39 2 Mineral Nutrition of Plants 



The marked effect of temperature is shown by the fact that a 5-degree 

 rise in temperature of the saturated intercellular spaces of the leaf has 

 the same effect in accelerating evaporation as a 30 to 40 per cent fall 

 in relative humidity (9). 



Increased transpiration usually results in some increased absorption 

 of mineral nutrients. Freeland (12) (Table I), Wright (yj), and others 

 have shown that the uptake of the macroelements is directly correlated 

 with the transpiration rate, but is not necessarily proportional to the 



TABLE I 



Data on the Amount of Mineral Absorption in Plants with High Transpiration 

 (H.T.) and Low Transpiration (L.T.) (from R. O. Freeland) 



Total 



Minerals Calcium Phosphorus Potassium Water 



Absorbed, Absorbed, Absorbed, Absorbed, Absorbed, 



g- g- g- g- cc. 



rate of water uptake. Broyer and Hoagland (4) have emphasized that 

 the metabolic condition of the plant is a very important factor in de- 

 termining the influence of transpiration on the uptake of salts by roots. 

 The salt uptake of young barley plants having an initially low salt, high 

 sugar composition was found to be principally dependent on aeration 

 and temperature, with the data showing only a slight increase in salt 

 uptake when the transpiration rate and water absorption were increased 

 by light and low relative humidity (Table II). High salt and low sugar 

 plants, on the other hand, took up potassium and bromium ions at a 

 considerably higher rate as the rate of transpiration and water absorp- 

 tion was increased. These data indicate that water uptake by roots and 

 salt absorption are relatively independent processes, but, under internal 

 root conditions which are unfavorable for salt uptake (as in high salt, 

 low sugar plants), an increased transpiration stream contributes ma- 

 terially to increased salt uptake. 



