362 



FACTORS AFFECTING PHOTOSYNTHESIS 



systems are indispensable for the occurrence of photosynthesis, although no 

 specific enzyme or enzymes has ever been identified with the process. 



5. Accumulation of the End Products of Photosynthesis. — During rapid 

 photosynthesis the carbohydrates produced in the process or in immediately 

 following secondary reactions accumulate in the photosynthesizing cells more 

 rapidly than they are translocated towards other tissues. Accumulation of 

 carbohydrates in chlorenchyma cells exerts a retarding effect upon photosyn- 

 thesis (Kurssanow, 1933). In excised leaves of the grape, for example, 

 photosynthesis ceases completely when the percentage of carbohydrates present 

 becomes equivalent to 17 to 25 per cent of the dry weight of the leaf 

 ( Saposchnikoff , 1891 ). The mechanism of this effect is not positively known, 

 but it seems probable that two principal causes are involved. Accumulation 

 of sugars in the cell sap may result in an increase in its diffusion pressure 

 deficit and hence in a decrease in the hydration of the protoplasm. Reduc- 

 tion in the hydration of the protoplasm, as we have already seen, results in 

 a diminished rate of photosynthesis. Furthermore accumulation of large 

 quantities of starch within the chloroplasts may interfere mechanically with 

 their efficient operation in photosynthesis. 



Daily Variations in the Rate of Photosynthesis. — Thomas and Hill 

 (1937) measured the daily variation in rates of apparent photosynthesis for 



6 A.M. 



6RM. 



6 A.M. 



6RM. 



Fig. 93. Daily variations in the rate of apparent photosynthesis of alfalfa. Data of 



Thomas and Hill (1937). 



small plots of alfalfa and wheat (Fig. 93). Well watered plots of alfalfa 

 six feet square were enclosed in transparent celluloid cabinets and air circulated 

 through each cabinet at rates ranging up to several hundred cubic feet per 

 minute. The consumption of carbon dioxide by the plants was determined 

 by measuring the difference in the concentration of this gas in the inflowing 

 and outflowing streams of air. No measurement was made of the quantity 

 of carbon dioxide liberated as a result of soil respiration in the enclosed plots, 

 and this source of error must be considered in evaluating the results. In 



