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Plant physiology Definition Summary

Botany for Horticulture
Terms: Plant physiology
Nature of life
Activator: Some textbooks refer to Cofactors as Activators. See Cofactors below.
Amino Acids: One of the organic, nitrogen-containing units from which proteins are synthesized. There are
about 20 in all proteins.
Cofactor: Inorganic molecules that activate enzymes. Sometimes these are called activators.
Energy: Technically this is the ability to do work. All energy comes from the sun. Plants change the form of
energy from sunlight into chemical bond energy.
Fat: An organic compound containing carbon, hydrogen, and oxygen, with proportionately less oxygen than
carbohydrates. These usually have a water-insoluble string of carbon atoms saturated with hydrogen.
Inorganic: A chemical compound that does not contain carbon.
Lipid: This is the generic term for fats, fatty substances and oils.
Organic: A chemical compound containing carbon.
Peptides: Amino acids linked together.
Proteins: A polymer composed of many amino acids linked together by a specific type of bond called a
peptide bond.
Starch: A carbohydrate formed of linked sugar molecules.
Suberin: A fatty substance found in the cell walls of cork cells and Casparian strips. It is water impermeable.
(suber, the cork oak)
Sugar: A molecule composed of carbon, hydrogen, and oxygen in approximate ratios of 1:2:1. This is the first
level of storage of energy reserves produced by photosynthesis.
Vitamin: Organic molecules that activate enzymes, produced by photosynthetic organisms.
Wax: A complex, water-insoluble compound.
Hormones and Horticulture
Abscisic acid: A plant hormone that brings about dormancy in buds, maintains dormancy in seeds, and brings
about stomatal closing. [abscissus, to cut off]
After-ripening: term applied to the metabolic changes that must occur in some dormant seeds before
germination can occur.
Apical dominance: The control of growth of lateral buds by the apical shoot.
Auxin: A class of plant hormones that control cell elongation. [auxein, to increase]
Bolting: The extremely rapid growth of internodes of a biennial in the second season of growth, followed by
flowering and death.
Circadian rhythms: Regular rhythms of growth and activity that occur on approximately a 24-hour basis.
(Circa, about + dies, a day)
Columnar: Type of growth typical of arborescent monocots (e.g. palms) where there are no lateral buds.
Sometimes this also applies to trees where the ratio of height to width is greater than for the average
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Botany for Horticulture
Cytokinin: A class of plant hormones that promotes cell division among other effects. [kytos, hollow vessel+
kinesis, motion]
Day-neutral plants: Plants that will flower under any daylength provided that they have received enough light
for growth.
Decurrent: In the majority of dicotyledonous trees, notably the oaks, hickories, maples, elms, and many others
the lateral branches grow as fast as or faster than the terminal shoot, giving rise to decurrent or
deliquescent growth habit where the central stem eventually disappears from repeated forking to form
a large spreading crown.
Deliquescent: Form of trees and shrubs where the terminal bud either aborts or is surpassed by laterals. Rhus
is one example of this.
Determinate: Growth of limited duration such as leaves of flowers, where the terminal will stop growing,
senesce and die.
Dormancy: The inability of a plant to grow even though conditions may support growth. (dormire, to sleep)
Ethylene: A simple hydrocarbon CH2=CH2 that is involved with the ripening of fruit and tissue maturation.
Etiolation: The stretching of shoots and loss of normal dark green colour. (etioler, to blanch)
Excurrent: In most coniferous species and in some dicotyledonous species the main stem or leader outgrows
the lateral branches beneath giving rise to cone-shaped crowns and a clearly defined central bole.
Germination: The first growth of the radical from a seed. (germinare, to sprout)
Gibberellin: A class of plant hormones whose best-know effect is stem elongation. [Gibberella, a fungal
Gravitropism: The growth of a plant organ towards gravity. (gravitas, grave + trope, a turning)
Hardening-off: The horticultural process of allowing a plant’s buds to become dormant, or decrease in
succulent plant growth that makes the plant more resilient.
Heliotropism: The growth of a plant organ towards sunlight. (helios, sun + trope, a turning)
Herbicide: A chemical that is used to severely restrict plant growth. (herba, grasses + cida, killer, slayer)
Hormones: Organic chemicals that are produced in minute amounts in one part of the plant and may have an
effect in other parts of the plant; are effective in very low concentrations; and have an effect on the
plant growth or development. (hormaein, to excite)
Indeterminate: Unrestricted or unlimited growth as in a vegetative apical meristem that produces an
unrestricted number of lateral organs indefinitely.
Intermediate-day plants: Plants that will not flower if days are too long or too short.
Long-day plants: Plants that will only flower if periods of light are longer than a critical length.
Nastic movements: Movements that occur in response to a stimulus but whose direction of movement is
independent of the position of the origin of the stimulus. One example is night closure of leaves.
