Control of Growth and Development

Chapter 7

Growth

•Increase in mass due to the division and enlargement of cells

•Types of growth in plants

–Determinate growth - reaches a certain size/form, then stops growing (only maintenance continues)

–Indeterminate growth - no pre-determined size or form, growth occurs as long as resources are available and mechanical limits are not exceeded.

•Differentiation of cells - become specialized for a specific function

•Development - the process of growth and differentiation of cells into tissues, organs, and organisms

 

Plant Hormones

•Defined: organic molecules produced in small quantities in active growing regions of plants that influence many aspects of plant development

•Uses: Many plant hormones have commercial applications, in agriculture or horticulture

•Five kinds of plant hormones have been identified:

–auxins

–gibberellins

–cytokinins

–abscisic acid

–ethylene

 

Auxins

Auxins

•Sites of production

–Apical meristems

–Buds

–Young leaves

–Other active young plant parts

•Auxins flow away from their source of synthesis

–high concentration near shoot tip

–lower away from tip

–cause expansion of cells in area of relocation

 

Natural and Synthetic Auxins

•Naturally occurring auxin

–Indole-3-acetic acid (IAA)

•Horticulture use of IAA or synthetic regulators applied to cuttings promotes root growth for plant propagation

•Fruit production

–for uniform flower and fruit set

–prevent formation of abscission layer (no early fruit drop)

–spray flowers to induce production of seedless fruit

Synthetic auxins

•Herbicides (2,4-D)

–common weed killer

•Agent Orange (mixture of 2,4-D and 2,4,5-T)

–used in Vietnam to defoliate the jungle, easier to see Vietcong

–use of 2,4,5-T banned in US by EPA because a dioxin (carcinogen) is present as a by-product in its formation

 

 

Gibberellins

•Giberellic Acids (GA) – many kinds

•Produced in immature seeds, root and shoot tips, young leaves, and fungi.

•Effects of gibberellins

–Increase stem growth without extra root growth – internode elongation

–when applied to stems of dwarf plants, can reach height of non-dwarf plants

 

Gibberellins

Gibberellins

•Effects of gibberellins (con’t)

–Breaking of dormancy of buds and seeds

•GA application can induce germination

•eliminate the cold period requirement

–GA applied to grapes increases fruit size and internode length (greater space between fruits inhibits mold growth)

–etiolated seedlings (grown in dark) produce excessive amounts of GA in trying to grow tall enough to reach light

Cytokinins

•Enhances cell division while in the presence of auxins

•Found in meristems and developing tissues (young fruit)

•Can extend the storage life of vegetables, but too expensive for commercial use

•Effects of cytokinins

–Enlarging of cells

–Differentiation of tissues

–Development of chloroplasts

–Delay of aging in leaves

–Causes tumor growth (galls) in plants, but not in animals

Cytokinins

Abscisic Acid (ABA)

•Substance present in buds which block the function of auxin,

•Initially called dormin, later called Abscisic Acid (ABA).

•Was thought to initiate leaf abscission, but later another substance (ethylene) was discovered to control that process

•Common in fleshy fruits

–Prevents seeds from germinating while still on the plant

•Induces bud dormancy

•Regulates stomatal opening

Ethylene

•Produced by fruits, flowers, seeds, leaves, roots

•Promotes ripening of fruits

•When fruits ready to ripen:

–green pigments replaced by red, orange, yellow pigments

–tannins break down

–starches break down into sugars

–ethylene causes membranes, cell walls, cuticles to degrade

Ethylene

•fruits are often picked green, stored cold and without oxygen until needed, then ripened in batches by pumping in ethylene

•wrapping individual fruit, or placing them in a bag retains naturally-produced ethylene to encourage ripening

 

Ethylene

•Induces the abscission of leaves

–trees lining streets with gas street lamps (which give off some ethylene) may drop their leaves early

–causes breakdown of cell walls and pectin in the abscission zone

 

Apical Dominance

•Suppression of the growth of lateral or axillary buds while promoting growth of the apical bud

•Auxin and/or cytokinin mediated

–auxin produced in apical bud

–inhibits growth of lateral buds as it flows down shoot

•Gardeners remove apical bud to encourage growth of lateral buds, resulting in bushier plant.

 

Apical Dominance

•Strong in trees with tall, conical form (true firs, Douglas-fir, cottonwood)

•Weak in trees with extensive branching (willows, maples, elms)

 

Plant Movements

•Tropisms

–growth movements

–response to external stimuli (light, gravity…)

–controlled by hormones

•auxin shifted to side away from stimulus

•Nastic movements

–growth movements

–triggered or controlled by internal processes

–controlled by hormones

Helical (Spiraling) movements

Nodding movements

•example: hypocotyl hook of emerging bean seedling nods side to side

Twining movements

Folding Nastic movements

•New leaves wave up and down as top side then bottom expand

•Flowers open and close

–cells on inside of petals elongate first, flower opens

–cells on bottom side of petals expand, flower closes

•Circadian rhythms

–daily cycle of movements in living organisms

 

Tropisms - Phototropism

•Response to light. Plant grows toward light (positive) by expanding stem growth on side away from light.

•Auxin shifts from light side to dark side of stem

Gravitropism (or Geotropism)

•positive (in the direction of gravity)

•roots grow down

Gravitropism (or Geotropism)

•negative (opposite from the direction of gravity)

•stems grow up

Gravitropism (or Geotropism)

•Auxins are translocated in stems to downward side

•Cells on that side expand

•Causes bending in upward direction

Other tropisms

•thigmotropism - contact (can respond with twining)

•hydrotropism - water (roots grow towards it)

•heliotropism - sun (flowers follow it)

–remember video of arctic poppies following sun as it circles the horizon, collecting solar heat like a satellite dish

 

Temperature control

•Plants need to control timing of events:

–Germination

–Budburst

–Reproduction

–Dormancy

•Why?

•Chilling period, followed by warmer temps and/or photoperiod cue

•Vernalization

•Giberellin often involved

 

Photoperiodism

•Plants sense the change in day length as cue to begin flowering

•Named for length of light or daytime, but it’s the length of night that affects plants

•Critical Day-length

–Short-day plants - flower in spring or fall

–Long-day plants - flower mid-summer

–Day-neutral plants - common in tropics where day length doesn't change much