Assimilate Partitioning• To utilize solar radiation efficiently and to store

the photosynthate, a plant must be able to transport to move assimilate from an area of synthesis to an area of mobilization

• Assimilate produced via Pn is translocated throughout the plant for growth, development, storage and cell maintenance

• Partitioning: Division of assimilate among these processes affect both the productivity and survival of a plant

Assimilate Partitioning (cont.)• Phloem transport: Materials are moved from

an area of source to an area of sink• Inter-organ translocation in the plant is

primarily through the vascular system (xylem and phloem)

• Movement in the xylem is essentially a one-way acropetal (upward) movement up through the roots via the transpiration stream

• Substances in the phloem have a bidirectional movement and may be acropetal (upward) or basipetal (downward)

Assimilate Partitioning (cont.)• Assimilate produced in the leaves move to

assimilate sinks while substances absorbed by the roots move upward (e.g., plant growth regulators/hormones: cytokinins)

• In both xylem and phloem exist plasmodesmata which allow some lateral movement

• The bulk of translocated substances, other then water is the result of photosynthesis or remobilization of assimilates in storage

• 90% of the total solids in the phloem consist of carbohydrates

• The majority of the carbohydrates are nonreducing sugars (sugars without an exposed aldehyde or ketone group such as sucrose and raffinose) which can make 10 to 15% of the sap

• Phloem sap also consists of nitrogenous substances; especially amino acids, amides, and ureides at concentrations of 0.03 to 0.4%

• Extremely low quantities of other compounds can be translocated in the phloem at extremely low concentrations which include plant growth regulators, inorganic nutrients and systemic pesticides

• Many compounds such as reducing sugars, contact herbicides, proteins, most polysaccharides, calcium, iron, and most micronutrients do not normally move via the phloem

Mass Flow or Pressure Flow Hypothesis

• Assimilates move in mass flow along a hydrostatic pressure gradient

• Active (metabolic) loading and unloading of assimilate in the source and sink regions are respectively responsible for differences in osmotic potential in the sieve tubes in these regions

Translocation Rates• Rate at which a compound is moved in

the phloem can be affected by the rate of acceptance by the sinks (phloem unloading), the chemical nature of the compound as it affects movement in phloem tissue, and the rate at which the source is moving the compound into sieve tube elements

• Can be measured by radioactive isotopes

Specific Mass Transfer• Weight of assimilate moved per cross-sectional

area of phloem per unit of time– Values of 3 to 5 g·cm-2·hr-1 are typical and may

inhibit the translocation rate

• Phloem size is dependent upon the size of the source or sink it is serving

• Different translocation rates occur among plant species especially between C3 and C4 plants

• C4 plants:– Higher CO2 exchange rates– Larger ratio of cross sectional phloem area to leaf

area– Greater translocation rates than C3 species– May be due to specialized anatomy with vascular

sheath cells containing chloroplasts

Phloem Loading and Unloading• Phloem loading (transfer of

photosynthate from the mesophyll cells of the leaf to the phloem sieve tube elements) and phloem unloading (transfer of photosynthate from phloem sieve tube elements to the cells of a sink) can be rate limiting and can affect translocation

• During phloem loading the mesophyll cells are typically at a lower osmotic potential (higher water potential) than the sieve tube elements

• Energy is required to move sugars against a concentration gradient

Phloem Loading and Unloading (cont.)

• Phloem loading provides the increased osmotic potential in the sieve tube elements supplying the driving force for the mass movement of assimilate

• Movement of sugars consists of symplast (mesophyll cells) into apoplast (cell walls) and then into symplast (phloem cells)

• When sugars move into sieve elements, the movement may be aided by adjacent companion cells

• The greater rate noted in C4 species may be due to chloroplasts in the sheath cells which provide the energy (ATP) needed for loading

Phloem Unloading

• Information is scarce: probably occurs by different mechanisms, may be a limiting factor, and may vary with the development of the sink

Assimilate Partitioning

• Partitioning of assimilate is generally to the sinks closest to the source

• Cross linking of sieve tube elements occurs in most species and allows for “alternate” transfer of assimilate

Sink-Source Relationships and Partitioning

• The steady buildup of sugars in the system may cause a feedback inhibition resulting in a reduced Pn rate

• Factors that control sink strength and partitioning–Plant growth regulators (hormones)–Environment

• Hormonal influences on initiation, development and abortion of flowers can have a significant effect on source-sink relationships

Assimilate Partitioning During Vegetative and Reproductive Phases

• If translocation is slow, assimilates can be converted into a form for storage

• Remainder is exported (translocated) to growth, maintenance, and storage functions

• During vegetative growth, roots, stems and leaves are competitive sinks for assimilate