Question

Delay in senescence is caused by the spray of
A. IBA
B. GA
C. Cytokinin
D. ABA

Answer 1

Plant hormones also known as plant growth regulators, are the chemical messengers produced within the plant body, which helps them in responding to external and internal stimuli and for proper physiological functioning. There are broadly two types of hormones, i.e., promoters and inhibitors. All the hormones have their own unique functions in the plant body.

Complete answer: The process of ageing in plants is defined as senescence. Various internal and external stress factors, accelerate the process of senescence. Cytokinin is a plant hormone that promotes the growth process and cell division in plant bodies. It delays or extends the process of senescence. In old leaves the concentration of cytokinins is low, hence they undergo cell death. Thus external spraying of cytokinins helps in delaying senescence, especially in the leaves by preventing the degradation and proteins stored in them.
Considering the other given options:
-IBA or Indole butyric acid is a type of auxin which is a plant hormone. This was a synthetic auxin. The major function of it is to promote apical dominance, which makes the central stem to grow in size and length more than the lateral branches.
-GA or gibberellic acid is a hormone present in plants, which makes them respond to stress and take proper response. It makes the stems grow in length. It arrests the dormancy in seeds, thereby accelerating the germination process.
ABA or Abscisic Acid is an inhibitory plant hormone. Even though the actual role of the hormone is in the closing of the stomatal pore, it also promotes the process of senescence.
Thus, the most appropriate answer is C, i.e., senescence can be prevented by spraying cytokinins.

Note: Another class of plant hormone is ethylene. The major function of it is to accelerate the ripening of fruits. Also makes the leaves, fruits, and flowers fall by promoting abscission. Thus it is an inhibitory hormone.