Plant Growth and Regulation

1. Plant Growth and Regulation Chapter 11

2. Circadian Rhythms • Photoperiodism helps plants respond to the time of the year and circadian rhythms help plants respond to daily changes • Photoreceptors help plants reset their biological clocks every 24 hours • One example of a circadian rhythm is plants opening and closing their stomata every 24 hours • These movements are not depended on light because they did open and close them when placed in the dark 24 hours

3. Circadian Rhythms • Another example of a circadian rhythm is sleep movements • During the day, leaves are horizontal for optimal light absorption and at night the leaves are in a vertical position • When placed in darkness, sleep movements still occur • Why circadian rhythms? • It is thought that they help plants to synchronize repeatable daily activities so that it carries them out at an appropriate time each day

4. Turgor Movements • The sensitive plant folds its leaves and droops in response to touch • How? • When touched an electrical impulse moves down the leaf to cells called the pulvinus at the end of the petiole • A pulvinus is a swollen joint that acts as a hinge • When the signal reaches the pulvinus, it induces a chemical signal that increases membrane permeability to certain ions • As potassium ions exit the permeable membrane, it causes water to leave too through osmosis. They plant loses its rigidity or turgor

6. Turgor Movements • Turgor movements are temporary and reversible • The plant goes back to its original state when ions and water diffuse back across the membrane • The same mechanism causes the Venus flytrap leaf to close • Turgor movements also cause solar tracking: the ability of leaves or flowers of certain plants to follow the sun’s movement • Changes in turgor in the cells of the pulvinus help position the leaf in its proper orientation relative to the sun

7. Tropism • Tropism: a permanent change in position of a plant because of an external stimulus such as light, gravity, touch • Phototropism: plants growing toward a light source • Auxin (hormone) causes cells on the shade side of a stem to elongate more than the light side causing bending • Gravitropism: growth in response to the direction of gravity • Special cells in the root cap contain large starch grains called statoliths that collect toward the bottoms of cells due to gravity. If the plant is turned upside down, the statoliths moves and elongation shortly occurs on the opposite side • Plant cells can sense changing in hydrostatic pressure in the cell wall and re-adjust growth

8. Tropism • Thigmotropism: growth response to a mechanical stimulus such as contact with a solid object • Work the same way as phototropism with cells on the far side elongating • Because all tropisms are growth responses, they cause permanent changes

9. Hormones • Plant hormones are organic chemical compounds produced in one part of a plant and transported to another part where they elicit some kind of physiological response • Hormones are difficult to study because they work in very small amounts, and they elicit many different responses • In addition, the effects of hormones overlap so that it is difficult to determine which hormone caused the response

10. Types of Hormones • Auxin • Gibberellins • Cytokinins • Ethylene • Abscisic Acid

11. Fan Project • How was it discovered? • Actions: What does it do? • Examples • Economic Use

12. End