2014-4-Alokasi Dan Partisi Biomassa

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PATTERNS OF TRANSLOCATION: SOURCE TO SINK

1. Phloem sap is not translocated exclusively in either an upwardor downward direction and is not influenced by gravity; phloem

translocation occurs from sourcetosink.

2. Sources include any exporting organ, typically mature leaves

exporting photosynthate.

3. Storage organs (roots, tubers, seeds, etc.) can also serve assources.

4. Sinks include any non-photosynthetic organ or tissue that does

not produce sufficient photosynthate to support its own

metabolic needs: roots, underground stems, buds, immatureleaves, flowers, fruits, etc.

How do plants transport carbohydrates?

Transport in plants

Mass-flow or pressure-flow hypothesis: Carbohydrates move from source(site produced or stored) to s ink(site

used)

Carbohydrates actively transported into phloem at source

High concentration of carbohydrates causes greater osmotic

pressure in phloem; water moves in from adjacent xylem byosmosis

Water influx creates (turgor) pressure inside phloem; pushingwater and dissolved carbohydrates through phloem

At sink, carbohydrates actively removed from phloem; reducing

osmotic pressure in phloem

water leaves phloem and reenters xylem, maintaining aosmotic pressure gradient between sources and sinks

The regulation of photoassimilate distribution into

plant organs governs their productivity

Important for yield of crop plants

Maximize yield of marketable part of crop plant

e.g., wheat farmers want more carbon to go tograinthan

to roots

We can define two plant physiology terms that define how C is

allocated to plant parts

These terms are important in agronomy

They govern the destination of C in plants: allocationandpartitioning

Allocation

The regulation of the distributed of fixed carbon intovarious metabolic pathways (i.e. the fate of fixed carbon)

include storage (starch), utilization (metabolic energy,

synthesis of other compound) and synthesis of transport

sugar (sucrose).

Allocation involves storage, metabolism and transport,

process which facilitate competition for photosynthates


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Synthesis of storage compounds. Starch is synthesized and

stored within chloroplasts and, in most species, is the primary

storage form that is mobil ized for translocation during the

night. Plants that store carbon primarily as starch are called

starch storers.

Metabolic utilization. Fixed carbon can be utilized within

various compartments of the photosynthesizing cell to meet

the energy needs of the cell or to provide carbon skeletons for

the synthesis of other compounds required by the cell.

Synthesis of transport compounds. Fixed carbon can be

incorporated into transport sugars for export to various sink

tissues. A portion of the transport sugar can also be stored

temporarily in the vacuole

Allocation of fixed carbon is a balancing act between

providing energy and C skeletons

The plants competing processes for C include:

Leaf metabolism

Respiration of glucose provides:

ATP Carbon skeletons for biosynthesis

Short-term storage

Plants can only photosynthesize in the light

But they need to grow at all times, therefore store carbon inleaves (form is species dependent)

Starch: dicots

Sucrose: sugarcane, sugarbeet

Sucrose polymers (fructans): grasses, monocots

Leaf carbon storage is a bufferagainst environmental factorsaffecting photosynthesis

Translokasi di dalam sel parenkimA large amount of C allocation is destined for export

from the leaf to metabolic sinks

Immediate transport (export)

About of fixed carbon

Regulation of export poorly understood

Shifts during development

Sink (young leaves)little export

Source (mature leaves)switch off/down regulatesucrose hydrolytic enzymes, increase SPS (sucrosephosphate synthase)

For most plants:

Starch (made in chloroplasts): stored

Sucrose (made in cytoplasm): transportedhigh synthesis(SPS) activity, high export rate

Export of sucrose ~constant at night

Source leaves regulate allocation

Increase in photosynthesis rate in a source leaf results in

increased translocation rate from the source

Control point for allocation

Starch syntesis

Sucrose synthesis (including distribution of sucrose

between transport and temporary storage)

Regeneration of intermediates in the reduction cycle of

the Calvin Cycle

Partitioning

The differential distribution of photosynthates within the plant ,e.g. into different sinks; efficiency of partitioning from

vegetative sinks into storage organs translates into productivity

for many crops.

The differential distribution of photosyntates within plants

- Various sinks partition sugars

- Distribution must be balanced

- Many cultivars are economically inportant because the

partition to edible plant parts (fruits, grains)


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Exported carbon is partitioned between various sinks

Partitioning refers to the relative amount of exported carbon used

by metabolic sinks within the plant

Sinks compete for carbon!

