Plant Growth and Development (14.2)

5.4

Plant growth regulators (PGRs) • plant hormones that affect the rate of division, elongation and differentiation of plant cells

Five well-characterized groups of PGRs • auxins, gibberellins, cytokinins, abscisic acid and ethylene

Auxins

• Produced in plants at the apical meristem

• Cause apical dominance; plant grows upward with few to no side branches

Auxins promote elongation of cells, stimulate growth and ripening of fruit, and also inhibit the dropping of fruit and leaves.

Synthetic auxins can be used to stimulate growth of fruit from unpollinated plants, resulting in, for example, seedless tomatoes.

Seedless tomatoes!

Gibberellins promote cell division and elongation.

Sometimes they cause a process called bolting where the stem of a plant rapid grows taller before flowering.

Many dwarf varieties of plants stay small because they do not produce normal levels of gibberellin.

Lettuce bolting after it has been treated with

gibberellins.

Cytokinins stimulate cell divison and leaf growth.

Commercially, cytokinins are used to extend the life of cut flowers.

Abscisic acid’s main role is to coordinate responses to stress in plants.

Usually it inhibits growth.

It can induce dormancy of seeds to protect them from harsh conditions and also regulates rate of transpiration.

Maple tree seeds. Some seeds can remain

dormant for years and still be viable in

newfound hospitable conditions.

Ethylene is a gas produced in many of the plant’s tissues.

It plays a role in flower death, fruit ripening and fruit loss.

Commercially, growers can delay ripening of fruit until they are about to be sold by controlling the levels of ethylene in the fruit.

Tomatoes before ripening. A spray of ethylene would have these ripe and red in

no time.

Learning Check

• Pg 595, Q 13-18

External Factors that Regulate Plant Development

Tropism • a change in the growth pattern or movement of a plant in response to an external stimulus

Nastic Response

• Plant response to a stimulus independent of the direction of the stimulus

• Examples– Flower opening petals during the day and closing

them at night– Mimosa plant

http://www.youtube.com/watch?v=g0LFBM3hOLs

Thigmotropism • a change in the growth pattern or movement of a plant in response to touch

For example, the tendrils of pea plants will grow around a supporting structure at the point of contact.

http://www.youtube.com/watch?v=dTljaIVseTc

Phototropism • a change in the growth pattern or movement of a plant in response to light

For example, a plant growing in uneven light will lean and bend its stem in order to become better exposed to the light.

http://www.youtube.com/watch?v=zHe7y8cy-7Y

Gravitropism • a change in the growth pattern or movement of a plant in response to gravity

If plants could not detect and respond to gravity, their roots would be as likely to grow up into the air as down into the ground.

This plant has been left to grow upside down.

You can see in the picture that the stem is

trying to grow back upwards and fight

gravity.

Photoperiodism • plants in regions where there are two or more seasons control their reproductive cycles based on the the duration of daylight, a.k.a the photoperiod.

Plants flowering in the spring means the most time available for seed and fruit development before winter.

Trees dropping their leaves in the fall means protection from the cold, dry conditions of winter.

NUTRIENTS IN ORDER OF PRIORITY

• CO2 and H2O needed for photosynthesis!

• N2 (from organic material) needed to produce proteins, nucleic acids, chlorophyll and leaf growth– NH3 from animal waste– NO2 and NO3 from decayed materials– Nitrogen fixing bacteria contain N2

gas and convert it to nitrates

• Others... (next slide)

NITROGEN CYCLE (DON’T MEMORIZE!)

OTHER ESSENTIAL NUTRIENTS

Nutrient Function Deficiency Symptoms

K Water balance, protein synthesis

P ATP (energy) syntesis, mitosis, cell division

Ca Part of cell walls, membrane permeability

Mg Part of chlorophyll and coenzymes for photosynthesis

S Part of proteins

You tell me! Brainstorm time!

OTHER ESSENTIAL NUTRIENTS

Nutrient Function Deficiency Symptoms

K Water balance, protein synthesis

Poor growth, weak stems, yellowing leaves

P ATP (energy) syntesis, mitosis, cell division

Stunted growth, poor seed/fruit development

Ca Part of cell walls, membrane permeability

Stunted growth

Mg Part of chlorophyll and coenzymes for photosynthesis

Yellowing leaves

S Part of proteins Stunted growth, yellowing leaves

FERTILIZERS

Natural Synthetic

Examples Manure, compost, sludge, crop rotation

Man-made chemical cocktails that contain ammonia and other nutrients

Advantage No harmful chemicals

Get exact nutrients

Disadvantage May not always get the correct amount of each nutrient

Costly, prone to leaching and runoff which alters chemical balance in water and soil