Ethylene (Plant Hormone)- Bioassay & Physiological Effects
Ethylene (Plant Hormone)
The first report that plant materials involve ethylene came from H.H. Cousins in 1910, who observed that when oranges and banana were stored together during shipping, some gas was eliminated from oranges which caused ripening of banana. In 1932, O.H. Elmer reported that the sprouting of potatoes could be inhibited by keeping them in close proximity of mature apples or pears. In 1934, R.Gane provided chemical proof that ethylene was produced by ripe apples. Since then, it has been shown that ethylene is produced from essentially all parts higher plants.
Ethylene is introduced into the atmosphere from both physical as well as vegetable sources. In coal gas, its concentration is in order of 10 ppm and in burning stubbles upto 4 ppm. In car exhausts it may be as high as 400 ppm. In most urban areas, its mean concentration does not normally exceed 0-50 ppb and in rural area it may be less than 5 ppb.
Bioassay of Ethylene
Because of its gaseous nature, ethylene is generally detected by gas chromatography. It can also be measured by manometric method. The gas is absorbed by mercuric perchlorate solution and liberated again by chlorides. For this, the gas or the test vapours are bubbled through a small volume of mercury perchlorate or through silica gel impregnated with mercury perchlorate. Then HCl or LiCl is added to it, which causes the liberation of ethylene gas. The volume of the gas can be measured and quantified. This procedure can be used measuring the amount of ethylene as well as for concentrating it.
Detection of ethylene using plant materials as tests, is also used in some investigation. The triple response of etiolated pea plant was developed by Neljubow (1901) is still used as bioassay. In most cases the plant material to be assayed for ethylene is enclosed in a small chamber, with an injection port. After incubation of weighed amounts of the tissue for required time, samples of gas are withdrawn with a hypodermic syringe and tested on other plants. Following bioassays are used.
Reduction in extension growth-
Plants enclosed in a chamber containing 0.01 ppm ethylene show reduction in extension growth. While at 1.0 ppm there is increase in horizontal growth and stem swelling takes place. Besides etiolated pea seedling, sunflower, ,tomato, potato, buckwheat and castor bean have also been used for this type of bioassay.
Acceleration of senescence-
Ethylene accelerates senescence of leave, flower, petals, etc. When leaves are enclosed in a chamber containing ethylene, loss in chlorophyll content can be recorded. Senescence of cotton cotyledons is also accelerated, and this is used as bioassay.
Induction of stem and leaf epinasty-
Epinastic responses into tomato plants is also used sometimes as ethylene bioassay.
Physiological Effects of Ethylene
Ethylene is known to break dormancy and induce germination of lettuce, ground nut, wheat, clover, and cocklebur seeds. It also causes the increased extension growth of the seedling in cocklebur. The extension growth is maximum at 0.3 ppm ethylene. The maximum germination however, is obtained at about 40-50 ppm ethylene. On the other hand, inhibition of germination by ethylene has been recorded in maize, Potentilla and some weeds.
Growth inhibition and morphogenetic effects-
In most cases exogenous application of ethylene inhibits plant growth. In most dicots the elongation growth of stems, roots and leaves is inhibited. But the hormone enhances radial growth, as a result both stem and root swell in response to ethylene. Auxin and ethylene when applied exogenously or produced in partially inhibitory concentrations, act synergistically to inhibit root elongation and swelling of the root tips (G. Bertell, E. Bolander and L. Eliasson 1990).
Growth of roots, when measured in terms of dry weight is unaffected by 10 ppm ethylene for 11 days in barley, although the same treatment causes stunting and feathery growth of the roots. Similar concentration of ethylene however, inhibits dry weight in tomato.
Reduction in growth may be due to inhibition of cell division, as has been observed in various tissues of etiolated pea plants. In most cases, inhibition of growth is proportional to the inhibition of cell division. In pea seedling, ethylene inhibits xylem vessel formation also. But no permanent damage is done to the cells; as soon as ethylene is removed, normal differentiation takes place.
Some growth promoting effects of ethylene are also known. The hormone appears to be involved in the increased coleoptile and shoot growth of submerged plants such as rice. Growth of rice coleoptile is greatly accelerated when rice plants are submerged in water. This is believed to be due to the combined influence of decreased oxygen supply and the accumulation of growth promoting concentration of ethylene and CO2. Each component appears to be acting independently. In rice plants growing in air. supply of ethylene greatly accelerates the growth of internodes.
One of the most important effects of ethylene is on hook opening. It inhibits hook opening in etiolated lettuce and bean seedlings.
Exposure to ethylene causes epinastic movement in petioles, as a result the leaves bend down. This is because of more growth on upper side than on lower side of the petiole. Epinasty is also caused when excess of auxin is applied. This is believed to be due to endogenous ethylene production. Stem epinasty is also caused due to ethylene, as in tomato.
Flowering inhibition and sex expression-
Ethylene inhibits flowering in most plants, although it is known to promote flowering in mango and pine apple. In Plumbago indica also, a short day plant, flowering can be induced by ethylene in long days. Flowering induction in pine apple with response to certain auxins has been attributed to endogenous ethylene production.
Inhibition of flowering by ethylene is controlled by the photoperiod. In Pharbitis nil, the hormone inhibits flowering only when it is applied during inductive dark period, and not when it is applied before or after inductive dark period. It is suggested that ethylene applied during the dark period produce some flower inhibiting substance.
Ethylene changes sex expression also in unisexual plants. It increases female flowers in several members of cucurbitaceae. It also induces male sterility in cucurbits and wheat.
Ripening of fruits-
Acceleration of fruit ripening was the first discovered effect of ethylene. The hormone is now known to accelerate ripening of mature fruits in most cases including banana, apple, tomato, avocado etc. In apple fruits, the softening of fruits increases with the increase in ethylene concentration, reaching maximum at 50 ppm ethylene.
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