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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. or transfer cells. apoplast: suc major selective transporter energy required: respiratory inhibitors, - PowerPoint PPT Presentation
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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