PLANT HORMONES, NUTRITION, AND TRANSPORT
A hormone
is any chemical produced in one part of the body that has a target elsewhere in
the body. Plants have five classes of hormones. Animals, especially chordates,
have a much larger number. Hormones and enzymes serve as control
chemicals in multicellular organisms. One important aspect of this is the
obtaining of food and/or nutrients
Auxins
Auxins promote stem elongation, inhibit growth of
lateral buds (maintains apical dominance). They are produced in the stem, buds,
and root tips. Example: Indole Acetic Acid (IA). Auxin is a plant hormone
produced in the stem tip that promotes cell elongation. Auxin moves to the
darker side of the plant, causing the cells there to grow larger than
corresponding cells on the lighter side of the plant. This produces a curving
of the plant stem tip toward the light, a plant movement known as
phototropism.
Auxin also
plays a role in maintaining apical dominance. Most plants have lateral
(sometimes called axillary) buds located at nodes (where leaves
attach to the stem). Buds are embryonic meristems maintained in a dormant state. Auxin maintains
this dormancy. As long as sufficient auxin is produced by the apical meristem,
the lateral buds remain dormant. If the apex of the shoot is removed (by a
browsing animal or a scientist), the auxin is no longer produced. This will
cause the lateral buds to break their dormancy and begin to grow. In effect,
the plant becomes bushier. When a gardener trims a hedge, they are applying
apical dominance.
Gibberellins
Gibberellins promote stem elongation. They are not produced
in stem tip. Gibberellic acid was the first of this class of hormone to be
discovered.
Cytokinins
Cytokinins promote cell division. They are produced in growing
areas, such as meristems at tip of the shoot. Zeatin is a hormone in this
class, and occurs in corn (Zea ).
Abscisic Acid
Abscisic Acid promotes seed dormancy by inhibiting cell
growth. It is also involved in opening and closing of stomata as leaves wilt.
Ethylene
Ethylene is a gas produced by ripe fruits. Why does one
bad apple spoil the whole bunch? Ethylene is used to ripen crops at the same
time. Sprayed on a field it will cause all fruits to ripen at the same time so
they can be harvested.
Plant Nutrition
Unlike animals
(which obrtain their food from what they eat) plants obtain their nutrition
from the soil and atmosphere. Using sunlight as an energy source, plants are
capable of making all the organic macromolecules they need by modifications of
the sugars they form by photosynthesis. However, plants must take up various minerals
through their root systems for use.
A
(plant) balanced diet
Carbon,
Hydrogen, and Oxygen are considered the essential elements. Nitrogen,
Potassium, and Phosphorous are obtained from the soil and are the primary
macronutrients. Calcium, Magnesium, and Sulfur are the secondary
macronutrients needed in lesser quantity. The micronutrients, needed
in very small quantities and toxic in large quantities, include Iron,
Manganese, Copper, Zinc, Boron, and Chlorine. A complete fertilizer provides
all three primary macronutrients and some of the secondary and micronutrients.
The label of the fertilizer will list numbers, for example 5-10-5, which refer
to the percent by weight of the primary macronutrients.
Soils
play a role
Soil is weathered,
decomposed rock and mineral (geological) fragments mixed with air and water.
Fertile soil contains the nutrients in a readily available form that plants
require for growth. The roots of the plant act as miners moving through the
soil and bringing needed minerals into the plant roots.
Structure of
soil, indicating presence of bacteria, inorganic, and organic matter, water,
and air. Image from Purves et al., Life: The Science of Biology, 4th
Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
Plants use
these minerals in:
1. Structural components in carbohydrates and
proteins
2. Organic molecules used in metabolism, such as
the Magnesium in chlorophyll and the Phosphorous found in ATP
3. Enzyme activators like potassium, which
activates possibly fifty enzymes
4. Maintaining osmotic balance
Mycorrhizae, bacteria, and minerals
Plants need
nitrogen for many important biological molecules including nucleotides and
proteins. However, the nitrogen in the atmosphere is not in a form that plants
can utilize. Many plants have a symbiotic relationship with bacteria
growing in their roots: organic nitrogen as rent for space to live. These
plants tend to have root nodules in which the nitrogen-fixing bacteria live. 
Development of
a root nodule, a place in the roots of certain plants, most notably legumes
(the pea family), where bacteria live symbiotically with the plant. Images from
Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer
Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
All the
nitrogen in living systems was at one time processed by these bacteria, who
took atmospheric nitrogen (N2) and modified it to a form that living
things could utilize (such as NO3 or NO4; or even as ammonia,
NH3 in the example shown below).
