126 



MINERAL SALTS ABSORPTION IN PLANTS 



readily observable just prior to leaf fall when the concentration of 

 certain ions, notably potassium and phosphate in the leaves, falls 

 rapidly. 



Growing tissues compete with one another to a certain extent for 

 the available salt supply. Arnon and Hoagland (1943) showed that 

 if tomato plants are grown in a complete nutrient medium until the 

 flowering stage, and then transferred to one lacking phosphate, some 

 fruits are formed at the expense of phosphorus withdrawn from 



3000 



1 1000 



0) 



« 



Q. 



c 



'e 



3 

 o 

 o 



100 



50 



12 3 4 5 



Position of leaves on stem 



Fig. 42. Amounts of ^-P detected in various leaves on the stem of Phaseolus 



vulgaris plants after absorption of labelled phosphate for 4 days. Oldest 



leaf = 1 ; youngest = 6. The vertical lines indicate the spread of points obtained 



in numerous determinations (redrawn from Biddulph, 1951). 



leaves and stems, but the yield is much reduced. Plants which were 

 de-flowered after transfer to the phosphate-free medium made more 

 vegetative growth, than did those which were allowed to fruit, and 

 growth was at the expense of salt absorbed from older tissues, 

 Williams (1948) found that when oat plants are grown under 

 conditions of low phosphate supply, phosphorus required for 

 development of the inflorescence is obtained mainly from the 

 medium, but when plants have been grown in the presence of an 

 abundant supply of phosphate, and thus have a high phosphorus 

 content, a larger amount is supplied from stems and leaves (Fig. 43). 



