Inhibition and Retardation of the Oxidation of lAA 



157 



and intensity of illumination. Figure 5 shows the progress ol the 

 oxidation with various kinds of illumination. As would be expected 

 from the absorption spectrum of riboflavin, blue light was most 

 effective, whereas red light was totally ineffective and white light 

 was required at a higher intensity to give the same effect as blue. In 

 systems containing 0.33 fxM riboflavinphosphate, light saturation was 

 attained at an intensity of 110 foot candles, under which conditions 

 the rate of oxidation attained a maximum equal to the dark rate 

 without riboflavin. Oxygen uptake always proceeded well past the 

 theoretical oxygen equivalence for lAA (Table 3). It is noteworthy 

 that after the light had been switched off (Figure 5), rapid oxidation 

 of lAA continued for some time, indicating a residual effect of light 

 except when lAA became limiting. 



The oxygen consumed in the breakdown of lAA by catalase 

 systems was observed to exceed the theoretical molar equivalence of 

 lAA when illuminated in the presence of riboflavin (8). This also 

 occurred with the wheat leaf system (Figure 6A and Table 3). In 

 the experiment (Table 3) carried on for 400 min., 199 fx\. of oxygen 

 were consumed by an illuminated standard system containing 6.0 

 /JVf riboflavin and the equivalence of 3.3 jxM or 79 /xl. lAA. The rate 

 of oxygen uptake showed no signs of abating, and similar results 

 were obtained when DCP or maleic hydrazide was present in place 

 of resorcinol. The amount of riboflavin used in this experiment was 

 approximately 30 times greater than that required to produce 50 

 per cent inhibition. 



In the absence of lAA (Figure 6C), oxygen was consumed by these 

 systems at a slower initial rate, but at the same final rate as with 

 lAA (Figure 6A). The difference between the oxygen consumed with 

 and without lAA (Figure 6B) always exceeded at equilibrium the 

 molar oxygen equivalence of lAA. The difference was greater at 



Table 3. Oxygen consumed by illuminated systems con- 

 taining riboflavin.* 



* Standard wheat leaf systems containing: 3.33 ixM lAA; 

 6 . /iA/ riboflavinphosphate; blue light, 220 foot candles. Molar 

 oxygen equivalence of lAA = 79 ^il.; riboflavinphosphate = 133 

 ix\.; resorcinol and dichlorophenol =35.5 ^1- maleic hydrazide 

 = 712 m1- 



