Plant Hormones (Unit 2)

Phototropism

growth movement in which plant shoots bend toward a light source due to uneven auxin distribution causing faster cell elongation on the shaded side

Heliotropism

reversible daily movement in which leaves or flowers track the sun’s position across the sky to maximize light absorption often using motor cells in pulvini

Gravitropism

directional growth response where roots grow toward gravity (positive) and shoots grow against gravity (negative) regulated by auxin redistribution in gravity-sensing cells

Hydrotropism

plant root growth directed toward areas of higher moisture concentration to improve water uptake

Thigmotropism

growth or movement response triggered by physical contact such as vines wrapping around supports or leaves folding when touched

Nastic Response

rapid or slow plant movements controlled by internal mechanisms that occur regardless of stimulus direction such as leaves opening and closing based on circadian rhythms

Hormones (general)

chemical messengers produced in small quantities that travel through plant tissues to regulate growth

Auxin

a major plant hormone that regulates phototropism

Auxin in Apical Dominance

suppresses lateral shoot growth by concentrating at the shoot tip to keep the main shoot dominant

Auxin in Root Branching

stimulates formation of lateral and adventitious roots allowing plants to expand their root network

Rooting Hormone

synthetic or concentrated auxin applied to cuttings to induce new roots to form from stems or leaves

Cytokinin

hormone that stimulates cell division

Cytokinin–Auxin Antagonism

regulatory relationship where high cytokinin promotes lateral shoot growth while high auxin suppresses shoots and maintains apical dominance

Cytokinin in Senescence

slows leaf aging by preserving proteins and chlorophyll allowing tissues to stay green longer

Gibberellin

hormone that promotes stem and shoot elongation by increasing the distance between nodes and triggering rapid growth known as bolting

Gibberellin in Fruit Growth

delays fruit maturation while increasing fruit size by stimulating cell expansion and elongation

Gibberellin in Seed Germination

breaks seed dormancy by activating enzymes that mobilize stored nutrients for the growing embryo

Abscisic Acid (ABA)

stress-response hormone that slows growth

ABA and Stomatal Closure

triggers guard cells to lose water and close stomata during dehydration to prevent further water loss

ABA in Seed Dormancy

maintains dormancy until environmental conditions remove or inactivate ABA allowing germination

Ethylene

gaseous hormone responsible for fruit ripening

Ethylene in Ripening

triggers a positive feedback loop where ripening produces more ethylene

Ethylene in Leaf Drop

promotes formation of an abscission layer when leaf auxin levels decline causing leaves to detach

Ethylene in Xylem Differentiation

induces programmed cell death allowing xylem cells to become hollow tubes for water transport