Growth Physiology Second Year - PowerPoint Presentation

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Growth Physiology

Second Year

Botany/Chemistry Students

Prof. Dr. Nemat M. Hassan

29-3-2020

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Plant hormones, phytohormones and plant growth regulators are all terms

They have been applied to the organic substances, produced naturally in higher plants, controlling growth or other physiological function at a site remote from its place of production and active in minute amounts.

Part IV

PHYTOHORMONES

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Five major hormones (or groups of hormones) are generally recognized as occurring in higher plants:

1) the auxins,

2) the gibberellins,

3) the cytokinins,

4) abscisic acid,

5) ethylene. In general terms, the auxins, gibberellins and cytokinins are plant growth promotors whilst abscisic acid and ethylene are plant growth inhibitors.

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AUXINS

The first of the plant hormones to be investigated were the auxins termed heteroauxin, or as it is known today, indole-3-acetic acid (IAA).

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DISTRIBUTION OF AUXINS:

Auxins appear to be synthesized mainly in meristemic tissues such as those of shoot and root apices, developing leaves, flowers and fruits

There are two general categories of auxins in plants: free and bound.

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Free auxins include diffusible auxins (which move out of the tissue quite readily)

Conversely, bound auxins are that released from plant tissues only after they are subjected to hydrolysis

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Free auxins include diffusible auxins (which move out of the tissue quite readily)

Conversely, bound auxins are that released from plant tissues only after they are subjected to hydrolysis

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PHYSIOLOGICAL EFFECTS OF AUXINS:

1) Cell elongation

The main target cells of auxin appear to be those in the process of differentiating close to the various meristems

The general response to auxin is cell elongation

Several theories were proposed to account for the action of auxin in cell elongation.

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It has been suggested that auxins decrease the osmotic potential of the cell,

increase the permeability of the cell to water,

induce the synthesis of RNA and protein (enzymes) for wall components,

and cause a reduction in wall pressure.

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(i.e., loosening or deformation), is the primary means by which auxins stimulate cell elongation

This in turn suggests that auxin stimulates the synthesis and deposition of new cell wall polysaccharides as well as increasing wall plasticity.

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2) Cell division and differentiation:

There is evidence that auxins are involved in cell division.

They appear to stimulate cambial activity.

It also appears to be responsible for the initiation of lateral roots, which originate from small groups of cambial cells in the pericycle.

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Auxin stimulates xylem and phloem differentiation in addition to the promotion of root initiation and the regulation of callus tissue morphogenesis.

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3) Callus formation:

In many tissue cultures where callus growth is quite normal, the addition of auxin is necessary for continued growth of such callus.

The amount of callus tissue formed is related to the concentration of auxin. The presence of auxin in addition to cytokinins is very important for callus development.

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4) Root initiation:

the concentrations of auxins that are stimulatory to stem growth are inhibitory to root growth.

roots are much more sensitive to auxins than are stems and real stimulation of root elongation may be achieved if low enough concentrations are

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5) Nucleic acid and protein changes:

the stimulation of growth by IAA may be preceded by an increase in RNA synthesis.

IAA might interact at the gene level, causing changes in RNA and protein synthesis.

It was found that inhibitors of RNA and protein synthesis would inhibit auxin-induced growth.

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6) Phototropism:

When a growing plant is illuminated by a unilateral light, it responds by bending toward the light

The bending of the plant is caused by cells on the shaded side elongating at a much greater rate than cells on the illuminated side.

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This differential growth response of the plant to light is called phototropism. This phenomenon is caused by an unequal distribution of auxin,

This unequal distribution of auxin might be a result of light-induced inactivation of auxin on the light side; light induced lateral transport of auxin, or a result of inhibition of basipetal transport of auxin.

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7) Geotropism:

If an intact seedling is placed in a horizontal position, it will respond to the earth's gravitational field with a particular pattern of growth.

The stem will curve upward until it is vertical again,

and the root system will curve downward until it too is vertical again.

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The differential growth exhibited by a horizontally placed stem, or root is due to the accumulation of auxin on the lower side.

This accumulation of auxin on the lower side of the horizontally placed stem causes accelerated growth on that lower side and results in stem curvature upward (negative geotropism).

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Conversely, a horizontally placed root will exhibit a positive geotropism when auxin concentrates on the lower side.

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8) Apical dominance:

Apical dominance means the dominance of apical over lateral growth in a great many species of plants.

The apical, or terminal bud of many vascular plants shows a great activity in growth and lateral buds remain inactive.

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The same phenomenon is also observed in the new shoot growth of many trees species.

Tall and unbranched plants reflect a strong influence

while short and shrubby plants give evidence of a weak influence of apical dominance.

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The strong influence of the apical bud on the growth of lateral buds is easily demonstrated by removing it from the plant

Apical dominance might be attributed to the auxin produced at the terminal bud and transported downward through the stem.

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9) Abscission:

Leaf abscission is an important dynamic process in the replacement of leaves during the plant vegetative cycle

The most important factor controlling abscission is the condition of the auxin gradient across the abscission zone

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Abscission occurs when the gradient becomes slight or neutral and is accelerated when the gradient is reversed.

auxin and ethylene appear to be the more hormones that control abscission. Ethylene is certainly the main promoter of abscission.

It is known that ethylene promotes abscission because of its direct effects on the stimulation of cellulase synthesis

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10) Parthenocarpy:

With pollination and the subsequent fertilization of the ovule of a flower, the complex growth patterns leading to fruit-set begin.

Growth of the ovary wall and, in some cases, the tissues associated with the receptacle is greatly accelerated.

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Growth of the ovary wall and, in some cases, the tissues associated with the receptacle is greatly accelerated.

Most of this acceleration of growth is due to cell enlargement, a phenomenon that is known to associate with auxins.

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Pollination and fertilization are connected with development of the fruit. However, fruit development in the absence of pollination occurs and is, in fact, relatively common in plants.

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The development of fruit in this manner is called parthenocarpic development and the fruit that is formed is called parthenocarpic fruit.

Natural parthenocarpic fruit development is common in plants.