APPENDIX 151 



Measurement of photosynthetic activity 



The rate of photosynthesis may be expressed in a number 

 of different units, such as mg. gas exchange per dry weight, 

 per fresh weight, or per chlorophyll. Comparison of the 

 rate in terms of only one unit may be misleading since the 

 percentage dry weight per fresh weight, and the percentage 

 chlorophyll per dry weight varies from plant to plant and 

 indeed (as discussed in Chapter 4) according to the age of 

 the leaf. For comparison with the activity of isolated enzyme 

 reactions the most convenient unit is that of ^al./mg. dry 

 weight/hour, i.e. the QOg value which since oxygen is 

 evolved in photosynthesis should be written as -Qo^- When 

 this value is referred to chlorophyll content, i.e. ^l./mg. 

 chlorophyll/hour, it is called the Q^^- Maximum 

 observed rates of photosynthesis expressed in these different 

 units are given in the table on page 152. 



Thus it can be seen that in Sambuciis 1 mg. chlorophyll 

 corresponds to 118 mg. dry weight or 450 mg. fresh weight. 

 In Chlorella 1 mg. chlorophyll corresponds to 20 mg. dry 

 weight or 100 [x\. cell volume. 



