1. Chapter 32 Plant Growth and Development

2. How Do Seeds Germinate?
Germination is the resumption of growth after a time of arrested embryonic development
Environmental factors influence germination
Spring rains provide the water amounts necessary to swell and rupture the seed coat (taking in water is imbibition)
Oxygen moves in and allows the embryo to switch to aerobic metabolism
Increase temperatures and number of daylight hours

3. Continue…
Repeated cell divisions produce a seedling with a primary root.

4. Genetic Programs, Environmental Cues
Patterns of germination and development have a heritable basis dictated by a plant’s genes
Early cell divisions may result in unequal distribution of cytoplasm
Cytoplasmic differences trigger variable gene expression, which may results in variations in hormone synthesis
Even though all cells have the same genes, it is the selective expression of those genes that results in cell differentiation.

5. Growth and Development
Growth and development are necessary for plants to survive
Growth is defined as an increase in the number, size, and volume of cells
Development is the emergence of specialized, morphologically different body parts

6. Plant Hormones
Plant hormones have central roles in the selective gene expression underlying cell differentiation and patterns of development.

8. Types of Plant Hormones
Gibberellins: Promote stem elongation
Help buds and seeds break dormancy and resume growth in the spring.
In some species, they influence the flowering process.
Cytokinins: stimulate cell division in root and shoot meristems, where they are most abundant; they are used commercially to prolong the life of stored vegetables and cut flowers

9. Grapes (Gibberellins)
With
Without

10. Continue (Plant Hormones)…
Auxins: affect lengthening of stems and coleoptiles (the protective cylinder that covers and protects the tender leaves during germination)
May participate in growth responses to light and gravity.
Indoleacetic acid (IAA) is applied to fruit trees to promote uniform flowering, set the fruit, and encourage synchronous development of fruit.
Synthetic auxins (such as 2,4-D) are used as herbicides

11. Auxins Picture

12. Continue (Plant Hormones)…
Abscisic Acid (ABA)inhibits cell growth, helps prevent water loss (by promoting stomata closure), and promoting seed and bud dormancy.
Ethylene stimulates the ripening of fruit and is used commercially for this purpose.
Other less well known hormones trigger flowering and inhibit the growth of lateral buds (apical dominance)

13. What are Tropisms? A plant tropisms is a growth response
Evidenced by a turning of a root or shoot toward or away from an environmental stimulus
Hormones mediate the shifts in rates at which different cells grow and elongate to cause the overall response

14. Types of Tropisms
Gravitropisms: is the growth response to gravity– shoots grow up, roots grow down.
Auxins, together with a growth—inhibiting hormone, may play role in promoting, or inhibiting, growth in strategic regions
Statoliths, which are unbound starch grains in the plastids, respond to gravity and may trigger the redistribution of auxin

15. Roots moving down toward gravity

16. Continue (Tropisms)… Phototropisms: is a growth response to light
Bending toward the light is caused by elongation of cells (auxin stimulation on the side of the palnt not exposed to light).
Flavoprotein, a pigment molecule probably plays a role because of its capacity to absorb blue wavelengths of light

18. Continue (Tropisms)…
Thigmotropism is shift in growth triggered by physical contact with surrounding objects.
Prevalent in climbing vines and in the tendrils that support some plants
Auxin and ethylene may have roles in the response

20. Response to Mechanical Stress
Response to the mechanical stress of strong winds explain why plants grown at higher mountain elevations are more stubby than their counterparts at lower elevations
Human interventions such as shaking can inhibit plant growth.

21. How Do Plants Known When to Flower?
Phytochrome: Alarm button for plants
Biological Clocks are internal time-measuring mechanisms that adjust daily and seasonal patterns of growth, development, and reproduction
Phytochrome– a blue-green pigment, is alarm button for some biological clocks in plants
Phytochrome- can absorb both read and far-red wavelengths with different results.
When is the pigment activated?
When is the pigment inactive?

22. Continue…
Some plants activities occur regularly in cycles of 24 hours (circadian rhythms) even when environmental conditions remain constant

23. Flowering – A case of photoperiodism
Photoperiodism is a biological response to a change in relative length of daylight and darkness in a 24-hour cycle; this resetting of the biological clocks is necessary to make seasonal adjustments

24. Continue…
The flowering process is keyed to changes in daylength throughout the year.
Short-day plants: flower in late summer or early autumn when daylength becomes shorter
Example: Poinsettias
Long-day plants: flower in the spring as daylength becomes longer
Example: Spinach
Day-Neutral Plants: flower when they are mature enough to do so
Example: Roses

26. Senescence
The dropping of leaves, flowers, fruits is called abscission
Senescence: is the sum total of the processes leading to the death of plant parts or the whole plant
Decrease of daylight hours trigger the reduction of auxin
Cells in abscission zones produce ethylene which causes cells to deposit suberin in their walls

27. Entering and Breaking Dormancy
Dormancy occurs in autumn when daylight shortens and growth stops in many trees and nonwoody perennials– it will not resume until spring
Strong cues for dormancy include in short days, cold nights, and dry, nitrogen-deficient soil.
Dormancy has great adaptive value in preventing plant growth on occasional warm autumn days only to be killed by later frost.

28. Vernalization
Vernalization is the stimulation of flowering only after plants have been exposed to lower temperatures (winter).
Deliberately exposing seeds to lower temperature to stimulate flowering the next season is common agricultural practice.

29. Breaking Dormancy
Dormancy is broken by milder temperatures, rains, and nutrients.
It probably involves gibberellins and abscisiic acid, and require exposure to specific periods of low temperatures.