Growth and Development of Horticultural Crops (1+1)

Physiological role

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 rate 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 rate of ripening process remains unaffected.
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 equalizing 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 absorbent such as KMnO4 the plumular hook opens.
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)
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.
5. Inhibition of Root Growth

• Ethylene is known to inhibit linear growth of roots of dicotyledonous plants.
6. Leaf Epinasty

• When the 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 concentration 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 (1-amino cyclopropane-1-carboxylic acid) (the immediate precursor of ethylene formation) in roots. ACC is then translocated to shoots along with transpiration stream where it is converted into ethylene in presence of oxygen and induces leaf epinasty.
7. Flowering

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

• Ethylene enhances senescence of leaves and flowers in plants. During senescence, concentration of endogenous ethylene increase with decrease in concentration of cytokinins and it is now generally held that a balance of 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.
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.
• The relative concentration 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, concentration 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 cellulases and pectinases. Activity of these enzymes results in cell wall loosening and cells separation ultimately leading to leaf abscission.
11. Breaking Dormancy of Seeds and Buds

• Ethylene is known to break dormancy and initiate germination of seeds. Seed dormancy is 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.