Transcript notes

CHAPTER 35
Plant Nutrition
Peter J. Russell • Paul E. Hertz • Beverly McMillan
www.cengage.com/biology/russell
Spanish Moss, an Epiphyte
Why It Matters…
• Tropical rainforests are among the most biologically diverse
ecosystems on Earth, containing countless thousands of
species of animals, fungi, protists, prokaryotes, and plants
• Tropical rainforests are demanding places for plants to survive
– the soil is chronically deficient in nutrients due to incessant
rain and high acidity
• In acid soil, minerals vital to plant metabolism (potassium,
calcium, magnesium, phosphorus) are subject to leaching –
“acid rain” exacerbates the leaching problem
Tropical Rain Forest
Figure 35-1 p794
35.1 Plant Nutritional Requirements
• The tissues of most plants are more than 90% water
• By growing plants in hydroponic culture, Julius von Sachs
deduced a list of six essential plant nutrients: nitrogen,
potassium, calcium, magnesium, phosphorus, and sulfur
• Many more essential plant nutrients were identified using a
modern hydroponic apparatus, in which the nutrient solution
is refreshed regularly, and air is bubbled into it to supply
oxygen to the roots
Research Method: Hydroponic Culture
A. Basic components of a hydroponic apparatus
Plant support
Air pumped
into bubbling
system
Nutrient
solution
B. Procedure for identifying elements essential
for proper plant nutrition
Plant thrives;
test element
may not be
essential
Transplantation
or
Lettuce plant
growing in complete
nutrient solution
Solution lacking
one element
Plant grows
abnormally;
test element
may be
essential
Figure 35-2 p795
Essential Nutrients
• An essential element is necessary for normal growth and
reproduction, cannot be functionally replaced by a different
element, and has one or more roles in plant metabolism
• With enough sunlight and 17 essential elements, plants can
synthesize all the compounds they need
• Nine essential elements are macronutrients that plants
incorporate into their tissues in relatively large amounts
• The rest are micronutrients, required only in trace amounts
Macronutrients
• Carbon, hydrogen, and oxygen, the main components of lipids
and carbohydrates, make up 96% of a plant’s dry mass
• Nitrogen is essential to proteins and nucleic acids
• Phosphorus is used in nucleic acids, ATP, and phospholipids
• Potassium functions include enzyme activation and
mechanisms that control the opening and closing of stomata
• Calcium, sulfur, and magnesium are also macronutrients
Micronutrients
• The essential micronutrients include copper, chlorine, nickel,
iron, boron, manganese, zinc, and molybdenum
• Some species of plants may require additional micronutrients:
• Many C4 plants require sodium
• A few plant species require selenium
• Horsetails and some grasses (such as wheat) require silicon
Minerals
• All essential elements (except carbon, oxygen, and hydrogen)
are minerals – elements or compounds that are formed by
geological processes and have a crystalline structure
• Minerals are available to plants through the soil as ions
dissolved in water – most are derived from the weathering of
rocks and inorganic particles in the Earth’s crust
• Many act as cofactors or coenzymes in protein synthesis,
starch synthesis, photosynthesis, and aerobic respiration
Essential Plant Nutrients
DO NOT MEMORIZE 
Figure 35-2a p795
Essential Plant Nutrients
DO NOT MEMORIZE 
Figure 35-2b p795
Nutrient Deficiencies
• The nutrient content of soils is an important factor in
determining which plants will grow well in a given location
• Plants differ in the quantity of each nutrient they require – an
adequate amount for one plant may be harmful to another
• Plants that are deficient in one or more essential elements
develop characteristic symptoms such as stunted growth,
abnormal leaf color, dead spots, or abnormal stems
35.2 Soil
• Soil anchors plant roots and is the main source of inorganic
nutrients
• Soil is the source of water for most plants, and of oxygen for
respiration in root cells
• The physical texture of soil is a factor in whether root systems
have access to sufficient water and dissolved oxygen
• Physical and chemical properties of soils have a major impact
on the ability of plants to grow, survive, and reproduce
Properties of a Soil
• Soil is a complex mix of mineral particles, chemical
compounds, ions, decomposing organic matter, air, water,
and assorted living organisms
• Soils develop from physical or chemical weathering of rock
• Soil particles range in size from sand (2.0–0.02 mm) to silt
(0.02–0.002 mm) to clay (diameter less than 0.002 mm)
Properties of a Soil (cont.)
