TRACE ELEMENTS IN PLANTS 119 



monocotyledons practically all the boron remains in solution. 

 In both groups the soluble calcium is directly related to the 

 soluble boron which is itself determined by the total boron, 

 which in its turn is determined by the concentration of the boron 

 in the medium. This much smaller proportion of soluble boron 

 in dicotyledons explains why the boron requirement of these 

 plants is so much higher (5-10 times) than that of mono- 

 cotyledons. 



That this explanation is not of general applicability, however, 

 appears from recent work on tomato by Reeve and Shive (1944), 

 who investigated the relations of boron to potassium and calcium 

 in this species. In their experiments on the relation of boron to 

 potassium twenty cultures, each containing three plants, were 

 grown in sand cultures which received a nutrient solution 

 supplied in a continuous flow. Five different potassium con- 

 centrations were used, namely, 10, 50, 89, 250 and 500 p.p.m., 

 there being thus four cultures receiving potassium in each one 

 of these concentrations. The four cultures at each potassium 

 level respectively received boron in the concentrations 0-001, 

 0-1, 0-5 and 5-0 p.p.m. The nutrient solution contained iron, 

 manganese and zinc in addition to the major nutrients. 



It was found that symptoms of boron deficiency appeared 

 first in the culture supplied with 0-001 p.p.m. of boron and 

 500 p.p.m. of potassium, and last in the culture supplied with 

 0-001 p.p.m. of boron and 10 p.p.m. of potassium and that, 

 in general, the severity of the symptoms of boron deficiency 

 increased with increase in the potassium concentration. No 

 symptoms, either of boron deficiency or excess, were observed 

 in any of the cultures receiving the intermediate concentrations 

 of boron (0-1 and 0-5 p.p.m.), butull cultures receiving 5 p.p.m. 

 of boron developed symptoms of boron toxicity. With these, the 

 severity of the symptoms increased with increase in potassium 

 concentration. Thus, increasing potassium concentration brings 

 about a progressive increase in the severity of the symptoms of 

 boron deficiency in low boron concentrations and of the 

 symptoms of boron excess in high boron concentrations. 



Analysis of the plants showed that at each level of supplied 

 boron the amount of both soluble and total boron in the 

 tissues increases with increase in the potassium concentration. 



