Transcript Soil - McGraw-Hill Education

Plant Nutrition and Soils
Chapter 39
Soils
Soil is the highly weathered outer layer of the
Earth’s crust
-A mixture of sand, rocks, clay, silt, minerals
and microorganisms
The Earth’s crust includes about 92 naturally
occurring elements
-Most are found in the form of inorganic
compounds called minerals
2
Soils
Most roots are found in topsoil
-A mixture of mineral particles of varying
sizes, living organisms and humus
-Humus consists of partly decayed
organic material
3
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Partially
decomposed
organic matter
Well decomposed
organic matter
Minerals leaching
from rocks and
accumulating
from above
Weathered
bedrock
material
Leaf litter and plant life
Topsoil
Subsoil
Bedrock
4
Water and Mineral Availability
Only minerals dissolved in water in spaces
among soil particles are available
-Organic soil particles tend to have negative
charges, and so attract positive ions
-Therefore, active transport is needed to
move positive ions into root hairs
5
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1. Soil particles tend to
+
–
have a negative charge.
+
–
+
–
+
–
+ –
–
Soil particle
+
–
–
–
+
+
–
–
+
–
ATP
+
2. Positive ions are
attracted to soil particles.
+
+
3. Negative ions stay in
–
–
+
–
+
–
+
–
–
–
–
Root hair
+
+
solution surrounding
roots, creating a charge
gradient that tends to
“pull” positive ions out
off the root cells.
+
4. Active transport is
Water
required to acquire and
maintain K+ and other
positive ions in the root.
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Water and Mineral Availability
About half of the soil volume is occupied by
pores, which may be filled with air or water
-Some of this water is unavailable because
it drains immediately due to gravity
-However, water that is held in small pores
is readily available to plants
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Water and Mineral Availability
8
Soil Loss
If topsoil is lost, soil’s water-holding capacity
and nutrient content are adversely affected
-Drought and poor farming lead to wind
erosion of farmland in the 1930s
-The southwestern
Great Plains of the
US became known
as the “Dust Bowl”
9
Soil Loss
Measures to prevent erosion include:
-Intercropping = Mixing of crops in field
-Conservation tillage = Minimal or even
no-till approaches to farming
Measures to prevent fertilizer runoff include:
-Site-specific farming
-Integrated nutrient management
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Acidic and Saline Soils
Acidic soils release minerals, such as
aluminum, that are toxic to plants
Saline soils alter water potential, leading to a
loss of water and turgor in plants
-Draining marshland
in southern Iraq
resulted in a salty
desert
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Plant Nutrients
The major source of plant nutrition is
photosynthesis
-Fixation of atmospheric CO2 into simple
sugars, using the energy of the sun
-However, this is not enough for the
synthesis of all the molecules needed
by the plant
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Plant Nutrients
Plants require a number of inorganic nutrients
-Macronutrients: Used in relatively large
amounts
-Nine = C, O, H, N, K, Ca, Mg, P & S
-Micronutrients: Used in minute amounts
-Seven = Cl, Fe, Mn, Zn, B, Cu & Mo
A deficiency of any one can have severe
effects on plant growth
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Plant Nutrients
14
Healthy wheat plant
Chlorine-deficient plant
Copper-deficient plant
Zinc-deficient plant
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Plant Nutrients
Assessment of nutritional requirements
-Plant seedling is first grown in a complete
nutrient solution
-Seedling is then transplanted to a solution
lacking one suspected essential nutrient
-Growth of the seedling is monitored for
presence of abnormal symptoms
16
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Suspected
nutrient is
not essential
Transplant
Normal growth
Monitor growth
Complete
nutrient
solution
Solution lacking
one suspected
essential nutrient
Suspected
nutrient is
essential
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Abnormal growth
Plant Nutrients
Hydroponic cultures
-Plant is suspended in air, with the root
rotating through a nutrient bath
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Plant Nutrients
Food security, avoiding starvation, is a
global issue
-Food fortification is an active research area
-Focuses on ways to increase a plant’s
uptake and storage of minerals
-Using genetically-modified plants
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Special Nutritional Strategies
Plants need ammonia (NH3) to build proteins
-However, they lack the biochemical
pathways necessary to convert N2 to NH3
Symbiotic relationships have evolved
between plants and nitrogen-fixing bacteria
-Legumes form nodules that house the
bacterium Rhizobium
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Special Nutritional Strategies
21
Special Nutritional Strategies
Nitrogen fixation is the most energetically
expensive reaction known to occur in cells
-Nitrogenase requires 16 ATPs to break the
triple bonds in N2 to form two NH3 molecules
Rhizobium fixes nitrogen in exchange for
carbohydrates
-Nodule formation requires extensive
signaling between bacterium and legume
22
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Epidermal cell1. Pea roots produce flavonoids
(a group of molecules used for
Cortex cells
plant defense and for making
Flavonoids
reddish pigment, among
numerous other functions).
Root hair
The flavonoids are transported
Rhizobium
into the rhizobial cells.
2. Flavonoids signal rhizobia
to produce sugar-containing
compounds called Nod
(nodulation) factors.
Nod factors
Nod factors
3. Nod factors bind to the surface
of root hairs and signal the
root hair to grow so that it curls
around the rhizobia.
4. Rhizobia make an infection
Infection thread
thread that grows in the root
hair and moves into the cortex
of the root. The rhizobia take
control of cell division in the
cortex and pericycle cells of
the root (see chapter 35).
5. Rhizobia change shape and
Vesicles with
differentiating
bacteroids
Cell division
starts making
nodule
are now called bacteroids.
