CHAPTER 



I O Light as a Modifying In- 

 fluence on the Mineral 

 Nutrition of Plants 



ROBERT B. WITHROW 



L 



iight is not known to play any direct role indispensable 

 for the absorption, movement, or metabolism of the mineral nutrients. 

 The ions of the mineral macroelements in the inorganic state have no 

 absorption bands making it possible for direct light interaction. A few 

 of the microelements in the ionized state, such as iron, manganese, and 

 copper, have absorption bands in the visible spectrum, but thus far no 

 strong evidence has appeared that light absorption by such ions is of 

 any physiological significance. The effect of light on mineral nutrition, 

 therefore, must be chiefly an indirect one resulting from the tempera- 

 ture rise concomitant with the absorption of light and from the basic 

 photochemical reactions occurring in plants, such as photosynthesis, 

 chlorophyll synthesis, photomorphogenesis, and photoperiodism. 



In many cases, the various mineral elements are essential for a photo- 

 chemical process, either as a constituent of an enzyme system or as a 

 component of a pigment or other substance involved in the photo- 

 chemical process. In such cases, it is apparent that the rate of the photo- 

 chemical process frequently will influence the mobilization and utili- 

 zation of the essential mineral elements. In addition, light can affect 

 the physiology of growth, of which mineral metabolism is a part, 

 through various rather poorly understood photochemical effects such 

 as alteration of permeability, changes in protoplasmic streaming, and 

 various photochemical oxidations. 



The most conspicuous indirect role of light in mineral metabolism 

 is through photosynthesis. A carbohydrate supply is necessary as a 



