GENERAL REVIEW 005 



Not satisfied with the qualitative similarity between the empirical 

 light curves and the broken lines predicted by the theory of limiting factors, 

 Blackman insisted on a quantitative agreement, and thus precipitated the 

 controversy to which we have referred in chapter 26. He insisted that no 

 decline in rate occurs at high light intensities, unless injuries are brought 

 about by overheating, and denied the gradual character of the transition from 

 the linearly ascending part to the horizontal part of the light curves. 



Blackman prolial)ly was right in suggesting that the inhibition of 

 photosynthesis by excessive light be attributed to destructi^•e processes 

 alien to the intrinsic kinetic mechanism of photosynthesis. (However, we 

 believe these processes to be photoxirlations, rather than thermal reactions 

 caused by overheating; this theory of light inhibition was discussed in 

 chapter 19, when we described the phot oxidation phenomena in living 

 plants.) It is, on the other hand, impossible to accept the second contention 

 of Blackman— that the linearly ascending part of the light curves goes over 

 abruptly into the horizontal part. All precise observations confirm that 

 light saturation is reached asijmptotically, sometime over an extended range 

 of fight intensities (cf. the early criticism of Blackman's interpretation by 

 Brown and Heise 1917, 1918). It was shoT^^l in chapter 20 (cf. also fig. 

 28.20) that the inhomogeneity of light absorption, which is inevitable even 

 in single chloroplasts, not to speak of multicellular systems, should in itself 

 suffice to make practical observation of Blackman's angular light curves 

 impossible — even if these curves correctly represented the relation between 

 light intensity and rate of photosynthesis in a uniformly illuminated vol- 

 ume element. Application of the general laws of reaction kinetics shows, 

 however, that even in the ideal case of completely uniform light absorption 

 Blackman's broken lines could represent only a first approximation, which 

 may be more or less satisfactory, depending on specific conditions. 



1. General Review 



Table 28.1 lists the most important determinations of the light curves of 

 photosynthesis carried out since the time of Blackman. 



A remark mu.st be made on the units of light intensity used in these measurements. 

 For white light, lux (meter candles), or foot candles, have been and still are widely used. 

 A foot candle is equal to 10.8 meter candles; one meter candle corresponds {cf. chapter 

 25, page 838) to about 4.5 erg, or 1.4 X lO'^ quanta, or 2.3 X 10^'^ einstein of photosyn- 

 thetically active light (400-700 mju), falling each second on a square centimeter of the 

 illuminated surface. For colored light, the intensity is usually given in ergs (or calories; 

 1 cal = 4.2 X 10^ erg) per square centimeter per second, or in watts per square centi- 

 meter (1 watt = 10^ erg/sec). We will use the abbreviations klux for thousand lux, 

 and kerg for thousand erg. In comparing the results obtained in light of different color, 

 the most appropriate measure of intensity is the number of incident quanta (A';,^), or the 

 number of einsteins (1 einstein = 6.1 X 10" quanta) falling per second on one .square 



