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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
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
• Formation of small drops of water in the morning on
edge of leaf blades
• Caused from root pressure
The Transpiration-Cohesion-Tension
• 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
• 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?
• Stomata needs to be open to take on CO2 and to
release O2 for photosynthesis
• While the stomata is open, water is lost via
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 &
• Results in evaporative cooling
What regulates the stomatal
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
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
• 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