218 Mineral Nutrition of Plants 



comitant increased loss of sugar which is reflected in the enhanced 

 respiratory activity of roots. If the supply of this material should be or 

 become low, the accumulation of inorganic solutes may be restricted. 

 A part of the limited ability of high-salt roots to accumulate inorganic 

 solutes may be related to this factor. It has been shown further that 

 such roots are usually what may be referred to as high-salt, low-sugar 



TABLE IV 

 Effect of Salt Status on Further Salt Absorption by Barley 



Expt. A Expt. B 



Experimental Conditions Absorbed from Absorbed from 



solution solution 



K Br K N0 3 



m.e./liter m.e./liter 



High-salt excised roots ... —0.20 0.50 0.67 0.89 



Low-salt excised roots ... 2.66 1.90 3 02 2 93 



High-salt entire plants ... 2.05 2.40 



Low-salt entire plants ... 3-66 3-49 



Note: Plants 3 weeks old. Nutrient solution during growth renewed daily for high-salt sets 

 and not renewed for low-salt sets. Composition of culture for experiment A: 0.0075 M KBr 

 initially; experimental time 20 hours at 25 C. Composition of culture for experiment B: 

 0.0025 M Ca(N0 3 ) 2 ; 0.005 M KNO3; 0.001 M MgS0 4 ; and 0.0005 M KH2PO4, initially; 

 experimental period 7 hours. 



systems. The latter factor is of secondary importance in the experiments 

 reported in Table IV. The substrate limitation can be more clearly 

 divorced from the high-salt factor by depletion of the carbohydrate in 

 root systems, through prolonged aeration of excised roots. When low- 

 salt roots are so treated, further accumulation is limited unless sugar 

 is concurrently supplied externally during the experimental study. 

 Although less effective than an inherent internal supply, an external 

 application of sugar will correct a substrate limitation to some degree 

 (Table V). Usually, this accentuated restriction is rather unnatural, 

 but the necessity of a ready substrate for the processes of respiration and 

 inorganic solute accumulation is emphasized. As will be pointed out 

 later, the respiratory quotient in roots is frequently approximately one, 