(nastos, pressed to)
Photoperiodism: Response to timing and duration of day and night. A mechanism evolved by organisms for
measuring seasonal time. (photos, light)
Photoperiodism: The response of plants to length of day or night. (photos, light)
Phototropism: The growth of plant organs towards light. (photos, light + troupe, turning)
Phytochrome: A light-sensitive pigment that is involved in photoperiodism and height control in plants.
Usually Pr (red sensitive) and Pfr (far red sensitive). [phyton, plant + chroma, color)
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Botany for Horticulture
Plasticity: The ability of plant cell to differentiate into different cell types, as a parenchyma cell can
differentiate into a sieve tube element and companion cell.
Protoplast pressure model of gravity detection: The theory that the pressure of the protoplasts determines
the gravitational force and gravitropism.
Quiescence: A state in which a seed or plant will not grow unless environmental conditions normally required
for growth are present.
Scarification: The wounding of the seed coat to allow penetration of water and gasses and leaching of
germination inhibitors. (skere, to cut)
Senescence: The breakdown of cell membrane and components that leads to death of the cell. (senescere, to
grow old)
Short-day plants: Plants that will not flower unless day lengths are shorter than a specified length.
Statolith model of gravity detection: The theory that starch grains in the central part of the primary root
(columnella) detect gravity and lead to gravitropism.
Stratification: The process of exposure of seeds to low temperatures for prolonged periods before attempting
to germinate at warmer temperatures. (stratus, to spread out)
Tensegrity model of gravity detection: The theory that the pull of actin fibres in the cell and their integrity is
responsible for gravity detection and gravitropism.
Thigmotropism: Or seismonastic movements are nastic movements resulting from mechanical stimulation of
plants, response to contact with a solid object, such as a tendril touching a wire trellis. (thigma, touch
+ trope, a turning)
Tropism: A response to an external stimulus in which the direction of movement is usually determined by the
direction from which he most intense stimulus comes. (trope, a turning)
Vernalization: The induction of flowering by a cold treatment. (vernalis, spring)
Water relations
Active transport: The movement of molecules “uphill” against a gradient from an area of lower concentration
to an area of higher concentration. Moving against the gradient requires energy. One example is the
movement of sugar from the mesophyll and bundle cells into the phloem, called phloem loading.
Adhesion: The sticking of water molecules to surfaces, in particular surfaces that are made of polar compounds
such as cell walls. (adhaerere, to stick to)
Apoplast: The cell wall continuum of plants and the interior of xylem. The movement of water and solutes
into the roots without entering cells, by moving through intercellular spaces and cell walls. This is the
primary means of water entry into plants. [apo, away + plastos, molded]
Brownian motion: This is the random vibration and movement of particles and molecules caused by the
energy of heat.
Capillary action: The movement of water up a narrow opening because of the forces of adhesion to the walls
of the opening and the cohesion of water molecules to each other. (from capillus, a hair)
Casparian strip: A ring of cells that have cells walls impregnated with waxy suberin to prevent water
movement apoplastically through the cell walls. Casparian strips are found in roots, forcing all water
through passage cells, and also in needles of many xeric conifers to reduce water loss.
Cavitation: The breaking of a column of water (by the presence or an air bubble) in a series of vessel elements
or a xylem vessel. This can be caused by hammering a nail into a tree, or by xylem feeding insects.
(cavus, a cave)
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Botany for Horticulture
Cohesion: The sticking of water molecules of water to each other. This is primarily the result of hydrogen
bonding between the hydrogen of one molecule to the oxygen of another molecule. (cohaerere, to
stick together)
Cohesion-Tension Theory: This is the theory that explains the movement of water up a plant.
Diffusion: This is the random movement of particles or molecules from a region of higher concentration to a
region of lower concentration, ultimately resulting in uniform distribution. (diffundere, to pour out)
Embolism: An air bubble in a xylem vessel or tracheid that prevents conduction of water. If the embolism is a
bubble of gaseous water it might be re-absorbed. Air bubbles are usually not absorbed. (from
emballein, to insert, to intercalate)
Guttation: The forcing of water out natural openings of the leaves such as hydathodes and wounds such as cut
grass, resulting from the occasional overall positive xylem pressure at late night/early morning. The
positive pressure results from low water stress combined with root pressure.
Imbibition: The absorption of water and swelling of colloidal materials because of adsorption of water onto
the internal surfaces of materials. (in - bibere, to drink in)
Osmosis: This is the diffusion of water or other solvent through a differentially permeable membrane from a
region of higher solute concentration to a region of lower concentration. (osmo, impulse or thrust)
Osmotic potential: This is the physical pressure that can be developed by water trying to dilute another
solution separated by a differentially permeable membrane (the pressure required to prevent osmosis
from taking place).
