Definition of Phloem Translocation: Phloem Translocation or Translocation of solute is the movement of organic food or organic solutes from site of synthesis (green parts) to site of utilization (non green parts) for consumption and storage.
Translocation of solute can be downward, upward or
lateral or radial
Upward from leaves to buds, flowers and fruits, Downward
from leaves to stem and roots, Lateral from Pith to cortex and epidermis
Two important terms for understanding Mass Flow
Hypothesis
- Source: site of synthesis or green parts of the plant like leaves
- Sink: Is the site of storage like fruits, seeds, tubers
Mass Flow Hypothesis (Munch, 1930):
Most accepted theory on Phloem translocation
- Mass flow of solute is
a passive process occurs from
source to sink along a hydrostatic pressure gradient or Turgor pressure
gradient
- HPG exists between
source and sink
- This difference drives
the movement of solutes from leaves
to other parts through phloem
- The movement is always
from the region high HPG to region of low HPG
Explanation
Step 1: Sugar concentration in Mesophyll cells increases by
photosynthesis. Water is absorbed from nearby cells increasing hydrostatic
pressure in mesophyll cell
Hydrostatic pressure or turgor pressure is the pressure exerted
on the wall due to entry of water by osmosis
Step 2: Phloem loading occurs. Transfer of organic food or sugar from
mesophyll cells to sieve tube of phloem due to turgor pressure or hydrostatic
pressure
Step 3: Water entry from xylem to sieve elements by
osmosis as the concentration inside sieve elements has increased by phloem
loading
Step 4:This hydrostatic pressure forces the solute to the
next sieve tube element. Thus hydrostatic
pressure difference (hydrostatic pressure gradient) drives solute movement from
sieve tube to sink
Step 5: Phloem unloading
Transfer of organic food from sieve tube to sink, the site
of storage
Step 6: This decreases the concentration in sieve elements
and water moves back to xylem vessels by osmosis and transpiration pulls up
water
This is a summarized video for better understanding
Demerits:
1. Recent studies suggest that
Phloem loading is an active process that requires ATP. Active phloem loading is the transport of sugars from the mesophyll cells
into the phloem against a concentration gradient.
2. This hypothesis proposes the unidirectional
flow of solutes but in plants bi directional flow is quite common.
3. The active role of cytoplasm in
phloem translocation is not considered.
4. Turgor pressure may not always be higher at the supply end.
