L28 Chapter 37
Plant Nutrition
Plant Nutrition
The __________ of Nutrients
Mineral Nutrients Essential to Plants
Soils and Plants
Nitrogen Fixation
Carnivorous and __________ Plants
The Acquisition of Nutrients
All living things need raw materials from the environment.
These nutrients include carbon, hydrogen, oxygen, and nitrogen.
Carbon comes from photosynthetic organisms or from CO2 in the air.
Hydrogen comes from water.
Carbon, oxygen, and hydrogen are plentiful, and enter the living world through photosynthesis.
The Acquisition of Nutrients
Nitrogen is in relatively __________ supply for plants.
Nitrogen enters living forms first in bacteria, which can convert N2 in air to forms that are useful to plants.
Other mineral nutrients essential for life include sulfur, phosphorus, potassium, magnesium, and iron.
Plants take up most nutrients as __________ solutes in the water of the soil, the soil solution.
The Acquisition of Nutrients
Plants are autotrophs. They make their own organic molecules from CO2, H2O, and minerals.
Heterotrophs require organic compounds as food and depend ultimately on autotrophs.
Most autotrophs photosynthesize.
A few autotrophs derive energy from reduced inorganic compounds, such as H2S. They are called chemosynthesizers.
All __________ are prokaryotes.
The Acquisition of Nutrients
Plants are sessile organisms. Nutrients and energy must be brought to them in some way.
A plant can extend itself by growing. The roots obtain most of the mineral nutrients needed.
Microenvironments in the soil may encourage or discourage the proliferation of a root system.
Growth of leaves helps the plant obtain and light and CO2.
Mineral Nutrients Essential to Plants
Some elements are essential to plants. Essential elements:
Are necessary for normal growth and development.
Can not be replaced by another element.
Are required directly.
Macronutrients are needed in concentrations of at least 1 g per kg of dry plant matter.
Micronutrients are those needed in concentrations of less than 100 mg per kg of dry matter.
Mineral Nutrients Essential to Plants
Plants that are deficient in a particular essential element show characteristic deficiency symptoms.
These symptoms can be used to determine which elements are lacking.
Appropriate fertilizers can be applied after diagnosing the specific deficiencies.
A fertilizer is an added source of mineral nutrients.
Mineral Nutrients Essential to Plants
Plants in natural environments are almost always deficient in nitrogen, but they seldom display deficiency symptoms.
The growth of these plants slows to match the available supply of nitrogen.
Crop plants, which have been bred to grow quickly, show symptoms when nitrogen is low.
Chlorosis, or yellowing of the leaves, is a visible symptom of nitrogen deficiency. Without nitrogen, chlorophyll can not be made, and the __________ carotenoid pigments become visible.
Mineral Nutrients Essential to Plants
Inadequate iron also causes chlorosis, but it tends to affect the youngest leaves.
Nitrogen can be readily translocated in the plant, but iron is more difficult to translocate.
Plants with insufficient nitrogen tend to move nitrogen from older leaves to younger leaves to favor their growth.
Plants with insufficient iron cannot move it to the younger leaves, where it is needed for chlorophyll synthesis.
Mineral Nutrients Essential to Plants
Essential elements often have several different roles in plant cells.
Magnesium is part of the chlorophyll molecule, and also acts as a cofactor for many enzymes in metabolic pathways.
Phosphorus is in phospholipids, nucleic acids, and energy transfer molecules such as ATP. Addition or removal of phosphate groups is used to activate many enzymes.
Calcium affects membranes and cytoskeletal activity and functions in the processing of hormonal and environmental cues.
Mineral Nutrients Essential to Plants
An element is considered to be essential if the plant fails to complete its life cycle or grows abnormally without it.
The essential elements for plants were identified by growing plants hydroponically, or without soil.
The essential __________ were more difficult to determine because they are required in such extremely small amounts.
Soils and Plants
Soil provides the minerals plants need.
Soil also provides water, mechanical support, microorganisms, and oxygen for roots.
Soils may also contain organisms that are harmful to plants.
Soils and Plants
Soils are complex mixtures of living and nonliving components, including bacteria, fungi, earthworms and other animals, particles of rock, clay, water, dissolved minerals, air spaces, and dead organic matter.
The inorganic component exists in three size classes: sand, silt, and clay (< 2 m in diameter).
The air spaces are a crucial source of oxygen to plant roots.
Soils and Plants
The structure of many soils changes with depth, revealing a soil profile.
Most soils have two or more horizontal layers, called horizons.