Plasmolysis: This is the shrinking in volume of the protoplasm of a cell and the separation of the protoplasm
from the cell wall due to loss of water via osmosis. It results in death of the cell. (plasma, form +
lysis, a loosening)
Pressure-flow hypothesis: This is the theory that food substances (sugars) in solution in plants move along
gradients from sources where the food is made to sinks where the food is utilized.
Root pressure: The uptake of water by the roots contributes a small positive pressure to the overall negative
pressure in the xylem. When there is little water stress the overall xylem pressure may become slightly
positive. The water is then forced out some natural openings such as hydathodes, nectaries, and
recently cut surfaces like the blades of grass.
Sink: This in a region of the plant where food reserves are consumed for growth and development or storage.
Examples include flowers and lateral buds that are growing.
Sources: These are the regions of the plant where food sources may arise. These include leaves where sugar is
produced, to regions where food is stored.
Symplast: The interconnected protoplasts and their plasmodesmata. The movement of water into plant roots
by entering the protoplasm of one cell and passing through plasmodesmata to other cells on its way to
the endodermis. [syn, together + plastos, molded]
Tension: The word tension is used here in the British sense: It means a vacuum or negative pressure.
Transcellular: The movement of water into plant roots by going into cells and through vacuoles. (trans, across
or beyond + cell)
Transpiration: The evaporation of water from plants, primarily through the open stomata.
Tropism: Growth toward some stimulant, gravity, light, etc. (trope, a turning)
Turgor pressure: Pressure inside of the cell caused by the uptake of water into the cell. The water within the
vacuole normally regulates it. (turgor, a swelling)
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Botany for Horticulture
Photosynthesis, respiration and metabolism
Aerobic respiration: The use of oxygen in the chemical processes of respiration. [aer, air + bios, life]
Anaerobic respiration: The absence of oxygen during the processes of respiration. [an, without + aer, air +
bios, life]
Assimilation: The cellular conversion of raw materials into cell protoplasm and cell walls. (ad, to + simulare,
to make similar)
ATP: This is adenosine tri-phosphate. There are millions of these molecules in cells. ATP is readily converted
to ADP (adenosine diphosphate) by loosing a phosphate group and providing chemical energy to the
cell. ATP is the immediately-available energy currency for the cell.
C3 plants: Most plants produce a 3-carbon compound, 3-phosphoglyceric acid during the initial steps of the
carbon-fixing reactions.
C4 plants: These plants produce a 4-carbon compound, oxaloacetic acid instead of 3-carbon compounds in the
initial steps of carbon-fixing reactions. These plants have photosynthetic rates that are 2-3 times
greater than C3 plants.
CAM: This type of carbon fixing reactions are similar to C4 reactions, however, malic acid accumulates in the
chlorenchyma tissues at night and is converted back to carbon dioxide during the day. Stomata can
remain closed during the day to conserve water. This type of carbon fixing reactions is found in plants
that need to tolerate both high light intensity and limited water supply.
Chemiosmosis: When H+ are pumped from the mitochondrial matrix to in-between the membranes. They
“diffuse” back across the membrane and do “work” making ADP into ATP.
Compensation point: This term is used to indicate when the amount of sugar produced is equal to that used up
normally by respiration, as affected by a single limiting factor such as light. So, a compensation point
for light occurs when there is enough light to make just as much sugar through photosynthesis as the
plant is burning up through respiration. Shade plants have lower light compensation points than do
sun-loving plants.
Digestion: An enzyme-controlled conversion of complex (usually insoluble) substances into simpler, soluble
ones. Because it is controlled by enzymes, it is temperature dependent. (dis, apart + gerere, to carry)
Fermentation: The breakdown of sugar and other foods where ethyl alcohol or lactic acid are produced.
Generally this in an inefficient method to burn energy.
Glycolysis: The splitting of glucose molecules into pyruvic acid without involving free oxygen. (glucose sugar
+ lysis to loosen)
Photo-oxidation: High light levels cause highly reactive oxygen that can attack chlorophyll. The resulting
products cause damage to the chloroplast membranes. This happens mostly to shade plants.
Photorespiration: CO2 and O2 compete for ribulose biphosphate. The enzyme rubisco uses either CO 2 or O2
depending upon relative proportions in the leaf. On hot dry days, the stomata are likely to close due to
water stress. This makes more O2 in the leaf instead of CO2. The net effect is to produce CO2 and uses
up O2. This happens principally to C3 sun plants. No ATP is made.
Photosynthesis: The conversion of light energy into chemical energy: water, carbon dioxide and chlorophyll
are essential for this process which ultimately produces carbohydrates with oxygen being released as a
Respiration: The cellular breakdown of sugar and other foods accompanied by the release of energy.
Sucrose: This is the disaccharide (sugar molecule made of two sugars chemically linked together) that the
plant uses to transport sugar produced by photosynthesis.
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