116 



MINERAL SALTS ABSORPTION IN PLANTS 



seem to pass through the origin indicating that salt absorption stops 

 at zero water absorption. No other investigator has reported such 

 complete dependence of salt uptake on concomitant absorption of 

 water. 



Hylmo (1953, 1955, 1958) observed a large effect of transpiration 

 on the absorption of calcium and chloride ions by 3-week old pea 

 plants, and concluded that a major part of the salt transported into 

 the shoot is carried across the root cortex in the transpiration stream 

 by mass flow. A part of the salt taken into the plants in these 

 experiments was retained by the roots, and of this a small fraction 

 was absorbed independently of water absorption, while the remainder 

 increased in amount as the rate of transpiration was increased. 



Table 10. Absorption of Water (ml) and Uptake of Various Ions (m eq.) 



BY Plants of Phaseolus vulgaris in 96 hr. 



(Wright, 1939). 



This last observation was taken to indicate that inner cells of the 

 root cortex receive a part of their salt supply via the transpiration 

 stream, and absorb salt less quickly when this supply is reduced 

 (see below). Brouwer (1954, 1956) concluded, in contrast to Hylmo, 

 that passive influx of salt in the transpiration stream plays a minor 

 role in rye, Zea mays and Vicia faba, because he found that dini- 

 trophenol (10~ ^ M) inhibits chloride absorption without a comparable 

 effect on water absorption, whereas increasing the osmotic pressure 

 of the external medium interferes with water absorption, but not 

 with that of salt. Such effects of transpiration on salt absorption as 

 Brouwer observed were attributed to the influence of water stress 

 on the "conductivity" of the root cortex, that is on the resistance 

 presented to metabolic transport. 



The relative importance of passive and active components in 

 the absorption of salts by plants probably varies between species, and 

 particularly with the conditions under which they are grown. High 



