640 5. Housley and B. J. Deverall 



radiation. Other literature relating GA to these light effects has 

 been discussed by Brian (1). Thus, it would appear the applied GA 

 in some systems cannot only by-pass the effects of red/near infrared 

 radiations but the two may mutually reinforce each other. To ac- 

 commodate this, an attractive modification of the scheme in Figure 5 

 (top) is to suggest that red/near infrared radiations are influencing 

 the naturally-occurring gibberellins (Figure 5, bottom). X now be- 

 comes Pa, a gibberellin precursor, while Y becomes G, its correspond- 

 ing gibberellin. The mechanism of increase in I under the influence 

 of G remains an unknown process. It may be noted that Brian (1) has 

 been led to postulate a similar relationship between gibberellin(s), 

 its precursor(s), and red/near infrared radiations as a residt of theo- 

 retical considerations on mechanisms of flowering in plants. 



Although it is possible to discuss further this scheme (Figure 5, 

 bottom) in relation to published literature, discussion becomes dif- 

 fuse due to lack of adequate biochemical data. Discussion also be- 

 comes diffuse when published data, which do not appear directly to 

 support the scheme of Figure 5, are considered. For example, choice 

 of the lAA-oxidase inhibitor for / may be objected to on the groimd 

 that growth interactions between hormone and GA are observed with 

 2,4-dichlorophenoxyacetic acid and 1-naphthaleneacetic acid and 

 these compounds are degraded less readily than lAA (5) and may not 

 go through the lAA-oxidase system. If, however, one accepts this 

 criticism without considering further the data in the literature on 

 which it rests, it does not alter the concept that GA operates through 

 an inhibitor system, but merely removes the scheme in Figure 5 with- 

 out replacing it with any known inhibitor system. In view of the 

 statements of Galston (8) on the relationship between GA and /, it 

 would seem preferable to use Figure 5 (bottom) to design biochem- 

 ical experiments to test the validity of the scheme in this figine. 



SUMMARY 



An examination has been made of the rates of disappearance of 

 indole-3-acetic acid (lAA) in the presence and absence of gibberellic 

 acid (GA) from solutions containing either lAA-oxidase enzymes from 

 pea or excised stem tissues of pea. lAA destruction with the enzyme 

 system in vitro was not consistently influenced by GA in darkness or 

 in light. With apical tissues (apex, young leaves -\- 10 mm. immature 

 stem) from light- and dark-grown plants, rate of lAA destruction in 

 light was decreased by GA, thus causing an lAA-sparing effect, while 

 in darkness GA had little or no effect upon rate of destruction. How- 

 ever, there was no sharp division between the light and dark treat- 

 ments, some experiments in light showing little or no sparing action 

 and thus resembling the dark experiments. 



