Organ abscission

Touring the past two decades intensive research has been conducted on the phenomenon of organ abscission in plants. A basic knowledge of the processes gives insight into the metabolism of cell walls which has applicability in attempts to control the ripening process in many fruit production programs. 

The majority of data on the physiology of abscission were obtained using leaves, and principal concepts evolved from these observations. Dijfferences among fruit, leaf, or other organ abscission are indicated as applicable to the discussion. Because of the vast number of articles published on abscission, neither time nor space will allow this to be an all-inclusive review, but hopefully the selected references allow an assessment of present knowledge of fruit abscission and chemical aids to harvest. Several excellent reviews are available on various phases of abscission (2-6) and alhed areas (7, 8, 9). 

Figure 6. The potential sites for modification of Type I primary cell walls by enzymes and/or expansins. The activities of the enzymes/expansins are associated with numerous aspects of plant growth and development including cell expansion, fruit ripening, and organ abscission.

Several inferences can be drawn from these data that may suggest the impacts to be expected at the consumer and decomposer levels. Accelerated leaf drop may influence the development of pests—namely, aphids, scale insects, and red citrus mites. Pest populations might be increased if injured leaves had higher concentrations of amino acids or free sugars before abscission (see Chapter 11) or diminished if leaves fell too rapidly. Leaf and fruit drop would provide a larger substrate for populations of decomposer organisms at the soil surface. 

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... 

Chemical aids to harvest and fruit abscission agents are reviewed and discussed in relation to structure, mode of action, and predictability of response. Also, an attempt is made to assess the physiological basis for different responses among fruits of differing physiological age and among various organs on the same plant. 

Attention in this review is directed toward abscission of mature fruits. Since plants abscise organs naturally, we examine the nature of the abscission process from the standpoint of structural changes and enzymes involved how plants seemingly regulate a selective control over abscission natural and synthetic chemicals affecting abscission whether... 

Ethylene also increases polygalacturonase activity in abscission zones, and this may also contribute to the abscission of some organs 31, 32). The polygalacturonases isolated from plants, however, have been the exo-types 31), and there have been some questions concerning separation of cellulases from polygalacturonase on the basis of a salting procedure (33). 

Ethylene is produced in measurable amounts in a number of fruits, leaves, and shoots under normal conditions (7, 46, 59, 72) and in large amounts after treatment with certain chemicals 14, 43, 44, 46), mechanical stresses (72), and adverse environmental factors (73). Thus, acceleration of abscission by many agents seems to occur via ethylene production—a fact that is being used to assay chemicals as potential accelerating agents for abscission 74). Apparently, this is the basis for the induction of abscission by placement of abscission chemicals on the surface of an organ such as an orange 41). 

Toxic to aquatic organisms for example, there was an inhibiting effect of 20-40 ppm aniline on the pigmentation of Xenopus laevis embryos, and of a concentration as low as 1 ppm on the body size of the young toads. Investigation of the death of pine trees in the United States found air pollution from aniline as the most likely causal agent for the needle necrosis and needle abscission. 

Use Organic synthesis, abscission of citrus fruit in harvesting. 

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. 

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