PLANT RESPONSE

Tropisms Plant growth toward or away from a stimulus Gravitropism gravity is “+” in roots and “-” in shoots – Plastids containing starch grains (statoliths) settle to the low point of the cell. Thigmotropism – touching stunts growth or affects shape – Difference in turgor pressure Phototropism light is “+” shoots

Discovery of Plant Hormones Classic experiments – Darwin – Boysen-Jensen – Went Phototropism – grasses bend toward the light because the cells on the dark side grow faster that the lighted side. – Darwin concluded that a plant hormone made in the coleoptile tip could somehow move down and induce other cells to elongate.

Auxin Produced in the tip of the shoot and transported down the stem. Promotes cell elongation – causes cells to pump H+ into their cell walls. – The higher pH activates enzymes to break cross linkages in the wall allowing cell elongation.

Cytokinins Produced in the root stimulate cytokinesis – Anti-aging effect Continues protein synthesis after leaves have been picked usually coupled with auxin. – When added to undifferentiated cell mass (callus) Higher auxin conc.  root Higher cytokinin conc.  shoot – Apical dominance Higher auxin conc.  terminal bud Higher cytokinin conc.  axillary bud

Gibberellin Originally gibberellin found in a fungus, causing rice seedlings to grow tall and spindly. – Stimulate cell elongation & division Stimulates the synthesis of hydrolytic enzymes during germination of seedlings in order to insure release of stored nutrients. – What are the practical uses?

Ethylene a gas and is produced by fruit Response to Stress – Growth around obstacles Triple response – Drought Leaf Abscission & Apoptosis Ripening of fruits – Why do organic fruits ripen slower? – Why are tomatoes picked green?

Abscisic Acid Slows growth induces winter dormancy by suppressing mRNA production. – auxin and gibberellins are no longer produced. enters guard cells during periods of water stress – outward transport of potassium ions (K+). – How will this help prevent the plant’s water loss?

Photoperiodism The response to changing lengths of light (day) and dark (night) Flowering is usually controlled by length of nights. – Chrysanthemums bloom in the fall when the nights are longer and the days are shorter. Leaves detect the longer nights and signal the flower buds. Mums are “short day” plants – Confirmed by interrupting the period of darkness with flashes of light Long Day-Short Night plants will begin to flower during winter Short Days-Long Night may be prevented from flowering

Phytochromes Pigments that control plant responses to light – responds reversibly to red (660nm ) and far red (730nm) light Pr is sensitive to red light (daytime) – necessary to reset the clock which counts the hours of darkness slowly accumulates spontaneously during the night. Pfr is sensitive to far-red light (nighttime) – Switches on physiological and developmental responses (germination, flowering, etc.) – Slowly converts to Pr

Circadian Rhythms ~24 hr. physiological signals that are innate but are set/ entrained by night and day. – Present in plants and animals a plant’s biological clock is reset by light through phytochromes – in dark all phytochrome are Pr – dawn light restores the balance of Pr : Pfr  clock is reset