68 



Britton Chance and Walter D. Bonner, Jr. 



directly from yoxing spinach and Swiss chard plants, and chloro- 

 plast preparations (kindness of Mr. Stephen Sikes). Etiolated 

 Mung bean leaves were obtained directly from the plant and were 

 greened for several hours with the red light prior to use. The 

 properties of these leaves have been reported on in more detail 

 by Dr. Bonner (8). The leaves are held lightly between two 

 lucite plates. These preparations were rapidly frozen by plung- 

 ing them into liquid nitrogen in an aluminxom holder of small 

 specific heat. Occasionally, the leaves crack at low temperat\ire^ 

 especially if they are held too tightly by the lucite plates. 



The leaf absorbancies are very high, not only due to their 

 chlorophyll content (0.3-0.^4- mmoles/kg wet weight) but also due 

 to the non-specific light absorption of the leaf. The effective 

 transmission in this apparatus is 1. 6 per cent at TOO mii and even 

 less at shorter wavelengths. Thus, nearly complete absorption of 

 the incident light occurs. 



An example of the responses of these fovir types of biological 

 materials is given in Fig. 1 where we display on a compressed 

 time scale the optical density decreases at 555 ^ measured with 

 respect to a nearby reference wavelength (5^0 or 5^^ ^V-) > ^^ 

 each one of the four records, it is seen that the optical density 

 change of the order of approximately 0.005 is obtained. It is 

 seen that a rapid downward deflection of the traces (corresponding 

 to a decreased absorbancy at 555 niii) is obtained when the actinic 

 light (680 rD\i) is tTxrned on. In each case, the leaf thicknesses 

 were between ,k and .6 mm. Two levels of actinic intensity are 

 employed (IX and 2X). In the case of the chloroplast suspension 

 and the Mung bean leaves the actinic light is interrupted period- 

 ically and the oxidation is seen to come to a halt. Thus, the 

 effects of the measviring light are not objectionable under the 

 experimental conditions. However, small effects can be observed 

 in the chloroplast suspension. 



Table I summarizes a number of properties of the cytochrcme f 

 responses in the four types of leaves and the one type of chloro- 

 plast preparation studied. As indicated in the Table, the opti- 

 cal density chants for the leaf thickness are in the range of 

 0,Oi4-i4- (Mung bean leaves) to O.OO69 for market spinach leaf. The 

 rates of cytochrome f oxidation, which will be discussed in more 

 detail below, when put on a basis of an arbitrary unit of light 

 intensity, are of a similar order of magnitude although the 

 fresh Swiss chard leaf seems to excel the other biological ma- 

 terials, even the chloroplast preparation. 



