Fruit abscission

Compounds that release ethylene when sprayed on plants have become of major economic importance, being used to accelerate diverse ethylene responses such as induction of flowering, stimulation of latex flow, leaf and branchlet abscission, fruit ripening, fruit abscission, and pod dehiscence. Novel aspects of application of these materials in agriculture are reviewed. 

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. 

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. 

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. 

Of the three jnonochlorophenoxyacetic acids, only the 4-chloro derivative is very active and is used to retard abscission of fruit. A study of the effects of the isomers on the carbohydrates of the bean plant showed that the alterations of the carbohydrates were not related to the growth-inhibiting mechanism.12 Reducing substances, starch, and sucrose decreased with all treatments, And the 2-chloro compound had the least effect polysacchar rides other than starch were not much altered. 

Okhuma, K., Lyon, J.L., Addicott, F.T., Smith, O.E. (1963). Abscission II, an abscission-accelerating substance from young cotton fruit. Science 142, 1592-1593. 

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,... 

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

An active abscission research program for citrus has been underway for a number of years in Florida by the Florida State Citrus Commission in collaboration with a number of chemical companies. This program was initiated originally because of a shortage of labor for handpicking citrus. Chemicals to speed up the removal of fruit by mechanical harvesters and to increase... 

The preharvest drop of citrus fruit can be reduced or prevented with 2,4-dichlorophenoxyacetic acid, which delays development in the abscission zone of the fruit stem, thus allowing the fruit to remain on the tree longer. Because of 2,4-D s herbicidal properties, however, care must be taken in spraying to prevent the chemical from damaging other crops, as well as the target crop... 

With the exception of 2,4-D and gibberellic acid treatments of a variety of citrus crops, where improved fruit set, reduced fruit drop, and the ability to delay harvesting without loss of fruit quality are recognized benefits, few applications have been developed for the use of hormone-type regulators in tropical fruits. Recent work has shown that gibberellic acid will delay postharvest ripenining and improve the quality of bananas and that napthaleneacetic acid treatment of oil palm to delay fruit abscission has increased both yield and oil content. 

Several materials have commercial use as abscission agents (6). In cotton, the products Accelerate (endothall), Folex (merphos) and DEF (oxidized form of merphos) have been identified as defoliants. Amid-thin continues to be used to thin fruit set. 3-CPA and Peach-thin 322, formerly used as peach thinners, serve as reminders of the disadvantages of highly specialized markets. RELEASE and PIK-OFF are under development as citrus abscission agents. 

With such cooperation, mutual support and obviously some "luck" (23), plant growth regulators, including abscission agents, may still be expected to be developed (24, 25) not only to enhance quality and yields of major crops e.g., cotton, corn, tobacco, but also important fruit crops e.g., apples, oranges (citrus), grapes, cherries, etc. 

ABA was discovered by three independent lines of research.548 Cams and coworkers found that an auxin inhibitor, abscisin, was involved in the abscission of immature cotton fruits. However, Ohkuma and other... 

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