Plant Responses

1. Plant Responses

2. Plant Hormones • Hormones are chemical messengers that coordinate the different parts of a multicellular organism. They are produced by one part of the body and transported to another. • Signal transduction is the series of steps between an organism's perception of an environmental change and the response to that change • Hormones help coordinate growth, development, and response to stimuli. • There are 5 main plant hormone classes: auxins (IAA), cytokinins, gibberellins, abscisic acid and ethylene. • See p.759

3. HormoneWhere Found in Plant Major Functions Auxin Seed embryo, young leaves, Stimulates cell elongation; involved in meristems of apical buds phototropism, gravitropism, apical dominance, and vascular differentiation; stimulates ethylene synthesis; regulates fruit development Cytokinin Synthesized in roots and Stimulates cell division, stimulates germination, transported to other organs growth, and flowering, delays leaf senescence Gibberellin Meristems of apical buds and Stimulates shoot elongation, stimulates roots, young leaves, embryo flowering and development of fruit, promotes seed and bud germination Ethylene Tissues of ripening fruits, Stimulates fruit ripening, leaf and flower nodes of stems, senescent senescence, opposes some auxin effects leaves and flowers Abscisic Acid Leaves, stems, green fruit Stimulates stomate closure during water stress, inhibits growth, promotes seed dormancy

4. The effect of gibberellin on Thompson seedless grapes

5. Light Responses • Tropism: A plant growth response that results in the plant growing either toward or away from a stimulus. • Phototropism: The growth of a shoot in a certain direction in response to light. • Positive phototropism is growth toward light; negative phototropism is growth away from light. • The direction of growth is due to the fact that cells on one side of the plant elongate more quickly than the cells on the other side (fig.35.2) • Responses to light are critical for plant success • Photomorphogenesis: The effects of light on plant morphology • Blue light has the greatest effect on plant growth and movement. • Phytochromes are pigments that are involved in many of a plant’s responses to light.

6. Photoperiods • Photoperiodism: A physiological response to a photoperiod (the relative lengths of night and day), such as flowering • Note: It is night length - NOT day length - that controls flowering and other responses • Short-day plants require a period of light shorter than a certain critical length in order to flower. • Long-day plants flower in the late spring or early summer; they require the most daylight to flower. • Day-neutral plants can flower in days of any length.

7. Midnight Noon Sleep movements of a bean plant (Phaseolusvulgaris): http://plantsinmotion.bio.indiana.edu/plantmotion/movements/leafmovements/bean/beansleep.html

8. Other Stimuli • Gravitropism: A response to gravity. • Roots show positive gravitropism and shoots show negative gravitropism • Thigmomorphogenesis: The change in form of a plant that results from mechanical disturbance (repeatedly touching a plant with a ruler to measure its height can affect its growth pattern) • Thigmotropism: Directional growth in a plant as a response to a touch. • Example: climbing tendrils of plants such as the sweet pea. The tendrils actually "feel" the solid object, which results in the coiling response. • Human skin can detect a thread weighing 0.002mg being drawn across it, but a feeding tentacle of the insectivorous sundew plant responds to a thread of 0.0008mg. • Thigmonasty: Touch-induced movement rather than growth (mimosa leaf) • Mimosa Video: http://plantsinmotion.bio.indiana.edu/plantmotion/movements/nastic/mimosa/mimosa.html

9. (a) Unstimulated (b) Stimulated Side of pulvinus with flaccid cells Leafletsafterstimulation Side of pulvinus with turgid cells Pulvinus(motororgan) Vein (c) Motor organs 0.5 m Rapid turgor movements by the sensitive plant (Mimosa pudica)

10. Other Responses • In times of drought, the guard cells lose turgor. This causes the stomata to close. Young leaves stop growing, and they will roll into a shape that slows transpiration rates. Deep roots continue to grow, while those near the surface grow slowly if at all. • In times of flooding, certain cells in the root cortex die, which creates air tubes that bring in oxygen and enable the plant to continue cellular respiration. • Plants respond to salt stress by producing compounds that keep the water potential of cells more negative than that of the soil solution, which encourages water to flow into the roots. Most plants cannot survive salt stress for long periods of time. • In heat stress environments, plants produce heat-shock proteins, which act to ensure that newly formed proteins are folded properly and mis-folded proteins are removed from the cell. • In cold stress environments, plants alter the lipid and protein composition of their cell membranes.