BIOCHEMISTRY OF PLANTS 



the essential inorganic elements, and especially certain "trace" ele- 

 ments, we have little or no guidance from studies on other organisms. 

 Boron is indispensable for all the higher plants so far properly investi- 

 gated. It has not been shown to be indispensable to the animal, 

 although little work has been done on this point. Boron is one of the 

 elements plants require in minute amounts, yet deficiencies even under 

 some soil conditions have assumed first-rate agricultural importance, 

 a fact which accentuates interest in the biochemical functions of boron. 

 Research by plant physiologists indicates that deficiency of boron often 

 results in a pathological state in the plant nearly the same as that caused 

 by a deficiency of calcium. One view is that an inadequate supply of 

 boron may limit the maintenance of an effective level of calcium in a 

 soluble or active form within the tissues of plants. Some workers think 

 that formation of pectin compounds does not proceed normally when 

 boron is deficient. Clearly these are questions which need the atten- 

 tion of skilled biochemists. Possibly productive leads might come from 

 comparative biochemistry. Certain groups of fungi, and perhaps some 

 algae, appear not to require boron. The same fungi also can grow 

 without calcium, or at least the amounts needed are too small to be 

 removed from the media by present methods of purification. Furthei 

 study of certain phases of organic metabolism in plant organisms with 

 different boron or calcium requirements might conceivably point to 

 biochemical reactions for which boron or calcium, or both, may be 

 indispensable. 



Another among the chemical elements eff"ective in micro 

 quantities to which an indispensable function in the growth of higher 

 green plants must be assigned is zinc. This element, like boron, is 

 not always adequately supplied by the soil, and the deficient plant 

 becomes diseased ("little-leaf," "motde-leaf" of trees, and pathological 

 ^conditions shown by other crop plants as a result of zinc deficiency). 

 Physiological studies under the control of artificial culture disclose 

 some of the effects of zinc deficiency. One such study in this laboratory 

 yielded evidence that, without an adequate supply of zinc, plant growth 

 substances of the auxin type are either not synthesized at a rate sufii- 

 cient for normal growth or else are destroyed too rapidly (12). But 

 it has also been learned that protein synthesis is retarded when the zinc 

 concentration in the plant falls below a critical level. Tomato plants 

 were grown with graduated supplies of zinc, so that some of the plants 



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