Chapter 37

Plant Nutrition

Nutrient Reservoirs Every organism

continually exchanges energy and materials with its environment

For plants…water and minerals come from the soil, while carbon dioxide comes from the air

The branching root system and shoot system of a vascular plant ensure extensive networking with both reservoirs of inorganic nutrients

Macronutrients and Micronutrients Plants derive most of their organic mass from the CO2 of air but they also depend on soil nutrients

More than 50 chemical elements have been identified among the inorganic substances in plants, but not all of these are essential

A chemical element is considered essential if it is required for a plant to complete a life cycle

How would you identify an essential nutrient? Hydroponic culture can be used to determine

which chemicals elements are essential

TECHNIQUE Plant roots are bathed in aerated solutions of known mineral composition. Aerating the water provides the roots with oxygen for cellular respiration. A particular mineral, such as potassium, can be omitted to test whether it is essential.

RESULTS If the omitted mineral is essential, mineral deficiency symptoms occur, such as stunted growth and discolored leaves. Deficiencies of different elements may have different symptoms, which can aid in diagnosing mineral deficiencies in soil.

Control: Solutioncontaining all minerals

Experimental: Solutionwithout potassium

APPLICATION In hydroponic culture, plants are grown in mineral solutions without soil. One use of hydroponic culture is to identify essential elements in plants.

Macronutrients and Micronutrients Nine of the essential elements are called macronutrients because plants require them in relatively large amounts C, O, H, N, K-Primary Ca, Mg, P, S -Secondary

The remaining eight essential elements are known as micronutrients because plants need them in very small amounts Cl, Fe, Zn, Mn, B, Cu, Mo

Primary Macronutrients

Nitrogen: Absorbed usually as NO3 or NH3

Essential for vegetable growth Deficiency causes Chlorosis

Phosphorous: Usually absorbed as PO4

Used in protein and nucleic acid production Deficiency causes purpling

Potassium: (K) introduced through inorganic salts

Maintains regular cell function Present in older plants moreso than younger—marginal firing

of leaves

Secondary Macronutrients

Calcium: (Ca2+) Essential to mitosis Deficiency causes malformed buds and no root growth

Magnesium: (Mg+2) Used in the creation of fats and sugars Deficiency causes yellowing between veins

Sulfur: Usually absorbed as sulfate (SO42-)

Used in formation of amino acids and taste of veg Deficiency causes chlorate foliage

Micronutrients

Boron: (B) Essential for mitosis Death of buds if deficient

Iron: (Fe) Component of chlorophyll Deficiency causes death of younger leaves

Manganese: (Mn+2) Used in synthesis of chlorophyll Deficiency leads similar to iron

Cont…

Zinc :Zn Enzyme activator Deficiency causes reduced leaf size

Copper: Cu2+

Chlorophyll synthesis Deficiency stunts plants and kills leaves

Chlorine: Cl Difficult to have deficiency Stunting and necrosis can occur from chlorine excess

Essential elements in plants

Mineral Deficiency

The symptoms of mineral deficiency Depend partly on the nutrient’s function Depend on the mobility of a nutrient within the plant

Deficiency of a mobile nutrient Usually affects older organs more than young ones (young tissue

can more efficiently draw minerals to it)

Deficiency of a less mobile nutrient Usually affects younger organs more than older ones (older tissue

has a store of minerals to fall back on when the mineral is in short supply)

Mineral Deficiency The most common deficiencies

Are those of nitrogen, potassium, and phosphorus

Phosphate-deficient

Healthy

Potassium-deficient

Nitrogen-deficient

“Firing”…drying along tips and margins of older leaves

Reddish-purple margins esp. on young leaves

Yellowing that starts at the tip and moves along the center of older leaves

Soil Characteristics Soil quality is a major determinant of plant distribution and

growth Along with climate

The major factors determining whether particular plants can grow well in a certain location are the texture and composition of the soil

Texture…is the soil’s general structure (sandy, clayey, etc) Composition…refers to the soil’s organic and inorganic

chemical components Various sizes of particles derived from the breakdown of rock are

found in soil along with organic material (humus) in various stages of decomposition

Topsoil… is the mixture of particles of rock and organic material

Soil Horizons The topsoil and other distinct soil layers, or

horizons are often visible in vertical profile where there is a road cut or deep hole

The A horizon is the topsoil, a mixture ofbroken-down rock of various textures, living organisms, and decaying organic matter.

