BOTANY Higher Secondary Second Year - Textbooks Online

Physiological effects of auxin
Auxins are well known to promote elongation of stem and coleoptile.
It promotes the growth by cell enlargement in stems, particularly by
elongation of cells behind the apical meristem.
Growth in lateral bud is inhibited when the apical bud of a tall plant
remains intact. However, the lateral bud grows rapidly on removal
of apical bud.
Suppression of growth in lateral bud by apical bud due to auxin
produced by apical bud is termed as apical dominance. The reason for
this is due to auxin produced in growing tip and it stimulates growth
but as it moves downward, suppresses growth in the stems below.
Auxin is responsible for initiation and promotion of cell division in
cambium, which is responsible for the secondary growth. This
property of induction of cell division has been exploited for tissue
culture techniques and for the formation of callus.
Auxin promotes growth of root only at extremely low concentrations.
At higher concentrations, it always inhibits growth of root.
When leaves and fruits mature, they shed from the stem. This is
called abscission. Auxin prevents abscission.
Seedless fruits are produced in tomato and apple, by external application
of auxin on flowers. Such seedless fruits are called parthenocarpic
fruits.
2,4 – Dichlorophenoxy aceticacid, a synthetic auxin is used to eradicate
weeds in the field.
Gibberellins
Gibberellin was first discovered in Japan by Kurusowa. He observed
from his field that some of the rice seedlings had grown much taller than
the others. On further observation, he found that such taller rice plants
had shown unusual internodal elongation. This internodal elongation is
known as the ‘bakanae’ or ‘foolish seedling’ disease of rice. Later, it
was discovered that the elongation was due to the action of a substance
produced by a fungus, Gibberella fujikuroi. This substance was successfully
isolated from the fungus and it was named as gibberellic acid.
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