Transcript Factors Affecting Plant Growth - hills

Factors Affecting Plant
Growth
External Factors - Light
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Terrestrial plants use pigments to capture light
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Chlorophyll
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Carotinoids
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Absorbs light in the red and blue areas of the spectrum
Absorb in the blue-green spectrum
Quantity of light depends on
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Geographical area
Competition from taller plants
Cloud cover, shading, time of day etc...
External Factors – Light cont’d
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Photoperiod
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The number of daylight hours
Triggers flowering and dormancy in plants
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Short-day plants
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Flower and reproduce when the photoperiods are shortening
ie, late summer
 E.g. chrysanthemum
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Long-day plants
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Flower and reproduce when the photoperiods are
lengthening i.e. spring.
 E.g. spinach
External Factors - Nutrients
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Soil Nutrients
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Requirements vary between species
Inorganic nutrients are released by the weathering of
rocks or the breakdown of dead organic matter by
bacteria and fungi
Absence can cause death or serious weakening
External Factors – Nutrients cont’d
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Macronutrients:
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9 nutrients required in relatively large quantities
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Carbon, oxygen, hydrogen make up 95% of the dry mass of
plants
Other 5%: nitrogen, potassium, phosphorus, calcium,
magnesium, sulfur
Micronutrients:
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8 nutrients needed in much smaller amounts
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Iron, chlorine, boron, manganese, zinc, copper, molybdenum,
and nickel
External Factors- Nitrogen
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Most needed macronutrient
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Important part of protein, high protein plants need a
lot of nitrogen
Dead organic material
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Bacteria and fungi breakdown organic nitrogen
compounds into a form plants can use – nitrate ions
(NO3-)
External Factors- Nitrogen
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cont`d
Atmosphere – Nitrogen Gas
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Nitrogen-fixing bacteria convert nitrogen gas into a usable form
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Bacteria live freely in the soil or form a symbiotic relationship with
plants
Symbiotic bacteria - Grow within plant root cells and cause bulges
called nodules
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Occurs most often in legumes (peas, beans, clover, and alfalfa)
Bacteria supply the legumes with nitrates in exchange for
carbohydrates from the plants
Most nitrates produced are not used by the plant but are instead
added to the soil
Internal Factors - Overview
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Growth controlled by
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Tropisms
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growth responses toward or away from unidirectional stimuli
Due to differential growth; positive response if toward
stimulus, negative if away
Allow plants to alter their growth so they can grow into more
favorable conditions
Hormones
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Internal chemical regulators
5 main hormones
Tropisms
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Phototropism
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Plants detect blue-light
in special receptors
called phototropins
Tropisms cont`d
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Gravitropism
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Plant will grow so that it stays oriented relative to the
source of gravity (the earth).
Dependent on the presence of starch-filled plastids
(amyloplasts) in specialized cells.
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Starch is heavy, plastids sink to the lower end of the cell,
allows plant to identify up and down!
Thigmotropisms
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Response to touch
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vines
Negative geotropism
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Fig 10.20
10-11
Hormones- Auxins
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Stimulate cell division and elongation in apical
buds
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Inhibit these same processes in lateral buds
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Influence fruit ripening and dropping of fruit
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Synthetic Auxin
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Can be used as a herbicide
Triggers production of fruit from unpollinated flowers
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E.g. tomatoes and cucumbers
Increases root production in cuttings
Hormones - Gibberellins
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Promote cell division and elongation in plant stems
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Can cause bolting
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Sudden growth of stem just before flowering
Raises flower above leafy structures
Synthetic Gibberellins
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Cause fruit to enlarge (are often used with grape crops, etc.)
Herbicide
Effects of gibberellins
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Fig 10.16
10-14
Hormones - Cytokinins
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Stimulates cell division
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High levels in endosperm and young fruit
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Stimulates the growth of lateral buds by blocking
apical dominance
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Wider not taller!
Synthetic Cytokinins
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Used to produce clones in tissue cultures
Can be sprayed on cut flowers to make them last
longer
Hormones - Ethylene Gas
Key in fruit development
 Causes
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Increased rate of ripening
Sweetness of fruit
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Sugar content
Colour change
Tissue softener
Can be applied externally to fruit to ripen faster
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Also why 1 bad apple spoils the bunch!
Functions of ethylene
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Fig 10.18
Hormones - Abscisic Acid
Stress Hormone
 Growth regulator, acts as an inhibitor
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Slows down and stops growth
Promotes the closure of stomata
 Induces seed and bud dormancy
 Helps resist water stress
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