218 



CHEMICAL AGENTS AND GROWTH 



Pisum 



C T C T C T 



Helionthus 



C T C T C T 



Phaseolus 

 (normal) 



C T C T C T 



Phaseolus 

 (dwarf) 



C T C T C T 



C T C T C T 



C T C T C T 



Fig. 1. Interaction of gibberellic acid and red light on stem growth in 

 several species. The effect on species grown in darkness (black bars); 

 low intensity red light (light shading); high-intensity red light (darker 

 shading). T, gibberellic acid treatments; C, untreated controls. 



may differ sufficiently from the natural gibberellin to make it physio- 

 logically less effective (cf. Phinney et al, 1957). 



In the case of Phaseolus vulgaris, application of gibberellic acid 

 reverses the light inhibition of stem growth and, in light, elicits a 

 growth which is much greater than that characteristic of dark-grown 

 plants (either of control or of gibberellic acid-treated plants). It has 

 been established that this promotion of growth by light (in the presence 

 of added gibberellic acid) is a red light response, which may be fully 

 reversed by subsequent irradiation in the far red (Fig. 2). If the far- 

 red radiation is not given immediately after the red, but is delayed 

 several hours, then the growth promotion is proportional to the time 

 the pigment remains in the far-red-absorbing (red hght-induced) form 

 (Table I). 



Dwarf and pole beans respond to gibberellic acid application in an 

 apparently identical manner. Neither responds in darkness. Both re- 

 spond to the application if also supplied with light, either as brief ex- 

 posures to red radiation or to greenhouse conditions with bright sun- 



