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There are five major types of plant hormones: auxin, cytokinins, gibberellins, abscisic acid and ethylene. Each hormone can interact with the other causes various result in the plant growth and plant development. These reactions can also be differed by the concentration of the hormone, the plants growth stage, as well as its site of action.
Ethylene is defined as the only gaseous plant hormone, responsible for the fruit ripening, growth inhibition, leaf abscission, and aging.
Ethylene is a gaseous by-product of kerosene combustion, which allows fruit to ripen in sheds. The ethylene is actually a hormone that is produced by the plant. This hormone produces more actions then just fruit ripening. Since this is the only plant hormone that is transported as a gas, it is allowed to diffuse it through the air spaces between the cells. Also this hormone can be transported in the cytosol. Here it travels from cell to cell in the symplast and phloem. Some times the ethylene synthesis is induced by a high concentration of auxin. Not only does ethylene act as a growth inhibitor but also has many effects on the aging of plant6s. Fruit ripening and leaf abscission are the two areas where the hormones have been studied the most.
The aging process is when the cell walls degrade, which makes the fruit softer as well as the chlorophyll content decrease and the fruit looses its greenness. Ethylene also causes ripened fruit to drop from the plant. The hormone triggers senescence, then the aging cells release more ethylene. Since ethylene is a gas it travels from fruit to fruit. Therefore one rotten fruit could ruin the whole bunch. If you leave fruit in a bag it allows the ethylene to accumulate. Some fruits are treated with carbon dioxide to retard the again. The air circulation prevents the accumulation of the aging hormone.
Each autumn leaves fall from the deciduous tree, to keep the tree alive when the roots cannot absorb water. Before the leaves fall the essential elements are stored in the tissue in the stem. These elements are recycled, so they can be used when the spring leaves change color and in the fall when chlorophyll is not made. The pigments that occur once the dark green chlorophyll has left create the fall colors.
The breaking point of the leaf is an abscission layer at the base of the petiole. There are parenchyma cells in this layer that have very thin walls, here there are no fiber cells around the vascular tissue. Next the abscission layer is weakened by enzymes hydrolyzed polysaccharides in the cell walls. Before the leafs fall the twig builds a protective scar so that pathogen cannot enter the plant.
The balance between auxin and ethylene controls abscission. When a leaf starts aging it starts producing less and less auxin. This makes the abscission more sensitive to ethylene. Then additional ethylene is produced which inhibits synthesis of auxin in the leaf. As this happens the enzymes produced by the cells digest the cellulose and other components of the cell walls.
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