THE SHOOT-ROOT RATIO 



619 



Some such correlative effects seem to be due to a virtual monopolization of 

 nitrogenous foods by the dominant organ {cf. Alurneek's results on tomato) ; 

 others may be due mainly to the diversion of carbohydrate foods to the de- 

 veloping flowers or fruits. 



The Shoot-Root Ratio. — A number of investigations have been made of 

 the so-called shoot-root ratios in crop plants. Such ratios are usually cal- 

 culated by dividing the dry weight of shoots by the dry weight of the roots 

 produced during the growth period under consideration. The shoot-root ratio 

 is influenced by reciprocal correlative influences between the aerial parts of 

 a plant and its roots. The kind and magnitude of these correlative effects 

 depend largely upon the environmental conditions to which the plant is ex- 

 posed. For example, the nitrate concentration of the substratum has been 

 shown to have a marked influence upon the shoot-root ratios of plants 

 (Table 58). 



TABLE 58 INFLUENCE OF NITRATE CONCENTRATION UPON THE SHOOT-ROOT RATIO OF 



BARLEY PLANTS. DURATION OF THE EXPERIMENT 49 DAYS (dATA OF TURNER, I922) 



The results of this experiment indicate a consistent increase in the shoot- 

 root ratio with increase in the nitrate concentration of the solution culture. 

 In this particular experiment there was also an absolute reduction in the 

 dry weight of the roots developed with increase in nitrate concentration, but 

 this was not found to be true in all the experiments performed by this in- 

 vestigator. Similar results have been obtained with a number of other species, 

 and by plants rooted in the soil as well as in solution cultures. 



The effect of nitrates upon the shoot-root ratio can be interpreted in terms 

 of their influence upon the internal food relations of plants. If the nitrate 

 concentration of the substratum in which the plant is rooted is low, most 

 of the nitrates absorbed are utilized in the synthesis of amino acids in the 

 roots, the carbohydrates necessary for this process being translocated down- 

 wards from the leaves. Most of these amino acids are used in the synthesis of 

 protoplasmic proteins during the growth of the roots. Only a small pro- 

 portion of the available nitrogenous compounds escapes utilization in the roots 

 and is translocated (either as nitrates or as amino acids and related com- 



