In this article we will discuss about the process of abscission in plants, explained with the help of suitable diagrams.
Abscission is a process that initiates the removal of a part of plant following the formation of a protective layer or scar tissue that prevents the entry of pathogen and protects the inner tissues from desiccation.
Plants are shedding organisms. Trees shed inactive xylem internally as heartwood. They shed organs externally.
Root, branch, twig, bark, fruit, flower, bud scales and trichomes. Leaf abscission is a very common phenomenon. In annual herbaceous plants abscission is confined to bracts and floral parts like calyx, corolla and androecium.
Perennials particularly the woody ones shed leaves, fruits, floral parts, branches etc. Some plants like Araucaria, Salix babylonica etc. abscise shoots, a process called cladoptosis (from the Greek clados = branch, ptosis = fall). Leaf abscission is the best example of shedding in trees.
Evergreen trees shed leaves in any season. Deciduous trees shed leaves in autumn, i.e. at the approach of winter. Gymnosperm and many dicotyledons show this periodic defoliation. Some changes occur at the base of shedding parts, which ultimately cause the separation. The exposed part is protected by the formation of protective layer.
The protective layer, and the region of separation, i.e. abscission layer are collectively termed as abscission zone. This zone initiates the process of abscission (Fig. 27.1). A single abscission zone occurs in dicot leaves. A fruit and monocot leaf may have more than one abscission zone. In palms the lamina is abscised by the first abscission zone. The second abscission zone abscises the leaf base. Abscission zone is designed and pre-positioned to facilitate shedding.
In leaves, abscission zone is formed at the base of simple leaves and compound leaves. In compound leaves this zone may also be formed at the base of petiolule of leaflet. In sessile leaf, this zone may be the only remnant of petiole. The abscission zones in the petioles may have different colour and groove or constriction by means of which they may be differentiated from the rest.
The cells in this zone are of same type as found in the other parts of a plant. Sclerenchyma cells are few or weakly developed. Collenchyma cells are absent. This zone can be between 15 to 40 cells wide. The separation layer is quite small. Parenchyma cells are thin walled, small and compactly set with no intercellular spaces.
A few cells may have remained in a cell division phase. These cells divide and the derivatives also prepare themselves for the abscission process. The abscission zone may not be active as soon as it is formed. It remains quiescent. It becomes active, when needed.
The cells in the abscission zone contain abundant starch grains. The cells are dense with cytoplasm and organelles. Each parenchyma cell is living and produces the materials needed for abscission. Dictyosomes play a major role in the development of abscission zone. Gilliland et al. (1976) observed that dictyosomes transport digestive enzymes for the destruction of cell walls of pedicel.
It is to be mentioned that dictyosomes transport materials for the construction of cells of pedicel. Dictyosomes, that transport wall materials at the time of construction, transport digestive enzymes for the destruction of same cell walls of pedicel at the time of abscission.
The abscission layer or separation layer is formed either earlier in the season or as the leaf approaches the time of falling from the plant. In the former case the abscission layer remains inactive until it is needed. It normally happens in those plants that grow in more variable climatic conditions. In the growing season if bad conditions arise, the trees shed their leaves.
The abscission layer consists of vascular bundle and parenchyma. The xylem, in most cases, consists of tracheids only. Fibres are few. When vessels are present, they are either narrow or discontinuous, e.g. Populus deltoides. The parenchyma is thin walled and is oriented transversely across the petiole. Some changes occur during the formation of abscission zone.
Prior to abscission, compounds like proteins, fats etc. are exported from the leaf. Chlorophyll production is stopped. The remaining chlorophyll breaks down and the nitrogen thus released is transported into the stem. Sugars, amino acids, minerals, nucleic acids etc. are transported from leaf to stem. The vessels gradually become nonfunctional due to deposition of gums and formation of tylosis.
The growth regulator auxin is produced in the leaf and is transported to the leaf base. As long as auxin is produced in the leaf and is transported across the abscission zone, cells of the abscission layer remain inactive. As the vessels gradually become inactive, leaves do not get adequate nutrients to produce auxin. As a result abscission layer begins to develop.
Moreover decline in auxin production and transport accelerates the activity of ethylene — the only gaseous plant hormone. Ethylene stimulates the development of abscission layer. Abscisic acid promotes abscission. Indole-3-acetic acid inhibits abscission. When the levels of auxin including Indole-3-acetic acid decrease in a leaf the principal accelerators of abscission, abscisic acid and ethylene become active.
The abscission process begins with the decline of auxin production and formation of abscisic acid and ethylene. In the cells of abscission layer the wall degrading enzymes like pectinase and cellulase are produced. These enzymes cause the degradation of the middle lamella, the intercementing material between the cells, and cause hydrolysis of cellulose in the walls.
The calcium bridges across the cell wall materials are removed. The cells in the abscission layer disconnect from each other. Starch, present in the cells, is converted to osmotically active substances and as a result cells absorb water. At turgid condition the cells expand. As cells expand a fracture line begins to develop in the separation layer.
