Solute Transport

1
Solute Transport
2
Translocation Source-Sink

• Sap in P moves from sources to sinks
• not always downward
• source area of supply or export
• mostly mature lvs, some storage areas
• produce excess photosynthates
• sink area of metabolism or storage
• roots/tubers/fruits/ immature lvs
• do NOT produce sufficient photosynthates b/c
  nonphotosynthetic/ storage/weakened

3
Source-Sink Pathways

• Specific sources supply specific
  sinks
• Depends on
• proximity distance btwn source/sink
• upper lvs feed new shoot, lower lvs feed roots
• major factor
• development during vegetative growth stage
  root/shoot tips are strong sinks
• during reproductive stage flowers and fruits
  become strong sinks
• vascular connections direct connections
  preferentially fed
• typically directly above or below
• twisting in some sp.
• modification of translocation pathways
  interference (wounding, pruning) w/ translocation
  alter proximity/vascular connections
• new routes emerge, highly plastic

4
Substances Moved in Phloem

• Substances found in P
• water
• most abundant substance in P
• carbohydrates
• sucrose most abundant sugar
• nonreducing sugars (i.e. sucrose) transported b/c
  less reactive
• reducing sugars (i.e. glucose, fructose) too
  reactive
• AA (i.e. glutamate and
  aspartate
  are common)
• hormones, nucleotides, proteins
• mobile ions
• K, Mg, P, Cl

5
Pressure-Flow Model

• P translocation in angiosperms
• Flow of sap in sieve elements driven by
  osmotically generated pressure gradient (?p) btwn
  source and sink
• mvmt in P 1 m h-1
• pressure due to P loading at source and P
  unloading at sink
• P loading mvmt of photosynthate from mesophyll
  cells into SE
• requires S
• symporter involved
• P unloading mvmt of photosynthate from SE to
  sink
• doesnt require S
• passively moved in P
• actively loaded into P which creates pressure
  gradient
• pressure gradient where ?p sink gt ?p source

6
Pressure-Flow Model

• At source end
• P loading (active) sugars accumulate in SE ?s
  decreases (goes away from 0) ?w decreases
  water enters SE ?p increases
• At sink end
• P unloading
  sugars
  removed from SE
  ?s
  increases
  (goes toward 0)
  
  ?w increases
  
  water exits SE
  
  ?p decreases
• Moves low to high ?w
• due to pressure gradient,
  not
  ?w gradient
• STUDY THIS IMAGE.

7
Allocation and Partitioning

• Allocation diversion of fixed C into metabolic
  paths
• at source end, fixed C can be allocated as
• syn. of storage cmpds
• starch
• metabolic utilization
• S needs of cell, C skeletons
• syn. of transport cmpds
• sugars for export
• at sink end, sugars can be allocated as
• stored as sucrose/hexose/starch
• remain as sucrose or be transformed into other
  cmpds
• used for R/syn. of other molecules

8
Allocation and Partitioning

• Partitioning differential distribution of
  photosynthates to various sinks in plant
• the greater the sinks ability to allocate
  imported sugars, the greater its ability to
  compete for photosynthate
• partitioning dictates growth patterns
• Ps vs water/mineral needs
• transplanting and rootshoot growth
• means of communication btwn source and sink
• ?p and chemical signals (hormones/nutrients)

9
Sink Strength

• Sink strength ability of sink to mobilize
  photosynthate toward itself
• sink strength sink size x sink activity
• sink size total wt of sink tissue
• sink activity rate of uptake of
  photosynthates per unit
  wt of sink tissue
• changing sink size or activity alters
  translocation
  pattern/partitioning
• effects of tree topping