Chapter 32 Plant Growth and Development

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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

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• Repeated cell divisions produce a seedling with a primary root.

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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.

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

Plant Hormones

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

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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

With

Grapes (Gibberellins)

Without

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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

Auxins Picture

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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)

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

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

Roots moving down toward gravity

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• 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

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• 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

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.

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?

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

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

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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

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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

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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.

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.

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.