Phloem loading: Sink/source vs. Productivity

Symplast vs. apoplast pathways:

distinguish at near the sieve element-companion cell complex

apoplast: suc majorselective transporterenergy required: respiratory inhibitors, active loading

Organic acids, hormones passive

s -1.3 MPa

s -3.0 MPa

symplast: suc and others, such as

raffinose and stachyose

or transfer cells

Sucrose-H+ symporter

SECC

in the apoplast of phloem loading

in the membrane of company cells

or sieve elements

dissipate proton energy to couple

the uptake of sucrose

Regulating sucrose loading by the sucrose-H+ symporter — are not completely clear

The turgor pressure of the sieve elements

below a threshold level, a compensatory increase loading?

Sucrose concentration in the apoplast

[Suc]apoplast?

The available No. of symporter molecules

feed Suc SUT1 transcription, SUT1 and itsmRNA rapidly degrade,

protein phosphorylation involved

a diurnal regulation Nutrient supply— potassium availability

enhance sucrose efflux into apoplast sink growth

Intermediary cells in the symplast of phloem loading¤ diffusion¤ raffinose, stachyose, in addition to sucrose¤ a polymer-trapping model

The type of phloem loading is corrected with plant family and with climate

herbaceous trees, shrubs, vines

temperate and arid tropic and subtropic

exceptions

Coexist

New loading ?

Symplastic loading advantages?

Phloem unloading and short-distance transport

— via symplast or apoplast

PCMB:

inhibit sucrose across membrane

The transition of a leaf from sink to source

sink source depend on species, 25% → 40-50%

The cessation of import and the initiation of export are independent events.

The extent of maturation and sampling position

Feed 14C on source leaf for 2 h

Squash

The base still a sink

sink leaf

source leaf