Allocation/Translocation L7 Part 2

Define Companion Cell

Present in sieve tube members, provides metabolic support. Angiosperms

Define Transfer Cell

Has folded walls for sucrose loading. Apoplastic movement

Define Intermediary Cells

Smooth walls with many plasmodesmata to collect solutes. Symplastic movement

What do P-proteins and callose do?

Form callus plugs in injured phloem

Define Reducing Sugars

Not translocated within phloem and have a free aldehyde/ketone group

• Glucose, mannose, fructose

Non-reducing Sugars

Can be translocated

• Sucrose

Describe the Pressure Flow Model

1. Phloem loading at source cell. Companion cell loads sugar into sieve tube

2. Water flows from xylem due to decreased water pressure in the seive tube

3. Pressure pushes fluid through sieve tubes to areas with lower water pressure potential, until arriving where the phloem occurs near the sink

4. Water flows back into the xylem due to change in seive tube pressure

Define Phloem Loading

Occurs at the source. Photosynthates are transported into sieve tube members

Define Phloem Unloading

Occurs at the sink. Photosynthates are transported out of sieve tube members

Define Sieve Element-Companion Cell Complex

Holds source cells

What are the two loading and unloading pathways?

• Symplastic: passive or assisted

• Apoplastic: Requires metabolic energy (active transport)

Sucrose H+ Symporter

• Transports sucrose into the se-cc complex

• Requires active transport (apoplastic)

What is the Polymer-Trapping Model?

• Sucrose moved into the se-cc is converted in oligosaccharides

• Prevents back-diffusion of sucrose into source cells and maintains the se-cc concentration gradient

• Bundle sheath cell → Intermediary cell → Sieve element

What are three uses for photosynthates?

1. Leaf metabolism and biomass maintenance

2. Short-term storage

3. Export to other plant parts

Define Partitioning

Distribution of photosynthates between sinks; determined by competition between sink tissues

Define Sink Strength

Decides which sink obtains more of the photosynthate pool

Define Sink Size

Dry weight biomass of sink tissue

Define Sink Activity

Rate of uptake per unit biomass per unit time

What is sink strength influenced by?

• Sink proximity to source

• Environmental factors (drought)

• Cell turgor

• Phytohormones