216 GROWTH OF PLANTS 



Inhibition of growth. The use of chemicals for inhibiting growth is often 

 as important as for accelerating growth. The same chemicals which stimu- 

 late root gro^vth (cause cell elongation, cell division, etc.) may be used also 

 to inhibit growth. Hitchcock ^^ in 1935 tested seven substances for com- 

 parative effectiveness in inducing epinasty. He listed them in decreasing 

 order of activity as follows: a-naphthaleneacetic, /3-indoleacetic, /3-indole- 

 butyric, /3-indolepropionic, phenylacetic, (3- (phenyl) -propionic, and phenyl- 

 acryhc (cinnamic) acids. The same order holds for inhibition. When 

 Hitchcock applied the substances to the cut surface of decapitated stems, 

 axillary buds down along the stem were inhibited. The buds nearest the 

 cut surface (that is, nearest the chemical) were inhibited most. Lanolin 

 preparations of two unsaturated hydrocarbon gases, ethylene and propyl- 

 ene, applied to the cut surface also inhibited the uppermost buds. 



Sprouts and seedlings treated with a-naphthaleneacetic acid were first 

 accelerated in growth and then inhibited (Fig. 80). The stems later in- 

 creased in diameter but not in length, and the buds failed to develop. ^^' ^^- ^* 

 Stem cuttings of some species, treated wdth a-naphthaleneacetic acid to 

 induce basal roots, showed inhibition of buds up along the stem after roots 

 were well established. 



Roots differed from stems in response to growth substances by being 

 inhibited immediately without a preliminary acceleration period.'*^ Aerial 

 roots of Cissus, which normally elongate 5 to 10 inches within a 24-hour 

 period, practically stopped when a lanolin preparation containing 0.1 to 1.0 

 per cent a-naphthaleneacetic acid was applied at the tip. 



Guthrie ^^ showed that methyl a-naphthaleneacetate applied as a vapor 

 was effective for inhibiting buds of potato tubers. He placed layers of 

 filter paper impregnated with the ester among the stored tubers, and effec- 

 tively prevented growth. About 500 mg of methyl a-naphthaleneacetate 

 per kg of potatoes were sufficient. Denny, Guthrie, and Thornton ^ im- 

 proved the method and worked out other methods for preventing potato 

 tubers from sprouting by use of methyl a-naphthaleneacetate. Dusting 

 potatoes with a talcum powder preparation of the ester was found to be 

 effective. Though not fully tested, the new aerosol-growth substance 

 method has been effective when used in the laboratory. 



Another use for bud inhibition is to delay flowering of fruit trees until 

 danger of frost is past. This is done by spraying in the previous summer and 

 autumn when the buds are forming. A solution containing 100 mg of 

 ct-naphthaleneacetic acid per liter of water applied in August with a sprayer 

 delayed flowering for two to three weeks. Some delay was accomplished 

 also by spraying branches when the buds started showing color in the spring, 

 but much higher concentrations were required. It was predicted that 

 tropical fruits and others could be staggered throughout the year by this 

 method. Flowering shrubs which have one flush of flowers in the spring 

 might also be staggered in time of flowering. 



