Carbon Assimilation

8/3/2019 Carbon Assimilation

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CARBON ASSIMILATION

AND

PRODUCTIVITY


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Carbon assimilation creates plant biomass, or

dry matter, which in turn supports humans andvirtually all other heterotrophic organisms in

the biosphere.

The physiology of photosynthesis light capture,energy conversion, and carbon assimilation

are at the root of productivity.

PRODUCTIVITY REFERS TO AN INCREASE IN BIOMASS

Although inorganic nutrients are a part of this dry

matter, by far the bulk of dry matter for any

organism consists of carbon.


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PHOTOSYNTHETIC CARBON REDUCTION CYCLE

(PCR)

The pathway by which all photosynthetic eukaryotic organisms

ultimately incorporate CO2 into carbohydrate is known as carbon

fixation or the photosynthetic carbon reduction (PCR) cycle. It is

also referred to as the Calvin cycle.

The PCR cycle can be divided into

three primary stages :

1. Carboxylation

2. Reduction

3. Regeneration


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THE CARBOXYLATION REACTION: How Calvin unraveled the path

of carbon in photosynthesis?


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The basic input into the biosphere is the conversion of solar

energy into organic matter by photosynthetic plants and

microorganisms, known as primary productivity (PP).

Total carbon assimilation is known as gross primary productivity (GPP).

Not all of the GPP is available for increased biomass - there

Is a respiratory cost that must be taken into account.

The principal focus of most productivity

studies is therefore net primary productivity (NPP).

NPP is determined by correcting GPP for energy and

carbon loss due to respiration.

NPP is a measure of the net increase in carbon, or carbon gain, and

reflects the additional biomass that is available for harvest by animals.


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CHLOROPLASTS OF C3 PLANTS ALSO EXHIBIT COMPETING

CARBON OXIDATION PROCESS

1. The rate of photosynthesis is to measure gas exchange-either CO2

uptake or O2 evolution.

2. Cellular (or mitochondrial) respiration (R) is an opposite gas

exchange,

since it results in an evolution of CO2 and uptake of O2.

3. Evolution of CO2 associated with photosynthetic metabolismcalled photorespiration (PR)

4. PR involves the reoxidation of products assimilated in

photosynthesis.

5. The photorespiratory pathway involves the activities of at least

three different cellular organelles (the chloroplast, the

peroxisome, and the mitochondrion)

6. CO2 evolved in PR results in a net loss of carbon from the cell.


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The measured CO2 uptake in the light is

termed apparent or net

photosynthesis (AP)

AP = GP - (R + PR)

True or gross photosynthesis (GP) is thus

calculated by adding amount of

mitochondrial = (respired CO2 +

photorespired CO2)

GP = AP + R + PR


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CARBON ECONOMY

Carbon economy is the term used to describe the balance

between carbon acquisition and its utilization.

Respiration is the principal counterbalance to photosynthesis.

Respiration consumes assimilated carbon in order to obtainthe energy required to increase and maintain biomass.

Respiratory loss of carbon constitutes one of the most

significant intrinsic limitations on plant productivity.


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PRODUCTIVITY IS INFLUENCED BY A VARIETY

OF GENETIC AND ENVIRONMENTAL FACTORS.

Environmental factors

1. light 5. Nutrient supply

2. CO2 6. Pathological conditions

3. Temperature 7. Pollutants

4. Soil water

Genetic factors

1. Carbon assimilation

pathway (C3,C4,CAM)

2. Leaf age3. Morphology

4. Leaf area index

5. Leaf angle

6. Leaf orientation


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PHOTON FLUENCE RATETotal number of photons (Np) incident

from all directions on a small sphere

divided by the cross-sectional area ofthe sphere and per time interval (m-2 s-1).

At very low fluence rates the rate of

CO2 evolution due to dark respiration

exceeds the rate of photosynthetic

CO2 uptake = negative CO2 uptake.

As fluence rate increases,photosynthesis also increases and

so does CO2 uptake until the rate of

CO2 exchange equals zero = the light

compensation point

At the light compensation point the photosynthesis and the respiration

are balanced. The light compensation point for most plants falls

somewhere in the range of 10 to 40 Qmol m2 s1.

At fluence rates above the compensation point, the rate of photo-

synthesis continues to increase until it reaches light saturation (LS).


