factors affecting plant growth. external factors - light terrestrial plants use pigments to capture...
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Factors Affecting Plant Growth
External Factors - Light Terrestrial plants use pigments to capture
light Chlorophyll
Absorbs light in the red and blue areas of the spectrum Carotinoids
Absorb in the blue-green spectrum
Quantity of light depends on Geographical area Competition from taller plants Cloud cover, shading, time of day etc...
External Factors – Light cont’d Photoperiod
The number of daylight hours Triggers flowering and dormancy in plants
Short-day plants Flower and reproduce when the photoperiods are
shortening ie, late summer E.g. chrysanthemum
Long-day plants Flower and reproduce when the photoperiods are
lengthening i.e. spring. E.g. spinach
External Factors - Nutrients Soil Nutrients
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
Macronutrients: 9 nutrients required in relatively large
quantities Carbon, oxygen, hydrogen make up 95% of the dry
mass of plants Other 5%: nitrogen, potassium, phosphorus, calcium,
magnesium, sulfur
Micronutrients: 8 nutrients needed in much smaller amounts
Iron, chlorine, boron, manganese, zinc, copper, molybdenum, and nickel
External Factors- Nitrogen Most needed macronutrient
Important part of protein, high protein plants need a lot of nitrogen
Dead organic material Bacteria and fungi breakdown organic nitrogen
compounds into a form plants can use – nitrate ions (NO3
-)
External Factors- Nitrogen cont`d Atmosphere – Nitrogen Gas
Nitrogen-fixing bacteria convert nitrogen gas into a usable form
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
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 Growth controlled by
Tropisms 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 Internal chemical regulators 5 main hormones
Tropisms Phototropism
Plants detect blue-light in special receptors called phototropins
Tropisms cont`d Gravitropism
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.
Starch is heavy, plastids sink to the lower end of the cell, allows plant to identify up and down!
Thigmotropisms Response to touch
vines
10-11
Negative geotropism
Fig 10.20
Hormones- Auxins Stimulate cell division and elongation in
apical buds Inhibit these same processes in lateral buds
Influence fruit ripening and dropping of fruit
Synthetic Auxin Can be used as a herbicide Triggers production of fruit from unpollinated
flowers E.g. tomatoes and cucumbers
Increases root production in cuttings
Hormones - Gibberellins Promote cell division and elongation in plant
stems
Can cause bolting Sudden growth of stem just before flowering Raises flower above leafy structures
Synthetic Gibberellins Cause fruit to enlarge (are often used with grape crops,
etc.) Herbicide
10-14
Effects of gibberellins
Fig 10.16
Hormones - Cytokinins Stimulates cell division
High levels in endosperm and young fruit
Stimulates the growth of lateral buds by blocking apical dominance Wider not taller!
Synthetic Cytokinins 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
Increased rate of ripening Sweetness of fruit
Sugar content Colour change Tissue softener
Can be applied externally to fruit to ripen faster Also why 1 bad apple spoils the bunch!
Functions of ethylene
Fig 10.18
Hormones - Abscisic Acid Stress Hormone Growth regulator, acts as an inhibitor
Slows down and stops growth Promotes the closure of stomata Induces seed and bud dormancy Helps resist water stress