634 5. Housley and B. J. Deverall 



DISCUSSION 



The present study has shown that GA at a concentration of 10 

 mg/1 has no influence on the lAA-oxiclase system of pea in vitro in 

 the dark, while in the light on some occasions it may result in an en- 

 hanced rate of lAA destruction. Results similar to the former have 

 been obtained by Brian and Hemming (5) and Kato and Katsumi 

 (16) using lAA-oxidase preparations from pea roots and shoots. 



The present study has also shown that GA can influence rate of 

 lAA destruction by excised pea tissues, and on some occasions the rate 

 is reduced thus producing a sparing of I A A. The sparing effect is 

 frequently not marked but varies considerably in magnitude from a 

 mere trend (Figure 3C) to an appreciable effect (Figure 4C). This 

 sparing accounts for the growth synergisms obtained by Brian and 

 Hemming (4); also, our range of variation is consistent with the range 

 of growth synergisms shown in Brian and Hemming's data, i.e., small 

 synergisms ranging to marked ones. 



Brian and Hemming (5) state that they have considered the possi- 

 bility that GA might reduce rate of auxin destruction, but have been 

 unable to detect any effect of GA on lAA destruction by pea inter- 

 node sections; however, they do not mention the method and tech- 

 niques used; therefore it is not possible to assess the reason for their 

 failure. On the other hand, Nitsch (24) has shown that prior appli- 

 cation oi GA to an intact plant (Rhus lyphina) causes the level of 

 endogenous auxin in the apical tissues to be raised, an observation 

 that may be accounted for by an auxin-sparing action of the type 

 described in the j)resent work. Nitsch's results (obtained by Avena 

 straight-growth bioassay of ether-soluble materials after separation by 

 paper chromatography) are important for they are a direct approach 

 to the examination of effects of GA on the naturally-occurring hor- 

 mone system of plants. 



The possible mechanisms whereby an auxin-sparing phenomenon 

 may be brought about by GA are next discussed. Before carrying out 

 our early experiments, the results of Brian and Hemming (A) were 

 considered in conjunction with those of Lockhart (20), and for a pre- 

 liminary hy[)othesis it was suggested that GA acts by interfering in 

 a light-mediated system of auxin destruction; the same hypothesis 

 has been speculatively put forward by Vlitos and Meudt (28). As it 

 is necessary to view hyjx)theses against the background of sparing 

 actions shown in Figures 2, 3, A, and 6, it is helpful first to summa- 

 rize the latter more concisely (Table 1). In assessing the scoring in 

 this table, the consistency of the sj)aring action through time was 

 taken into account as well as the magnitude of the action. It may 



