THE ROLE OF WATER IN PHOTOSYNTHESIS 357 



(5) High temperatures may have a destructive effect on chlorophyll or 

 possibly on some other protoplasmic factor besides chlorophyll or enzymes. 



The first two of the above suggestions appear to be more probable explana- 

 tions of this phenomenon than the others. It is also quite possible that this 

 effect may result from the composite influence of several factors. 



The time factor eft'ect in photosynthesis has also been demonstrated in 

 terrestrial plants. In fact this principle was first pointed out by Blackman 

 and Matthaei (1905) in their classical paper on temperature effects upon 

 the rate of photosynthesis in the cherry laurel and Jerusalem artichoke. In 

 their investigations corrections were made for respiration, indicating that the 

 observed results could not be due to a differential effect of temperature upon 

 photosynthesis and respiration. 



The foregoing discussion of the time-temperature relations of photosyn- 

 thesis indicates that there can be no optimum temperature for this process 

 in the usual sense of a certain temperature for each species at which photo- 

 synthesis will proceed most rapidly, when no other factors are limiting. The 

 optimum temperature can be defined only with reference to the element of 

 time, and from this standpoint might be considered as the highest temperature 

 at which the initial rate of photosynthesis is maintained for a relatively long 

 period. The optimum temperature for photosynthesis, defined in this sense, 

 varies considerably with different species, and is usually higher in tropical 

 species than in those native to temperate regions. 



3. Influence of Light Intensity upon Temperature Effects on Photosyn- 

 thesis. — Temperature effects upon photosynthesis are further complicated by 

 the fact that they may differ according to the light intensity to which the 

 plants are exposed. As pointed out in the last chapter the temperature co- 

 efficient of photosynthesis at relatively low light intensities approximates one, 

 while at higher light intensities it approximates two. At relatively low -light 

 intensities, therefore, raising the temperature, at least within the range of 

 about 15-30° C, (Warburg, 1919), has little effect on the rate of photo- 

 synthesis, because the rate of the process is limited by its photochemical phase. 

 At relatively high light intensities, however, the rate of photosynthesis is ap- 

 proximately doubled for each 10° C. rise in temperature, subject to the limita- 

 tions imposed by the time factor effect. A logical conclusion which may be 

 drawn from this finding is that temperature has very little effect on the rate 

 of photosynthesis in plants growing in deep shade or in plants exposed to the 

 low light intensity of cloudy or foggy days. 



The Role of Water in Photosynthesis. — Less than i per cent of the 

 water absorbed by a plant is used in photosynthesis. It therefore seems prob- 

 able that the indirect effects of the water factor upon photosynthesis are more 