• Organic components of soil include humus – decomposing
parts of plants and animals, animal droppings, and other
organic matter
• Humus has a loose texture and retains water well – organic
molecules in humus are reservoirs of nutrients, including
nitrogen, phosphorus, and sulfur
• Well-aerated soils that contain roughly equal proportions of
humus, sand, silt, and clay are loams – the soils in which most
plants do best
Influences of Living Organisms
• A square meter of fertile soil contains trillions of bacteria,
hundreds of millions of fungi, several million nematodes,
and other worms and insects
• Bacteria and fungi decompose dead plant parts and other
organic matter, and earthworms aerate the soil
• Plant roots and other tissues help shape the characteristics
and composition of soil, including the abundance of soildwelling organisms
Water Availability
• Gravity pulls water down through spaces between soil
particles into deeper soil layers
• The soil solution (water and dissolved substances) coats soil
particles and partially fills pore spaces
• Clay particles and organic components in soil (especially
proteins) bear negatively charged ions on their surfaces that
attract polar water molecules, which form hydrogen bonds
with the soil particles
Soil Solution
Water film
around
soil particles
Clay
particle
Air space in
soil (pore)
Sand
particle
Figure 35-5 p799
Water Availability (cont.)
• The amount of water in soil solution depends largely on the
amount and pattern of precipitation (rain or snow) in a region
• Water available to plants depends on soil composition:
• Sandy soil has large air spaces, so water drains rapidly
• Humus-rich soil contains air spaces and holds water
• Clay has few air spaces and tends to hold water tightly
• Good agricultural soils tend to be sandy or silty loams, which
contain a mix of humus and coarse and fine particles
Water Potential
• Differences in water potential govern the osmotic movement
of water through root hairs into plant roots
• Soil solution usually has fewer dissolved solutes than water in
root cells – water tends to move from wet soil (higher water
potential) into roots (lower water potential)
• Plants in deserts or salty soils have adaptations that allow
roots to absorb water under unfavorable conditions
• Water potential in clay soils is lower than in other soil types,
even when clay is wet
Mineral Availability
• Mineral nutrients enter roots as cations (positively charged
ions) or anions (negatively charged ions)
• Although both cations and anions are present in soil solutions,
they are not equally available to plants
• Cations (Mg2+, Ca2+, K+) are reversibly bound by net negative
charges on the surfaces of soil particles (adsorption), and
can’t easily enter roots
Mineral Availability (cont.)
• Roots acquire cations through cation exchange, in which one
cation (usually H+) replaces a soil cation
• Hydrogen ions come from two main sources:
• CO2 from root cells dissolves in soil solution, yielding
carbonic acid (H2CO3), which is ionized to HCO3– + H+
• Reactions involving organic acids inside roots also produce
H+, which is excreted
• H+ in the soil solution displaces adsorbed mineral cations
attached to clay and humus, freeing them to move into roots
Cation Exchange
A. Adsorption of cations to a
clay particle
B. Adding gypsum to the soil
Root hair
Clay
particle
Figure 35-6 p800
Mineral Availability (cont.)
• Anions in the soil solution (NO3–, SO42–, PO4–) are only weakly
bound to soil particle – they move freely into root hairs
• However, because anions are so weakly bound compared with
cations, they are more subject to loss by leaching
• pH of soil also affects the availability of some mineral ions
(e.g. formation of calcium phosphate in alkaline soil):
• In areas with heavy rainfall, soils tend to become acidic
• In arid regions, soil is often alkaline
Mineral Availability (cont.)
• The mineral ions that plants take up from the soil must be
replenished naturally or artificially
• Over the long run, some mineral nutrients enter soil from
ongoing weathering of rocks – in the short term, minerals,
carbon, and other nutrients are returned to the soil by the
decomposition of organisms and their wastes
• Nitrogen and phosphorus also enter soil in agricultural
chemicals (fertilizers)
35.3 Obtaining and Absorbing Nutrients
• In natural habitats, wide variations in soil minerals, humus,
pH, the presence of other organisms, and other factors
influence the availability of essential elements
• Roots continue to branch and grow as long as a plant lives –
extensive root systems allow plants to locate nutrients in their
immediate environment
• Root hairs and ion-specific transport proteins are two major
adaptation for the uptake of mineral ions and water
Root Secretions
• Roots of various plant species release organic compounds into
soil – including carbohydrates, amino acids, organic and fatty
acids, enzymes and other proteins
• These “root exudates” may improve a plant’s access to
particular nutrients
• Example: Roots of A. thaliana secrete organic compounds that
help determine which species of symbiotic soil fungi live near
the roots
Nutrients Move by Several Routes
• Some mineral ions enter root cells immediately – others travel
in the apoplast and are actively transported into endodermal
