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7/31/2019 Ecological Significance
1/19
Unlike C3 plants, photosynthesis of C4
plants is not inhibited by O2 and they
exhibit no post-illumination CO2 burst
and have a very low CO2 Compensation
concentration (Table 1).
Ecological Significance of the C4
Syndrome
Resource: Introduction to Plant Physiology/ Williams G. Hopkins
7/31/2019 Ecological Significance
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C3 C4
Photorespiration Yes No
CO2 Compensation (1 CO2 1-1) 20-100 0-5
Temperature optimum (oC)
photosynthesis
Rubisco
PEPcase
20-25
20-25
30-45
30-35
Quantum yield as a function of temperature Declining Steady
Transpiration ratio 500 1000 200 350
Light saturation (mole photon m-2 s-1) 400 500 Does not
saturate
Table 1: A comparison significant features of C3 and C4 plants
Resource: Introduction to Plant Physiology/ Williams G. Hopkins
7/31/2019 Ecological Significance
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CO2 Compensation
In a close environment, CO2 compensation concentrationwould be the stable CO2 concentration in air when CO2uptake and evolution have come to equilibrium.
For C3 plants, values fall into the range of 20 to 100 1CO2 per liter.
Comparable values for C4 plants are in range of 0-5 1-1
In conclusion, photorespiration is either absent from C4
plants or that the process is suppressed.
Resource: Introduction to Plant Physiology/ Williams G. Hopkins
7/31/2019 Ecological Significance
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The high level of CO2 developed in the bundle
sheath cells would tend to suppressphotorespiration by out-competing O2 forbinding to Rubisco.
In addition, the anatomical and biochemicaladaptation of C4 leaves ensure that any CO2that might escape the bundle-sheath cell is
trapped and re-assimilated by PEPcase inmesophyll cells, before it has the opportunityto escape from the leaves.
Introduction to Plant Physiology/ Williams G. Hopkins
7/31/2019 Ecological Significance
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Temperature optimum
Most C4 plants tend to have a higher
temperature optimum (30-45oC) than C3
plants (20-25oC)
Due to higher temperature stability of some of
the C4 cycle enzymes.
Maximal activity of:
PEPcase -> 30-35oC
Rubisco -> 20-25oC
Introduction to Plant Physiology/ Williams G. Hopkins
7/31/2019 Ecological Significance
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As a result, the quantum yield of
photosynthesis in C3 plants tends to declinewith the increasing leaf temperatures due to
decrease carboxylation activity of Rubisco at
the higher temperatures
While the quantum yield of C4 plants remains
essentially constant.
Introduction to Plant Physiology/ WilliamsG. Hopkins
7/31/2019 Ecological Significance
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Effect of temperature on quantum yield of
photosynthesis in C3 and C4 plants(From Hopkins, W.G, Introduction of Plant Physiology. 203, 1995)
Quantum yield (mol CO2 / mol quantana)
15oC 25oC 35oC
C3 Plants
Triticum aestivum 0.055 0.050 0.041
Encelia californica 0.067 0.059 0.046
C4 Plants Zea mays 0.057 0.059 0.058
Antriplex rosea 0.054 0.054 0.054
Introduction to Plant Physiology/ WilliamsG. Hopkins
7/31/2019 Ecological Significance
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Disadvantage of C4:
Low-temperature sensitivity
While the are some cold-tolerance c4 species,most perform poorly if at all, at lowtemperature.
Zea mays, for example, will not grow attemperature below 12-15oC.
This lower limit for growth is probably set by
the enzyme pyruvate, phosphate dikinase,which is cold labile and experiences asubstantial loss of activity below 12oC
Introduction to Plant Physiology/ WilliamsG. Hopkins
7/31/2019 Ecological Significance
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Photosynthesis in most situation is limited byavailability ofCO2 and water.
In C3 plants, even moderate water stress willinitiate closure of the stomata and reduce theavailable supply of CO2.
