INTERPRETATION OP LIGHT CURVES 1007 



after having analyzed the foundations of this estimate, we feel certain that 

 its order of magnitude, at least, is secure — even though the figure given may 

 be in error bj^ as much as a factor of two. 



7. Interpretation of Light Curves 



(a) Influence of Inhomogeneity of Light Absorption 



The nonimiform illumination and supply of reactants in photosynthesis 

 were discussed in general terms in chapter 26 (section 2). In chapter 27, 

 while dealing with the carbon dioxide curves, we noted that these curves 

 can be strongly affected by concentration gradients, which arise, particu- 

 larly during intense photosynthesis, between the external medium and the 

 immediate neighborhood of the chloroplasts. Similarly, the light curves 

 may present a strongly distorted picture of the intrinsic relationship be- 

 tween the rate of light absorption and the yield of photosynthesis, because 

 a considerable gradient of light intensity often must exist between the light- 

 exposed and the shaded chlorophyll molecules. Even within a single 

 chloroplast the rate of light absorption may decrease by a factor of five or 

 ten from the light-exposed to the shaded side ; or, in the case of diffuse il- 

 lumination, from the surface to the center of the plastid. In suspensions 

 containing millions of cells, as well as in leaves or thalli, the heterogeneous 

 nature of light absorption is further enhanced by the mutual shading of 

 the numerous chloroplasts (of. fig. 26.5). Consequently, the light curves 

 of different specimens of one organism, even if they all have the same con- 

 tent of all the relevant pigments and catalysts and are investigated under 

 the same external conditions, maj^ nevertheless differ in shape, depending 

 on optical density {i. e., the number of cells per square centimeter in an 

 algal suspension, or the thickness of a leaf or thallus). The assumption of 

 equal content of catalysts may itself be incorrect; for example, the cells 

 of the spongy parenchyma may be adapted to weaker light (and thus con- 

 tain less of certain catalysts) than the leaves of the palisade tissue. 



Let us consider, as the simplest example, two suspensions of identical 

 cells — one optically thin (e. g., transmitting 80% of incident light), the other 

 optically dense (e. g., absorbing 80% of incident light). There is no reason 

 (aside from the phenomena of "self-inhibition" by metabolic products 

 mentioned in chapter 25) why these suspensions should differ in the maxi- 

 mum quantum yield at low light intensities, or in the maximum yield per 

 chlorophyll molecule in strong light. However, the transition from the 

 linearly ascending part to the horizontal part of the light curves wll be 

 sharper in the optically thin system (where saturation occurs more or less 

 simultaneously in all cells), and more gradual in the optically dense system, 



