Control Systems in Plants Chapter 22 Control Systems in Plants

22.1 Hormones Charles and Francis Darwin Plant tip experiments

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

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”

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

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

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

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 touch Climbing plants Tendrils coil and grasp objects Mechanical stress Bend to avoid damage Leaf folding

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

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

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

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

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 Chemicals 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

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

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