Transcript Document

TRANSPORT in
PLANTS
From Roots to Shoots
The Big Picture
Transport of Xylem Sap
• Xylem sap = water + minerals absorbed
by roots
• Is this sap pushed upward from roots or
pulled upward by the leaves?
Root Pressure
• At night
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–
–
–
–
Transpiration is low or zero
Root cells are still taking in water/minerals
Stele does not allow ions to seep out
New ions seep into stele
This lowers water potential and water moves
into xylem forcing fluid upward
Guttation
• Formation of small drops of water in the morning on edge of
leaf blades
• Caused from root pressure
The Transpiration-Cohesion-Tension
Mechanism
• Air has lower water potential than leaf—water moves out of leaf via
the stomata
• Water in the spaces of leaves moves to replace lost water creating a
meniscus
• Water “sticks” to the side of walls of the tracheids by adhesion
• Water “sticks” to other molecules by cohesion
The Transpiration-Cohesion-Tension
Mechanism
• This results in a continuous column of water flowing from
the roots to the stomata of the leaves
• This is “bulk flow”
• The polarity of water and its subsequent hydrogen bonding
explain this phenomenon
Ascent of
Xylem Sap in
a Tree
What Controls Transpiration?
Photosynthesis-Transpiration Compromise
• Stomata needs to be open to take on CO2 and to release O2
for photosynthesis
• While the stomata is open, water is lost via transpiration
The Benefits of Transpiration
• It is the process that results in the upward flow of xylem sap
• Assists in delivery of minerals from roots to shoots & leaves
• Results in evaporative cooling
What regulates the stomatal
opening?
The Role of Potassium
Factors that affect Stomatal Opening
• Light—open during day; closed at night
• Concentration of Co2 within air spaces of leaves—low level CO2
opens stomata
• Internal clock located in guard cells—stomata have a daily rhythm of
opening and closing (circadian rhythm)
• Environmental stresses—water deficiency, abscisic acid; high
temperature
Translocation of Phloem Sap
• Big Idea—Transport the product of photosynthesis (sugar)
throughout the plant.
• Phloem Sap = A sugar solution (usually sucrose). May be
30% by weight sugar—and syrupy thick.
• Translocation = process of moving phloem sap
Direction of Translocation
• From SUGAR SOURCE—Sugar producer (usually mature
leaves) by photosynthesis or breakdown of starch
• To SUGAR SINK—Organ that is a consumer or storer of
sugar (root, fruit, tuber, bulb)
Phloem Loading
Translocation is caused by pressure flow
• Phloem movement is too fast to be accounted for by
diffusion or cytoplasmic streaming
• Phloem moves by “bulk” pressure flow
Pressure Flow of
Phloem