Bacteroids produce an
O2-binding heme group that
combines with a globin group
from the pea to make
leghemoglobin. Leghemoglobin
gives a pinkish tinge to the
nodule, and its function is
much like that of hemoglobin,
bringing O2 to the rapidly
respiring bacteroids, but
isolating O2 from nitrogenase.
Differentiated
bacteroids 6. Bacteroids produce
fixing nitrogen
nitrogenase and begin fixing
atmospheric nitrogen for the
plant’s use. In return, the plant
provides organic compounds.
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Mature nodule
Special Nutritional Strategies
Symbiotic associations with mycorrhizal fungi
are found in about 90% of vascular plants
-Substantially expand the surface area
available for nutrient uptake
-Enhance uptake of phosphorus and
micronutrients
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Carnivorous Plants
Often grow in acidic soils that lack nitrogen
-Trap and digest small animals, primarily
insects, to extract additional nutrients
-Have modified leaves adapted for
luring and trapping prey
-Prey is digested with enzymes
secreted from specialized glands
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Carnivorous Plants
Pitcher plants = Have pitcher-shaped leaves
with cavity filled with digestive fluid
Venus flytrap = When hairs are touched, the
two halves of the leaf snap together
Sundews = Glandular trichomes secrete both
sticky mucilage and digestive enzymes
Waterwheel = Uses trigger hairs and snaps
to capture and digest small aquatic animals
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27
Carnivorous Plants
The phylogenetic relationship is:
Pitcher plants are not
related to this clade
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Parasitic Plants
May be photosynthetic or non-photosynthetic
-At least 3,000 types of plants
Dodder (non-photosynthetic)
-Wraps around its host
-Relies on it for its nutritional needs
Indian pipe (non-photosynthetic)
-Hooks into host trees through mycorrhizae
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Parasitic Plants
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Carbon-Nitrogen Balance
The Intergovernmental Panel on Climate
Change (IPCC) has concluded that CO2 is
maybe at its highest level in 20 million years
-Associated with increased temperatures
The C-N ratio in a plant is important for the
health of the plant and the herbivore
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Carbon-Nitrogen Balance
Ribulose 1,5-bisphosphate
carboxylase/oxygenase (rubisco) catalyzes
the first step of the Calvin cycle
-Can bind CO2 or O2
-If CO2 binds, a 3-C sugar is made, that
can is used to make glucose and sucrose
-If O2 binds, photorespiration occurs
-Neither nutrient nor energy storage
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Photorespiration (no sugars)
O2
CO2
CO2
Rubisco
Rubisco
CO2
Ribulose 1,5-bisphosphate
Calvin
Cycle
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Glucose and other sugars
Carbon-Nitrogen Balance
C3 photosynthesis occurs in mesophyll cells
C4 photosynthesis uses an extra pathway to
shuttle carbon deep within the leaf
-This reduces photorespiration by limiting
the Calvin cycle to cells surrounding the
vascular tissue where O2 levels are low
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Vascular tissue
CO2
Mesophyll
cell
RuBP
Calvin
Cycle
Glucose
3PG
(C3)
Sucrose (in phloem)
a. C3 leaf
CO2
Mesophyll
cell C4 pathway
Bundle sheath
Vascular tissue
Bundle CO2
sheath
Calvin
cell
Cycle
Glucose
Sucrose (in phloem)
b. C4 leaf (Kranz anatomy)
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Carbon-Nitrogen Balance
In C3 plants, as CO2 increases, the Calvin
cycle becomes more efficient
-Increased photosynthesis and plant growth
-However, the plants have less nitrogen
and minerals per unit mass
-Resulting in lower nutritional value
for herbivores
-More plant matter must be eaten
to obtain same amount of nutrients
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Carbon-Nitrogen Balance
One possible outcome of increased CO2 in atmosphere:
Photosynthesis
CO2
Plant growth
Relative levels of
protein and
minerals
in plant tissue
Herbivory
Human
nutrition
As ambient temperature increases, respiration
rate will also increase
-Result is more changes in plant nutrient
balance
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Phytoremediation
Phytoremediation is the use of plants to
concentrate or breakdown pollutants
-Phytodegradation: Contaminant is taken
up from soil and broken down
-Phytovolatilization: Contaminant is taken
up from soil and released through stomata
-Phytoaccumulation: Contaminant is taken
up from soil and concentrated in shoots
-These are later harvested
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Phytoremediation
Trichloroethylene (TCE) may be removed
from the soil by poplar trees
-Degraded into CO2 and chlorine
-A fraction moves rapidly through the xylem
and is released through stomata
Trinitrotoluene (TNT) may be removed from
soil and degraded by poplar and bean plants
-But at high concentrations, it is toxic to
these plants
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Phytoremediation
Heavy metals, including cadmium and lead,
are toxic to animals in even small quantities
-400 plant species have the ability to
hyperaccumulate toxic metals from soil
-However, a concern is that animals
eating these plants will be exposed to
high concentrations of toxic compounds
40
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Phytovolatilization
Phytodegradation
Phytoaccumulation
a.
TCE
CO2
b.
Lead
Not
degraded
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Phytoremediation
Phytoremediation is a promising technique
-Costs 50-80% lower than cleanup methods
involving mechanical removal of
contaminated soil
An illustrative example comes from the 1998
accident at the Aznalcóllar mine in Spain
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Dike of a holding lagoon
for mine waste broke
Large amounts of
sludge were removed
mechanically
Hyperaccumulating
native plant species are
now populating the area
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