The B horizon contains much less organicmatter than the A horizon and is lessweathered.

The C horizon, composed mainly of partiallybroken-down rock, serves as the “parent”material for the upper layers of soil.

A

B

C

Availability of Soil Water After a rainfall, water drains away from the larger

spaces of soil but smaller spaces retain water because of its attraction to surfaces of clay and other particles.

The film of loosely bound water is usually available to plants

Soil water. A plant cannot extract all the water in the soil because some of it is tightly held by hydrophilic soil particles. Water bound less tightly to soil particles can be absorbed by the root.

Soil particle surrounded byfilm of water

Root hair

Water available to plant

Air space

Cation Exchange Acids derived from roots contribute to a

plant’s uptake of minerals when H+ displaces mineral cations from clay particles

Cation exchange in soil. Hydrogen ions (H+) help make nutrients available by displacing positively charged minerals (cations such as Ca2+) that were bound tightly to the surface of negatively charged soil particles. Plants contribute H+ by secreting it from root hairsand also by cellular respiration, which releases CO2 into the soil solution, where it reacts with H2O to form carbonic acid (H2CO3). Dissociation of this acid adds H+ to the soil solution.

H2O + CO2 H2CO3 HCO3– +

Root hair

K+

Cu2+Ca2+

Mg2+K+

K+

H+

H+

Soil particle–

–– –

– – –––

Agriculture

Conventional agriculture In contrast to natural ecosystems agriculture depletes the

mineral content of the soil, taxes water reserves, and encourages erosion

Sustainable agriculture Is ecologically sound Is economically viable Is socially just Is humane.

Fertilizers

Commercially produced fertilizers contain minerals that are either mined or prepared by industrial processes

“Organic” fertilizers are composed of manure, fishmeal, or compost

Irrigation Is a huge drain on water resources when

used for farming in arid regions Can change the chemical makeup of soil

Salinization (salt buildup)

drip

Ditch…trench

sprinkler

Erosion Topsoil from thousands of acres of farmland

Is lost to water and wind erosion each year in the United States

Erosion on conventionally tilled field

The U.S. Soil Conservation Service reports that more than 4 million acres of cropland are being lost to erosion in this country every year. That's an area greater than the size of Connecticut. Our annual topsoil loss amounts to 7 billion tons. That is 60,000 pounds for each member of the population.

Prevention of topsoil loss Strip cropping: practice of growing field crops in narrow strips

either at right angles to the direction of the prevailing wind, or following the natural contours of the terrain to prevent wind and water erosion of the soil

Contour tillage (slows water runoff and erosion)

Prevention of topsoil loss Terraces

Cover Crops

Cover crop in an orchard

Cover crop in vegetable garden

Conservation tillage (Min-till)

A minimum tillage system may involve quicker and fewer passes

at a shallower depth

Soil Reclamation Some areas are unfit for agriculture

Because of contamination of soil or groundwater with toxic pollutants

Phytoremediation: is a biological, nondestructive technology that seeks to reclaim contaminated areas by using the ability of some plants to remove soil pollutants

Nitrogen

Nitrogen is often the mineral that has the greatest effect on plant growth

Plants require nitrogen as a component of proteins, nucleic acids, chlorophyll, and a host of other important organic molecules

Soil Bacteria and Nitrogen Availability Nitrogen-fixing bacteria convert atmospheric N2

to nitrogenous minerals that plants can absorb as a nitrogen source for organic synthesis

Atmosphere

N2

Soil

N2 N2

Nitrogen-fixingbacteria

Organicmaterial (humus)

NH3

(ammonia)

NH4+

(ammonium)

H+

(From soil)

NO3–

(nitrate)Nitrifyingbacteria

Denitrifyingbacteria

Root

NH4+

Soil

Atmosphere

Nitrate and nitrogenous

organiccompoundsexported in

xylem toshoot system

Ammonifyingbacteria

The Role of Bacteria in Symbiotic Nitrogen Fixation

Symbiotic relationships with nitrogen-fixing bacteria provide some plant species with a built-in source of fixed nitrogen

From an agricultural standpoint the most important and efficient symbioses between plants and nitrogen-fixing bacteria occur in the legume family (peas, beans, and other similar plants)

Root Nodules Along a legumes roots are

swellings called nodules composed of plant cells that have been “infected” by nitrogen-fixing Rhizobium bacteria

The bacteria of a nodule obtain sugar from the plant and supply the plant with fixed nitrogen

Each legume is associated with a particular strain of Rhizobium

Pea plant root. The bumps onthis pea plant root are nodules containing Rhizobium bacteria.The bacteria fix nitrogen and obtain photosynthetic productssupplied by the plant.

