Leaves, abscission

Hydrogen fluoride Tip and margin bums, dwarfing, leaf abscission narrow brown-red band separafes necrotic from Youngest leaves most sensitive Epidermis and mesophyll cells 0.1 (ppb) 0.08 wi. i... 

Bleaching between veins, tip and margin burn, leaf abscission marking often similar to that of ozone... 

The phenomenon of leaf and fruit drop is known as abscission and has been suspected for some time to be under hormonal control. Dr. Addicott s description of the structure and properties of Abscisin II, the abscission hormone isolated from cotton, climaxes over 12 years of his own investigations on the subject and represents perhaps the greatest advance in plant hormone research in the past decade. It also provides an unusual and unsuspected structural model to guide... 

Abscisin II is a plant hormone which accelerates (in interaction with other factors) the abscission of young fruit of cotton. It can accelerate leaf senescence and abscission, inhibit flowering, and induce dormancy. It has no activity as an auxin or a gibberellin but counteracts the action of these hormones. Abscisin II was isolated from the acid fraction of an acetone extract by chromatographic procedures guided by an abscission bioassay. Its structure was determined from elemental analysis, mass spectrum, and infrared, ultraviolet, and nuclear magnetic resonance spectra. Comparisons of these with relevant spectra of isophorone and sorbic acid derivatives confirmed that abscisin II is 3-methyl-5-(1-hydroxy-4-oxo-2, 6, 6-trimethyl-2-cyclohexen-l-yl)-c s, trans-2, 4-pen-tadienoic acid. This carbon skeleton is shown to be unique among the known sesquiterpenes. 

Taylor,J.E., Tucker,G.A., Lasslett,Y., Smith,C.J.S., Amold,C.M., Watson,C.F., Schuch,W., Grierson,D and Roberts,J.A (1990) Polygalacturonase expression during leaf abscission of normal and transgenic tomato plants. Planta. 183.133-138. 

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. 

White phosphorus exposure to plants results in a variety of deleterious effects which are based upon the species of plant, the smoke concentration, the duration of exposure, the relative humidity, and the wind speed. These effects can include leaf tip bum, leaf curl, leaf abscission and drop, floral abortion, chlorosis, neucrotic spotting, wilting, dessication, and dieback. Other factors influencing the effects of white phosphorus upon plants are whether or not there is a post-exposure rainfall and whether the exposure is a large acute dose or several lower chronic doses (Van Voris et al. 1987). 

Induce or prevent leaf and/or fruit drop (abscission). 

Chemical harvest aids are presently used on more than 75% of the cotton acreage in the U.S.. The amount of foliage may be reduced either by the use of defoliants or the use of desiccants. Defoliants induce leaf fall and must be applied one to two weeks before harvest so that the abscission process may be complete. Desiccants cause the foliage to lose water and sometimes the leaves and stems are killed so rapidly by desiccants that an abscission layer has insufficient time to develop and the drying leaves remain attached to the plant. Desiccants usually require one to three days to act before harvest can be started. The obvious advantage of desiccants over defoliants is that they may be applied at a later date, thus gaining additional time during which the leaves continue to function and to contribute to see the fiber quality. Hundreds of chemicals have been evaluated as defoliants and desiccants. Only a few are in commercial use. 

Plant Growth Regulator absorbed by the leaves which stimulates formation of an abscission layer between the plant stem and leaf petioles, causing dropping of the entire green leaves... 

Induces cADPR-mediated RY-R opening [leaf abscission, bud dormancy, stomatal closure]... 

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