11 Major Physiological Effects of Ethylene | Plants

The following points highlight the eleven major physiological effects of ethylene. The physiological effects are: 1. Fruit Ripening 2. Plumular Hook Formation 3. Triple Response 4. Formation of Adventitious Roots and Root Hairs 5. Inhibition of Root Growth 6. Leaf Epinasty 7. Flowering 8. Sex Expression 9. Senescence 10. Abscission of Leaves and 11. Breaking Dormancy of Seeds and Buds.

Physiological Effect # 1. Fruit Ripening:

One of the most pronounced effects of ethylene is in ripening of fruits and therefore, ethylene is also known as fruit ripening hormone. Different types of fruits react differently with exogenous application of ethylene. In climacteric fruits such as apples, bananas, tomatoes etc., exposure of mature fruits to ethylene result in respiration climacteric (marked increase in respiration during initiation of ripening) followed by additional production of ethylene leading to hastening of ripening process.

Addi­tional production of ethylene by ripening fruits is autocatalytic. But, in non-climacteric fruits such as citrus fruits and grapes, ethylene treatment does not cause respiration climacteric and additional ethylene production and the ripening process remains unaffected.

However, minimum threshold level of endogenous ethylene is essential for all types of fruits for ripening. This has been confirmed by experiments with transgenic tomatoes in which ethylene production was completely blocked by making expression of antisense version of ACC synthase or ACC-oxidase.

Ripening process was completely checked in these transgenic tomatoes which could be restored only by exogenous application of ethylene. In never ripe mutant of tomato also, ripening process is completely blocked due to mutation in ethylene receptor making it unable to bind with ethylene and preventing the latter to exert its hormonal effect.

Physiological Effect # 2. Plumular Hook Formation:

In etiolated dicot seedlings, the plumular tip (i.e., shoot apex) is usually bent like a hook. This hook shape is advantageous to seedling for penetration through the soil, protecting the tender apical growing point from being injured.

The plumular hook formation and its maintenance in etiolated (dark grown) seedling are due to formation of ethylene in that region which causes asymmetric or unequal growth on the two sides of plumular tip. Ethylene causes more rapid elongation of outer side of plumular tip than on its inner side. When the seedling is exposed to white light, formation of ethylene decreases, the inner side of the hook also elongates rapidly equalising the growth on two sides and the hook opens.

Red light is more effective in opening of plumular hook. This effect is reversed by expos­ing the seedling to far-red light. This red/far-red reversibility is indicative of the role of the pigment phytochrome in it.

When etiolated seedlings are exposed to light in presence of ethylene, the plumular hook fails to open. On the other hand, if seedlings are grown in dark along with an ethylene absorbant such as KMnO₄, the plumular hook opens.

It is believed that asymmetric growth on two sides of plumular tip resulting in hook formation and its maintenance in etiolated dicot seedlings is probably due to an ethylene de­pendent auxin gradient similar to that which develops during phototropic curvature.

Physiological Effect # 3. Triple Response:

Ethylene causes ‘triple response’ of etiolated seedling such as in pea which consists of:

(i) Inhibition of stem elongation,

(ii) Stimulation of radial swelling of stems and

(iii) Horizon­tal growth of stems with respect to gravity (i.e., diageotropism)

Triple response effects of etiolated seedlings were the first to be related to beginning of discovery of ethylene as natural plant growth hormone.

Physiological Effect # 4. Formation of Adventitious Roots and Root Hairs:

Ethylene induces formation of adventitious roots in plants from different plant parts such as leaf, stem, peduncle and even other roots. In many plants especially Arabidopsis, ethylene treatment promotes initiation of root hairs

Physiological Effect # 5. Inhibition of Root Growth:

Ethylene is known to inhibit linear growth of roots of dicotyledonous plants.

Physiological Effect # 6. Leaf Epinasty:

When upper side (adaxial side) of the petiole of the leaf grows faster than the lower side (abaxial side), the leaf curves downward. This is called as epinasty. Ethylene causes leaf epinasty in tomato and other dicot plants such as potato, pea and sunflower. Young leaves are more sensitive than the older leaves. However, monocots do not exhibit this response.

Higher concs of auxin, stress conditions such as salt stress, water-logging and pathogen infection also induce leaf epinasty indirectly through increased ethylene formation. In tomato and other plants, water-logging creates anaerobic condition around the roots resulting in ac­cumulation of ACC (the immediate precursor of ethylene formation) in roots. ACC is then trans located to shoots along with transpiration stream where it is converted into ethylene in presence of oxygen and induces leaf epinasty.

Physiological Effect # 7. Flowering:

Ethylene is known to inhibit flowering in plants. However, in pineapple and its allies (Family Bromeliaceae) and also mango, it induces flowering. Ethylene is used commercially to synchro­nize flowering and fruit set in pineapple. Plumbago indica (a Short Day Plant) can be made to flower even under non-inductive long days with the application of ethylene.

Physiological Effect # 8. Sex Expression:

In monoecious species (with separate male and female flowers on the same plant) especially some cucurbits like cucumber, pumpkin, squash and melon; ethylene strongly promotes formation of female flowers thereby suppressing the number of male flowers considerably.

Physiological Effect # 9. Senescence:

Ethylene enhances senescence of leaves and flowers in plants. In senescence, concentra­tion of endogenous ethylene increases with decrease in conc. of cytokinins and it is now generally held that a balance between these two phytohormones controls senescence.

Freshly cut carnation flowers when held in water in a conical flask, loose colour of their petals and wither (i.e., senescence) within a few days. But, if the cut carnations are held in conical flask containing silver thiosulphate solution, they remain fresh for many weeks. This is because silver thiosulphate is potent inhibitior of ethylene action. Role of ethylene in enhanc­ing senescence has now been confirmed by studies with transgenic plants also. (ABA (Abscisic acid) has also been implicated in regulation of senescence. Its conc. also increases during the process).

Physiological Effect # 10. Abscission of Leaves:

Ethylene promotes abscission of leaves in plants. Older leaves are more sensitive than the younger ones. Fumigating the wild type birch tree (Betula pendula) with 50 ppm ethylene re­sults in rapid defoliation of the tree within few days. Contrary to this, transgenic birch tree with a mutated version of Arabidopsis ethylene receptor ETR1-1, does not respond to ethyl­ene treatment and therefore, does not defoliate.

The relative conc. of auxin on two sides of the abscission layer has regulatory influence on the production of ethylene that stimulates leaf abscission. At the time of abscission, conc. of auxin in laminar region decreases with simultaneous increase in ethylene production. This also increases sensitivity of cells of abscission zone to ethylene which now synthesize cell wall degrading enzymes such as celluloses and pectinases.

Activity of these enzymes results in cell wall loosening and cells separation ultimately leading to leaf abscission. (Besides auxin and ethylene, ABA has also been implicated in the process of leaf abscis­sion. Its conc. increases in leaf at the time of abscission).

Physiological Effect # 11. Breaking Dormancy of Seeds and Buds:

Ethylene is known to break dormancy and initiate germination of seeds in barley and other cereals. Seed dormancy is also overcome in strawberry, apple and other plants by treatment with ethylene. Non-dormant varieties of seeds produce more ethylene than those of dormant varieties.

In many plants, rate of seed germination is increased by ethylene and a close correlation has been found between ethylene formation and seed germination in peanuts (Arachis hypogaea). In many plants, dormancy of buds can also be broken by ethylene treatment. Some­times, potato tubers are exposed to ethylene in order to sprout the dormant buds.