208 Mineral Nutrition of Plants 



during pretreatments with two conditions of temperature and aeration. 

 The lower graph (12B) shows the experimental measurements of 

 isotopic exchange under aerobic and anaerobic conditions. The ex- 

 change rate was not enhanced by aeration (compare with upper graph). 

 On the contrary, an even lower rate of exchange was obtained with 

 aeration, which is probably related to concurrent reaccumulation by 

 metabolic influx or internal translocation of previously adsorbed ions 

 from the effective loci of exchange. That exchange does occur is clear 

 from these experiments. It should be mentioned, in passing, that while 

 this limited exchange of isotopic cations was proceeding, a pronounced 

 net influx of total potassium occurred where oxidative metabolism was 

 favored (i.e., with aeration and at higher temperatures). These observa- 

 tions suggest two modes of movement of inorganic solute apart from 

 diffusion phenomena, namely, an ionic exchange, more or less inde- 

 pendent of metabolism, and an accumulation mechanism, the latter 

 directly related to concurrent metabolic activity. 



Water supply 



A supply of water to the living system is obviously necessary. How- 

 ever, it may be less obvious that the relative supply may alter or de- 

 termine the rate of internal processes, especially the accumulation of 

 inorganic solutes. The influence is indirect. The movement of water 

 into tissues will dilute the concentration of solutes within the system, 

 thus lowering the accumulation level therein. Since the maximum 

 inorganic solute level of tissues is limited by hereditary potentialities 

 and rates of processes, further movement of such solutes inward may 

 occur through dilution within, or by translocation from, the loci of 

 initial absorption and accumulation to other parts of the plant (30). 

 (Compare with the discussion of internal solute concentration.) If, as 

 is considered to be the case by most investigators, the movement of 

 inorganic compounds is in the form of ions or ion pairs, water is 

 obviously essential to this dissociation. Certainly ion exchange phe- 

 nomena are of this sort. The water factor in mineral nutrition is dis- 

 cussed elsewhere in this monograph by Biddulph, Burstrom, and Wad- 

 leigh. 



