Lecture N: Translocation and Phloem

In dicots, where is phloem in relation to the xylem?

It is outside the xylem

Phloem vs. xylem contents

Phloem: contains lots of sucrose, amino acids, and potassium

Xylem: mostly contains water

Girdling

The practice of cutting off phloem from the outside of the tree to either kill it or increase yield

Phloem vs. xylem: positive pressure/tension, and does osmosis matter?

Phloem: under positive pressure, osmosis matters

Xylem: under tension, doesn’t have solute potential so osmosis doesn’t matter

Water will come out of the phloem if the stem is cut, but not the xylem

What cells make up the phloem? How do they compare to the xylem’s cells?

The phloem consists of sieve-tube elements and companion cells. The companion cells produce materials like ATP to sustain the sieve-tube elements and plasmodesmata connect the two cells. Sieve elements are connected to each other by sieve plates

Unlike the xylem, the phloem is made up of living cells.

What are the 3 types of phloem loading?

Diffusion (passive symplastic)

Apoplastic loading (active apoplastic)

Polymer trapping (active symplastic)

What is the cellular direction of movement of phloem loading?

Mesophyll (M) —> Companion cell (CC) —> Sieve element (SE)

Phloem loading: diffusion

(passive symplastic)

Sucrose moves from high to low concentration, and there is no apparent accumulation of sucrose in veins

Phloem loading: apoplastic loading

(active apoplastic)

Using ATP to create a proton gradient, sucrose is unloaded into the cell wall (why it’s called apoplastic) then pumped into the companion cell. This causes sugars to be in high concentration in the veins

Phloem loading: polymer trapping

(active symplastic)

High concentrations of long sugar polymers are concentrated in the phloem and can’t move out of the companion and SE cells. Polysaccharides can’t diffuse back but can diffuse into the sieve element symplastically

Source-sink in terms of sucrose concentration

Generally the source has the highest sucrose concentration and the sink has a much lower concentration.

Harvest index

Ratio of commercial or edible yield to total shoot yield