1. Control Systems in Plants
Chapter 22
Control Systems in Plants

2. 22.1 Hormones
Charles and Francis Darwin
Plant tip experiments

3. Auxins Promote plant growth Produced in apical meristem of shoots
Cell elongation
Auxin builds up on shaded side
Elongation on shaded side causes plant to bend toward light
Stimulate secondary growth
Secreted by seeds to stimulate development of ovary to fruit
Farmers use this to grow seedless plants

4. Cytokinins Stimulate cell division
Produced in embryos, roots, and fruits
Slow aging of flowers and fruits
Affected by auxins
High levels of auxins inhibits branching
Travels from terminal bud
Bushier plant = “pinch back”

5. Gibberellins Stimulate growth of stems
Cell division and elongation(similar to auxins)
With auxins, influence fruit development
Promotes seed germination

6. Abscisic Acid (ABA) Halts primary and secondary growth Stress hormone
Promotes dormancy during winter or drought
More likely to survive when it becomes dormant
Stress hormone
Dehydration - ABA causes stomata to close

7. Ethylene Stimulates fruit ripening Promote “leaf drop”
Adaptation helps trees from drying out in the winter
High ethylene; low auxins - cooler temps and shorter days

8. 22.2 Plant Response
Tropisms: growth responses that cause parts of a plant to grow slowly toward or away from a stimulus
Rapid plant movements
Plants can respond quickly to touch to avoid being eaten
Leaf folding

9. Thigmotropism Change in plant growth due to touch Climbing plants
Tendrils coil and grasp objects
Mechanical stress
Bend to avoid damage
Leaf folding

10. Phototropism Growth of a plant part toward or away from light
Light on a shoot tip
Protein with light-absorbing molecule
Signals molecules that affect
auxin transport
Auxin lengthens shade side

11. Gravitropism Response to gravity
Seedling’s shoot grows upward, root downward
Hypothesis: gravity pulls organelles to low points of cells
Uneven distribution may signal cells to move auxins

12. Drought Causes wilting and reduces photosynthesis Succulents (cacti)
Respond by conserving water
Succulents (cacti)
Thick cuticle and fleshy stems
Spines instead of leaves
Arctic Plants
Small leaves and low to the ground
Reduce transpiration

13. Flooding Overwatered plants can suffocate Plants in wet areas
Lack of oxygen spaces in soil
Plants in wet areas
Mangroves
Roots partially above ground to provide oxygen
Release ethylene
Some cells die to form air tubes

14. Salt Stress Too much salt; lose water to soil through osmosis
halophytes: salt-tolerant plants
Salt glands
Pump out salt across leaf epidermis
Rain washes salt away
Pickleweed
Excess salt to stems; shed stems

15. Disease Defense Epidermis - barrier to pathogens Chemicals
Pathogens cross by wounds or stomata
Chemicals
Antimicrobial
Signal lignin production- seals off invader
Ability to recognize and attack certain pathogens

16. 22.3 Light and Seasons
circadian rhythm: biological cycle that occurs about every 24 hours
Daily signals; light
Plants produce flowers during different times of the year based on day/night length

17. Photoperiodism
Ability to use lengths of day to time seasonal activities
short-day plants: flower in fall or winter when darkness is a certain length
Long-day plants: flower in spring or summer when days are longer
Spinach, irises
day-neutral plant: flowers when it reaches a certain stage of maturity
Dandelions, tomatoes, rice
Florists can change this by flashing light in the night

18. Phytochromes Pigment proteins that detect sunrise and sunset
Sunrise= red light
Change to active form that triggers responses
Sunset - change back to inactive form