phloem part 3

What materials are translocated in the phloem

Carbohydrates, amino acids, hormones, inorganic ions, specialized metabolites, sugars in their non reducing form, macromolecules (rna and proteins)

P- proteins

Plug pores of damaged sieve elements immediately from positive pressure

Callose

Long term dealing of sieve plate pores. Deposition occurs between the cell membrane and cell wall

How can callose be reversed

Callose hydrolyzing enzymes can reverse deposition

Pressure gradient in sink

Made from phloem translocation in sugar loading at source and sugar unloading at sink

What happens to water and solute potentials during the pressure gradient at the sink

Phloem unloading lowers sugar concentrations in sieve elements generates generating higher (more +ve) pressure potential. This causes a rise in water potential. As that rises, water leaves phloem reducing tutor pressure in sieve element of the sink.

Sieve plate at sink

Maintains pressure potential. In an open tube a stepwise pressure gradient cannot be maintained

Phloem unloading

Import of photosynthate into sinks (tubers, developing roots, reproductive structures)

What does phloem unloading require

1. Sugars leaving sieve elements into sink tissues

2. Post sieve element short distance transport— sugars move to sink cells

3. Storage and metabolism of sugars in sink cells

Symplasmic unloading

Symplastic pathway predominate in elongating vegetative tissues ( most tissues).

What kind of transport in symplasmic unloading

Transport is passive along the sugars concentration gradient.

Apoplastic unloading

Occur in sink cells with high sucrose accumulation (fruits, seeds, storage organs). Sugars enter the apoplast at a point away from the sieve element - cc complex

How is sugar moved in Apoplastic unloading

Primary and secondary transporters move sucrose into the apoplast and into the next cell. Unloading mech is dependent on the sink

Sink to source transition

Occurs later in development. As leaf matures, it exports more sugar than it takes in

Is becoming a source from a sink an independent event?

The cessation of import and beginning of export are independent

How are the leaf veins involved in sink to source transition

As leaves mature, the major veins capable of unloading lost that ability

How do leaves develop the ability to load

1. Need to produce excess photosynthate

2. Minor veins need to mature

3. Sieve element - cc complex needs sucrose - H+ symporters (in some cases)