Chapter 39 Plant Responses to Internal and External

Chapter 39: Plant Responses to Internal and External Signals 39. 2 Admit Slip 3. List 3 words you think of when you look at the picture/diagram 2. Write 2 ideas you have based on the picture and your words. If possible, use your words as you write your ideas. 1. Write 1 question you have.

Hormones • Plant hormones help coordinate growth, development, and responses to stimuli – Hormones: chemical messengers that coordinate the different parts of a multicellular organism (produced in one location and transported to another in the body)

Tropism • A plant growth response from hormones that results in the plant growing either toward or away from a stimulus – Phototropism: growth of a shoot in a certain direction in response to light • Positive phototropism: growth toward light • Negative phototropism: growth away from light

Greening response to light in potatoes

Types of hormones • • • Auxin Cytokinins Gibberellins Abscisic acid ethylene

AUXINS • Natural auxin in plants is idoleacetic acid (IAA) • Plays a key role in phototropisms and gravitropisms • Stimulate elongation of cells in young shoots – Produced in apical meristems and activate proton pumps in plasma membrane (lowers p. H) – Cell wall is weakened, turgor pressure expands cell wall=cell elongation Apical dominace: apical bud (left) apical bud removed (right)

How Auxins Cause Cell Elongation • Increase in H+ activates expansins • Weakened cell wall allows more water to enter H+ pump

Synthetic Auxins • Used as herbicides – Monocots (grasses) can quickly inactivate synthetic auxins but dicots cannot – High concentrations therefore kill broadleaf (dicot) weeds while grasses (like corn and turf grass) are not harmed

Other Hormones • Cytokinins: stimulate cytokinesis or cell division (aide in growth). Work with auxins to stimulate cell division and differentiation • Gibberellins: work with auxins to stimulate cell elongation by loosening cell walls, allowing for expansion of cells and stems. Effects: fruit growth, stem elongation, seed germination

Other Hormones • Abscisic Acid: slows growth , promotes seed dormancy, drought tolerance (water stress=close stomata to save water) • Ethylene: gas, role in programmed cell death (apoptosis) such as shedding of leaves/death of annual after flowering. Promotes ripening of fruits which triggers more ethylene (positive feedback loop)-1 rotten apple can spoil the bunch…

Went Experiments

Chapter 39: Plant Responses to Internal and External Signals 39. 3

Plants and Light • Plants detect presence, direction, intensity, and wavelength of light • Red and blue light are the most important to plants – Blue-light receptors initiate plant response such as phototropisms and opening of stomata – Red-light receptors=Phytochromes

Phytochrome photoreceptors • Molecular switch reaction to red light – conversion of Pr Pfr in sunlight stimulates germination, flowering, branching… – conversion of Pfr Pr in dark inhibits response, & stimulates other responses: growth in height Light induced Chromophore Photorecptor Kinase activity Phytochrome Response: Vertical growth 2007 -2008

Practical Application • Why do you plant lettuce seed by scattering them on the ground instead of burying seed? • What is the evolutionary advantage to lettuce seeds? 2007 -2008

Phytochromes and Circadian Rhythyms • Circadian Rhythms: physiological clocks that usually run on a 24 hour cycle (not paced on environmental cues) • Surge of Pfr at dawn resets clock • Combination of phytochrome system and biological clock allow plant to asses the amount of daylight/darkness and time of year

Photoperiodism • Photoperiod: relative length of night and day • A physiological response (such as flowering) to a photoperiod=photoperiodism • Controls when plants flower • Length of night is more critical than length of day – Old terminology remains!! We refer to plants by day-length!

Day-length Plants • Short-day plants: require a period of continuous darkness longer than a critical period to flower. – Flower in early spring or fall – Actually ‘Long-night” plants • Long-day plants: require a period of continuous darkness sorter than critical period to flower – Flower in late spring/early summer – Actually “short-night” plants • Day-neutral plants: flower in days of any length

Figure 39. 22 Photoperiodic control of flowering

Figure 39. 23 Reversible effects of red and far-red light on photoperiodic response