90 ANlSrUAL REPORT SMITHSONTAN INSTITUTION, 1962 



examined. The results of these experiments showed that cobalt in- 

 hibited the destruction of ATP. This effect was observed both in the 

 presence and absence of 2,4-dinitrophenoL Thus, it is indicated that 

 cobalt exerts its growth-promoting influence by inhibiting the activity 

 of the enzyme ATPase. 



Studies on light- dependent chloroplast maturation have been con- 

 ducted, using chloramphenicol (antibiotic) to determine the participa- 

 tion of protein synthesis in the maturation process. Etiolated plants 

 that have been treated with chloramphenicol do not develop photo- 

 synthetic activity when irradiated with white light. Synthesis of 

 chlorophyll is markedly inhibited and cannot be altered by large 

 changes in the intensity of irradiation. In contrast, other light-de- 

 pendent responses such as leaf expansion, opening of the hypocotyl 

 hook, and anthocyanin formation are not inhibited by the antibiotic. 



Measurements of excitation of chlorophyll fluorescence in intact con- 

 trol and treated leaves show that chloramphenicol does not prevent 

 development of the ability of carotenoid pigments to transfer energy 

 to chlorophyll. Measurements of Hill reaction and photosynthetic 

 phosphorylation of chloroplasts of treated and control leaves show 

 that antibiotic prevents development of these photosynthetic activities. 

 Light-dependent increase of the photosynthetic enzyme TPN-linked 

 glyceraldehyde-phosphate dehydrogenase was inhibited by chloram- 

 phenicol but that of another, carboxydismutase, was not. Increase in 

 leaf protein which is associated with chloroplast maturation was par- 

 tially inhibited by chloramphenicol. 



These results indicate that chloramphenicol prevents synthesis of 

 substances necessary for dark reactions of photosynthesis. At least 

 one of the substances lacking in treated leaves and necessary for 

 photosynthesis is associated with chloroplasts. Inhibition of develop- 

 ment of photosynthetic activity of leaves can be accounted for by the 

 ability of chloramphenicol to inhibit protein synthesis in leaves. 



The physiological basis for changes in sensitivity of maize chromo- 

 somes to X-rays during seed germination has been studied, using 

 somatic mutation technics. X-ray damage was markedly enhanced 

 by oxygen, i.e., plants grown from seeds irradiated under anoxia 

 (helium) showed a two- to three-fold reduction in sector frequencies 

 as compared to plants grown from seed irradiated in air. Oxygen 

 enhancement, which was virtually nil for dry seed, attained its initial 

 expression after 4-5 hours hydration. Irradiation in two atmospheres 

 of pure oxygen, rather than in air, effected no increase in sector fre- 

 quencies during these first 4-5 hours. Apparently, the "oxygen effect" 

 is not influenced primarily by factors governing availability of oxygen 

 to the embryo, but owes its inception to other biochemical or biophysi- 

 cal changes during this early period of germination. 



