160 PHOTOSYNTHESIS 



There is as yet not sufficient information to enable a clear conception 

 to be formed as to the manner in which the accumulation of starch in- 

 hibits the photosynthetic activity of the chloroplasts. In fact, in view of 

 the behavior of plants which form no starch and show an inhibition 

 through high concentration of soluble carbohydrates, it is possible that the 

 accumulation of starch is not directly associated with the cause of the 

 inhibition, but is merely an accompanying phenomenon. At the same 

 time, if we accept the theories of Wurmser, of Briggs, and of Warburg, 

 that the primary photochemical action takes place on the surface of the 

 chloroplast, it is conceivable that the accumulation of starch in the chloro- 

 plast would materially afifect this action. In starch-laden leaves, the 

 starch grains are frequently larger than the chloroplasts, the latter being 

 partially covered by the starch. Under these conditions the surface of 

 the chlorophyll solution is materially reduced. As a consequence as much 

 carbon dioxide could not reach the chlorophyll as when the chloroplasts are 

 starch-free, and the amount of carbon dioxide entering the photochemical 

 reaction would be reduced with a resulting decrease in photosynthesis. 

 Thus, any substance which displaces carbon dioxide from the surface 

 would tend to inhibit the rate of photosynthesis. 



Miss Henrici ^~^ has described some interesting curves showing the 

 indirect effect which temperature and light may exert on the photo- 

 synthetic rate. These curves for alpine phanerogamic shade plants and 

 lichens do not show the single maximum, either as a function of tempera- 

 ture or illumination intensity when only one factor is changed. There 

 are, in fact, two maxima. These plants form starch under conditions of 

 higher light intensity or higher temperature than normally occur in the 

 winter. For example, while at 0° in these alpine plants there is never 

 any starch, when these are exposed to light of high intensity starch ap- 

 pears. The threshold of temperature and illumination intensity for photo- 

 synthesis is much lower than that of starch formation. As a result when 

 either factor is increased the rate of photosynthesis rises to a maximum; 

 during this time no starch is formed. With continued increase of either 

 factor the rate decreases ; this decrease is synchronous with the appearance 

 of starch in the chloroplasts. With further increase of either Hght or 

 temperature the rate of photosynthesis passes a minimum value, then rises 

 to a second maximum and finally drops rapidly again. Plants which 

 habitually form no starch do not behave in this manner and, as has been 

 stated, the feeding of soluble sugar and consequent starch formation, 

 rapidly decreases the photosynthetic rate. These effects are apparently 

 closely associated with those observed by Ursprung. 



Prolonged exposure to sunlight may cause temporary inhibition of 

 the photosynthetic capacity of plants. The causes of this action are still 

 obscure, though it appears that the plasma of the chloroplasts is more 

 sensitive under certain conditions than the protoplasm of the cell.^"'' 



""Henrici, Vcrh. Naturf. Ges. Basel, 32, 107 (1921). 

 "' Pringsheim, Jahrh. zviss. Bot., 12, 326 (1882). 



