1. Factors Affecting Plant Growth

2. 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...

3. 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

4. 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

5. 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

6. 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-)

7. 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

8. 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

9. Tropisms Phototropism
Plants detect blue-light in special receptors called phototropins

10. Tropisms cont`d Gravitropism Thigmotropisms
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

11. Negative geotropism
Fig 10.20

12. 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

13. 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

14. Effects of gibberellins
Fig 10.16

15. 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

16. 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!

17. Functions of ethylene
Fig 10.18

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