NO. 20 GROWTH SUBSTANCE BURKHOLDER AND JOHNSTON II 



DISCUSSION 



The relation of light to growth substance activity in plants is not 

 well understood, although considerable work has been done on various 

 aspects of the problem. Apparently under certain conditions, light 

 enhances the formation of growth substance ; effective action of the 

 substance is greater, however, in darkness than in light (Thimann 

 and Skoog', 1934). An increase in the amount of growth substance 

 due to light exposure has been reported for a number of plants by 

 various workers. Lupinus seedlings subjected to weak Mazda light 

 (Navez, 1933), Raphamis seedlings kept in the greenhouse (Van 

 Overbeek, 1932), young Nicotiana in daylight (Avery, 1935), and 

 Vicia growing in light (Thimann and Skoog, 1934), have been found 

 richer in growth substance than similar control plants kept in darkness. 

 The production of root-forming materials occurs in orange-red light, 

 though their activity apparently is inhibited by light (Went, 1935). 

 Avena coleoptiles and Raphamis hypocotyls exposed to light exhibit 

 less response to given amounts of growth substance than darkened 

 control plants (DuBuy, 1933; Van Overbeek, 1933). 



Various theories have been proposed to explain the depressing effect 

 of light upon growth. A change in the ability of cells to respond to 

 growth substance may occur in Hght (Van Overbeek, 1933 ; cf. Boysen 

 Jensen, 1936 a), or perhaps light hastens senescence (DuBuy, 1933). 

 Laibach (1936) postulates the formation of an excitatory substance 

 in darkened plant cells which furthers the action of growth substance. 

 Absence of this cooperating stuff in illuminated tissues is supposed 

 to account for the observed decrease in growth rates. 



It is well known that decapitated Avena test objects possess all the 

 requirements necessary for growth except growth substance. Experi- 

 ments reported in the present paper indicate that the growth-pro- 

 moting property of excised and illuminated coleoptile tips is de- 

 creased below the value of excised darkened tips when they are 

 tested upon decapitated coleoptiles in darkness. Likewise, irradiated 

 growth substance in agar blocks lost a large part of its activity. It 

 appears, therefore, that growth substance itself, not some other 

 material, is inactivated by irradiation in our experiments. Preliminary 

 investigations by Navez (1936) indicate that photo-destruction of 

 growth substance takes place through the photodynamic action of 

 certain plant pigments. 



Many of the instances on record concerning the more rapid growth 

 of plants at night are explainable, perhaps, on the basis of growth- 

 substance synthesis in the daytime and its increased effectiveness in 



