Botany II (Plant Growth and Development)

Despite the significance of botany to our economy and quality of life, it is neglected all too often in our education system. In one respect this is a tragedy; but in another respect, it presents a career and commercial advantage to those who do study botany.

Plants have the potential to solve major problems - they can be used to create petrol for our cars, to reduce global warming, to purify polluted soil and air, to overcome food shortages. To achieve these and other things with plants though, we need to keep growing our knowledge of botany, and how to apply that knowledge to the problems of our modern and rapidly changing world.

Lessons include: Flower physiology; phytoperiodism; control of flower bud initiation and development; dormancy; effects of plant associations and competition; respiration and post harvest physiology; post harvest storage, transport, retailing and shelf life; endogenous and synthetic growth regulators; risks involved with plant growth manipulation.

Lesson Structure

There are 10 lessons in this course:

1. Flower physiology
   • Introduction
   • The flowering response
   • Genes control flowering
   • Physiological age
   • Minimum leaf number
   • Photoperiodism
   • Terminology
2. Phytochrome
   • Light sensing systems
   • Blue light responses
   • Red light responses
   • Other light responses
   • Phytochrome
   • Photoreceptor forms: Pr, Pfr
   • How molecules change
   • Relevance to commercial horticulture
   • Controlling light
   • Terminology
3. Photoperiodism
   • Light
   • Measuring light
   • What wavelengths do plants need
   • Typical photoperiod responses
   • Photoperiodic responses in seasonal flowering plants
   • Photoperiodic classification of plants: short day plants, long day plants, day neutral plants
   • Detection of photoperiod
   • Critical photoperiod and flowering
   • Research facts
   • Other photoperiodic effects
   • Terminology
4. Control of flower bud initiation and development
   • Stages in flower bud growth
   • What can affect flower bud initiation
   • Differentiation
   • Development
   • Anthesis
   • Effect of temperature on growth and flowering
   • Vernalisation
   • Thermoperiodism
   • Research reports or reviews of specific plants
   • Terminology
5. Dormancy
   • Dormancy in plants
   • Abscisic acid and dormancy
   • Breaking dormancy
   • Dormancy in seeds
   • Factors affecting seed dormancy
   • Breaking seed dormancy
   • Terminology
6. Effects of plant associations and competition
   • Introduction
   • Competition
   • Parasitism
   • Coevolution
   • Mutualism
   • Plant herbivore and pathogen interactions
   • Crop spacing and crop yields
   • Crop canopy and plant density
   • Impact of weeds
   • Protected environments
   • Greenhouses
   • Shadehouses
7. Respiration and post harvest physiology
   • Respiration
   • Glycolysis
   • Aerobic respiration
   • Anaerobic respiration
   • Bioluminescence and Fluorescence
   • Post harvest respiration
   • Terminology
8. Post harvest storage, transport, retailing and shelf life
   • Effect of growing conditions on post harvest life
   • Controlled storage conditions: temperature, atmosphere, humidity
   • Normal atmospheric conditions
   • Controlled and modified atmospheres
   • Effect of oxygen levels Effect of carbon dioxide levels
   • Ethylene
   • Controlling ethylene levels
   • Modified Atmosphere Packaging
   • Commodity transport
   • Retailing and shelf life
9. Endogenous and synthetic growth regulators
   • Nature of plant hormones
   • Auxins: IAA, IBA, NAA
   • Gibberellins: natural and synthetic
   • Cytokinins: over 130 different types
   • Abscisic acid
   • Ethylene
   • Other hormones: anti auxins, growth inhibitors, growth retardants, defoliants, growth Stimulators, non standard hormones
   • Controlled ripening and degreening
   • Waxing
10. Risks involved with plant growth manipulation
    • Commercial risks
    • Human health and safety risks
    • Plant pathology risks
    • Ecological risks
    • Genetic modification
    • Benefits
    • environmental hazards
    • Human hazards
    • Terminology

Each lesson culminates in an assignment which is submitted to the school, marked by the school's tutors and returned to you with any relevant suggestions, comments, and if necessary, extra reading.

Aims

• Investigate the physiology of growth development and flowering.
• Examine the nature of phytochrome and its effect on flowering in the phytochrome reaction.
• Examine the photoperiodic responses of flowering plants to differing dark and light periods.
• Examine the effect of temperature on the onset of flowering and flower development.
• Understand and describe the causes of dormancy in seeds and plants and describe the methods of breaking dormancy.
• Understand plant associations and competition and their effects on quality and marketable yield.
• Explain the process of respiration in plant cells and its effect on post-harvest storage and transportation of crops.
• Describe physiological processes in post-harvest crops in relation to the storage conditions.
• Investigate the effect on plants of endogenous and synthetic growth regulators.
• Understand risk assessments relevant to plant growth manipulation.

