GROWTH REGULATION 283 



Antiauxins 



A variety of substances are capable oi inhibiting growth even in the 

 presence ol auxins. Such compounds, called antiauxins, serve to reduce 

 the effectiveness of auxins, though it is not clear whether they directly 

 antagonize the action of the auxin or whether they inhibit growth by 

 an indirect means. 



Maleic hydrazide, an example of an antiauxin, is capable of in- 

 hibiting plant growth without causing any obvious morphological 

 abnormalities. It appears to produce effects opposite to those induced 

 by indoleacetic acid. Thus, maleic hydrazide inhibits terminal growth 

 and stem elongation and destroys apical dominance. Recent experi- 

 ments indicate that the inhibitory effects displayed by this compound 

 can be neutralized by the addition of more auxin. Thus it is believed 

 that the original growth inhibition results when maleic hydrazide 

 stimulates an enzymatic destruction of indoleacetic acid. 



Although maleic hydrazide inhibits growth (auxin) activity, it 

 stimulates maturation processes such as production of lateral buds ancf 

 initiation of leaf and fruit abscission. Apparently, then, maleic 

 hydrazide is a true growth-inhibiting and maturity-inducing substance. 



Flower-forming Hormones 



The economic importance of controlling flower formation must not 

 be underestimated. For example, in order for crops such as celery, 

 spinach, cabbage, lettuce, beets, and carrots to have market acceptance, 

 they must remain in the vegetative state. On the other hand, it is 

 ecjually important that certain other plants be encouraged to flower 

 and to produce fruits and seeds. Included in this category are the 

 cereals, various fruits, vegetables, and ornamentals. 



At the time that plants change from vegetative to reproductive 

 development, the naturally occurring growth regulators, such as 3-in- 

 doleacetic acid, reach a low level in the young active regions of the 

 plant. From this behavior it seems reasonable that external applica- 

 tion of growth regulators could delay or inhibit flower formation. In 

 actual practice, flowering has been delayed or entirely prevented in 

 petunias, snapdragons, larkspurs, salvia, stocks, and iris following 

 application of 3-indoleacetic acid. Thus growth regulators are used 

 to supplement temperature and day-length adjustments in regulating 

 flowering in many floricultural crops. 



As described in a previous section, flowering depends to a large 

 extent on the exposure of a plant to alternate periods of light and 

 darkness, and seems to be controlled by hormonal substances called 



