PLANT RESPONSES TO EXTERNAL SIGNALS Ch 39

A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots After exposure to light, a potato undergoes changes in which shoots and roots grow normally A potato’s response to light is an example of cell-signaling

I. General Features of Plant response The stages are reception, transduction, and response

Reception: Internal and external signals bind receptors Transduction: Second messengers transfer and amplify signals from receptors to proteins that cause responses Response: A signal transduction pathway leads to regulation of one or more cellular activities

II. Plant Germination, Growth, and Fruit Production Under the control of growth hormones: auxins, cytokinins, gibberellins Many of the hormones work together

Effect of these hormones: – Growth: Stimulate cell division, elongation and phototropism

Pinching of apical bud induces growth of lateral buds under influence of cytokinins Auxin induces cell elongation thru uptake of water in vacuole and loosening of cell wall

Effect of auxin on phototropism:

Germination: Stimulate the breakdown of stored energy in seed to produce glucose for cell respiration

Production of Fruit: After seed formation, fruit forms to aid in seed dispersal

Under what conditions might these hormones be produced?

III. Dormancy and Fruit Ripening Under control of hormones that inhibit growth: – Ethylene – Abscisic acid

Effect of these Hormones – Seed dormancy: stops germination so seed won’t germinate in winter – Fruit ripening: Speed up fruit ripening to aid in seed dispersal – Leaf abscission: Chlorophyll in leaf, and cellulose at base of leaf is broken down

Why drop leaf? Why break down chlorophyll? Why don’t evergreens lose their leaves? Ethylene is a gas. Why does “one bad apple spoil the whole bunch?”

IV. Plant Responses to Light are critical to their success A.Phytochromes and circadian rhythms in plants

Pr →Pfr occurs during the day Pfr →Pr occurs at night Since length of day/night changes over a year (except where?) plants use this to adjust activities Plants use photoperiod (how many hours of light/day a plant is exposed to) to detect time of year

B. Effect of the Daylength on Flowering What is this illustration telling?

– Flowering in plants is controlled by the amount of DARK – Short day plants flower when amount of dark is greater than the critical period – Long day plants flower when the amount of dark is less than the critical period t/chp39/ html t/chp39/ html

Explain effect of red and far red light