Stimuli In Plants

Question 1

Define photomorphogenesis?

Answer 1

• Light triggers many key events in plant growth and development, collectively known as photomorphogenesis.

Question 2

What does a plant detect from light

Answer 2

• Plants detect not only light signals, but also the direction, intensity and wave length (Color)

Question 3

State a graph which shows the dependency of plants on light to fulfill its functions like photosynthesis

Answer 3

Action spectrum

Question 4

2 important colors in regulating photomorphogenesis

Answer 4

Red

Blue

Question 5

How can a plant measure the passage of days and seasons

Answer 5

By light reception

Question 6

Define an action spectrum

Answer 6

An action spectrum is a graph showing the effectiveness of different wave lengths of light in stimulating the photosynthesis or any other plant processes

Question 7

2 major classes of light receptors

Answer 7

Blue light receptors

Phytochromes ( red wavelength is absorbed)

Question 8

State 3 functions initiated by blue light receptors

Answer 8

phototropism,
the light induced opening of stomata ,
light induced slowing of hypocotyl elongation that occurs when a seedling breaks ground.

Question 9

State 2 functions regulated by phytochrome

Answer 9

Seed germination

Shade avoidance

Question 10

4 processes in plants which is effected by the light.

And explain how or why do they depend on light

Answer 10

Seed germination

• As the nutrient reserves are limited, many types of seeds germinate only when the light environment and other conditions are nearly optimal.
• Such seeds often remain dormant for years until light conditions change.
(e.g.- Plowing a field or a death of a shady tree may create a favorable light environment for germination)

Plant spacing

• Phytochromes provide the plant with information about the quality of light which enables the plant to get adapted to changes in outside light conditions.
e.g. “Shade Avoidance” response of a forest tree (below the canopy) that requires relatively high light intensity.
As the forest canopy absorbs more red light allowing only far red light to pass through, the tree below the canopy will allocate more of its resources to grow taller.
• In contrast, exposure to direct sunlight increases the proportion of far red: red light and thereby stimulates branching and inhibits vertical growth.

Flowering

• Photoperiod is the interval in a 24hour period in which the plant gets exposed to light.
• Photoperiod controls flowering in many types of plants.

Shoot elongation

• The growth of a shoot towards light (positive) or away from it (negative) is called phototropism.
• Positive phototropism strengthen photosynthesis.
• This response results from a differential growth of cells on opposite sides of the shoot; the cells in the darker side elongate faster than the cells on the brighter side.

Question 11

State the photoreceptor which provide the plant with information about the quality of light which enables the plant to get adapted to changes in outside light conditions.

Answer 11

Phytochrome

Question 12

What is meant by photoperiod

Answer 12

• Photoperiod is the interval in a 24hour period in which the plant gets exposed to light.

Question 13

Define phototropism

Answer 13

• The growth of a shoot towards light (positive) or away from it (negative) is called phototropism.

Question 14

State whether shoot elongation a growth reponse shown by plant would consume ATP

Answer 14

Yes

It is a growth response which consumes ATP

Question 15

Explain phototropic movements shown by plants

Answer 15

Explain how IAA would deposit in the zone of cell elongation and how growth occur

Question 16

Explain statoliths hypothesis which describes the positive gravitropic movement in plants

Answer 16

The statolith hypothesis:
The aggregation of statoliths at the low points of root cap cells triggers re-distribution of Ca2+ which causse lateral transport of auxin within the root.
As a result, Ca and auxin get accumulated at lower side of elongation zone of root.
At high concentration of auxin, cell elongation is inhibited resulting slow growth on lower side and more rapid elongation on upper side. Consequently, the root grows downwards.

Question 17

State the tropic movement which occur 1st during seed germination

Answer 17

• Gravitropism occurs as soon as a seed germinates. This ensure that the root grows into the soil and shoot grows towards sunlight.

Question 18

Define Thigmomorphogenisis

Answer 18

Trees grow in windy environment normally have shorter stockier trunks than same species growing in normal environmental conditions.
Advantage of this is that the tree could stand high winds. This exhibits the sensitivity of mechanical stress of plants.

the changes in plant form due to mechanical disturbances is called thigmomorphogenesis.

Question 19

Define thigmotropism?

Answer 19

Climbing plants have tendrils that coil rapidly around support.
Tendril usually grows straight until it touches a support.
The contact stimulates differential growth on opposite sides of the tendril.

The directional growth of tendril towards support is called thigmotropism.

Question 20

Define thigmonasty

Answer 20

Touching results in a sudden loss of turgor of cells in a specialized motor organ called pulvini.
causing the leaflets to collapse.
This response is called thigmonasty.

Question 21

Example for nastic movement shown by plants

Answer 21

respond to touch by rapid leaf movements. E.g. Mimosa pudica collapses its leaflets when touched.

At low light intensity Sesbenia grandiflora collapse it’s leaves

Question 22

What are statoliths

Answer 22

• Statoliths of vascular plants are specialized plastids containing dense starch grains.

Question 23

Location of statoliths

Answer 23

• They can settle under the gravity, to the lower portions of the cell.
• In roots, they are located in certain cells of the root cap.