AB.SOLTITK MAXIMUM RATE 9S9 



Bohning (1949) noted that the rate of photosynthesis of shade-adapted 

 leaves on trees of Pyrus malus decHned in continuous ilkimination of 32 

 klux from an initial value of about 20 mg. to < 5 mg. C02/(hr. X 100 cm. 2) 

 after 20 days. Sun-adapted trees, on the other hand, showed no dechne 

 during a similar period of continuous illumination, even in 50 klux. Kramer 

 and Decker (1944) compared the light curves of white pine with those of 

 three hardwood trees, and noted that the first one behaves as a heliophile 

 and the deciduous trees as umbrophiles. This supports a previously sug- 

 gested explanation of the fact that young deciduous trees "squeeze out" 

 young pine trees on the floor of a forest. 



Table 28.IV 



Photo.synthesi.'; and Respiration of Shade Leaves and Sun Leaves 



OP THE Same Plant (after Boysen-Jensen and Muller 1929) 



" Respiration {R) and photosynthesis {P), in nig. CO^/lOO cm."- hr. 



Lubimenko (1928) and Montfort (1934) found that some species have 

 "rigid" umbrophihc or heliophihc characteristics, i. e., they are unable to 

 adapt themselves to illuminations different from those to which the species 

 as a whole has become adapted in its phylogenesis, whereas other species 

 are capable of individual readjustment, as shown in figures 28.17 and 28.18 

 and in Table 28.IV. 



Umbrophihc character is typical also of algae that have been adapted — 

 phylogenetically or individually — to weak light. Chlorella cultures grown 

 in dim hght are richer in chlorophyll than those grown in strong light (c/. 

 Table 25.1); the light curves of these "shade-adapted cells" rise more 

 steeply, and reach saturation earlier than those of the "light-adapted cells." 

 Similarly, van der Paauw (1932) found that Hormidium cells grown in dim 

 light (2000 lux) become light-saturated in comparatively weak light (5000 

 lux), and show light inhibition in only slightly stronger light. 



Algae that live deep under the sea, particularly the red ones, behave as 

 extreme umbrophiles, and their photosynthesis, too, reaches saturation at 

 light intensities of the order of a thousand lux. 



5. Absolute Maximum Rate 



It was mentioned before that simultaneous increase of carbon dioxide 

 supply and light supply usually leads to saturation of photosynthesis long 



