Nutrition and
Transport in Plants
Ch. 26 – Plant Nutrition & Transport
Essential Inorganic Nutrients
 About 95% of a plant’s dry weight is carbon, hydrogen, and oxygen
 Primary nutrients are carbon dioxide and water
Minerals
•Know why plants need the following minerals:
N, C, H, O, P, K, Mg
• Know the difference between macronutrients & micronutrients
Use book slogan to remember the names of all macro & micronutrients.
“C Hopkins Café Managed by Mine Cousin Clyde Mo”
1

Overview of Plant Nutrition 2
Except for CO
2 and O other minerals are
2 all absorbed as minerals by roots

Nutrition and
Transport in Plants
Plant Nutrition & Transport
A Nutrient is an Essential Nutrient:
 If it has an identifiable role
 No other nutrient can substitute for it
 A deficiency of this nutrient causes plant to die
Hydroponics
•Water culture
•Can be used to determine if a nutrient is essential.
•Eliminate mineral from the water given to plants and see if they can survive.
3

Nitrogen
Phosphorus
Calcium
Nutrient Deficiencies 4

Water and Mineral Uptake
Nutrition and
Transport in Plants
5
Water moves from soil to roots by osmosis, or by flowing along cell walls.
• If water moves by traveling along porous cell walls it is following the apoplast pathway:
-
Water will move into roots because they have lower osmotic pressure than the soil solution does.
-
Water moves inward towards center of root & the
Casparian strip.
- At this point the water is forced to enter the endodermal cells to get into the xylem.

Water and Mineral Uptake 6
Apoplast
Symplast

Water and Mineral Uptake
Nutrition and
Transport in Plants
7
• If water moves by osmosis from cell to cell it is following the symplast pathway:
-
Concentration of substances dissolved in cells is higher than in the soil, so water moves in through root hairs.
-
Water dilutes cell concentration compared to adjacent cells, so water moves into next cell.
-
This continues to xylem where the Casparian strip is encountered and water must continue through endodermal cells to get into xylem.

Water and Mineral Uptake 8
Apoplast
Symplast

Water and Mineral Uptake
Nutrition and
Transport in Plants
9
• In contrast to water, minerals are actively taken up by plant cells.
-
The concentration of minerals in roots is as much as 10,000 times greater than in surrounding soil.
-
Following uptake by root cells, minerals move into xylem & are transported with the water.
-
Along the way, minerals can exit the xylem & enter cells that require them.

Nutrition and
Transport in Plants
Adaptation of Roots for Mineral Uptake
10
Important Symbiotic Relationships
 Rhizobium bacteria fix atmospheric nitrogen (N
2
)
-
Live in root nodules
-
Found in legumes such as soybeans & alfalfa
-
Turn NH
3 into NH
4
+
-
Plants can then take up the NH
4
+ compounds to make organic
-
The bacteria are supplied with food by the host plant
-
This is an example of mutualism .

Root Nodules 11

Nutrition and
Transport in Plants
Adaptation of Roots for Mineral Uptake
12
Mycorrhizae
 An association between fungi and plant roots
-
Fungus increases surface area available for mineral and water uptake & breaks down organic matter
-
Root furnishes fungus with sugars & amino acids
-
Plants are extremely dependent on mycorrhizae
Example: orchid seeds don’t germinate until mycorrhizal fungus has invaded cells

Mycorrhizae 13

Nutrition and
Transport in Plants
Adaptation of Roots for Mineral Uptake
14
Epiphytes
 Some plants have poorly developed roots or no roots at all.
Epiphytes are “air plants”
-
They grow on larger plants which give them support
But they don’t receive nutrients from their host.
Instead they might catch rain and minerals in special pockets at base of their leaves.
-
Example:
Spanish moss

Spanish Moss 15

Nutrition and
Transport in Plants
Transport Mechanisms in Xylem
16
The watery contents of xylem and phloem vessels is called sap.
 Xylem sap transports water & dissolved minerals from roots to leaves
 Phloem sap transports organic nutrients to all parts of the plants

Plant Transport System 17
Xylem is continuous from roots through stem to leaves

Nutrition and
Transport in Plants
Transport Mechanisms in Xylem
18
Water flows passively from an area of higher water potential to an area of lower water potential
 Water potential is the energy of water. It depends on two factors:
-
Water pressure across membrane. Water moves from high pressure to low pressure.
-
Solute concentration across membrane (osmotic pressure). Water moves from area of low solute concentration to area of high solute concentration.

Nutrition and
Transport in Plants
Transport Mechanisms in Xylem
19
Increasing water pressure will counter the tendency of water to enter a cell because of solutes.
 A common situation in plants.
-
As water enters a plant cell by osmosis, pressure will build up inside due to the strong cell wall.
-
When will water stop entering the cell?
-
When pressure inside the cell increases to the point that it balances the osmotic pressure from outside.
-
This leads to turgor pressure.

Water Transport in Xylem
Nutrition and
Transport in Plants
20
Water entering roots creates a positive pressure
(root pressure)
 Occurs primarily at night
 Tends to push xylem sap upward
 May be responsible for guttation
-
Water forced out vein endings along edges of leaves
Root pressure is not likely to be mechanism by which water rises to tops of very tall trees. Can only push water up to about 10m.

