39° Mineral Nutrition of Plants 



source of energy for the accumulation of mineral nutrients by roots 

 and for carrying out all other chemical reactions driven energetically 

 by respiratory reactions. The mineral elements enter into synthesis with 

 carbohydrate-derived materials to form the metabolic and structural 

 components of plants. It therefore follows that the mineral nutrient 

 requirement of plants is directly dependent upon the carbohydrate 

 supply and, in turn, upon photosynthesis. 



While there are many photochemical reactions in plants necessary 

 for normal growth and development, photosynthesis is the principal 

 energy-converting reaction and consequently requires relatively high 

 light intensities to maintain optimum growth. So far as is now known, 

 all the other photochemical reactions are saturated at relatively low 

 light intensities of the order of 20 foot-candles or less. In the case of 

 photosynthesis in higher plants, on the other hand, saturation does not 

 occur until light intensities of several thousand foot-candles have been 

 attained. In the case of wheat, Hoover et al. (22) have shown that the 

 relation between the rate of photosynthesis and light intensity is very 

 nearly a linear one up to about 1000 foot-candles at normal atmospheric 

 carbon dioxide concentrations of 0.03 volume per cent. Above 1000-2000 

 foot-candles, photosynthesis increases with increase in light intensity, 

 but less rapidly than at lower values. 



For the other photochemical processes such as chlorophyll synthesis, 

 photoperiodism, and phototropism, the intensity of daylight between 

 sunrise and sunset seldom becomes a factor critically limiting the rate 

 of reaction. For photoperiodism and certain morphogenic effects, the 

 daylight factor that becomes limiting is the duration of the lighted 

 period. 



In view of these considerations, the relative length of day and night 

 and incident solar energy become important in relation to mineral nu- 

 trition. At 40 degrees latitude in the temperate zone, the natural day 

 length between sunrise and sunset varies from about nine hours to a 

 little over fifteen hours. The range is greater in the more northern lati- 

 tudes and less in the southern regions. The average daily total solar 

 and sky radiation on a horizontal surface at a 40-degrees latitude station 

 (79), however, is 80 per cent less during the winter months than during 

 June and July. Since the reduction in day length is only about 40 per 