Apical meristems Young leaves

Fruit

Dependent on: Vascular connections between source and sinks (best to have

direct links (vascular traces) Same side of stem

As close as possible

How close sink is to source physically

Sink strength

Kemampuan relatif dari organ untuk menarik fotosintat

tergantung pada: ukuran & stadia pertumbuhan

Kedua faktor dan jarak menentukan laju aliran

fotosintat

Jarak dan laju menentukan gradien source: sink

Sink strength depends on sink size and activity

Sink strength = sink size x activity

sink size = total biomass of the sink tissue

sink activity = rate of uptake of photosynthates per unit

biomass of sink tissue

Sink strength

Kemanpuan sinkuntuk memobilisasi fotosintat tergantung padaukuran dan aktivitas sink

Urutan kekuatan sinkuntuk hasil fotosintesis:

Biji > daging buah = pucuk daun muda > kambium > akar >

organ penyimpan lain

Source-sink pathways follow patterns

Proximity:of source to sink is a significant factor.

Upper nature leaves usually provide photosynthesis products togrowing shoot tip and young, immature leaves

Lower leaves supply predominantly the root system

Intermediate leaves export in both directions

Development:Importance of various sinks may shift during plant

development

Roots and shoots major sinks during vegetative growth

But fruits become dominant sinks during reproductive

development


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Source-sink pathways follow patterns

Vascular connections:Source leaves preferentially supply sinks

with direct vascular connections

A given leaf is connected via vascular system to leaves above andbelow it on the stem

Modifi cations of translocation pathways: - Interference with a

translocation pathway by mechanical wounding (or pruning)

vascular interconnections can provide alternate pathways forphloem transport

SOURCE TO SINK PATHWAYS FOLLOW ANATOMICAL AND

DEVELOPMENTAL PATTERNS

1. Proximity is important: upper mature leaves supply photosynthate to

growing shoot tip; lower leaves supply the root; middle leaves supply both.

2. Development influences transport:

a. Young leaves begin as sinks, gradually become sources.

b. Reproductive structures become dominant sinks during flowering.

3.Vascular connections important; source leaves preferentially supply sinks

to which they have vascular connections; typically, sources leaves supplysinks along the same vertical row ororthostichy.

4. Phloem interconnections (anastomoses) can provide alternative pathways

in the event of wounding or pruning.

The sink strength of a plant organ is a measure of its

capacity to absorb photoassimilate

Sink strength= sink size(mass) Xsink activity(rate of uptake)

The sink strength of an organ definesits capacity to assimilatemacromolecules

Why sink strength changes with timeor what determines it: unknown

Good: High unloading rate

High sequestration rate into storage

Being close to source

Developing grain: very strong sink

No yield reduction even at low [CO2],carbon stolen from roots and otherless strong sinks

Factors Affecting the Translocation

Temperature

Increased temperatureincreased loading & unloading optimum 20 -30oC

Chilling Sensitive Plants (most)

Chilling Tolerant Plants (beets)

Can acclimate translocation of photosynthate to increasingly cold conditions

Factors Affecting the Translocation of Sap

Light

In the dark root translocation of photosynthate is favored

over stem translocation.

At least one study shows that the translocation of sap in

the stem was increased by BLUE and RED light.


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Factors Affecting the Translocation of Sap

Hormones

Both cell division (cytokinins) and cell elongation (auxins)

creates sinks absorbs sap.

Bud break

Increased GA, decreased ABA

Partitioning control is a complex process

The steps involved in C flow to a

sink: phloem loading in a

source leaf, phloem transp ort

into the sink, unloading andshort distance transport with in

the sink and then metabol ism

and storage in the sink.

- Molecule signalling: sucrose , hormone- The status of various nutrient elements

affects the partitioning of photosynthate. n

The status of various nutrient elements affects the

partitioning of photosynthate

When plant growth is limited by a low supply of the

elements N, P, S or Fe, the ratio of root to shoot mass

rises, more organic C being delivered to the roots,

whereas limitation of the supply of Mg, Mn or K leads

to a decrease of the root : shoot mass ratio. Water

stress increases the root : shoot ratio, root growth

being stimulated while shoot growth is reduced.

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2014-4-Alokasi Dan Partisi Biomassa