318 A. R. KRALL 



to be almost at equilibrium with the P*^ in the inorganic phosphate 

 pool since longer experiments give essentially the same distribution 

 of P^^ among the various separable components. The esters in the 

 inhibited tissue were not at equilibrium with the inorganic P^-, 

 since longer time experiments gave very much higher P'^ levels in 

 phosphoglycerate and hexose diphosphate and certain as yet un- 

 identified fractions which were isolated by the separation method. 

 Only ATP seemed to possess a constant P'^ content upon continued 

 exposure to the label under the same conditions. Therefore "% P^^ 

 in ATP" is used here as a measure of ATP content of the tissue while 

 "counts per minute P^^ in hexose monophosphate" is used as an 

 approximate measure of hexose monophosphate synthesis. The leaves 

 illuminated by both red and yellow light were the only ones under 

 carbon monoxide which were labeled with P^^ at approximately a 

 normal rate. If the effects of red and yellow light above that of dark- 

 ness were additive, one would expect about 80 X 10^ counts P^^ in 

 the HMP — instead there were 195 X 10'' counts per minute. Thus 

 illumination with the two lights together has a greater effect in 

 stimulating P^^ incorporation into organic esters of phosphorus than 

 the arithmetic sum of the effects of the two lights applied separately. 



DISCUSSION AND CONCLUSIONS 



The observations to be explained are these: Exposure of barley 

 leaves to carbon monoxide containing small amounts of oxygen allows 

 only minimal incorporation of P^^ into organic esters of phosphate 

 under either the red or yellow lights used. The rate of HMP syn- 

 thesis was higher in red than yellow light and the ATP level was 

 lower. As long as the PO2 was low, white light, w^hich may be looked 

 upon as including both red and yellow wavelengths, was no more 

 effective than the yellow light alone in raising the P^^ level in the 

 organic phosphates above that found in the darkened controls. 



At higher oxygen levels the ATP content was almost normal under 

 the yellow light, but P^^ incorporation into hexose phosphates was 

 still very slow. Under red light more P^^ was found in the hexose 

 phosphates but the ATP level was only 40% of that found in the 

 darkened inhibited plant. Illumination with a combination of red 

 and yellow light at 5% oxygen gave the nearest approximation, 

 under CO, to the phosphate distribution pattern found in tissue 

 photosynthesizing in air under bright white light. 



