636 5. Housley and B. J. D ever all 



increases auxin in the plant by increasing the amount formed does 

 not readily lend itself to the interpretation of our data in terms of 

 any known scheme. The possibility that GA retards auxin destruction 

 is considered by Brian and Hemming (5), but since they were unable 

 to obtain a sparing action they took the matter no further. To sum 

 up our own views on one aspect of this hypothesis, it is felt that there 

 is no close relationship between light treatment and GA sparing ac- 

 tions, and that any relationship between the two involves another 

 f actor (s). 



An hypothesis favored by Brian and Hemming is that GA affects 

 some metabolic process which normally limits growth even though 

 auxin is present in non-limiting amounts. They discuss this idea at 

 length in relation to their own and other data and are led to suppose 

 that in the intact plant there is an inhibitory system which limits 

 growth rate and that GA treatment can be envisaged as a neutraliza- 

 tion of this inhibitory system. Thus, GA plays no direct or positive 

 part in cell extension, but that, by neutralizing an inhibition of ex- 

 tension, it releases the full potentialities of the auxins present. This 

 hypothesis is attractive as it is potentially versatile enough to account 

 for our own data; it is compatible with our own views and has some 

 supporting evidence cited by Brian and Hemming (5). 



A naturally-occurring system which answers to the above require- 

 ments is the lAA-oxidase enzymes and their inhibitor(s). One may 

 raise arguments objecting to this choice; however, it is profitable to 

 discuss the system and to defer objections for later discussion. The 

 facts required to construct a theory have been briefly simimarized by 

 Galston (8), but only limited data have been published in detail (12). 

 In light-grown 'Alaska' pea considerable lAA-oxidase activity can be 

 demonstrated in young stem and bud tissues while in the leaf a large 

 amount of inhibitor is present. Some inhibitor is present also in the 

 stem, the highest concentration occurring in apical tissues and a 

 gradient existing down the plant (presumably the stem too). The level 

 of inliibitor in the youngest leaves (i.e., part of the apical tissues) may 

 be raised by treatment of the entire plant for 2 days with 10-^M GA 

 (administered via the roots), while inhibitor level in leaves may also 

 be raised progressively by increasing the length of exposure of the 

 jilant to light. 1 his light effect on inhibitor level has been further 

 investigated by Hillman and Galston (12) who have shown that in 

 vitro lAA-oxidase activity of dark-grown 'Alaska' plants is greatly 

 inhibited by red light given to the intact plants before harvest. The 

 inhibition is reversible by near infrared radiation, i.e., far-red (29, p. 

 384), given immediately after the red light, but not more than 1 hr. 

 afterwards. 