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In most C3 plants at normal

atmospheric CO2 levels,

photosynthesis saturates

with light levels of about 500 to 1000

Qmol photons m2 s1, that is, about

one-quarter to one-half of full sunlight.

C4 plants never really achieve LS.

There are also a small number of C3plants, such as peanut (Arachis

hypogea), which do not light saturate.

The light saturated rate of photosynthesis, for example, is lower in leaves

that have acclimated to growth at low irradiance (shade leaves) than in

those that have acclimated to higher irradiance (sun leaves).

Between dawn and dusk, the rate of photosynthesis gradually increases,

reaching a maximum near midday, and then declines.


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The CO2 concentration of the

atmosphere is relatively low and

reasonably stable at about 0.035

percent by volume or 350 Ql/l.350 Ql/l is well below the CO2

saturation level for most C3 plants

at normal fluence rates, which

means that availability of CO2 is

often a limiting factor inphotosynthesis.

In C3 plants, increased photosynthetic rates with higher CO2 levels

results from two factors:

1. Increased substrate for the carboxylation reaction2. Competition with oxygen, reduced photorespiration

At higher fluence rates, both the maximum rate of photosynthesis and

the CO2 saturation level increase. Interestingly, most C4 plants appear

to saturate at CO2 levels at or just above normal atmosphericconcentrations, regardless of fluence rate.


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The rate of photosynthesis is

actually determined not by the

ambient CO2 concentration, as

much as by the intracellular CO2concentration, that is, the supply

of CO2 at the carboxylation site

in the chloroplast.

It assumed that the intracellular

CO2 concentration is inequilibrium with the intercellular

spaces.


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Since CO2 diffusion rates depend

in part on concentration

gradients, the primary effect of

increasing ambient CO2 levelswould be to increase the

intercellular CO2 concentration

by increasing the rate of diffusion

into the leaf. Here it is assumed

the water supply is adequate and,

consequently, stomatal CO2

conductance is not limiting.


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Photosynthetic capacity is

determined by the balance

between carboxylation capacity

and electron transport capacity

At low CO2 concentrations, the

rate of photosynthesis is

limited and, hence, the

carboxylation capacity of the

system, but is saturated with

respect to availability of the

acceptor molecule RuBP

However, any excessgeneration of RuBP, which is in

turn dependent on the electron

transport reactions, over that

required to support

carboxylation would represent

an inefficient use of resources.


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TEMPERATURE

Photosynthesis, like most other

biological processes, is sensitive

to temperature.The temperature response curve

can be characterized by three

cardinal points: the minimum and

maximum temperatures (Tmin and

Tmax, respectively) at which thereaction can proceed and the

optimum temperature (Topt)

The temperature response of chemical and biological reactions can generally be

characterized by comparing the rate of the reaction at two temperatures 10oC

apart, a value known as the Q10: Q10 = RT +10

RT

The value of Q10 for enzyme-catalyzed reactions is about 2, meaning that the

rate of the reaction will approximately double for each 10oC rise in temperature.

This value for Q10 applies primarily to stimulation of the reaction by

temperatures between Tmin and Topt. Once the optimum is reached, the reaction

rate may decline sharply due to enzyme inactivation


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The temperature characteristics of

C3 and C4 photosynthesis seem

to be dominated by the

temperature response curves for

Rubisco and PEPcarboxylase,respectively.

The low temperature sensitivity of

C4 photosynthesis probably

reflects, in part, the low

temperature inactivation of theenzyme pyruvate, phosphate di-

kinase.


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SOIL WATER POTENTIAL

The rate of photosynthesis declines under conditions of water stress,

and in cases of severe water stress may cease completely.

Stomatal closure and the resultant decrease in CO2 supply due to water

stress imposes a major limitation on photosynthesis. When this occurs

in the presence of light for prolonged periods of time, this lack of CO2

supply may lead to photoinhibition of photosynthesis.

Photorespiration may protect the photosynthetic apparatus from

excess light under such conditions because the energy absorbed can

be used to fix O2 when the CO2 supply is limiting due to stomatal

closure.

C4 plants enjoy some advantage over C3 plants with respect to

photosynthesis and water stress because of their higher water use

efficiency.


