1. Factors affecting plant growth ABT-320
Dr. Rabia Amir
Lecture 2

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. Light Photoperiod The number of daylight hours
Triggers flowering and dormancy in plants
1. Short-day plants
Flower and reproduce when the photoperiods are shortening ie, late summer
E.g. chrysanthemum
2. Long-day plants
Flower and reproduce when the photoperiods are lengthening i.e. spring.
E.g. spinach

4. Short day vs long day

5. Nutrients Macronutrients: Micronutrients:
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. Macronutrients; 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. Nitrogen Atmosphere – Nitrogen Gas
1. Nitrogen-fixing bacteria convert nitrogen gas into a usable form
Bacteria live freely in the soil or form a symbiotic relationship with plants
2. 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. Nitrogen Cycle

9. Internal factors 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

10. Tropism
Phototropism
Plants detect blue-light in special receptors called phototropins

11. Tropism
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!

13. Negative Geotropism

14. Thigmotropism Thigmotropism
Movement in which a plant moves or grows in response to touch or contact stimuli.

15. 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 un-pollinated flowers
E.g. tomatoes and cucumbers
Increases root production in cuttings

16. 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

17. Effects of Gibberellins

18. 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

19. 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

20. Functions of Ethylene

21. 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

22. Thank You !