Abscission chemicals

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). 

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... 

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. 

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... 

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... 

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). 

Auxins (67), gibberellins (68), cytokinins (69), and abscisic acid (67) can enhance the production of ethylene if added in concentrations that are generally considered stronger than tissue levels. Paradoxically, ethylene was one of the first chemicals identified as a potent defoliant (70), and now it has been shown to be a natural product of plant tissues that seems to regulate abscission (68, 71) as well as an influence on a host of other physiological reactions (7). 

A recent development in the field of chemical abscission formulations has been the use of mixtures of chemicals to induce and to control fruit loosening. CHI and Release have been shown to have a synergistic influence on fruit abscission (108). 

The considerations given above still leave the question open as to which of the two models is actually valid. A qualitative method of dhscriminating between the two models is provided by the J o versus InfCj,) plot, with the intercept on the absciss providing an answer. If physical-chemical reasons suggest that gelation should occur, the gel layer model may be valid and the intercept gives c = Cg. If, on the other hand, the osmotic pressure at point c is equal to the applied pressure [AP = n(c)], then Cj, = c ,. However, it should be noted that often, in practice, the phenomena are much more complex than those described here. Thus, adsorption and other phenomena (see figure Vn - 2) have not been taken into account. Even the way in which the pressure increments occur can lead to other results which cannot be predicted or even described by the two theories advanced above. 

These first quantitative studies of cell differentiation under the influence of hormones are among the few cases in the literature where exact substitution of synthetic hormone for the normal hormone source has been demonstrated to give full and exact replacement of the hormone source in terms of a physiological effect. The prevention of abscission by lAA is another such case (Chap. 8 in Jacobs 1979). In both cases, the full effect of the leaves was replaceable by lAA alone. The results confirmed the hypothesis that, of all the chemicals undoubtedly required to differentiate a strand of regenerated tracheary cells, lAA from the leaves was the one that acted as a limiting factor in the process. Furthermore, the results with the polarity of the stem (Fig. 4.2) supported the view that the ability of the stem to transport lAA was a further check on how much lAA reached the regenerating area (Jacobs 1954). 

---