56 PHOTOSYNTHESIS 



Since the time of Warburg's experiments the effect of 

 other poisons on photosynthesis has been investigated. 

 Hydroxylamine inhibits photosynthesis in Chlorella to about 

 the same extent at all light intensities (Fig. 4.8). It does not 

 inhibit photosynthesis in the photosynthetic bacteria, nor 

 does it inhibit the photochemical reduction of carbon 

 dioxide by green algae using hydrogen (reactions which will 

 be discussed in Chapter 5) to the same extent as photo- 

 synthesis. Thus hydroxylamine cannot be classed with 

 urethane as a narcotic but is presumed to be a poison which 

 combines with an enzyme the concentration of which is 

 postulated to be proportional to the light intensity. It is sup- 

 posed that the enzyme catalyses a dark reaction resulting in 

 the liberation of oxygen. Other poisons include iodoacetate, 

 which like cyanide inhibits photosynthesis to a greater 

 extent at high light intensities, and o-phenanthroline which 

 is similar in action to hydroxylamine. As we shall see in 

 Chapter 7 cyanide has no effect on the photochemical reduc- 

 tion reactions catalysed by isolated chloroplasts whereas 

 hydroxylamine and o-phenanthroline are powerful inhibi- 

 tors of these reactions. Since the chloroplast reactions 

 involve only the evolution of oxygen and do not involve 

 carbon dioxide this evidence confirms the view that 

 hydroxylamine and o-phenanthroline are effective upon the 

 reaction in which oxygen is liberated. 



High concentrations of carbon dioxide (greater than 6%) 

 and very high concentrations of oxygen (greater than 21%) 

 depress the rate of photosynthesis. The depression due to 

 high concentrations of oxygen is only in part due to oxygen 

 consumption in photo-oxidative reactions catalysed by 

 chlorophyll. {In vitro photo-oxidative reactions were dis- 

 cussed in Chapter 3.) Such photo-oxidative reactions can be 

 demonstrated in vivo with cells in high concentrations of 

 carbon dioxide only at very high light intensities — far in ex- 

 cess of those at which photosynthesis is rapidly approaching 

 'saturation'. If the concentration of carbon dioxide is reduced 

 photo-oxidative consumption of oxygen occurs at lower 

 light intensities. The depression of photosynthesis due to 

 high oxygen concentrations is greater in magnitude than the 