Pathway for
converting (fixing) atmospheric nitrogen, N2, into organic nitrogen,
NH3. Images from Purves et al., Life: The Science of Biology,
4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
Nitrogen uptake
and conversion by various soil bacteria. Images from Purves et al., Life:
The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com), used with permission.
Roots have extensions of
the root epidemal cells known as root hairs. While root hairs greatly
enhance the surface area (hence absorbtion surface), the addition of symbiotic mycorrhizae
fungi vastly increases the area of the root for absorbing water and minerals
from the soil.
Role of the
root hairs in increasing the surface area of roots to promote increased uptake
of water and minerals from the soil. Image from Purves et al., Life: The
Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com), used with permission.
Animals have a circulatory
system that transports fluids, chemicals, and nutrients around within the
animal body. Some plants have an analogous system: the vascular system
in vascular plants; trumpet hyphae in bryophytes.
Root hairs are
thin-walled extensions of the epidermal cells in roots. They provide increased
surface area and thus more efficient absorption of water and minerals. Water
and dissolved mineral nutrients enter the plant via two routes.
Water and selected
solutes pass through only the cell membrane of the epidermis of the root hair
and then through plasmodesmata on every cell until they reach the xylem:
intracellular route (apoplastic). Water and solutes enter the cell wall
of the root hair and pass between the wall and plasma membrane until the
encounter the endodermis, a layer of cells that they must pass through
to enter the xylem: extracellular route (symplastic).
The paths of
water into the xylem of a root. Image from Purves et al., Life: The Science
of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and
WH Freeman (www.whfreeman.com), used with permission.
The endodermis has a
strip of water-proof material (containing suberin) known as the Casparian
strip that forces water through the endodermal cell and in such a way regulates
the amount of water getting to the xylem. Only when water concentrations inside
the endodermal cell fall below that of the cortex parenchyma cells does water
flow into the endodermis and on into the xylem.
Details of the
Casparian strip. Image from Purves et al., Life: The Science of Biology,
4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
Xylem and Transport
Xylem is the water transporting tissue in plants
that is dead when it reaches functional maturity. Tracheids are long,
tapered cells of xylem that have end plates on the cells that contain a great
many crossbars. Tracheid walls are festooned with pits. Vessels, an
improved form of tracheid, have no (or very few) obstructions (crossbars) on
the top or bottom of the cell. The functional diameter of vessels is greater
than that of tracheids.
Water is pulled
up the xylem by the force of transpiration, water loss from leaves.
Mature corn plants can each transpire four gallons of water per week.
Transpiration rates in arid-region plants can be even higher. Water molecules
are hydrogen bonded to each other. Water lost from the leaves causes diffusion
of additional water molecules out of the leaf vein xylem, creating a tug on
water molecules along the water columns within the xylem. This "tug"
causes water molecules to rise up from the roots to eventually the leaves. The
loss of water from the root xylem allows additional water to pass throught the
endodermis into the root xylem.
Cohesion is the ability of molecules of the same kind
to stick together. Water molecules are polar, having slight positive and
negative sides, which causes their cohesion. Inside the xylem, water molecules
are in a long chain extending from the roots to the leaves.
Adhesion is the tendency of molecules of different
kinds to stick together. Water sticks to the cellulose molecules in the walls
of the xylem, counteracting the force of gravity and aiding the rise of water
within the xylem.
Cohesion-Adhesion
Theory
Transpiration
exerts a pull on the water column within the xylem. The lost water molecules
are replaced by water from the xylem of the leaf veins, causing a tug on water
in the xylem. Adhesion of water to the cell walls of the xylem facilitates
movement of water upward within the xylem. This combination of cohesive and
adhesive forces is referred to as the Cohesion-Adhesion Theory.
Guard Cells Regulate Transpiration
In most environments,
the water concentration outside the leaf is less than that inside the leaf,
causing a loss of water through openings in the leaf known as stomata (singular
= stoma). Guard cells are crescent-shaped cells of the epidermis that
flank the stoma and regulate the size of the opening. Together, the guard cells
and stoma comprise the stomatal apparatus. The inner wall of the guard
cell is thicker than the rest of the wall. When a guard cell takes up potassium
ions, water moves into the cell, causing the cell to become turgid and swell,
opening the stoma. When the potassium leaves the guard cell, the water also
leaves, causing plasmolysis of the cells, and a closing of the stoma.
Stomata occupy 1% of the leaf surface, but account for 90% of the water lost in
transpiration.
Ions and
stomatal function. Image from Purves et al., Life: The Science of Biology,
4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
Transportation and Storage of Nutrients
Plants make
sugar by photosynthesis, usually in their leaves. Some of this sugar is
directly used for the metabolism of the plant, some for the synthesis of
proteins and lipids, some stored as starch. Other parts of the plant also need
energy but are not photosynthetic, such as the roots. Food must therefore be
transported in from a source, an action accomplished by the phloem tissue.
