162 CHEMICAL AGENTS AND GROWTH 



ception of the light by the plant are identical. In fact, it would be diffi- 

 cult to explain all these different responses on the basis of a series of 

 identical reactions. Only when we have a thorough understanding of 

 the underlying biochemistry will we be able to show definitely the 

 differences and similarities in reactions. 



Although the biochemistry of each of these responses is important, 

 in this paper I will concentrate on one of them, that of leaf expansion, 

 and more specifically on the biochemistry of the light-induced reac- 

 tions. This area has been selected because of oui research on this prob- 

 lem for the past three years. To what extent these results can be 

 extrapolated to the other light-controlled growth processes in seedlings 

 is unknown at this time, so that one can only hazard a guess concern- 

 ing the possible implications of the findings. 



LIGHT CONTROL OF LEAF GROWTH 



A brief review of the effect of light in controlling leaf growth will 

 be helpful in introducing the problem. It has been known for many 

 years that leaves on plants grown in the dark expand only slightly un- 

 less given a short exposure to fight, after which they grow at approxi- 

 mately the maximum rate. This appears to be true whether the leaves 

 are on intact seedlings (Downs, 1955) or are removed and grown as 

 leaf discs on a substrate to serve as an energy source (Miller, 1952; 

 Scott, 1957). The action spectrum for the fight requirement for expan- 

 sion of leaves on intact plants was indicated in the pioneering experi- 

 ments of Parker et al, 1949) for the Alaska pea. Downs (1955) ex- 

 tended these experiments to the red kidney bean and showed that the 

 far-red portion of the spectrum was very active in reversing the red- 

 light promotion. Although such a detailed spectrum has not been 

 worked out for expansion of the leaf disc, it was shown in 1953 

 (Liverman et al., 1955) that the red-far red system controlled the ex- 

 pansion of discs from leaves of the dwarf stringless greenpod bean. It 

 appears, therefore, that the light requirement for expansion of the disc 

 and for the intact leaf are identical. 



With these early experiments as a basis, we turned in my laboratory 

 to a detailed study of the biochemical nature of the light-induced 

 expansion of leaf discs. This system permits an assay for the effect of 



