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Govindjee 



reaction with a certain oxidant- the long wave form of chlorophyll 

 a (Chi a 680/Chl a 690) were simply inactive, one would still 

 get a "red drop".' 



Conversely, the absence of a "red drop" cannot be taken as 

 proof of the absence of two light reactions, because extensive 

 overlapping of the spectral bands of the two systems could make 

 separation of their effects difficult. 



However, the presence of both "red drop" and an enhancement 

 effect clearly suggests the existence of two photochemical 

 systems acting cooperatively in the overall process of photo- 

 synthesis. 



II. "LIGHT CURVES" OF PHOTOSYNTHESIS IN MONOCHROMATIC LIGHT 



Studies of the enhancement phenomena are reliable only if the 

 light curves (the plot of the rate of photosynthesis, R, as a 

 function of light intensity, I) are linear in the intensity 

 range of the experiment, so that by combining the two beams, one 

 does not get into the saturating (or at least less sloping) ranges 

 suggesting "negative" Emerson effect. Similarly a "positive" 

 effect can be wrongly inferred when one of the light curves is 

 sigmoid in shape (cf. below). This necessitated a careful 

 investigation of the light curves under different conditions. 



A. Light Curve with "Knick" 



McCloud (as well as Govindjee ' ) had observed that the 

 saturation rates of photosynthesis in various algae were depen- 

 dent on the wavelength of light; at the longer wavelengths- 

 beyond 680 m|j. - an "early" saturation on a relatively low level 

 was reached. These observations were very difficult to inter- 

 pret, since saturation was supposed to be imposed by the availa- 

 bility of a limiting enzyme, which should be the same whatever 

 the wavelength of light. Because of the narrow range of light 

 intensities used in my earlier experiments, which had led to 

 the above conclusions, we repeated these measurements on 

 Porphyridium over a wide range of intensities. Two wavelengths 

 were selected -- one absorbed primarily by phycoerythrin (546 

 m\i) and the other by the "long wavelength" form of chlorophyll 

 a (700 m|i) , Advantage was taken of the well-known fact that 

 saturation levels are lower at the lower temperatures. Measure- 

 ments were made at 3-5°C (see figure 3). The 700 m|a light curve 

 seems to approach the same saturation level as that obtained in 



