The Maximum Efficiency of Photosynthesis : A Rediscovery 95 



When the steady State in the white light was reached, a beam of red light of 

 measured intensity was sent through the bottom of the vessel into the cell Suspen- 

 sion, the cross section of the beam being 3 cm. 2 and the total energy flux about 

 0.25 microeinsteins/min. It was completely absorbed in the cell Suspension. The 

 increase of positive pressure efTected by the red light and the intensity of the red 

 light were the two magnitudes from which the efficiency of the energy transforma- 

 tion of the added red light was calculated. The efficiency so found with red light 

 proved to be as high, at and several fold above the compensation point with white 

 light, as the efficiency at zero or low white light intensities below the compensation 

 point (3 to 5 quanta per Oz molecule developed; see Examples 1 — 5). 



This result raised the whole level of certainty in this field of investigation, and 

 is probably owing in large part to the rapid motion of a great amount of cells. 

 The time of illumination with red light of relatively high intensity must be so short 

 for every cell that no Blackman or other dark reaction limits the action of the light; 

 in other words the product of light intensity and time (i X t) must be tco small to 

 alter the concentrations of dark reactions. The observed efficiency is, of course, a 

 fact that is independent of all theories about what happens in the cell suspensions, 

 chemically or physically. 



Another factor that might limit the certainty of efficiency determinations is time 

 of illumination. The shorter the time periods, the greater is the danger that the 

 energy of some dark reaction contributes to the oxygen production. The longer the 

 time periods, the more certain are we that we have reached the thermodynamically 

 desirable State in which the concentrations of all cell constituents are kept constant. 



From 10 min. in 1923 * and 15 min. in 1948 5 we have now extended the time 

 periods to more than 20 hr. of continuous illumination with white light, producing 

 contmuous positive pressure, and have varied the periods of the efficiency deter- 

 minations with added red light from 5 to 60 min. This great improvement was 

 possible because the horizontal motion of the rectangular vessels did not damage 

 the photosynthetic capacity of the cells and because the white light, when it over- 

 compensated respiration, stabilized the chemical conditions in the cells. In fact, 

 the efficiency of the energy transformation has now been measured under the 

 conditions of growth, so that very likcly the experiments can be extended to any 

 length of time. 



Nonaction of Light on Respiration 



If the manometric efficiency of the red light is the same when the respiration of the 

 cell suspensions is noncompensated, compensated, or several fold overcompensated 

 by added unmeasured white light, then obviously every theory should be rejected 

 that assumes that light acts on the process of respiration anticatalytically or stoichio- 

 metrically. This conclusion has now been confirmed independently by the follow- 

 ing type of experimentation : 



A rectangular vessel was set up to contain NaOH and glass beads in two side 

 arms, 300 mm. 3 of cells in 7 ml. of culture medium (pH 4.5—5) in the main com- 

 partment, and air in the gas phase. The vessel was shaken at 20° C. slowly or ra- 



