NOTES FOR BIOLOGY 1002

SECTIONS 004, 005, 006

Spring 2006

DR. STEVEN POMARICO

CHAPTER 32

PLANT GROWTH AND DEVELOPMENT

The plant embryo contains three parts (see fig 32.4 and 32.5)

          -hypocotyl (root)

          -epicotyl (shoot)

          -cotyledons (seed leaves)

In dicots there are 2 cotyledons in the embryo and monocots have only 1 cotyledon

In the monocots the cotyledon is protected by a tough sheath called the coleoptile

Plants perceive and respond to environmental stimuli by regulating growth and development through the action of chemicals called plant hormones or growth regulators.

What are the stimuli that plants receive from their environment?

          Direction of gravity.

          Direction, intensity, and duration of sunlight.

          Strength of the wind.

Plant Hormones and Their Action

Hormones are chemicals produced in one location and transported to other regions where they exert specific effects.

This definition can be applied to all organisms plants and animals.

There are 5 major types of plant hormones (See table 32.1)

          -Auxins

          -Gibberelins

          -Cytokinins

          -Ethylene

          -Abscisic acid

Auxins

          -Regulate plant responses to light and gravity

          -Promote cell elongation in shoots

          -Prevent growth of lateral buds

          -In roots

                     low concentrations stimulate elongation

                     higher concentrations inhibit elongation.

                     stimulate root branching.

          -Stimulate fruit development

Cytokinins

          -Promote cell division including lateral meristems

          -Stimulate overall metabolism, delaying the aging of leaves.

Abscisic acid is an inhibitory hormone

          -Causes stomata to close when water is scarce.

-Maintains dormancy in buds and seeds in bad weather by inhibiting the action of other hormones especially gibberellin.

Gibberellins

          -Promote cell elongation in stems.

          -Stimulate flowering, fruit development, seed germination, and bud growth

Ethylene, the only known gaseous hormone:

          -Causes fruit to ripen.

          -Causes breakdown of cell walls in abscission layers, allowing leaves fruit,                      and flowers to drop off at appropriate times.

The balance between these hormones helps to shape a plant

Auxin produce at the shoot tip apical meristem give that part of the plant an apical dominance as you move away from the shoot tip that dominance lessens