Chapter 22

Control Systems in Plants

22.1 Hormones

Charles and Francis Darwin Plant tip experiments

AuxinsPromote plant growthProduced in apical meristem of shootsCell elongation

Auxin builds up on shaded side Elongation on shaded side causes plant to bend

toward lightStimulate secondary growth Secreted by seeds to stimulate development

of ovary to fruit Farmers use this to grow seedless plants

Cytokinins

Stimulate cell divisionProduced in embryos, roots, and fruitsSlow aging of flowers and fruitsAffected by auxins

High levels of auxins inhibits branching Travels from terminal bud Bushier plant = “pinch back”

Gibberellins

Stimulate growth of stems Cell division and elongation(similar to

auxins)

With auxins, influence fruit developmentPromotes seed germination

Abscisic Acid (ABA)

Halts primary and secondary growth Promotes dormancy during winter or

drought More likely to survive when it becomes

dormant

Stress hormone Dehydration - ABA causes stomata to close

Ethylene

Stimulates fruit ripeningPromote “leaf drop”

Adaptation helps trees from drying out in the winter

High ethylene; low auxins - cooler temps and shorter days

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

Thigmotropism

Change in plant growth due to touchClimbing plants

Tendrils coil and grasp objects

Mechanical stress Bend to avoid damage

Leaf folding

PhototropismGrowth of a plant part toward or away

from lightLight on a shoot tip

Protein with light-absorbing molecule Signals molecules that affect

auxin transport Auxin lengthens shade side

Gravitropism

Response to gravitySeedling’s shoot grows

upward, root downwardHypothesis: gravity pulls

organelles to low points of cells Uneven distribution may

signal cells to move auxins

Drought

Causes wilting and reduces photosynthesis 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

Flooding

Overwatered plants can suffocate 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

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

Disease Defense

Epidermis - barrier to pathogens Pathogens cross by wounds or stomata

Chemicals Antimicrobial Signal lignin production- seals off invader

Ability to recognize and attack certain pathogens

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

PhotoperiodismAbility 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

Phytochromes

Pigment proteins that detect sunrise and sunset

Sunrise= red light Change to active form that triggers

responses

Sunset - change back to inactive form