Phloem loading: Sink/source vs. Productivity
Symplast vs. apoplast pathways:
distinguish at near the sieve element-companion cell complex
s -1.3 MPa
s -3.0 MPa
symplast: suc and others, such as
raffinose and stachyose
or transfer cells
apoplast: suc major
selective transporter
energy required: respiratory inhibitors,
active loading
Organic acids, hormones  passive
Sucrose-H+ symporter
 in the apoplast of phloem loading
 in the membrane of company cells
or sieve elements
 dissipate proton energy to couple
the uptake of sucrose
CC
SE
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
exceptions
Coexist
herbaceous
trees, shrubs, vines
temperate and arid
tropic and subtropic
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
Squash
Feed 14C on source leaf for 2 h
source leaf
sink leaf
The base still a sink
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