ACS conduct two botany courses Botany I and Botany II, if you would like further information on one or both of these courses

    click here for Botany I 
or click here for Botany II

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Example of home study lesson

FLOWER PHYSIOLOGY

 

 

LESSON AIM

 

Investigate the physiology of growth development and flowering.

 

 

FLOWER PHYSIOLOGY

Plant growth is the process by which a plant increases in size, creating more leaves and stems. Plant development is the process by which plants change from one stage of growth to the next. These stages include juvenility, maturity, flowering and seeding. Plant development involves differentiation of the plant into specialised parts. This may include visible changes such as the structural organisation of the plant and new patterns of growth as well as less visible changes such as localised biochemical and metabolic activity.

 

Growth and development are characteristics not only of the entire plant, but also of each cell within that plant. Active cell division occurs in parts of the plant called meristems. Cell division results in growth in length and diameter of the plant, and in the differentiation of specialised plant organs. Differentiation results in the development of individual plant parts such as stems, leaves, new shoots, flowers, fruits, seed and other structures.

 

 

THE FLOWERING RESPONSE

 

For plants to flower they must first go through a vegetative phase, during which the main processes are elongation of the stem and roots and increase in stem girth. The end of the vegetative phase is marked by flower initiation, whereby the vegetative shoot apex undergoes a sequence of physiological and structural changes to become a reproductive apex.

 

The stimuli for flower induction includes hormonal changes and environmental changes, such as day length (photoperiod) and temperature.

 

Physiological age

The plant’s physiological age may be a determining factor in whether a plant is able to form flowers. Physiological age refers to the plant’s stage of development. The stages are embryonic growth, juvenility, transition stage, maturity, senescence and death:

· Embryonic growth – the growth and development of the seed within the parent plant

· Juvenility – following germination the plant increases in size as the cells enlarge and differentiate to form stems, leaves and roots. In some woody plants, this stage is characterised by the plant’s inability to form flowers, or the loss or reduction in the ability of cuttings to form adventitious roots.

· Maturity – this phase is marked by the formation and development of the sexual organs (the flower buds, flowers, fruit and seed).

· Senescence – the decline of the plant due to physiological change (due to the decline of cell division and reproduction) or environmental stress or pathogenic attack.

 

Many perennial species have two phases with distinctive morphological and physiological characteristics. The juvenile phase in such plants has the following characteristics:

  • the leaves may be morphologically different to those of the adult or mature phase
  • flowering cannot be induced
  • there is an increased ability of stem cuttings to form adventitious roots.

 

The adult or mature phase has the following characteristics:

  • the leaves may be morphologically different to those of the juvenile phase
  • flowering and reproduction can occur
  • rooting ability is diminished or lost

 

The juvenile phase may last from one year up to 40 or more years, but commonly in trees lasts for 5 to 10 years.

 

The English Ivy (Hedera helix) is a well known example of a plant showing distinct juvenile and mature phases. In the juvenile phase it grows as a creeping vine, with palmate, lobed leaves. In the mature phase Hedera becomes shrub-like and forms flowers and berry fruits. The leaves are entire and ovate. Frequent pruning prevents the formation of flowers and keeps the plant in a prolonged juvenile (vegetative) phase, hence ivy is most commonly seen in gardens as a creeper. (For this reason, propagators frequently prune mother stock plants, keeping them in a juvenile, vegetative stage which ensures high rooting ability in the cuttings taken from those plants.)

 

Juvenility may be induced in mature plants by treating shoots that develop from lateral buds with GA3 (discussed later in the course).

 

A similar condition is known as ‘ripeness to respond’ or ‘ripeness to flower’, whereby some species can only commence flowering in response to their environment (particularly to day length and temperature) when the organs that detect the environmental change (usually leaves and meristems) have reached physiological maturity.

 

Minimum leaf number

‘Minimum leaf number’ is the term used to describe the minimum number of leaves from seedling to earliest flower under the most ideal conditions for flowering.

 

The number of leaves on a plant is frequently used to monitor plant growth in cropping situations, particularly in cereals. Annual plants will frequently develop flowers early when under stress and the quality of plant development can thus be monitored by assessing the number of leaves at the outset of flowering. Various scientific methods exist to do this. Benchmarks vary with different crops.

 

Photoperiodism

Many kinds of plants are induced to flower in response to changes in the duration of light and darkness the plant receives (the ‘photoperiod’). Some plants will only flower when the periods of light exceed a critical length – these are known as ‘long day plants’. ‘Short day plants’ only flower when periods of light are less than a critical period. Plants that flower regardless of day length are known as ‘day neural plants’. Photoperiodism, and the pigment responsible for detection of light, phytochrome, are discussed in detail in the following two lessons.


 

TERMINOLOGY

Search for definitions of each of the following, either in a dictionary, your text book or through an internet search.

Hint: If doing an internet search, type the word you seek a definition for, followed by the word “definition"

Phytochrome:

Meristems:

Minimum leaf number:

 

SET TASK

Contact a production nursery. If this is not possible, contact any horticultural business dealing with growing plants from seeds or seedlings to the flowering stage. Ask them about species they grow, physiological age and minimum number of leaves to onset the flowering, and other factors affecting the flowering.

 

 

ASSIGNMENT

1. Explain how physiological age, minimum leaf number, and phytochrome are responsible for determining plant growth and development.

 

2. Explain in your own words the difference between plant growth and plant development.

 

3. Describe physiological age and its relationship to flowering.

 

4. List 10 different plants with minimum leaf number to the onset of flowering for each of them. Include plants grown by the business you visited in the Set Task .

 

5. List five different plants with different critical day length in relation to flower induction and state the critical day length for each of them. Include plants grown by the business you visited.