Plant growth and development

The need to explain tropisms:re-direction of growth in response to

light, PHOTOTROPISMgravity, GEOTROPISMtouch, THIGMOTROPISM

The need to explain patterns of development:production of flowers,

development of fruits,senescence of foliage,response to wounds

PHOTOMORPHOGENESIS

The cress shoot was grown in unidirectional light from the right. There is a rapid curving response in the apical region which moves down the stem. Straightening of the upper stem in the later stages of the sequence makes the shoot appear to stop curving, but close examination of the lower stem shows there is still a response towards the light source.

http://www-users.york.ac.uk/~drf1/tropism/tropisms.htm

Phototropism

180 frames filmed over 4 hours at 0.013 f.p.s.

A young, vertically growing, sunflower seedling shoot placed in a horizontal position. The shoot curves until it is once again growing vertically. This gravitropic response is first observed in the apical region of the shoot. Rapid curving at the tip progresses along the whole stem. The rate of curvature is not necessarily constant along the stem and is complicated by subsequent straightening of curved areas known as autotropism.

Negative gravitropism in a shoot

180 frames filmed over 5 hours at 0.01 f.p.s.

Fig. 33.1C

Experimental analysis of shoot phototropism

A sequence of experiments started by the Darwins and continued by Boysen-Jensen

Components of experimentsAn experiment has:

2. Treatment

4. Control

5. Replication

And must be

6. Repeated

1. Hypothesis

3. Measurement

A statement predicting alternative responses: “If this is done that will happen otherwise it will not.”

A specific, designed, manipulation

sufficiently accurate to detect response to the treatment

The same measurement is made but the treatment is not applied. This provides the essential contrast.

Enables the degree of response to be defined and helps to protect against obtaining results by chance

Required to establish the degree of certainty that can be attributed to a result, e.g., repetition with the same and different species

The initial observation The first experiment

2. Treatment

4. Control

5. Replication

And must be

6. Repeated

1. Hypothesis

3. Measurement

Deficiencies?

to remove the effect

Expts 2 and 3

2. Treatment

4. Control

5. Replication

And must be

6. Repeated

1. Hypothesis

3. Measurement

The initial observation

Does the postulate change for Expt 2 and 3?

What are the improvements over the first experiment?

What are the treatments and what the controls in Expt2 and Expt3?

Expt 1 Expt 2 Expt 3

Expt4

The initial observation What type of an experiment is this?

2. Treatment

4. Control

5. Replication

And must be

6. Repeated

1. Hypothesis

3. Measurement

Has the postulate changed from that of Expt 4?

What are the improvements over Expt4 ?

Which is the treatments and what are the controls?

Boysen-Jensen’s experiment

2. Treatment

4. Control

5. Replication

And must be

6. Repeated

1. Hypothesis

3. Measurement

The initial observation

Fig. 33.1D

Went’s experiments

Is this really a control?

Chemicals are produced in small quantities that change the rate at which growth takes place and/or the types of cells that are produced.

These chemicals are usually produced by meristematic tissue and are actively transported from that tissue. The chemicals influence the development of cells according to the concentration that accumulates in the developing cells.

Plant growth substances, the text book calls them hormones

Auxin

Cytokinin

Ethylene

Abscisic Acid

Gibberellin

Meristems of apical buds,embryo of seed, young leaves

Synthesized in roots and transported to other organs

Tissues of ripening fruits, nodes of stems, senescent leaves and flowers

Leaves, stems, green fruit

Meristems of apical buds and roots, young leaves, embryo

Stimulates cell elongation; involved in phototropism, gravitropism, apical domincance, and vascular differentiation; stimulates ethylene synthesis and induces adventitious roots on cuttings

Stimulates cell division, reverse apical dominance, involved in shoot growth, delay leaf sequence

Stimulates fruit ripening, leaf and flower senescence, and abscission

Inhibits growth, stimulates stomatal closure, maintains dormancy

Stimulates shoot elongation, stimulates bolting and flowering in biennials, regulates production of hydrolytic enzymes in grains

Major Functions Where ProducedFive plant growth substances and their functions

Fig. 33.1B

Auxin and phototropism

Auxin, indole acetic acid (IAA), is transported downwards from the shoot apex.

In a seedling exposed to light from the side the concentration of IAA is greater on the shaded side of the shoot

Results of experiments that applied IAA

Fig. 33.3B

1. The chemical may have different effects at different concentrations

2. It can affect different tissues differently

Polar transport of auxinTransport at ~1 cm/hr implies active transport

Picks up a hydrogen ion at the acid wall environment

Passes across membrane as a neutral molecule

Gives off the H+ into the cell which induces the proton pump

Auxin can only exit the cell at its basal end where there are specific carrier proteins

The acid growth hypothesis

Gibberellins

Gibberellins act in the elongation of intact plants as opposed to stem section elongation by auxin.

Much research on plant gibberellins has been possible due to gibberellin sensitive mutants. They have adequate levels of GA1 (the GA species most likely to be responsible for stem elongation) but can not respond to it. This may be due to lack of receptor protiens.

•Reverses dwarfism – the first discovery of gibberellin •Seed Germination--Barley de novo amylase synthesis (Varner 1964) •Can cause bolting in biennials •Control of sex expression •Can enhance fruit growth – e.g., seedless grapes •Delays Senescence

Transport is non polar, has been found in both the transpiration and

translocation stream. It can occur more rapidly, 5 cm/hr, than auxin

There are many gibberellins – closely related chemically

BioassayUsing an organism or other living material to determine the level of an environmental condition

1. Establish pattern of response under standardized conditions

2. Use the established pattern in analysis of treatments

Advantages: cost, useful where the treatment has not been precisely defined yet, e.g., substances ‘like’ gibberellin.

Fig. 33.11

The effect of day length on flowering

Autumn flowering plants,e.g., chrysanthemums

Summer flowering plants, e.g., iris

Fig. 33.12A

Flowering response can be manipulated by short periods of red or far-red radiation applied during the dark period of a long night regime

PhytochromeThe control of flowering is determined by a substance called phytochrome that exists in two forms.

660 nm

730 nm

All plants contain phytochrome – but they may respond differently to the relative amounts of the two forms

Phytochrome is involved in other plant growth process in addition to flowering

Control through the relative amounts of different Plant Growth Substances

Abscissic acid as a growth inhibitor gibberellin as a promoter

For cells growing in culture:

Cytokinins added have no effect on their own.

Cytokinins plus auxin cause cells to divivde.

If the concentrations are about equal the cells continue to grow and form a callus but there is no cell differentiation.

If there is more cytokinin than auxin then shoot buds develop.

If there is more auxin than cytokinin then roots develop

This hormone concept as developed for animals has some distinct differences from what we know of the production, distribution and function of plant growth substances.

Are plant growth substances ‘hormones’?

3. Plants no equivalent to the central nervous system that integrates and co-ordinates physiological activities.

1. Transport in plants is very different from that in animals. Its is frequently polar

A hormone is a regulatory chemical that travels in the blood from its production site and affects other sites in the body often at some distance. Hormones are made and secreted by organs called endocrine glands.

2. The range of plant growth substances that we know are produced and possibly distributed in different ways

Sections you need to have read

33.1 through 33.12

Courses that deal with this topic

Botany 371/372 Plant physiology laboratory