LESSON 7: GROWTH RESPONSES AND REGULATION OF GROWTH

Internal Genetic Factors

The ultimate control of plant growth and development is genetic. If the genes required for development of a particular trait are not present, that characteristic does not develop.

The location of a cell in the young plant body also has a profound effect on gene expression during development

Plant hormones

Organic compounds that act as chemical signals between cells.

External environmental Factors

Environmental cues, such as:

temperatures, exert an important influence changing day length and changing on gene expression and hormone.

Tropism

Directional growth response results

in a change in the position of a plant part.

Are irreversible and are positive (the

plant grows toward the stimulus) or negative

(away from it).

Phototropism

Growth of a plant caused by light. Blue light (wavelengths between 400 and 500 nm) triggers the bending response.

Photoreceptor

Light-sensitive substance that absorbs blue light and triggers the phototropic response and other blue-light responses such as stomatal opening is a family of yellow pigments called phototropins.

Gravitropism

Plant growth in response to the direction of

gravity.

Root cap

Site of gravity perception in roots. It contains

amyloplasts which initiate the gravitropic response.

Thigmotropism

Plant growth in response to contact with a

solid object. Example is the twining or curling growth of tendrils or stems, which helps attach a climbing plant such as a vine to some type of support.

Plant hormones

A plant hormone is an organic chemical compound that acts as a chemical signal, eliciting a variety of responses that regulate growth and development.

Photoperiodism

Describe a plant's ability to flower in response to changes in the photoperiod: the relative lengths of day and night.

Wightman Garner and Henry Allard

Conducted experiments to test the effect of day length on flowering. They discovered that plants such as barley flowered when the day length was longer than a certain critical length.

Short - day

Long - day

Intermediate- day

Day - neutral plant

Plants are classified into 4 main

groups

Short - day

Also called long-night plants

Flower when the night length is equal to or greater than some critical

period.

The minimum critical night length falls between 12 and 14 hours for many.

Examples of short-day plants are florist’s chrysanthemum and poinsettia.

Long - day

also called short-night plants

flower when the night length is equal to or less than some critical length

Radish, Spinach, Lettuce are example

Intermediate - day

cannot be categorized as either short- or long-day plants and thus are termed

intermediate-day plants. They need longer days than short-day plants, but shorter

days than long-day plants in order to flower. Examples are grasses and sugar canes.

Day - neutral plant

Do not initiate flowering in response to seasonal changes in the period of daylight and darkness but instead respond to some other type of stimulus, external or internal.

Tomato, dandelion, string bean are examples of day-neutral plants. Many of these plants originated in the tropics, where day length does not vary appreciably during the year.

Phytochrome

A blue-green proteinaceous pigment involved in many plant responses to light, independent of photosynthesis.

Pr

(for red-absorbing phytochrome), strongly absorbs red light with a relatively short wavelength (660 nm).

Pfr

Is red light with a relatively long wavelength (730 nm. The active form of phytochrome, triggers or inhibit responses such as flowering.

Vernalization

The low-temperature requirement for flowering in some plant species.

Some examples include beets, onions, winter wheat, cabbage, and turnips.

Circadian Rhythms

A biological activity with an internal rhythm that approximates the 24-hour day. From the Latin circum, “around,” and diurn, “daily”.

Help an organism respond to the time of day, whereas photoperiodism enables an organism to detect the time of year.

In plants affect such biological processes as gene expression, the rate of photosynthesis, and the opening and closing of stomata.

Sleep movements observed in the common bean and other plants are another example.

Phytochrome

Cryptochrome

2 photoreceptors

Phytochrome

Red light - absorbing

Cryptochrome

Violet light - absorbing

Turgor Movements

Reversible changes in the position of plant parts due to a change in internal water pressure and are often, but not always, started by contact with objects outside of the plant.

Examples of turgor movement in plants is the Leaves of Mimosa plant folds and droops down when anyone touch them.