The low CO2 compensation concentration of C4plant means they can maintain higher rates ofphotosynthesis at lower CO2 level.
Thus C4 plants gain an advantages over C3 plantswhen the stomata are partially closed to conserveduring a period of water stress.
Introduction to Plant Physiology/ WilliamsG. Hopkins
In Availability of CO2 and Water
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Transpiration Ratio
The transpiration ratio (TR) relates to uptakes
of CO2 to the loss of water by evaporation
(transpiration) from the leaves.
Introduction to Plant Physiology/ WilliamsG. Hopkins
7/31/2019 Ecological Significance
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Transpiration ratios for C4 plants are typically
in the range of 200 350
While for C3 plants values the range of 500
1000
The low transpiration ratio for C4 plants
reflect their capacity to maintain high rates ofphotosynthesis while effectively conserving
water.
Introduction to Plant Physiology/ WilliamsG. Hopkins
7/31/2019 Ecological Significance
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Even under ideal condition, CO2 supply limits
photosynthesis in C3 plants to the extent thatlight saturation occurs at fluence rate about
25% of sull sunlight.
C4 plants, on the other hand never really
saturate, even at full sunlight.
Introduction to Plant Physiology/ WilliamsG. Hopkins
7/31/2019 Ecological Significance
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Disadvantage of Respiration ration of
C4 Plants
C4 is not necessarily more efficient than C3
photosynthesis
At leaf temperature below 30oC, the quantum
yield for C4 plants is actually lower than C3
plant that is C4 photosynthesis is lessefficient.
Introduction to Plant Physiology/ WilliamsG. Hopkins
7/31/2019 Ecological Significance
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How can the lower photosynthetic
efficiency of C4 plants be reconciled with
their apparent higher activity?
Recall that C4 plants are native to tropical orsubtropical habitats where there is usually anabundance of light.
They can take advantage of some of this excess
light to generate the ATP needed to run the C4cycle, concentration of CO2, and increase netcarbon assimilation.
Introduction to Plant Physiology/ WilliamsG. Hopkins
7/31/2019 Ecological Significance
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Although C4 plants are not competitive in all
situations some C3 plant may even equal or
exceed C4 plants in productivity given the
right combination of high temperature, highlight and low water, the C4 syndrome confers
a definite advantage.
Introduction to Plant Physiology/ WilliamsG. Hopkins
7/31/2019 Ecological Significance
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Prove?
The frustration of home owner and farmers as
Many of our aggressive weeds are C4 species :
crabgrass (Digitaria sanguinalis), Russian Thristle
(Salsola kali), and several species of pigweed(Amaranthus) that often take over during the hot dry
month in the middle of summer.
Many of the more highly productive crop species alsofall within the C4 group, including sugarcane
(Saccharum officinarum), sorghum (Sorghum bicolor,
maize (Zea mays) and millet (Panicum miliaceum)Introduction to Plant Physiology/ Williams
G. Hopkins
7/31/2019 Ecological Significance
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Terminologies
Terms meaning
1. CO2 compensation concentration Ambient CO2 concentration at which the
rate of CO2 uptake (for photosynthesis) is
balanced by the rate of CO2 evolution (by
respiration)
2. Post-illuminating CO2 burst (PIB) A short-lived continuation of a strong light
dependent CO2 evolving process in thedark (e.g. Dekker 1975, Tregunna et al.
1966)
Later was taken as a measure of photo
respiration rate and has been referred to
glycolate pathway (e.g Chollet & Ogren
1975)
3. Peroxisomes A cell organelle containing enzymes, such
as catalase and oxidase, that catalyze the
production and breakdown of hydrogen
peroxide.
Introduction to Plant Physiology/ Williams G. Hopkins
7/31/2019 Ecological Significance
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Terms Meaning
4. Quantum yield Quantum Yield can also be defined in
fluorescence (emission) in which case the
quantum yield is a measure of theefficiency with which absorbed light
produces some effect
Introduction to Plant Physiology/ WilliamsG. Hopkins