348 5. M. Siegel and F. Porto 



used successfully to protect ciliates, rodents, and vascular plants 

 against oxygen poisoning. The culture of Clostridium tetani, an 

 obligate anaerobe, in air can be accomplished in the presence of C0+2 

 salts (3). In this case, the antioxidant protects the organism against 

 ordinary atmospheric levels of oxidant. 



Both growth and enzyme levels in embryos of Phaseolus vulgaris 

 ('Red Kidney') are reduced by incubation at elevated oxygen ten- 

 sions (20, 23). Other signs of oxygen damage in a variety of plants sug- 

 gest disturbance of the protoplast membrane and accelerated aging 

 of leaf tissue (6). Pea root tissues damaged by incubation in pure Oo 

 show increased peroxide production and increased lAA destruction 

 (7). The presence of C0+2 suppresses excessive oxidations and reduces 

 visible injury. Mitochondria can be protected against harmful levels 

 of oxygen by thyroxine (15). Enzymes whose activity is thiol-dependent 

 are sometimes readily inactivated by oxygen, even at atmospheric 

 level (14). Tyrosyl groups may be reasonably put forth as an addi- 

 tional oxidation-sensitive member of the peptide chain. Peroxides 

 participate in the depolymerization of deoxyribonucleic acid and 

 even more drastic chemical changes occur at elevated oxygen ten- 

 sions (4, 12). Organic peroxides formed from cellular components 

 can in turn attack other constituents such as thiols (5). 



Experimental Studies 



The study of oxygen and oxidant toxicity consists of simple 

 germination and growth tests as previously described. Experimentally, 

 responses of plant systems both to increased oxidant and to temporary 

 removal of oxidant stress will be considered. The recognition of 

 peroxides as probable intermediates in oxygen poisoning has led to 

 their use in experiments. 



Damage to lettuce seed {Lactuca sativa, 'Iceberg') incubated in hy- 

 drogen peroxide was reduced markedly in the presence of hydrazine, 

 although the latter alone at the concentration used was also toxic 

 (Table 3). A reciprocal relationship is indicated in the mutual can- 

 cellation of toxic effects of oxidant and antioxidant. A similar re- 

 sponse is the removal of inhibitory effects of high levels of lAA by 

 elevated oxygen tension. Organic peroxides, which are not decom- 

 posed by catalase, are generally more toxic than HoOo. Nevertheless, 

 it is possible to obtain an appreciable amount of protection against 

 their effects by the use of suitable antioxidants. For example, indole 

 has been employed as a partial protectant against p-menthane hydro- 

 peroxide. 



