236 Plant Biology 



of the leaves, thus inhibiting the normal entrance of carbon dioxide. 

 Water is also necessary to transport the soil salts in solution. 



4. Temperature: In general, the rate of photosynthesis rises in a 

 geometrical way as the temperature rises from the minimum toward the 

 maximum. For every 10 degree rise in temperature, the rate of photo- 

 synthesis increases an average of 2.4 times, until a maximum of about 

 35° C. is reached, beyond which no increase in the rate occurs. In fact, 

 bevond 35° C. the rate mav even decrease. Certain conifers accus- 

 tomcd to cold climates may photosynthesize at -25° C, while the mini- 

 mum for most plants is about 0° G. The maximum for most plants is 

 about 45° C. 



5. Soil Salts, Including Magnesium and Iron: Iron salts in the cells 

 probably act as catalyzers in the photosynthesis process, although iron 

 does not enter into the composition of chlorophyll. Magnesium is the 

 central constituent of chlorophyll, and consequently the quantity avail- 

 able wall influence the formation of chlorophyll, and this in turn will 

 determine the rate of photosynthesis. Excesses of salts in the soil retard 

 photosynthesis by inhibiting the osmosis of water by the root hairs. Salts 

 in the plant liquids also may influence the normal functioning of the leaf 

 stomata, and hence influence the entrance of carbon dioxide. 



6. Internal Factors, Including Chloroplasts and Enzymes: Chloro- 

 phyll is absolutely essential for photosynthesis, and the amount of car- 

 bohydrate manufactured varies almost directly with the amount of 

 chlorophyll in the chloroplasts. Fruits and other plant products are 

 directly influenced by the number and size of the leaves with their 

 contained chlorophyll. The removal of some of the leaves shows the 

 important quantitative relation between chlorophyll and the food manu- 

 factured. Damage to leaves produced by hail, storms, and insects 

 causes a corresponding decrease in photosynthesis. 



Willstatter and Stoll theorize that a specific enzyme is associated with 

 chlorophyll in photosynthesis. The action of this enzyme is accelerated 

 as temperature rises, which may explain in part the eff^ect of increased 

 temperature. However, acceleration of the enzyme leads to increased 

 photosynthesis only when an abundant supply of chlorophyll is present, 

 the latter absorbing more of the necessary light energy. Probably in 

 plants with a minimum of chlorophyll it is the lack of light absorption 

 that limits the rate of photosynthesis, while in plants high in chlorophyll 

 content the activity of the enzyme may be the limiting factor. Many 

 factors operate simultaneously in the photosynthesis process and the 