Minerals tend to leach, or be carried away by water from the upper horizons, and sink into deeper horizons.
Soil scientists recognize three major horizons:
A, the topsoil
B, the subsoil
C, the parent rock
Soils and Plants
Most organic matter is in the topsoil, as are roots, earthworms, insects, nematodes, and microorganisms.
Agriculture depends on adequate topsoil.
The best topsoil is loam, a mixture of clay, sand, and silt in equal proportions.
The B horizon or subsoil is the zone of accumulation of leached materials from above.
The C horizon is the parent rock from which soil is derived.
Soils and Plants
The type of soil in a given area depends on the type of rock from which it forms, the climate, landscape features, organisms living on it, and time.
Rocks are broken down by mechanical weathering, or physical breakdown; and chemical weathering.
Soils and Plants
The amount and type of __________ particles, which bind mineral nutrients, determines many physical and chemical properties of soils.
Several types of chemical weathering are required to produce clay:
Oxidation by atmospheric oxygen makes some essential elements more available to plants.
Reaction with water (hydrolysis) releases some mineral nutrients from the rock.
Acids, particularly carbonic acid, free some essential elements from their parent salts.
Soils and Plants
The availability of mineral nutrients to plant roots depends on the presence of clay particles in the soil.
The weathering processes leave clay particles with many negatively charged groups.
Clay particles bind the cations needed by plants, such as Mg2+, Ca2+, and K+.
To be accessible to plants, cations must disassociate from the clay.
Soils and Plants
Protons (H+) are released by roots; they also release CO2 through respiration.
CO2 dissolves in water to form carbonic acid, which then dissociates to bicarbonate ions and free protons.
CO2 + H2O H2CO3 H+ + HCO3
The protons bind to the clay particles more strongly than the mineral cations, so they trade places with the cations. This is called ion exchange.
The cations in solution can then be taken up by the plant.
Soils and Plants
The capacity of soil to support plant growth, called soil fertility, is partly dependent on ion exchange.
Clay particles hold cations and so they are retained in the A horizon.
Nitrogen, sulfur, and phosphorus are found as anions and there is no comparable mechanism for holding them. They tend to be leached rapidly.
The main reservoir for nitrogen in the topsoil is in the organic matter.
Soils and Plants
Agricultural soils often require fertilizer because irrigation and rainwater leach minerals from the soil, and the crop harvest removes nutrients.
Three elements commonly used in fertilizers are nitrogen, phosphorus, and potassium.
N-P-K percentages are often labeled on fertilizer bags. A 5-10-10 fertilizer has 5% nitrogen, 10% phosphate (P2O5), and 10% potash (K2O) by weight.
Soils and Plants
Organic fertilizers, such as manure, release nutrients slowly, which results in less leaching than occurs with inorganic fertilizers.
Organic fertilizers also contain residues of plant or animal materials that help improve the structure of the soil.
Inorganic fertilizers provide an immediate supply of plant nutrients, and can be formulated to the requirements of a particular crop or soil type.
Soils and Plants
The availability of nutrient ions is influenced by soil pH. pH 6.5 is optimal for most crops.
In the process of liming, compounds such as calcium carbonate, calcium hydroxide, or magnesium carbonate are added to acidic soil to raise the pH.
The pH of soil can be lowered by adding sulfur, which soil bacteria convert to sulfuric acid.
Soils and Plants
Nutrients can also be added directly to leaves.
Plants take up more copper, iron, and manganese when they are applied as a foliar (leaf) spray.
Soils and Plants
Plants also have an effect on soils.
Types of plants affect the type of soil that forms in a particular place.
Plant litter is a major source of humus, or organic matter, in the soil.
Soil bacteria and fungi produce humus by breaking down plant litter, animal feces, and other organic materials.
Humus is rich in minerals, especially nitrogen. Its texture helps to provide roots with oxygen.
Soils and Plants
Plants influence the soil pH by the absorption of cations and anions.
If they absorb more cations than anions, they excrete H+ ions and lower the soil pH.
If they absorb more anions than cations, they excrete OH or HCO3 ions and raise the soil pH.
Nitrogen Fixation
Earths atmosphere is about 78% nitrogen in the form of N2 gas.
N2 is very stable. A great deal of energy is required to break the triple bond.
Some bacteria have an enzyme that allows them to break the bond and convert N2 into a more usable form, NH3.
The process is called nitrogen fixation.
There are only a few species of these essential nitrogen fixers.
Nitrogen Fixation
Most nitrogen fixation is done by bacteria.
Humans fix nitrogen industrially, and a small amount is fixed by lightning, forest fires, and volcanoes.