cells and xylem for transport throughout the plant
• Inside cells, most mineral ions enter vacuoles or remain in the
cytoplasm where they are immediately available for metabolic
reactions
• Some nutrients, such as nitrogen-containing ions, move in
phloem from site to site in the plant as dictated by growth
and seasonal needs
Mycorrhizae
• Mycorrhizae are symbiotic associations between a fungus and
the roots of a plant that promote the uptake of water and
ions (especially phosphate)
• Hyphal filaments of the fungal partner grow around or into
the plant’s roots, and provide a huge surface area for
absorption – aided by hyphal transport proteins
• Some of the plant’s sugars and nitrogenous compounds
nourish the fungus and, as the root grows, it uses some of the
minerals obtained by the fungus
Hyphae in a Leek Root
Hyphae
p803
Nitrogen: A Limiting Factor
• Lack of nitrogen is the single most common limit to plant
growth – there usually is not enough nitrogen available in
usable (ionic) forms to meet plants’ ongoing needs
• Plants can’t extract gaseous nitrogen (N2) from air because
they lack the enzyme necessary to break apart the three
covalent bonds in the N2 molecule (N=N)
• Plants can absorb nitrogen from the soil in the form of nitrate
(NO3–) or ammonium (NH4+), which are produced by bacteria
as part of the nitrogen cycle
Nitrogen Fixation
• The incorporation of atmospheric nitrogen into compounds
that plants can take up is called nitrogen fixation
• Nitrogen-fixing bacteria in soil add hydrogen to N2, producing
two molecules of NH3 (ammonia) and one H2 (requires ATP,
catalyzed by nitrogenase)
• H2O and NH3 react, forming NH4+ (ammonium) and OH–
Ammonification and Nitrification
• Ammonification produces ammonium (NH4+) when
ammonifying bacteria break down decaying organic matter
• Nitrification, in which NH4+ is oxidized to nitrate (NO3–), is
carried out by nitrifying bacteria
• Because of ongoing nitrification, nitrate is more abundant
than ammonium in most soils
• Plant roots may take up ammonium directly in highly acidic
soils (e.g. bogs) where low pH is toxic to nitrifying bacteria
Nitrogen Assimilation
• In root cells, absorbed NO3– is converted by a multistep
process back to NH4+, which is used to synthesize organic
molecules such as amino acids
• These molecules pass into the xylem, which transports them
throughout the plant
• In some plants, nitrogen-rich precursors travel in xylem to
leaves, where different organic molecules are synthesized,
then travel to other plant cells in the phloem
How Plants Obtain Nitrogen from Soil
Atmospheric
nitrogen (N2)
Decaying
organic matter
Nitrogen-fixing
bacteria convert
N2 to ammonia
(NH3) which
dissolves to
form ammonium
(NH4+)
Ammonium
(NH4+)
Ammonifying
bacteria
NO3– converted to
NH4, which is moved
via xylem to the
shoot system
Nitrifying bacteria
Nitrate (NO3–)
Figure 35-7 p804
Plant-Bacteria Associations
• Most nitrogen is fixed by the nitrogen-fixing bacteria
Rhizobium and Bradyrhizobium, which form mutualistic
associations with roots of legumes
• The host plant supplies bacteria with organic molecules for
cellular respiration – the bacteria supply the plant with NH4+
used to produce proteins and other nitrogenous molecules
• In legumes, nitrogen-fixing bacteria reside in root nodules,
localized swellings on roots
Plant-Bacteria Associations (cont.)
• Usually, a single species of nitrogen-fixing bacteria colonizes a
single legume species, drawn by chemical attractants
(flavonoids) that the roots secrete
• Example: Soybean plant and Bradyrhizobium japonicum
• Flavonoid is released by soybean roots, bacterial nod
genes are expressed, products trigger release of bacterial
enzymes that break down root hair cell wall
• Bacteria enter the cell, plasma membrane forms an
infection thread, bacteria invade cortex cells
• Bacteroids stimulate nodule development
Root Nodules
Root nodule
Figure 35-8a p805
Experiment: Soybeans and Rhizobium
Figure 35-8b p805
Bacteroids
Figure 35-8c p805
Root Nodule Formation in Legumes
A. Root signal and bacterial response
Soil particles
Root hair
B. Bacterial signal and root
response
Effects of
the nod
gene
Bacteria
Flavonoid
Root
Secreted
from root
Root cortex
Bacterial nod hair
genes expressed
C. Integration of bacteria
D. Micrograph of a
developing root nodule
Infection
thread
Swelling
bacteroid in
cortex cell
Root nodule
Infection
thread
Stepped Art
Figure 35-9 p806
Plant-Bacteria Associations (cont.)
• Inside bacteroids, N2 is reduced to NH4+ using ATP produced
by cellular respiration (catalyzed by nitrogenase)
• Toxic NH4+ is moved out of bacteroids into surrounding nodule
cells and converted to other compounds
• Nod genes stimulate nodule cells to produce leghemoglobin,
which carries oxygen from the cell surface to bacteroids
• Leghemoglobin delivers just enough oxygen to maintain
bacteroid respiration without shutting down nitrogenase
Unusual Modes of Nutrition
• “Carnivorous” plants survive in nitrogen-deficient
environments such as bogs and sand through elaborate
mechanisms for extracellular digestion and absorption
• Venus flytrap and sundews capture and digest insects
• Cobra lily and tropical pitcher plants form a “pitcher” with
downward-pointing leaf hairs and a slick coating that speeds
larger prey into a pool of digestive enzymes