The fracture line gradually grows by the pushing and pulling of expanding cells, gravity and wind tugging. The fracture line follows the path of middle lamella between cells (Fig. 27.2). Normally, a single fracture line occurs in the abscission layer. The xylem elements are not degraded. With the formation of fracture line the separation layer disappears.
The xylem elements only remain to hold the leaf. The leaf ultimately falls to the ground when the xylem elements break. This happens when the leaf sways by wind and the weight of the leaf exceeds the strength of vascular element. Animal actions also can break the xylem elements to allow leaf fall.
After the fall, tissues on the tree side are exposed. The vascular tissues are clearly visible and the wound is more or less smooth. They are subjected to moisture loss and attack of pathogenic organisms. The protective layer, formed in the abscission zone, protects the tissues from desiccation and the entry of fungal spores, bacteria, pests and other pathogenic organisms. The protective layer is formed in the abscission zone next to separation layer on the tree side.
This layer consists of a group of cells some of which are able to undergo cell division. These cells divide and form a sealing layer. After the completion of wall formation, the walls become filled with gum, suberin, lignin etc. In this region tylosis is formed in the vascular elements. Thus this layer forms an impenetrable corky barrier and protects the inner tissues on the tree side. The cells, situated on leaf side and exterior to protective layer, die.
The photoperiod and temperature are among the factors that cause the formation of abscission layer. Long night, frost, below freezing air temperature activate the formation of abscission layer. Leaf senescence is dependent on abscission, which is triggered by plant hormone. Abscission is the last step of leaf senescence.
Senescence, also called aging, refers to the process that naturally terminates in the death of organ. In this process the quite inefficient leaves that cannot perform photosynthesis because of shading by the upper portion of shoot are shed. In the senescent leaf catabolism exceeds anabolism.
In these leaves there is massive export of soluble metabolites to the stem. Frosts and heavy freezes at night may damage the living tissues of leaves. In this case the plants use the senescence sequence to remove the valuable resources from leaves to stems before they die and shed.
In autumn, when the duration of sunlight decreases, the dark period increases and cold weather sets in, the deciduous trees start shedding their leaves en masse. This phenomenon is so striking that it gives autumn another name – ‘autumn fall’ or simply ‘fall’. Before fall, leaves change their colour. This is due to absence of chlorophyll.
The colour of carotenoids is expressed as the green chlorophyll previously masked it. A wide spectrum of leaf colour ranging from yellow to red occurs. This is due to the presence of yellow xanthophyll’s and orange carotene. Microscopical examination of autumnal leaves shows the pigments. Chemical analysis detects many toxic chemicals like heavy metals in the shedding leaves.
Opinion varies regarding the fall of leaves of deciduous trees in fall. The common belief is that by this phenomenon plants conserve moisture in winter. In this season the ground water freezes, the plants do not get adequate water; so water conservation becomes necessary. By the fall the broad leaves of deciduous trees will not be able to give off large amount of moisture through transpiration.
Opponents of this view cite the example of those plants, which do not shed their leaves at all. Moreover, the leaves of evergreen trees, which are anatomically adapted to survive extremes of cold temperature, do shed their leaves at the different seasons. Another popular view is that the leaves are shed because of competition for nutrients.
But studies on abscission in the leaves of tropical rain forest reveal that they are regularly shed from those plants that are in the richest soil. The other view says that the leaves are shed to prevent shading of those of lower down and when they are damaged. But in palms the leaves are shed as it grows.
Naturally question arises why do leaves fall in fall? The answer is now emerging — a plant sheds its leaves to relieve the accumulated wastes. Excretion is one of the essential properties of all living organism. Like animals plants do not possess any excretory mechanisms. The fall is the excretory mechanism of a plant.
Moreover by fall the plant conserves the valuable resources. It was previously mentioned that many compounds are transported to the stem before leaf fall. Leaf abscission is highly ordered series of events, which prepare a plant for a resting period in winter. It helps trees to survive.
It is now revealed that apart from photosynthesis and transpiration, leaves have another function, i.e. excretion. This property is now utilized to purify contaminated soil. A new line of study has developed called phytoremediation (also known as bioremediation or phytobioremediation).
It is a process to purify the contaminated soil by plants. In this line of study, the deciduous plants that are metal tolerant are planted in the contaminated soil. The plants grow and absorb the toxic metals. Thus the soil gets purified. The toxic metals are excreted from the plant through the process of abscission.
To sum up, the process of abscission is now considered as a strategy for survival to the trees. It was previously mentioned that during the process the trees conserve the valuable resources needed to them.
In abscission, the leaves are abscised passing through the senescence process. So they are designed to be disposable. During the ‘fall’ leaves change colour. The great diversity of colours makes the trees most popular for enjoying autumn colour. The fall of leaves, to people, is the beauty of ‘fall’.