Cyanobacteria are the principle nitrogen fixers in aquatic ecosystems.
Some nitrogen fixers live in close association with plant roots in a mutualistic relationship.
Nitrogen __________
Rhizobium fix nitrogen only in close association with the roots of legumes.
These bacteria infect plant roots, causing the roots to develop nodules.
Farmers coat legume seeds with specific species of Rhizobium.
Nitrogen Fixation
Some cyanobacteria fix nitrogen in association with fungi as lichens; or with ferns, cycads, and nontracheophytes.
Rice farmers increase fixed nitrogen by growing the water fern Azolla in their rice paddies.
A species of actinomycete fixes nitrogen in association with root nodules on woody species such as alder and mountain lilacs.
Nitrogen Fixation
Nitrogen fixation is the reduction of nitrogen gas by the stepwise addition of three pairs of hydrogen atoms to the N2.
Three things are required for fixation:
A strong reducing agent to transfer the hydrogen atoms to N2.
Energy, supplied by ATP.
The enzyme nitrogenase.
Nitrogen Fixation
Nitrogenase is strongly inhibited by O2.
Many nitrogen fixers are __________. But others, such as Rhizobium, are not.
In root nodules, low O2 levels are maintained that can support respiration, but not inhibit nitrogenase activity.
The protein leghemoglobin produced by nodule cells carries O2 to the bacteroids.
Nitrogen Fixation
Neither free-living Rhizobium nor uninfected legumes can fix nitrogen.
Establishment of this symbiosis requires a complex series of steps.
The root attracts Rhizobium with chemicals called flavonoids.
These trigger bacterial transcription of nod genes, which are translated into Nod factors that influence nodule growth.
Within the nodule the bacteria take the form of bacteroids.
Bacteroids are swollen, deformed bacteria that can fix nitrogen; in effect, nitrogen-fixing organelles.
Nitrogen Fixation
Another type of symbiosis in which plants depend on another organism for their nutrition is that of mycorrhizae, the root-fungus association.
There is growing evidence that nodule formation depends on some of the same genes and mechanisms that allow mycorrhizae to develop.
Nitrogen Fixation
More nitrogen is needed today by agriculture than is available from bacterial nitrogen fixation.
The Haber process is currently being used by industry, but it is energy intensive.
Nitrogen-containing fertilizers require larger energy investments than any other aspect of modern agriculture.
Some scientists are trying to genetically engineer plants to fix their own nitrogen.
Currently there are attempts to transfer genes from Rhizobium into bacteria that already live in the roots of cereal plants.
Nitrogen Fixation
The nitrogen cycle includes the process of nitrogen fixation, nitrification, nitrate reduction, and denitrification.
Nitrogen fixers in the soil release NH3 and NH4+. Ammonia is toxic to plants, but some ammonium ions can be taken up at low concentrations.
Soil bacteria called nitrifiers oxidize NH3 to nitrite (NO2) and nitrate ions (NO3) in a process called nitrification.
Nitrogen Fixation
Nitrate __________ is carried out by plants using their own enzymes, and reduces nitrate back to ammonia. The ammonia is used to produce amino acids.
Animals can not reduce nitrate, and depend on plants for reduced nitrogen compounds.
Bacteria called denitrifiers return the nitrogen from animal wastes and dead organisms back to N2 gas in a process called denitrification.
Carnivorous and Heterotrophic Plants
Some plants that grow in acidic, nitrogen-poor environments trap and digest __________ to help augment nitrogen and phosphorus supplies.
These carnivorous plants include sundews, Venus flytraps, and pitcher plants.
These plants have adaptations to capture small animals and digest the proteins.
Carnivorous plants can survive without __________ on insects, but they grow much faster in their natural habitats when they succeed in capturing insects.
Carnivorous and Heterotrophic Plants
Some plants are heterotrophic parasites that get nutrients directly from other plants.
Mistletoes are green and photosynthesize, but get water and mineral nutrients from other plants.
Dodder is another parasitic plant, which obtains all its food via absorptive organs called haustoria which invade the hosts vascular tissue.
Indian pipe was once thought to be saprobic, but is now known to get its nutrients from living plants.
Parasitic plants can be serious problems for commercial crops and timber.
Animation 37.1 Nitrogen and Iron Deficiencies
Video 37.1 The bladderwort, Utriculari intermedia, traps mosquito larvae
Video 37.2 The long-leaf sundew, Drosera anglica, traps a horsefly
Video 37.3 The Venus flytrap, Dionaea