Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 29

Plant hormones affect growth and development and may interact in complex ways. Auxins stimulate cell elongation in shoot tips, embryos, young leaves, flowers, fruits, and pollen. Gibberellins stimulate cell division and elongation but act more slowly than auxins. Cytokinins stimulate mitosis in actively developing plant parts. Ethylene speeds ripening. Abscisic acid inhibits the growth-inducing effects of other hormones.
A tropism is a growth response toward or away from an environmental stimulus, usually caused when different parts of an organ or structure grow at different rates. In phototropism, light sends auxin to the shaded portion of the plant, stimulating growth towards the light. Shoot growth is a negative gravitropism, and root growth is a positive gravitropism. Auxin accumulation can cause these opposite responses because the responding tissues differ in their sensitivities to it. Thigmotropism is a response to touch.
Nastic movements are not oriented toward a stimulus. Thigmonasty is response to contact. Nastic response to light and dark is photonasty, caused by osmotic changes that differentially alter cell volume. Thigmomorphogenesis is a growth-inhibiting response to mechanical disturbance.
Plants sense seasonal and other environmental changes. Photoperiodism is the ability of a plant to measure length of day and night. Flowering can depend upon photoperiodism. Short-day plants flower only when the duration of light is less than a critical length, whereas long-day plants require a light period longer than a critical length. Intermediate-day plants require days of intermediate length to flower. Day-neutral plants do not use light or dark cues to flower. The type of plant determines the season when it flowers. Plants requiring a precise photoperiod display obligate photoperiodism, whereas plants whose flowering is hastened by a certain photoperiod display facultative photoperiodism. Plants may respond to length of darkness rather than length of daylight.
A plant pigment, phytochrome, controls response to light. The inactive form, Pr, absorbs red light to become Pfr, the active form. Pfr promotes flowering of long-day plants and inhibits flowering of short-day plants. It is reconverted to Pr by absorbing far-red light. The ratio between these two forms provides information about daylight because sunlight has more red than far-red light. Phytochrome also controls early seedling growth, provides information about shading, and helps direct shoot phototropism and seed germination.
Senescence is an active and passive cessation of growth. Growth becomes dormant during cold or dry times and resumes when environmental conditions are more favorable. Internal biological clocks control daily responses, or circadian rhythms. Environmental changes can alter, or entrain, these clocks.