Ecological Significance

7/31/2019 Ecological Significance

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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


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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



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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.



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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


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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



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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


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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



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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



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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.


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.



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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.



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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.



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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.



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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.



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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.



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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


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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.



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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


Ecological Significance

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