What is a hormone? Hormones- chemical messengers Chemical Control in Plants depend on : The hormone involved Concentration [ ] Tissue affected by the chemical

Main types of hormones in plants Auxins- “plant regulators” chemicals that stimulate or slow(inhibit) growth by influencing cell division, elongation, and cell differentiation (specialization of cells).

Example of an auxin IAA (Indole acetic acid)- [High] in the terminal buds, causes a lateral bud to grow, explaining why plants become bushier when cutting off terminal buds. Terminal bud lateral bud

Gibberellins-affect plant growth of stems, roots, & leaves. stimulate flowering and increases size of fruits. [ 50 or more have been found in plants, algae, & fungi] Sprayed with Gibberellins

Other hormones Cytokinins- stimulate cell division and growth during seed germination. Ethylene- stimulates ripening of fruits (bananas). Also, plays a role in abscission (slowing down of plant activities & dropping of leaves, flowers, or fruits). Abscisic acid- associated with shedding leaves and seasonal slowing down of plant activities. [Higher] in the fall.

Why use plant regulators? Weed killers – chemicals causes dandelions to grow abnormally & die. Affects monocots (grass) less than dicots. Used to rid (Monocot) corn fields of weeds. Stimulate root formation for cuttings- rooting powders aid in propagation To produce seedless fruit- auxins applied to ovaries of some flowers causes them to develop into fruits w/out pollination. Prevent fruit from falling before maturity

Types of Tropisms Tropisms- a growth of part of a plant in a specific direction in response to a stimulus. Positive – growth toward a stimulus Negative- growth away from a stimulus Phototropism – light Plants always grow toward the light [positive phototropism]

Geotropism- (gravity) Roots generally grow down into the ground [positive geotropism]

Chemotropism- chemicals Thigmotropism- touch ( gravevine winds around stem of another plant) Hydrotropism – water Willow trees roots grow toward water

Nastic Movements- plant movements in response to a stimulus, but independent of the direction of the stimulus. Leaves of a prayer plant are spread out flat during day, but become vertical at night.

Sensitive plant- if touched collapse due to a change in osmotic pressure. [ions] drops when touched forcing water out and cells lose turgor & collapse. Venus fly trap- when trigger hairs on leaves are touched, cells lose solute & water moves out of cells. When water pressure is restored plants open.

Photoperiodism- the response of a plant to changes in the length of day or night. Long day (short night)- flower when shorter periods of darkness. Ex. Clover, potato, beet, gladiolus, poppy [bloom in summer]

Short day (long night)- flower when longer periods of darkness. Ex Short day (long night)- flower when longer periods of darkness. Ex. Morning glory, tulip, forsythia [bloom in late spring, late summer or fall] Day neutral plants- are unaffected by lengths of light & dark Ex. String beans, corn, tomato, cucumber