868 LIGHT AND LIFE 



synthetic rate was far below the rate at saturation with light of shorter 

 wavelengths. It had long been thought that at high light intensities, 

 where the photosynthetic rate is limited by some "dark" reaction, the 

 intensity or color of the light could have no effects; but this turns 

 out not to be so. An action spectrum can be obtained at saturating 

 light intensities, and recent experiments by McLeod with Chlorella 

 show a peak of 650 m^, whereas the customary action spectra obtained 

 at low light intensities have a maximum at 680 m^u.. French sug- 

 gests that this effect is also a product of the Emerson enhancement 

 effect, involving activation by 650 vajx light of both chlorophylls a 

 (Ca695) and b, whereas at 680 m/A only chlorophyll a is activated. At 

 the latter wavelength only unenhanced photosynthesis woidd conse- 

 quently be expected. 



The critical problem in photosynthesis, as compared with other 

 well-understood photochemical reactions, is to account for the way 

 in which a reaction that seems to be a single step — the splitting of a 

 water molecule — and yet requires more energy than is contained in a 

 single quantum of visible light, gathers the necessary energy for the 

 process. The several phenomena attributable to the accessory pig- 

 ments seem related to this process; and since 0^670 itself acts as an 

 accessory pigment, the problem may possibly be resolved into that of 

 understanding the respective photosynthetic roles of the different 

 forms of chlorophyll a. Is the enhancement of the effect of Ca695 

 through interaction within the pigment system itself? or do the 

 various pigments produce different chemical products that then in- 

 teract? The latter is the preferred theory, because the interaction still 

 occurs even when the beams of the tw^o different wavelengths are 

 separated in time, as well as because of the different time course of 

 respiration after illumination respectively at 650 m/x and at 700 mfi. 

 French proposes a scheme in which there are two pathways of photo- 

 synthesis: (a) a main path, depending on the forms of chlorophyll a 

 absorbing at 673 and 684 m^, which may be activated by energy 

 transfer from the accessory pigments; and (b) the enhancement path, 

 depending on the form of chlorophyll a absorbing at 695 ni/x, which 

 produces a chemical product that interacts with some chemical prod 

 uct of the accessory pigments to release oxygen. The half-lives of both 

 these chemical products, although not identical, are of the order of 

 5 seconds. 



Like French, Eugene Rabinowitch and Govindjee have emphasized 

 the need arising from the enhancement phenomena to investigate 

 the effect on photosynthesis of combinations of monochromatic beams 