Nodules

Roots

Development of a soybean root nodule

Infectionthread

Rhizobiumbacteria

Dividing cellsin root cortex

Bacteroid

2 The bacteria penetrate the cortex within the Infection thread. Cells of the cortex and pericycle begin dividing, and vesicles containing the bacteria bud into cortical cells from the branching infection thread. This process results in the formation of bacteroids.

Bacteroid

Bacteroid

Developingroot nodule

Dividing cells in pericycle

Infectedroot hair

1

2

3

Nodulevasculartissue

4

3 Growth continues in the affected regions of the cortex and pericycle, and these two masses of dividing cells fuse, forming the nodule.

Roots emit chemical signals that attract Rhizobium bacteria. The bacteria then emit signals that stimulate root hairs to elongate and to form an infection thread by an invagination of the plasma membrane.

1

4 The nodule develops vascular tissue that supplies nutrients to the nodule and carries nitrogenous compounds into the vascular cylinder for distribution throughout the plant.

Symbiotic Nitrogen Fixation and Agriculture The agriculture benefits of symbiotic nitrogen fixation are the basis for crop rotation

In this practice a non-legume such as maize is planted one year, and the following year a legume is planted to restore the concentration of nitrogen in the soil

Mycorrhizae and Plant Nutrition Mycorrhizae: are modified roots consisting of mutualistic associations of fungi and roots

The fungus benefits from a steady supply of sugar donated by the host plant

In return, the fungus increases the surface area of water uptake and mineral absorption and supplies water and minerals to the host plant

Agricultural importance: Farmers and foresters often inoculate seeds with spores of mycorrhizal fungi to promote the formation of mycorrhizae

Ectomycorrhizae In ectomycorrhizae the mycelium of the fungus

forms a dense sheath over the surface of the root

a Ectomycorrhizae. The mantle of the fungal mycelium ensheathes the root. Fungal hyphae extend from the mantle into the soil, absorbing water and minerals, especially phosphate. Hyphae also extend into the extracellular spaces of the root cortex, providing extensive surface area for nutrient exchange between the fungus and its host plant.

Mantle(fungal sheath)

Epidermis Cortex Mantle(fungalsheath)

Endodermis

Fungalhyphaebetweencorticalcells (colorized SEM)

100 m(a)

Endomycorrhizae In endomycorrhizae the microscopic fungal

hyphae extend into the root

Epidermis Cortex

Fungalhyphae

Roothair

10 m

(LM, stained specimen)

Cortical cells

Endodermis

Vesicle

Casparianstrip

Arbuscules

2 Endomycorrhizae. No mantle forms around the root, but microscopic fungal hyphae extend into the root. Within the root cortex, the fungus makes extensive contact with the plant through branching of hyphae that form arbuscules, providing an enormous surface area for nutrient swapping. The hyphae penetrate the cell walls, but not the plasma membranes, of cells within the cortex.

(b)

Epiphytes, Parasitic Plants, and Carnivorous Plants

Some plants have nutritional adaptations that use other organisms in nonmutualistic ways

Staghorn fern, an epiphyte

EPIPHYTES

PARASITIC PLANTS

CARNIVOROUS PLANTS

Mistletoe, a photosynthetic parasite Dodder, a nonphotosynthetic parasite

Host’s phloem

Haustoria

Indian pipe, a nonphotosynthetic parasite

Venus’ flytrapPitcher plants Sundews

Dodder

Epiphytes use a host for support but do not extract nutrients from the host

Carnivorous plant movie

Improving the Protein Yield of Crops

Plant breeding research has resulted in new varieties of maize, wheat, and rice that are enriched in protein

Such research addresses the most widespread form of human malnutrition: protein deficiency

Many of the projects creating GMOs (genetically modified organisms) are aimed at protein enrichment of crops.

High lysine corn