
This procedure builds upon and applies a number
of basic concepts—diffusion, chemical equilibria,
pH, organic indicators, respiration, and photosynthesis—which usually have been presented previously in chemistry, physics, and biology courses.
The basic principles which should be stressed to
5. The Universal Indicator is a complex of pHspecific color indicators that show the pH of the
equilibrium buffer solution and, thus, indicate the
concentration of CO2 in the atmosphere of the container.
Plant material selected should be grown under
soil, water, and environmental conditions that
would encourage open stomata. With open stomata
and in a closed system, the atmospheric CO2 concentration will be reduced to compensation levels
by photosynthesis within about 15 min if sufficient
leaf area is present. Gentle swirling of the jar will
help mix the indicator solution and speed the
equilibrium. Time needed for color change will depend on rate of CO2 exchange with the solution
and usually will occur in less than 4 hours. The
leaves maintain equilibrium for several days. Alternatively, the lights can be turned off to demonstrate respiration. With lights turned on again, the
original equilibrium returns.
The effect of oxygen on photosynthesis and CO2
compensation value (1) can also be demonstrated in
the following way. Set up two of the jars containing similar leaves of a photorespiring species. Flush
one jar well with nitrogen before closing the lid.
The leaves in the oxygen-free environment will
create a lowPCQ 2 value (blue color) demonstrating
blockage of photorespiratory processes, while the
leaves in normal air will photorespire and create a
students are:
high/*CO2 value (green color).
color of the solution indicates photosynthetically
active tissue andPcO 2 reduction. A green solution
indicates a PCO2 °f greater than 50 ppm (photorespiring plant), whereas a blue color indicates a
low PCO2 °f IGSS than 5 ppm (non-photorespiring
plant). The system can be checked by placing a
sodium hydroxide solution in the beaker instead of
water. Since sodium hydroxide creates a low/ ) CO 2 >
the indicator will be blue at equilibrium. Once this
experiment is assembled, the demonstration can be
set up in a few minutes.
RESULTS AND DISCUSSION
1. Under equilibrium conditions, the PCQ2 m
the closed atmosphere will depend upon the physiology of the species being tested (photorespirer or
non-photorespirer).
2. The concentration of CO2 in the atmosphere
of the closed container and the concentration of
CO2 dissolved in the indicator solution will attain
an equilibrium due to simple diffusion into or out
of the solution.
3. The final (equilibrium) concentration of CO2
in the indicator solution will be directly proportional to the concentration of CO2 in the container
atmosphere.
4. The dissolved CO2 will chemically react with
the water to produce carbonic acid:
CO2 + H2O ^ H 2 CO 3 - H+ + HCO3" - H+ + CO32~