PHOTOSYNTHESIS AND RESPIRATION 1939 



exchange, followed by an outburst, also a secondary "wave" — first a CO2 

 gulp, then a "burst." A short "compensation period" now appears also 

 at the beginning of each light interval. 



These experiments clearly show the complex course of the carbon di- 

 oxide exchange — and indicate that it is due to the influence of light on the 

 respiration mechanism, rather than to the complexity of photosynthesis 

 itself. 



While the exact mechanism of mutual interaction of respiration and 

 photosynthesis remains to be elucidated, experiments with tracer carbon, 

 described in chapter 36, indicate that certain compounds, known to occur 

 as intermediates in respiration (such as phosphoglyceric acid, and malic 

 acid) also appear as intermediates (or, in the case of malic acid, as rapidly 

 formed side-products) in photosynthesis. 



Unless spatial segregation intervenes, these compounds should provide 

 a cross-link between catabolic and anabolic processes— reservoirs fed both 

 by photosynthetic CO2 fixation, and by the Krebs cycle in respiration, and 

 into which both the reductive processes of photosynthesis and the oxida- 

 tive processes of respiration could dip. Depending on the relative rates 

 of the two processes, this cross linkage may lead either to the reoxidation 

 of intermediate products of photosynthesis, or to the utilization of respira- 

 tion for reduction in light. 



Imagine that a reservoir, situated midway on a gravity waterway (res- 

 piration), is also used as midway reservoir of an uphill pumping system 

 (photosynthesis). When the latter pumps much faster than the stream 

 runs down {i. e., when illumination is strong), the water level in the com- 

 mon reservoir will be determined practically completely by the uphill 

 movement of water ; the situation will be reversed when the gravity stream 

 is fast compared to the pumping velocity (weak light) . If the stationary 

 level of water in the reservoir as determined by pumping alone is lower 

 than it would be if it were determined by gravity stream alone, strong 

 pumping will cause the level of water in the reservoir to drop, and the 

 amount of water running down from the reservoir to decline (i. e., strong 

 light will inhibit respiration). If, to the contrary, strong pumping causes 

 the halfway reservoir to be filled to a higher level than it would have as- 

 sumed under the influence of the downward stream alone, then pumping 

 will increase the downhill flow of water from the reservoir {i. e., strong 

 photosynthesis will accelerate respiration). Similarly, in weak light (near 

 or below the compensation point) photosynthesis (defined as the effect of 

 light on the uptake of CO2 and liberation of O2) could be either increased or 

 decreased by respiration, depending on the sign of the difference between 

 the steady levels of the common reservoir as determined by each process 

 separatel3^ 



