Lecture 2 — 31 — Micronutrient Elements 



of such experiments has since been thoroughly revised. 

 The possibility still seems to remain that manganese 

 added to the culture solution, over the impurities 

 usually present, may increase efficiency of photosyn- 

 thesis. 



No other micronutrient element has received so 

 much attention as boron and still the mechanism of 

 the function of this element in plant growth eludes us. 

 Conceivably some clue will come from studies of the 

 comparative biochemistry of different types of plants. 

 As stated above, the fungus Aspergillus appears not 

 to require boron and suggestively, perhaps, this organ- 

 ism has no essential need for calcium within the limits 

 of positive demonstration by the techniques so far 

 employed. A boron requirement on the part of Chlo- 

 rella has not been shown although this may be merely 

 a matter of inadequate technique. One idea concern- 

 ing boron is that it has a role in the formation of 

 pectin compounds. These contain galactose derivatives, 

 which requires an inversion of H and OH on one of 

 the carbon atoms, if they are formed from glucose. 

 At any rate, boron does form compounds with some 

 sugars or organic acids through adjacent OH groups 

 and this property of boron needs consideration in 

 further research. 



Greatest emphasis has been given to possible inter- 

 relations between calcium and boron. Often there is 

 a marked similarity or even practical identity of symp- 

 toms of calcium and boron deficiencies in plants. Both 

 deficiencies are strikingly reflected in the failure of 

 growth in meristematic regions of the plant. Shive 

 (1941) and his co-workers have presented recently 

 some evidence that boron in a still unknown way has 

 a role in the determination of the state of calcium in 

 the tissue, that is, the amount of calcium present in 

 dissolved or easily soluble form. Several phases of 

 organic metabolism need exploration before a good 

 hypothesis for this effect of boron can be advanced. 