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NUTRIENT SUPPLY, PATHOLOGY, AND POLLUTANTS

Photosynthetic capacity is particularly sensitive to nitrogen supply. As a basic

constituent of chlorophyll, redox carriers in the photosynthetic electrontransport chain, and all of the enzymes involved in carbon metabolism,

nitrogen plays a critical role in primary productivity. In a C3 species, Rubisco

alone will account for more than half of the total leaf nitrogen. In one

study of C3 and C4 grasses, net photosynthesis increased linearly with

nitrogen content. In barley seedlings, a 5-fold increase in nitrate supply

stimulated a 25-fold increase in net photosynthesis.

Photosynthesis energy is used

directly in nitrate reduction andnitrogen assimilation provides

amino acids for the synthesis of

enzymes and proteins involved in

both photosynthesis and

respiration.


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LEAF FACTORS

During initial development and the

rapid growth phase, the

photosynthetic capacity of leaf alsoincreases. Leaf photosynthetic

capacity then declines as the aging

leaf undergoes senescence, a

progressive deterioration of the leaf

characterized in part by the loss ofchlorophyll and photosynthetic

enzymes.

Different types of leaves may also have different photosynthetic

capacities. Evergreen leaves, for example, have a lower photosynthetic

capacity than deciduous leaves.

The young, growing leaves at the top are exposed to full sunlight while

older leaves further down may be heavily shaded. Very often the fluence

rate reaching leaves lowermost in a canopy may fall below the light

compensation point for a large part of the day.


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The ratio of photosynthetic leaf area to

covered ground area is known as the leaf

area index (LAI).

Because both leaf surface and the coveredground are measured as areas (m2), LAI is

dimensionless. Values of LAI in

productive agricultural ecosystems

typically fall in the range of 3 to 5.

The optimum LAI for a given stand of plants depends on the anglebetween the leaf and the stem. Horizontal leaves, typical of beans

(Phaseolus) and similar crops, are efficient light absorbers because of

the broad surface presented to the sun, but they also more effectively

shade leaves lower down in the canopy. Erect leaves, typical of grasses

like wheat (Triticum) and maize (Zea mays), produce less shading but,

because of their steeper angle, are not as efficient at intercepting light.

Field studies have confirmed that canopies with predominantly

horizontal leaves have LAI values of 2 or less, while vertical leaf

canopies support LAI values of 3 to 7.


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PRIMARY PRODUCTIVITY ON A GLOBAL SCALE

Total global productivity is estimated to be approximately 172 billion

metric tons of dry organic matter per year.

Approximately 68 percent (117.5 v 109 t yr1) of that total is accounted

for by terrestrial ecosystems. The remaining 32 percent is accounted

for by marine ecosystems.

Productivity on land is more than twice that of the oceans on an area

less than half as large. On an area basis, land-based production is

about five times greater than the oceans. This difference is at least

partly explained by differences in nutrient supply.

Over the major portion of the oceans, dead organisms and sinking

particles carry nutrients out of the lighted zone near the surface wherephotosynthesis occurs.

Terrestrial ecosystems, on the other hand, retain a much larger

nutrient capital in the soil and litter where they can continue to support

growth and productivity. Agriculture accounts for only 5.3 percent of

global productivity.


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SUMMARY1. Carbon assimilation by plants creates the plant biomass that supports

humans and virtually all other heterotrophic organisms.

2. Overall carbon gain depends on net primary productivity-the balance

between carbon uptake by photosynthesis and carbon loss respiration.

3. Carbon loss to respiration, can be divided into the carbon cost of

growth, or growth respiration, and the cost of simply maintaining

structure and processes that do not result in a net increase in drymatter.

4. Several studies have shown a negative correlation between respiration

and growth rate.

5. Productivity is also influenced by a variety of genetic and environmental

factors that influence photosynthesis. These include light, available

carbon dioxide, temperature, soil water, nutrients, and canopy

structure.


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6. Productivity is also influenced by a variety of genetic and

environmental factors that influence photosynthesis. These include

light, available carbon dioxide, temperature, soil water, nutrients, and

canopy structure.

7. Plants also require an adequate water and nitrogen supply in order to

maximize their leaf photosynthetic capacity.

8. Productivity depends on the pattern of leaf senescence and the

structure of the canopy. The ideal canopy maximizes the efficiency oflight interception and carbon gain by balancing leaf area, leaf angle,

leaf orientation, plant density, and senescence of older leaves.

Carbon Assimilation

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