Guttation 21

Cohesion-Tension Model of Xylem Transport
Nutrition and
Transport in Plants
22
Passive mechanism of xylem transport
 Water molecules tend to cling together (cohesion)
- Due to hydrogen bonding between water molecules. Water forms a continuous column in xylem from roots to leaves.
 Polarity of water allows interaction with molecules of vessel walls (adhesion)
-
This gives the water column extra strength & prevents it from slipping back down the xylem.

Cohesion-Tension Model of Xylem Transport
Nutrition and
Transport in Plants
23
Water column primarily moves upward passively due to transpiration.
 Transpiration is the evaporation of water from leaf cells. 90% of water taken up by roots is lost by transpiration eventually
-
Water evaporates from mesophyll cells of leaves into intercellular spaces and then exits the leaf through stomata
-
H
2
0 molecules lost from mesophyll cells are replaced by H
2 veins.
0 molecules from xylem of leaf
-
This exerts a force, or tension , that draws the water column up in vessels from roots to leaves.

Cohesion-Tension Model of Xylem Transport
Nutrition and
Transport in Plants
24
Tension can reach from the leaves to the root only if the water column is continuous.
 If the water column is broken, as when you cut a stem, the water column “snaps back” making it more difficult for transport to occur.
-
This is why it is best to cut flowers under water.
-
When plants are under water stress, the stomata close.
-
Now the plant loses little water because of the waxy cuticle. However, plants can’t get CO
2 for the
Calvin Cycle of photosynthesis.
-
So plants need abundant water to be healthy.

Cohesion-tension Model of
Xylem Transport
25

Nutrition and
Transport in Plants
Opening and Closing of Stomata
26
Each stoma , or pore, in leaf epidermis is bordered by guard cells
 When water enters guard cells & turgor pressure increases, the stoma opens
 When water exits guard cells & turgor pressure decreases, the stoma closes
 Guard cells attached to each other at ends; inner walls thicker than outer walls.
 Cellulose microfibrils in walls prevent radial expansion but they can expand lengthwise.
 When this occurs the guard cells buckle outward and stoma opens.

Opening and Closing of Stomata 27

Nutrition and
Transport in Plants
Opening and Closing of Stomata
28
What causes water to end or leave guard cells?
 Potassium ions (K + ) accumulate within guard cells when stomata open
 Active transport of K + into guard cells causes water to follow by osmosis & stomata open.
 H + ions accumulate outside guard cells as K + moves into them.
 A proton pump run by the hydrolysis of ATP transports H + to outside of cell. This creates an electrochemical gradient that allows K + to enter by way of a channel protein.

Opening and Closing of Stomata 29

Nutrition and
Transport in Plants
Opening and Closing of Stomata
30
What regulates opening & closing of stomata?
 Blue-light component of sunlight is a signal that can cause stomata to open.
- A flavin pigment may absorb blue light & then sets in motion a response that activates the proton pump .
- Could also be due to a receptor that inactivates pump when CO
2 levels rise
- Abscisic acid (ABA) produced by wilting leaves also causes stomata to close

Nutrition and
Transport in Plants
Opening and Closing of Stomata
31
Circadian rhythms are behaviors that occur nearly every 24 hours
 Plants kept in dark open & close stomata every
24 hours.
- This means there is some sort of internal biological clock that is keeping time.

Nutrition and
Transport in Plants
Transport Mechanisms in Phloem
32
Phloem transports organic materials to any parts of plants that need them.
 This could be young leaves, flowers & fruits & roots
-
Girdling a tree
 Stripping bark from part of tree or leaving a metal band around the circumference for a long time
 Girdling of tree below the level of leaves causes bark to swell just above the cut
 Sugar will accumulate in the swollen tissue

Organic Nutrient Transport
Nutrition and
Transport in Plants
33
How to get samples of phloem sap to study?
 Difficult to get phloem sap from plants without injuring the phloem or having the phloem tubes get clogged up.
 Scientists use small insects called aphids .
- These have a long mouthpart called a stylet .
- They inject their stylet into phloem like a hypodermic needle.
- Can let the aphids attach to a plant & then cut their bodies off & use stylet as a funnel to collect phloem sap.

Excess phloem sap (honeydew) passing out of aphid’s body
Acquiring Phloem Sap 34
Cut aphid head off and leave stylus attached to plant to collect the sap

Nutrition and
Transport in Plants
Pressure-Flow Model of Phloem Transport
35
Positive pressure drives sap in sieve tubes
 Photosynthesizing leaves produce sugar. They are a source of sugar.

Sucrose is actively transported into sieve tubes of phloem. Sugar is carried across in conjunction with H + ions.
 Water follows passively by osmosis
 Increase in volume creates flow that moves water and sucrose to a sink
 Roots, etc. are sinks for sugar. They are removing sugar from phloem & using it in cell respiration.
 Sugar is actively transported OUT of sieve tubes, water exits passively and enters xylem. Water travels back up to the leaves.

Pressure-flow Model of Phloem Transport 36
1. Sugar actively transported into sieve tubes
2. Water follows by osmosis
3. Positive pressure causes phloem contents to flow from source to sink
4. At sink, sugar is actively transported out of sieve tubes
5. Some water returns to xylem, where it mixes with more water absorbed from soil
6. Xylem transports water to mesophyll of leaf
7. Most water is transpired, some is used for photosynthesis, and some reenters phloem by osmosis.