General Ecology: Lecture 3
Physiological Ecology: Plant Adaptations to the Light Environment
Reading for this lecture:
 Review of photosynthesis: pp. 86-90
 Response of plants to differing light environments: pp. 90-93.
Reading for Monday, October 1 through Monday October 8: I highly recommend reading
these chapters as soon as possible (i.e. stay ahead of the lectures!)
 The next several lectures are tied somewhat closely to the text in terms of coverage and depth.
 We will cover Chapter 10 fairly thoroughly; except that we will only briefly cover the section
on Life Tables.
 We will cover virtually all of Chapter 11, except for the life table-related equations on the first
two pages of the chapter.
I.
II.
III.
Overview: My goal: provide one major example of physiological/morphological
adaptations of organisms to the abiotic environment
Photosynthesis as a function of variation in PAR
A.
What is PAR?
B.
A “typical” photosynthesis-irradiance curve, (or P-I curve) [Fig. 6.6]
1.
Photosynthetic rate, measured as net CO2 uptake (y-axis), is graphed as a
function of light (specifically PAR) intensity (x-axis).
a)
Be sure you can accurately label the x and y-axes of a P-I curve!
2.
The light compensation point is the amount of light needed so that
photosynthesis matches respiration
3.
The light saturation point is the light level at which maximum
photosynthetic rate is achieved
a)
Additional light will not increase the rate
b)
Photoinhibition: In some cases, additional light inhibits
photosynthesis [Fig. 6.9]
Individual plant responses to reduced PAR
A.
Short term response (of individuals)
1.
The plant’s rate of photosynthesis is reduced, as expected from the P-I curve
B.
Physiological/morphological responses (of individuals): longer time scale
1.
Quick review of photosynthesis
a)
What do the light-dependent reactions (C-B cycle) accomplish?

Know what is transferred from the light dep. to light-indep.
reactions!
b)
What do the light-independent reactions accomplish? Know!
2.
Reduction in the amount of the enzyme rubisco produced
a)
The enzyme “rubisco” catalyzes the reaction that fixes the CO2 in the
C-B cycle.
b)
Reduction of rubisco helps because:

With lower light levels, amount of CO2 fixed/sugar produced is
limited by the products of the light-dependent reactions
(energy, H+, e-) not by rubisco.
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
IV.
V.
VI.
By reducing rubisco production, less cellular respiration
occurs (why?—be sure you understand!), and the
compensation point (gross photosynthesis – cellular
respiration) is shifted to lower light levels [Fig. 6.7]

What is the cost of reducing rubisco? (HINT: Look at the light
saturation points of the two curves) [Fig. 6.7]
c)
Key concept: due to physiological constraints, trade-offs always
occur!
3.
Increases in chlorophyll and accessory photopigment concentration
a)
Allows for more effective light absorption
4.
Changes in leaf morphology
a)
Two examples….
5.
Changes in leaf orientation
a)
Which leaf orientation is an adaptation for low light? [Fig. 3.7]
6.
Changes in overall plant morphology
a)
What plant shape/leaf position tends to occur in low light levels, and
why?

Begins to occur even with minimal shading by other plants.
Plants can detect the shift from red to far-IR associated with
the filtering of light by a nearby plant.
Species differences: shade-intolerant vs. shade-tolerant plants [Fig. 6.9]—result of
population responses/natural selection
A.
Again, think about the concept of trade-offs.
Responses to ultraviolet (UV) radiation:
A.
Overview: What is the relationship between the ozone layer and UV-B?
B.
Lab studies show that UV-B radiation can damage DNA, partially inhibit
photosynthesis, alter growth forms and reduce yield (directly from text).
1.
In situ studies in the Antarctic ozone hole (1990) suggested that the presence
of the ozone hole decreased primary production in coastal Antarctic waters
by 6-12% per year, and that different species were affected differently.
2.
Minimal field studies on terrestrial plants
C.
Plant defenses against UV-B
1.
A variety of compounds absorb UV-B but allow PAR to pass through.
a)
In some cases, PAR may also be partially blocked
2.
Tropical and alpine plants are more capable of blocking UV-B radiation
a)
Makes adaptive sense, since they are more exposed to UV-B
b)
In conifers, UV-B blocking compounds are found within cell walls as
well as in the interior of the cells.
3.
Plants with short-lived seasonal leaves were less effective at blocking UV-B
rays.
a)
Not as cost-effective to develop these defenses as it is for plants with
longer-lasting leaves.
Summary: Don’t forget that this was just one example of physiological ecology. Plants
and animal have various adaptations to the full range of abiotic factors.
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Study questions
1.
Draw a P-I curve and explain it. Be especially sure that
a. you clearly and accurately label your axes
b. you appropriately show the shape of the curve
c. indicate clearly where it crosses the x and y axes
d. identify and explain the light compensation point
e. identify and explain the light saturation point.
2.
Review of photosynthesis: make sure you know what important “products” of the lightdependent reactions are transferred to the light-independent reactions (i.e. Calvin-Benson
cycle.)
3.
What specific reaction does the enzyme rubisco catalyze in Calvin-Benson cycle? Explain
why reducing the production of rubisco helps a plant adapt to low light conditions. Be sure
to explain what happens to the light compensation point and why. Also explain why, at low
light levels, having more rubisco would not increase the overall rate of photosynthesis.
4.
What is the potential cost of reducing rubisco production? HINT: Be sure to explain what
happens to the light-saturation point and why (physiologically) this happens.
5.
Describe several other long-term adaptations to low light levels in addition to the reduction
of rubisco production.
6.
Draw a P-I curve of a shade-tolerant species compared to that of a shade-intolerant species.
Be sure that you clearly show a comparison of both the light-compensation point as well as
the light-saturation point. (See question 1 as a guide for what to include in your sketch.)
7.
What are the trade-offs (i.e. benefits and costs) to having adaptations to shade vs. not having
them?
8.
What are the negative effects of uv radiation on primary production? (List several…)
Which types of plants tend to be most effective at blocking uv, and how do they do so?
Also, adaptively speaking, why did these types of plants develop greater defenses than
others? Are there associated costs? (Speculate—think about rubisco example.)
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