220 CHEMICAL AGENTS AND GROWTH 



Mohr (1957) has recently distinguished between a low-energy (red 

 and far-red) inhibition and a high-energy (blue and red) inhibition of 

 stem elongation. He found that stem growth of Sinapis alba is not in- 

 hibited by brief exposures to red light, but after long exposures to rela- 

 tively high-intensity light, inhibition of stem elongation is clear, but 

 both with red and with blue radiation. In the present experiments it 

 was found that the application of gibberellic acid (4.0 /xg/plant) to 

 this species does not reverse the inhibition of stem elongation by light. 

 It should be noted that in all other plants studied, the high-energy in- 

 hibition of stem growth is partially or completely reversed by gibber- 

 ellic acid treatment. 



The gibberellic acid-induced reversal of light inhibition may be most 

 readily demonstrated by comparison of the responses of plants grown 

 in continuous light and of those grown in complete darkness. Growth 

 inhibition is much less pronounced in the case of plants given inter- 

 mittent light exposures owing, in part, to compensatory growth. When 

 a plant (e.g., Pisum) is given a single exposure to light, an inhibition 

 of growth results. After return of the plant to darkness, however, its 

 growth rate exceeds that of the normal until the height of the light- 

 treated plants is approximately equal to that of the plants which re- 

 mained in complete darkness (Went, 1941). The compensatory 

 growth phenomenon was originally demonstrated on a variety of dwarf 

 pea (Little Marvel) whose growth has since been found to be pro- 

 moted by gibberellic acid (in both light and dark). Apparently, then, 

 the growth of this variety is limited in both light and darkness by the 

 endogenous gibberellin. The phenomenon of compensatory growth 

 means that the endogenous gibberellin has not been destroyed by light, 

 but rather it is "saved" for use at a later time. Since the plant can 

 respond to added gibberellic acid even in light (and at almost equally 

 low doses), it appears that the capacity of the plant to respond to and 

 utilize the gibberellin is not impaired in light. Apparently the gibber- 

 elHn is simply not present in a usable form during irradiation. Some 

 precursor (or alternate product) presumably builds up during irradia- 

 tion. During the following dark period this material is then converted 

 to the active gibberellin in unusually large amounts. The amount of the 

 precursor should be just sufficient to restore the height of the treated 

 plants to that of the controls. 



