GROWTH REGULATION 287 



o£ seeds, fiiiits, and oLher reproductive structures, the shedding of 

 leaves under drought conditions in order to reduce rate of water loss, 

 the dropping of excess young fruits to permit the remaining fruits 

 to attain greater size and retention to maturity, and the shedding of 

 bark, leaves, and other portions as a part of the regular excretion 

 process. An impressive amount of evidence points to the fact that 

 abscission of an organ is inhibited so long as its auxin concentration 

 remains high, but that the abscission process is initiated as soon as 

 the auxin level decreases materially. 



The regulation of the abscission process has attained great commer- 

 cial significance to producers of agricultural products. Whereas in 

 some instances it is desirable to delay the process, in others it is im- 

 portant that abscission processes be accelerated. Following are several 

 examples to illustrate the significance of regulation of this natural 

 process. 



A number of growth regulators have proved valuable in inhibiting 

 or retarding leaf-fall. For example, 2,4-D applied as a preharvest spray 

 inhibits the defoliation of cabbage and cauliflower during storage. 

 In a similar fashion the inclusion of 2,4-D in the oil sprays required 

 for the control of red scale on citrus largely prevents the defoliation 

 which follows the use of the oil sprays alone. In holly, defoliation 

 is often induced by the high humidities and traces of ethylene which 

 are frequently encountered during the shipment of the refrigerated 

 product. Such defoliation can be completely avoided by spraying the 

 plants with NAA before shipment. 



In the Northern Hemisphere during the month of June a consider- 

 able amount of fruit is lost owing to the process of premature abscis- 

 sion. In a number of instances, June drop can be controlled by the 

 judicious use of certain plant regulators. For example, the addition 

 of small quantities of NAA to sprays of lead arsenate or dispersible 

 sulfur greatly reduced June drop of a number of varieties of apples. 

 In other instances the addition of 2,4-D to the spray had the same 

 desirable effect. 



In some cases, controlled defoliation of a plant is desirable. For 

 instance, the removal of leaves from the cotton plant before harvesting 

 the cotton is desirable to prevent staining of the fiber. Moreover, such 

 defoliation reduces the possibility of insect damage and boll rot. In 

 general, monosodium cyanamid and calcium cyanamid are most effec- 

 tive for this purpose. These compounds are also used to bring on 

 defoliation of young ninsery stock before digging and storing. In this 

 case, the removal of leaves is an important steji in j^rcventing the 

 plants from becoming infected during storage. 