WHY IS IT IMPORTANT TO UNDERSTAND BIOCHEMISTRY?

There are lots of reasons:

• Our bodies are biochemical machines - understanding biochemistry is to understand critical aspects of our existance.
• Most of what we eat comes from plants, either directly or indirectly. Even meat we eat comes from animals that eat plants. We need plants to grow well, in order to maintain our food supply; and by understanding how they grow, we can grow them better.
• Plants affect our environment. Without them, we would have greater extremes of temperature, poorer air quality and perhaps an unliveable environment
• Plants give us fibres for clothes, medicines for when we are sick, building materials and more 

Extract from Course Notes

There are two forms of photoreceptors:

1. “Pr” - Pr absorbs red light
2. “Pfr” - Pfr absorbs far-red light.

During sunlight much of the phytochrome in a plant exists as Pfr. In darkness, as in the night, Pfr levels decline. The relative levels of the two forms of the pigment give a plant a way of detecting the light-dark transition.

When a molecule of Pr absorbs a photon of 660-nanometre (the wavelength involved in the flowering response) light, the molecule is converted to Pfr in a matter of milliseconds. When a molecule of Pfr absorbs a photon of 730-nanometre (in the far-red region) light, it is quickly reconverted to the Pr form. These are called photo conversion reactions.

Pfr is biologically active and will trigger certain processes such as seed germination, whereas Pr is inactive which cancels the effect of the prior red light. The Pfr and Pr act as a biological switch turning certain processes on or off.

How Pr and Pfr Molecules Change

When a molecule of Pr absorbs red light (660nm), it is converted to Pfr. When a Pfr molecule absorbs far-red light (730nm), this molecule is converted back to Pr. This process is called photo conversion reaction.

In seed germination, red light is converted to far-red light, thereby inducing germination. Since Pr absorbs red light most efficiently, this light will convert a high proportion of the molecules to the Pfr form, in so doing, inducing germination. Successive far-red light absorbed by Pfr will convert all of the molecules back to Pr, cancelling the effect of the prior red light.

In flowering plants, the amount of net transformation from the inactive to active affects the flowering mechanism during the course of a day. White light contains both red and far-red wavelengths therefore both forms of the pigment are exposed at the same time to photons that promote their photoconversion. After a few minutes in the light an equilibrium is established where the rates or conversion of Pr to Pfr and Pfr to Pr are equal and the proportion of each is constant.

When plants are switched to darkness, the level of Pfr declines over a period of several hours. If a high level of Pfr is regenerated by pulse irradiation with red light in the middle of the dark period it will inhibit flowering in short-day plants that would have flowered and promotes flowering in long-day plants that would not have flowered. 

Phytochrome is synthesised in the Pr form and accumulates in this form in dark-grown plants. Pr changes to Pfr when exposed to red light, which is present in sunlight. 

When a seedling tip emerges into the light, the etiolated growth pattern gives way to normal plant growth. In dicots, the hook unbends, the growth rate of the stem may slow down somewhat, and leaf growth begins. In grasses, growth of the mesocotyl stops, the stem elongates, and the leaves open. A dark-green bean seedling that receives five minutes of red light a day will show these effects on the fourth day. If this exposure of red light is followed by a five-minute exposure to far-red, none of the changes usually produced by the red light will appear.

Plants growing in a natural environment can detect shading by other plants due to the presence of phytochrome. Radiation of wavelength below 700 nanometres is almost completely reflected or absorbed by vegetation, whereas that between 700 and 800 nanometres (far-red) is transmitted. Therefore shaded plants have a higher equilibrium ratio of Pr to Pfr (more Pfr is converted to Pr) which results in a rapid increase of internodal elongation. With the competition for light under the canopy in a forest a plant’s ability to sense the level of light and adjust its growth accordingly has an important adaptive significance.

When the existence of phytochrome was discovered it was thought that its behaviour might explain the phenomenon of photoperiodism. However, the time measuring attribute of photoperiodism is not controlled by phytochrome and a more complex process was found to be occurring.

Why is an understanding of Light and Phytochromes important in Commercial Horticulture?

The quantity and nature of light is directly related to not only the rate at which plants grow; but also the type of growth that occurs in plants. Phytochrome is the most important chemical involved in a plants capture and utilization of energy from light.

 By understanding these processes, we can do many things to facilitate better commercial plant production, for example:

• diagnose problems that result from a deficiency of appropriate light
• control rate of plant growth by controlling light
• manipulate type of plant growth by manipulating light
• ensure proper provision of light requirements when selecting where and how to grow crops
• provide artificial lighting systems for plants that provide the specific quantity and quality of light which is required by those plants
  

WHERE TO AFTER STUDY?