Abscisic Acid - Leaf Abscess

Leaf fall from a deciduous plant may occur in response to environmental signals, such as short or low temperatures in the fall, or due to adverse conditions to plant development.

The young leaf has the ability to synthesize relatively high auxin levels; during senescence, auxin synthesis in leaf limb decreases considerably, which promotes rupture of the petiole in the abscission layer.

During senescence, while decreasing the flow of auxins in the petiole, there is an increase in the production of ethylene in the abscission region. The drop in auxin level apparently makes cells in the abscission region more sensitive to ethylene action. Ethylene also inhibits the transport of auxins in the petiole and causes the synthesis and transport of enzymes that act on the cell wall (cellulases) and the middle lamella (pectinases).

Partial or total dissolution of the cell wall and the middle lamella makes the abscission region mechanically weakened. At this time a moderate wind is sufficient to cause the vascular bundle to break and complete the separation of the leaf from the rest of the plant.

Fruit abscission is very similar to leaf abscission, except that in fruits and some leaves there is, before abscission, an increase in the level of abscisic acid. This plant hormone could promote the synthesis of ethylene and, possibly, the synthesis of enzymes that act on the cell wall and middle lamella.

Gaseous in nature, ethylene is an unsaturated growth-regulating hydrocarbon that acts as a hormone. Its production in a normal plant occurs in virtually all cells and becomes more abundant in flowers after pollination and ripening fruits. Its synthesis is also verified in damaged cells.

For example, a ripe banana placed next to other greens accelerates the ripening of others because of the ethylene it gives off. For this reason, growers usually store fruits in chambers where the accumulation of ethylene in the air is avoided, thus delaying ripening.

Fruit ripening can also be prevented by enriching the warehouse air with carbon dioxide (as this gas antagonizes the effects of ethylene) or by preventing fruit oxygenation (the low oxygen level reduces the ethylene synthesis rate).

Ethylene is also involved in the fall - abscission - of leaves and fruits. This process begins with the reduction of the leaf IAA content, followed by ethylene production. It stimulates the synthesis of cellulase, the enzyme that digests the cellulosic walls, in the petiole's abscission region. In this region arises an abscission meristem, in which the derived cells organize a scar that will close the gap produced by the fall of the leaf or fruit.