In plant physiology, abscission refers to the shedding of leaves , including needles or coniferous leaves, fruits and other plant parts such as leaf and flower buds, twigs, spines, thorns or inflorescences . One speaks therefore of leaf fall , fruit fall , leaf fall , bud fall or flower fall . The deciduous fall of deciduous deciduous trees in autumn, but also of some conifers such as larches , is likely to be the most noticeable phenomenon of abscission in temperate climates. The abscission is preceded by a shift of reserve materials ( allocation (botany) ) into storage organs, starch is shifted from the leaves into the trunk and in winter - as frost protection - reversibly converted into sugar.
Function and physiology
Abscission can be associated with self-cleaning for the plant by removing old, injured or diseased parts. Furthermore, an excretory function is possible by rejecting organs in which metabolic waste products have accumulated. Abscission also plays a role in the spread of fruits and asexual reproductive bodies.
The shedding of the plant parts is preceded by characteristic morphological and anatomical changes in the separation zones. There is often a pronounced separating fabric. This is located at the base of the leaf or fruit stalk and consists of particularly small parenchymal cells with dense protoplasm . Here the separation process is prepared in that the middle lamellae and / or primary walls or whole cells dissolve in a two to three cell layer wide separation layer , which differs from plant species to plant species. For this correlatively controlled active process, air, oxygen and a respiratory substrate are required. Respiratory toxins inhibit abscission. Ribonucleic acid and protein synthesis are also necessary , especially the synthesis of cellulase and pectinase . Pectinase, which makes protopectin water-soluble, is actively secreted from the protoplasm into the cell wall. Auxin , senescence factors and ethylene including abscisic acid are involved in the correlative control of abscission .
The plant hormone auxin, which is formed in intact, not yet aged leaves and fruits and migrates through the leaf stalk, prevents abscission. Accordingly, premature fruit fall is often suppressed in horticultural practice by spraying the plants with auxin solutions. On the other hand, they stimulate senescence factors that migrate from aging leaves and flowers. With many fruits and some leaves, e.g. B. the lupine , there is a temporal correlation between abscisic acid production and abscission. According to the senescence hypothesis of abscission, the readiness of the separating tissue to abscise is regulated by the ratio of auxin to senescence factors.
Factors that delay leaf and fruit senescence, such as cytokinins , also delay abscission accordingly. Ethylene is the direct regulator that induces it when the separating tissue is ready for abscission, among other things through the formation of ribonucleic acid , cellulase and / or pectinase. Differential gene activation is seen as the primary effect of this ethylene effect. In addition, ethylene inhibits auxin synthesis and transport, so it stimulates abscission in two ways. Ethylene-releasing preparations are often used to accelerate or synchronize fruit ripening. This is important for the mechanization of the harvesting processes.
Deciduous woody plants
Most deciduous trees and shrubs shed their leaves in autumn before or during the first frosts or in the dry season ("leaf fall"). This reduces the likelihood that they will dry up in a winter frosty season or dry season. The color comes about as follows: The production of chlorophyll (green) is stopped and other colorings become visible (yellow and orange carotenoids and xanthophylls). In addition, anthocyanins can be formed which turn the leaves red.