HESSEL KOK AND CEOIU'.E HOCH 



399 



sensitized by 700 ni^/, lioht may be increased about twice by addition 

 of suj)pleniental light. I'his niaxiinmn enhancement requires an in- 

 tensity of su])])lcmcntal Ught ^\•hich by itself gives about twice the 

 rate of photosynthesis produced by the 700 m^A light alone. Dr. 

 French will discuss these matters in more detail. 



Figs. 1 and 2 show some data we have obtained with the blue- 

 green alga Anocystis nidulayis and may serve to illustrate the en- 

 hancement. The action spectrum (Fig. 1) clearly shows the relative 

 inefficiency in oxygen evolution and CO2 uptake of chlorophyll a if 

 it alone is excited. The rate of oxygen production mediated by 

 chlorophyll a (700 ni/x) is increased by a simultaneous excitation of 

 phycocyanin (638 m^) . Fig. 2 plots the magnitude of the enhance- 

 ment versus the ratio of rates induced by the two beams separately. 



Thus, together with the observations discussed in the previous 

 section, these data lead to a rather embarrassing picture. Chlorophyll 

 a — the most abundant and the only universally occurring pigment — 

 is unable to sensitize photosynthesis efficiently (and to fluoresce) un- 



150 



100- 



Rate of 0_ Evolution 



and CO, Consumption 



(arbitrary units) 



T 1 I 1 1 1 1 1 1 I 



550 60 70 80 90 600 10 20 30 40 50 60 70 80 90 700 710 



Wavelength (m^i) 



Fig. 1. Action spectrum of photosynthesis by Anacystis nidnlans. Rate of gas 

 uptake or evolution measured with a mass spectrometer. Circles, oxygen evolu- 

 tion crosses; CO™ consumption. 



