Hormonal Mechanism of Growth Inhibition by Radiation 549 



Table 1 . Response of Pisum sativum, 'Alaska' to red radiation and gibberellic acid 

 treatment. 



* Both dark and irradiated plants were kept in rooms in which the air was filtered 

 through activated carbon. The dark plants were kept in a chamber in which the air 

 was also bubbled through alkaline potassium permanganate. 



t Irradiation consisted of filtered red fluorescent radiation (ca. 150 ergs -cm. ^• 

 sec.^i) for the duration of the experiment. 



t Gibberellic acid was applied as a 4 /ihethanolic drop giving a dose of 1 Mg- pcr 

 plant. Limits are standard deviations of the means. 



tion (254 mp) is completely unaffected by gibberellic acid (and auxin) 

 treatments (18). 



It is possible to measure the level of the nongibberellin factors 

 controlling stem growth. This is done by saturating the growth sys- 

 tem with gibberellic acid. Growth, then, will be limited by the sum 

 of the other growth factors. In this manner, the level of nongibberellin 

 growth factors may be compared in irradiated and dark-grown plants. 

 In Pisum, saturation with gibberellin results in equal growth of 

 irradiated and dark-grown plants. Thus the level of nongibberellin 

 growth factors is unchanged by irradiation. 



Where N is the effective sum of the nongibberellin growth fac- 

 tors and g z= growth, or growth rate. 

 In dark: N^ • natural GA^ = ga- 

 in light: Nj ■ natural GA^ = gj, 



and gi < ga. 

 Since g, < ga, then Nj < iV^ or G^^ < GA^ (or both). 

 Now, saturate the plants with gibberellic acid. 

 Growth is now equal •' ■ gi = gd- 

 Since gibberellic acid is saturating: GAj r= GA^- 

 Therefore A^^ must be equal to N/, and the change in growth due 

 to irradiation must be due to a change in endogenous gibberellin. 



The next question is, then, how irradiation affects the endogenous 

 gibberellin system. In general terms, it could affect (I) synthesis, (2) 

 effective endogenous level of the hormone, or (3) response of the 

 plant to a given level of hormone. Two lines of evidence make pos- 

 sible a partial answer to this question. 



It has already been pointed out that dwarf peas respond to gib- 

 berellic acid treatment even when grown in darkness. It is possible, 

 then, to compare the responsiveness of irradiated and dark-grown 

 plants to various concentrations of added gibberellic acid. When 

 this was done (20) it was found that the responsiveness under the two 



