92 Fundamentals of Auxin Action 



vitro with eosin, lumichrome, and even chlorophyll (Skoog, 1935; 

 Galston, 1949; Brauner, 1953). It is interesting to note that in vitro 

 suspensions of auxin with ^-carotene do not destroy indoleacetic acid 

 in light (Reinert, 1952). 



Long ago, eosin was observed to have no appreciable effect on 

 plant growth if applied in the dark, but upon exposure of eosin-treated 

 plants to light all growth was blocked and the responses of the plants 

 to phototropism were lost (Boas and Merkenschlager, 1925). The 

 studies of Skoog (1935) indicate that this growth inhibition by light 

 was attributable to the destruction of auxin. 



Ferri (1951) pointed out that the various dyes which are capable 

 of oxidizing auxin are all fluorescing substances. He has suggested 

 that the destruction of auxin may be more nearly a function of the 

 fluorescing characteristics that a simple light-absorbing quality of the 

 dye. He has produced rather dramatic support for this suggestion by 

 demonstrating that quinine, a colorless substance which fluoresces 

 under illumination, is capable of destroying indoleacetic acid in vitro 

 in the light. 



Another means by which auxins can be destroyed is by x-irradia- 

 tion. Early workers observed that x-rays could cause large increases in 

 branching (Johnson, 1936). As we know, apical dominance or the 

 natural control of branching is a function of the growth hormone. 

 With this idea in mind Skoog (1935) found that x-rays could indeed 

 destroy auxin. Working with simple solutions of indoleacetic acid, 

 Skoog was able to determine that 25 roentgens could destroy as much 

 as 30 per cent of the auxin activity. The x-rays apparently cause a rup- 

 ture of the indole ring (Gordon, 1953). While the great bulk of ex- 

 perimentation on x-ray inactivation of auxin has been carried on in 

 vitro, evidence of auxin destruction by this agent in intact plants has 

 also been obtained (Skoog, 1934, 1935). A further effect of x-rays is 

 an apparent interference with the enzyme system which converts 

 indoleacetaldehyde into the acid auxin (Gordon, 1953). 



Another means by which auxin can be inactivated in the plant, or 

 perhaps more accurately by which the auxin supply can be lowered 

 in the plant, is heat treatment. Working with sugar cane, Brandes 

 and van Overbeek (1949) found that placing stem sections in hot water 

 (52° C) for 20 minutes caused breaking of essentially all lateral buds 

 and a loss of geotropic sensitivity of the stem. Both apical dominance 

 and geotropic sensitivity could be restored after the treatment by 

 supplying auxins artificially. Analysis of the auxin content of the 

 stem sections indicated that the heat treatment resulted in a 50 per 

 cent decrease in free auxin content 24 to 48 hours after treatment. The 