Phloem,
Sugar, and Translocation
Phloem consists of several types of cells: sieve tube
cells (aka sieve elements), companion cells, and the vascular parenchyma.
Sieve cells are tubular cells with endwalls known as sieve plates. Most
lose their nuclei but remain alive, leaving an empty cell with a functioning
plasma membrane.
Companion cells
load sugar into the sieve element (sieve elements are connected into sieve
tubes). Fluids can move up or down within the phloem, and are translocated
from one place to another. Sources are places where sugars are being produced. Sinks are places where sugar is being consumed or
stored.
Food moves
through the phloem by a Pressure-Flow Mechanism. Sugar moves (by an
energy-requiring step) from a source (usually leaves) to a sink (usually roots)
by osmotic pressure. Translocation of sugar into a sieve element causes
water to enter that cell, increasing the pressure of the sugar/water mix
(phloem sap). The pressure causes the sap to flow toward an area of lower
pressure, the sink. In the sink, the sugar is removed from the phloem by
another energy-requiring step and usually converted into starch or metabolized.
One plant
response to environmental stimulus involves plant parts moving toward or away
from the stimulus, a movement known as a tropism. Nastic movements
are plant movements independent of the direction of the stimulus.
Alterations
in Growth Patterns Generate Tropisms
Charles Darwin and his
son Francis studied the familiar reaction of plants growing toward light: phototropism.
The Darwins discovered that the tips of the plant curved first, and that the
curve extended gradually down the stem. By covering the tips with foil, they
prevented the plant from curving. They concluded that some factor was
transmitted from the tip of the plant to the lower regions, causing the plant
to bend.
Phototropism in
the coleoptile of a monocot. Image from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH
Freeman (www.whfreeman.com), used with permission.
We now know,
from the 1926 experiments of Frits Went, that auxin, a plant hormone produced
in the stem tip (auxins promote cell elongation), moves to the darker side of
the plant, causing the cells there to grow larger than corresponding cells on
the lighter side of the plant. This produces a curving of the plant stem tip
toward the light, a plant movement known as phototropism.
Geotropism is plant response to
gravity. Roots of plants show positive geotropism, shoots show negative
geotropism. Geotropism was once thought a result of gravity influencing auxin
concentration. Several new hypotheses are currently under investigation.
Geotropism.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by
Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
Germination of
corn seeds occurs regardless of the seed orientation. The above image is
reduced in size from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/External_Factors_and_Plant_Growth/Gravitropism/Corn%2C_+_gravitropism.
Thigmotropism is plant response to
contact with a solid object. Tendrils of vines warp around objects, allowing
the vine to grow upward.
Curling of a
tenbdril around a metal support, an example of thigmotropism. Note the tendril
of this passion flower wrapping around the metal rod. The above image is
cropped and reduced from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/External_Factors_and_Plant_Growth/Thigmotropism_Passion_flower.
Nastic
movements, such as nyctinasty, result from several types of stimuli,
including light and touch. Legumes turn their leaves in response to day/night
conditions. Mimosa , also known as the sensitive plant, has its leaves close up
when touched.
Photoperiodism is the plant response to the relative amounts
of light and dark in a 24 hour period, and controls the flowering of many
plants. Short-day plants flower during early spring or fall, when the
nights are relatively longer and the days are relatively shorter. Long-day
plants flower mostly in summer, when the nights are relatively shorter and
the days are relatively longer. Day-neutral plants flower without respect for
the day length. Phytochrome is a plant pigment in the leaves of plants
that detects the day length and generates a response.
Plant Secondary Compounds
Plants produce primary
compounds important in their metabolism. They also produce secondary
compounds that serve to attract pollinators, kill parasites, and prevent
infectious diseases. Pea plants produce pisatin, a chemical that protects them
from most strains of parasitic fungi. Some strains of the fungus (Fusarium)
contain enzymes that inactivate pisatin, allowing them to infect pea plants.
Some plants
produce natural insecticides, such as pyrethrum, a chemical produced by
chrysanthemums that is also commercially available to gardeners. Antinutrients
are chemical produced when plants are under attack. These compounds inhibit the
action of enzymes in the insect's digestive system.
More that 10,000
defensive chemicals have been identified, including caffeine, phenol, tannin,
nicotine, and morphine.
Some plant
secondary compounds are useful to humans as
1. pesticides
2. medicines (salicylic acid, the main component
in aspirin)
3. stimulants (caffeine, the most widely used
psychoactive drug in the world)
4. chewing gum (chicle, a
compound from the sapodilla tree in Mexico was used in the first chewing gum).
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