Physiological role of Auxins

A. Cell division:

Auxin has been found to be responsible for initiating and promoting cell division
in certain tissues eg. Cambium. Whenever wound is caused in the plant a swelling called
callus is developed because of the proliferation of the parenchyma cells stimulated by
auxin and a chemical substance traumatic acid. This can be put to practical use in grafting
where the callus plays an important role in strengthening the union between stock and
scion. Hence, during grafting of grapes, it was found that immersion of stock and scion in
0.1 percent of IAA resulted in quick growth of callus and success of graft union.

B. Root initiation:

The stem cuttings of some plants readily form adventitious roots when put in the
soil. Adventitious root formation will takes place at the basal end of stem cuttings.
Cuttings of such plants which do not readily root, form abundant roots when treated with
auxins. Synthetic auxins like IBA, NAA are particularly very effective. In general cuttings
of herbaceous plants readily respond to auxins while those of woody perennials like
eucalyptus, mango and others fail to respond to auxin application.
In air layering of guava auxins like IBA at 500 ppm is used for root initiation.

C. Apical dominance :

The presence of apical bud causes a complete or partial inhibition of lateral buds.
This is due to the presence of higher concentration of auxin at the apical bud, which causes
a preferential movement of nutrients towards it.

In plants like sunflower, the main stem continues to grow and the lateral buds in the
axils of leaves do not emerge and grow into branches. However, when the tip of the main
stem is cutoff or when it terminates into an inflorescence, the lateral buds emerge into
branches.

In either case the influence of the shoot apex on the lateral bud is lost. This
phenomenon of inhibition of laterals by the shoot apex is termed as apical dominance.
In plants like potato and tomato apical dominance is weak consequently the apical
growing point of the main stem fails to suppress the emergence of lateral buds. Such plants
are therefore extensively branched and bushy.

D. Inhibition of abscission layer:

Abscission is a process of dissolution of the middle lamella and primary walls of the
cells at the base of the petiole, pedicle or peduncle. Abscission refers to detachment of
plant organs. It is a balance between the inhibition of auxin and promotion of substances
like ABA, Ethylene etc. Several auxins (2,4-D, IAA, NAA) inhibits the abscission of both
leaves and fruits. Ex: application of NAA at 20 to 30 ppm twice at 15 days interval reduces
flower and fruit drop in chillies and cotton. 2,4-D prevents defoliation in cabbage and
cauliflower that often occurs during harvest.

E. Flower initiation :

Application of auxin inhibits flowering in several photoperiodically sensitive plants
such as xanthium, soybean and others. But the exception is pineapple (Ananas comosus),
a day neutral plant. This plant can be made to bloom promptly with the application of
NAA or 2,4-D. However in this plant the effect of auxin is not direct but is mediated
through ethylene formation.

Application of auxin also alters the sex ratio of flowers. In monoecious cucurbits
increase the number of female flowers but the decrease the number of male flowers.
Similarly in dioecious plants like cannabis, the male plants start producing female flowers
when auxin is applied. Here again the effect of auxin is not shown to be direct but is
mediated through ethylene formation.

F. Production of parthenocarpic fruits:

It is a general observation that in the absence of pollination and fertilization the ovary
of the flower does not develop into the fruit, but the flower abscises and falls. However
application of auxin causes development of ovary into the fruit in several plants such as
tomato, brinjal and others. Such fruits are seedless as these have developed without the
normal process of fertilization these are known as parthenocarpic fruits. The presence of
large number of seeds in a fruit lowers its commercial value in the canning industry.

G. Eradication of weeds:

Plant roots are extremely sensitive to auxins. Very high concentration of auxins over
stimulates growth promoting activities of root cells resulting in distorted roots with
blocked sieve tubes. The roots ultimately decay and the plant are killed. 2,4D and 2,4,5-T
are effective weedicides at higher concentration of 1 to 3 percent. 2,4D is selective weed
killer. It is highly toxic to broad leaved plants or dicotyledons while relatively nontoxic to
narrow leaved plants or monocot.

H. Growth in thickness:

The stem of dicots and gymnosperms not increase in length but also in thickness.
Increase in thickness of the stem and root is due to radial growth. The process is termed
as secondary growth.

I. Vascular differentiation:

Not only the activation of cambial rings but also the differentiation of cambial
derivatives into xylem and phloem is also under the control of hormones. Interaction of
both auxin and GA is involved in this.

J. Prevention of lodging:

Naphthyl acetamide when applied on the base of oats and flax, they grow stiff, woody
and erect. Thus, lodging in these crop plants is prevented.

Leave a Reply