Page 263 - Biology_F5

Biology for Advanced Level Secondary Schools

(a) They are used to prolong the shelf life Abscisic Acid (ABA)
of fresh crops, such as cabbage and ABA is also known as stress hormone;
lettuce. because its production is stimulated by
(b) They break bud and seed dormancy. environmental stress or other adverse
(c) They keep flowers fresh. conditions, such as drought and water
logging. In contrast to other growth
FOR ONLINE READING ONLY
Ethene (Ethylene) substances such as auxins, gibberellins,
Unlike other plant hormones, ethene is in and cytokinins, which are plant growth
promoters, ABA is a growth inhibitor;
gas state. It is formed from the amino acid hence it acts antagonistically to the
methionine and released from ripening growth promoters. It is produced by

fruits, nodes of stems, ageing leaves, and any tissue containing chloroplasts. It is
flowers. It is produced as a metabolic concentrated in leaves, fruits, and seeds.
byproduct of most plant organs. It plays The term abscisic acid originates from the
the following role in plants: belief that it was a direct cause of leaf fall
(a) It stimulates ripening of fruits. (abscission), although this is now known to

(b) It is involved in axillary bud inhibition. be not true. Abscisic acid has the following
This occurs when auxins are role in plants:
transported from the apical meristem (a) It promotes dormancy and inhibits
of the stem downward, stimulating growth.
the production of ethylene, which (b) It causes abscission (fall of leaves and
suppresses axillary bud development. fruits). When fruit ripens, the level
of auxins which inhibits abscission
(c) It suppresses stem and root elongation, falls, while that of abscisic acid which
especially during physiological stress promotes abscission increases.
such as during drought.
(c) It promotes closing of stomata under
(d) It breaks bud dormancy. water stress conditions.

Application of synthetic ethylene in
crop production Application of synthetic abscisic acid
Synthetic ethylene has the following in crop production
application in agriculture: Abscisic acid can be sprayed on tree crops
(a) It promotes fruit ripening within a to regulate fruit fall at the end of the season.
relatively short period of time. This helps to ensure that fruits fall at the
(b) It is applied to citrus fruits to attain same time; hence avoiding the need for
attracting colours before being repeating picking of fruits over a long time
presented for sale. span.
(c) It induces flowering in pineapples.

(d) It stimulates the latex flow in rubber
trees.

Form Five Student’s Book
256