82 The Quantum Efficiency of Photosynthesis 



IV. Comments on the 2-Vessel Manometric Method 



If the yield q> and the assimilatory quotient, y = ^ ' , are to be determined 



simultaneously, two vessels must be employed. If H be the pressure change in 

 vessel I and H' that in vessel II, the xo 2 and jcco-2 values can be calculated by well 

 known equations (see 1 and section 8). 



The 2-vessel method, simple when the gas-exchanges in the dark are deter- 

 mined, requires special attention when applied to illuminated cells. As will be 

 shown later, the illumination of the cells is an illumination with intermittent light. 

 This intermittency should be equal in the two vessels, and this is attainable if the 

 liquid volumes are equal in both vessels. Furthermore, the respiration in most cell 

 suspensions gradually changes with time, so that the pressure changes in light will 

 also change with time, Thus the two vessels should be darkened and illuminated 

 simultaneously so that the conditions of the aforementioned equations are fulfilled, 

 namely 



3 Xo 2 = x o 2 



xco 2 = x'co 2 



where the primed magnitudes refer to one vessel and the non-primed to the other. 

 These conditions may be satisfactorily met by the method of alternately shifting 

 the mirror under the two vessels at periods of, e.g., 10 minutes, as indicated in 

 Fig. 1, and discussed in the next section. After two or more cycles, the pressure 

 readings for each vessel for light and dark periods may be averaged and the light 

 action calculated from the differences between the pressure changes in light and 

 dark. A possible error involving noncomparability of time periods is thus eliminated. 

 This error has been one of the main sources of difficulty in C///on?//a-photosynthesis 

 experiments with the 2-vessel method. 



V. Procedure 



Simple Haldane-Barcroft constant-volume manometers with small capillaries 

 (0.8 mm diameter) with rectangular vessels attached were shaken horizontally 

 (not by are motion) at 140 — 180 (usually 150) cycles per minute at an amplitude of 

 2.0 cm in a water bath at 20°C. The two rectangular vessels of about 2.2 ;■; 3.8 cm 

 inside width and length and 13 — 14 and 18 — 19 ml volume respectively, were 

 filled with 200 — 400 u\ cells in 7 ml, thus the liquid volumes were identical and the 

 gas spaces differed. The vessels (with capillary sidearm vents) were gassed on the 

 bath, simultaneously with aid of a manifold, and with shaking. The horizontal 

 (not are) shaking was so effective that physical after-effects of gas equilibration in 

 the transition periods of dark to light and vice versa were not appreciable even 

 when the illumination produced photosynthesis far above the compensation point 

 and pressure changes of 5 — 10 mm per minute were involved. The manometers 

 were usually read without stopping. The end of the manometer male Joint was not 

 flat rough but coneave and polished, so that bubble formation in the cappillary did 

 not oeeur; nor did foaming. 



As indicated in Fig. 1 a beam of red light (630 — 660 m/i) of about 3 — 4 cm 2 area, 



