Abscission of leaves

Abscisic acid, as the name suggests, has been implicated in the control of abscission of leaves, flowers and fruits, as well as with the function of stomata in response to water stress (Figure 5.4). Abscission involves the synthesis of cellulase in the ageing process and it is thought that abscisic acid influences the rate at which this proceeds. 

Ethylene plays an important role in a number of plant developmental processes, including senescence and abscission of leaves and flowers, responses to wounding, and the ripening of climacteric fruits (Abeles, 1973). In each case ethylene is produced from methionine (Fig. 1). The two enzymes specific to the pathway, ACC synthase and ethylene forming enzyme, increase in activity in response to wounding and during ripening,... 

There are few data on eventual risks for plants. Fluckiger (1979) reports an increase of peroxydase activity and of ethylcn synthesis by birches (Betula pcndula) growing near highways. An early abscission of leaves is observed too. 

Abscisic acid is a general plant-growth inhibitor. It induces dormancy and prevents seeds from germinating causes abscission of leaves, fruits, and flowers and causes stomata to close. High concentrations of abscisic acid in guard cells during periods of stress induced by drought probably have a role in stomatal closure. 

Other growth processes like enlargement of leaves, abscission of leaves, growth and ripening of seeds are also controlled by the ratio of phytohormones and growth inhibitors. When growth is retarded, the content of phytohormones drops abruptly, while the activity of natural growth inhibitors is promoted. Such processes as leaf abscission, and the inception of dormancy in seeds and buds take place at a minimum level of phytohormones. 

SO Bronchoconstriction. cough. Cellular injury, chlorosis, withering of leaves and abscission. Precursor to acid rain acidification of surface waters with community shifts and mortality of some aquatic organisms. Possible effect on uptake of Al and other toxic metals by plant roots. Weathering and corrosion. Defacing of monuments... 

Leaves with symptoms of acute injury usually drop prematurely. Abscission layer development may be stimulated by a rapid reaction of leaves to a high concentration of toxicant, resulting in heavy defoliation without detectable necrosis or chlorosis. Exposure to very high concentrations of nitrogen dioxide, chlorine, or hydrogen chloride may cause extensive defoliation within a few hours. Much lower concentrations may cause gradual development of typical symptoms of senescence, followed by premature dropping of affected leaves. 

Anatomy of the abscission zone at the base of mature fruits has been investigated in apples (10), cherries (11), citrus (12), and olives (13) and has been discussed as related to mechanical fruit removal (2, 14), The histochemistry of the separation layer of mature fruit is very similar to that of leaves of citrus (15) and bean (16,17), The actual separation occurs through one or more processes in the separation zone (a) a weakening of the cementing ability of the middle lamella between cells, and (b) a softening of the entire cells (17), In some plants, the abscission zone is structurally differentiated as a layer of compact cells or as a zone of localized cell division in other species, abscission may occur across a transect of cells which show little or no visible diflFerentiation (18). Thus, major components of the separation process involve the dissolution of middle lamella and lysis of cell walls or entire cells in the separation zone, but this is not necessarily associated with distinctive morphological characteristics (19). 

Lilium sp. (Liliaceae) Lilium virescence Zig-Zag line pattern in leaves of Lilium Leaf scorch or leaf bum of Lilium Virescence and alteration of reproductive structure Zig-zag lines between midribs of leaves, dwarfism Leaf malformation and necrosis and flower bud abscission Bertaccini and Marani (1982) Cortes-Martinez et al. (2007) Kaminska and Sliwa (2008b)... 

The genetically mandated senescence that causes leaves to die involves many coordinated steps. Loss of leaves in temperate zones is meant to protect against desiccation of the plant in winter when the roots cannot absorb water from the frozen ground. The separation of leaves at their points of attachment is called leaf abscission, but before this happens, many of their essential proteins, carbohydrates, and nucleic acids need to be transported back into the plant via the phloem (Taiz and Zeiger, 1998). 

Abscission, the dropping of leaves and other plant parts, is correlated with diminished production of auxin in those parts. By treating evergreen holly (Ilex aquifolium) with auxin, leaf and berry drop during shipment is minimized. Auxins also prevents preharvest drop of citrus fruits. 

The shedding of old mainstern leaves helped to alleviate water stress of the upper canopy, as indicated by midday LWP data (not shown). Abscission of old leaves and young fruit was well correlated to the degree of stress. S plants shed their leaves at an accelerated rate, and "M" plants shed only a few leaves at a linear rate (Fig. 3). "S" plants abscissed during 11 days of water stress 29%, 95% and 21% of their leaf DW, small fruit and large bolls, whereas "C" plants, shed only 3%, 48% and 10%, respectively, of these organs. 

Ethylene also induces abscission the falling of leaves or flowers, by promoting the formation of the enzyme cellulase that weakens cell walls by destroying the cellulose from which they are made. A weakened abscission layer forms at the base of the leaf or flower, and wind or rain then breaks the stem. Indeed, it was this phenomenon that indirectly led... 

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