Chapter 26, Problem 1TC

Summary Introduction

To determine:

The observation could make to support the hypothesis that “abscisic acid (ABA) is responsible for the turgor pressure changes that permit a plant to track the sun”.

Introduction:

The plant hormones also called as phytohormones. The plant hormones are chemical substances that regulate the growth of plants. There are five types of hormones present in plants: Auxin, Gibberellins, cytokinins, abscisic acid and ethylene. Auxins regulate the growth of plants, Gibberellic acid regulates the elongation of stems, cyokinin regulate the cell division in plants, abscisic acid helps the plant to survive under stress condition and ethylene causes the ripening of fruit.

Explanation of Solution

Abscisic acid is also called as a plant stress hormone. Its main role is in abscission and dormancy. Abscissic acid function as inhibitor of plant growth and its metabolism. It also prevents the germination of plant. Abscissic acid, also called as the stress hormone because it helps plants to adapt in stress condition.

Abscisic acid causes the closing of stomata under water stress condition. The movement of water from higher concentration to lower concentration causes the turgor pressure. This movement of water is stimulated by the influence of temperature or thermal stimulation. It is believed that the sun is responsible for causing the thermal stimulation in many plants. This thermal stimulation result in the dry and warm environmental condition. The presence of abscisic acid helps the plant to survive under unfavorable or stress condition. It causes the closure of stomata in water stress condition to prevent the water loss.

The hypothesis “ABA is responsible for causing the turgor pressure” can be tested by following steps:

(i) Plants contain photosensitive sensors which cause the phototropism (movement of plant towards the sun). Thus, by changing the position of light an individual can test whether plants show phototropism or not.

(ii) The light source cause thermal stimulation in plants which results in the closing of stomata by the action of ABA. To test this response of plant, an individual could cover the plant stem with dark colored paper. This results that the plant will not respond against the thermal stimulation and an individual could check whether the response of ABA occurs or not.

(iii) The function of Gibberellins or abscisic acids is antagonistic. The Gibberellic acid promotes the germination of seeds while the ABA inhibits the seed germination. By incorporating the Gibberellic acid in plants, an individual could test whether the plant shows response against the Gibberellic acid or not. If plant responds to Gibberellic acid, it means that ABA is not present in plants.

Conclusion

The thermal stimulation causes the turgor movement of water in plants. This results the plant close their stomata by the action of ABA.