620 GROWTH CORRELATIONS 



pounds) to the aerial portions of the plant. The tops are therefore relatively 

 deficient in proteins. Hence the growth rate of the aerial portions of the 

 plant will be relatively slow and the shoot-root ratio relatively low. 



When the supply of nitrates is more abundant, however, a smaller pro- 

 portion of the total quantity absorbed is utilized in the roots. A larger 

 proportion of the nitrogen, in one form or another, is translocated into the 

 aerial portions of the plant, where much or all of it is usually utilized in the 

 synthesis of protoplasmic proteins. The enhanced vegetative development 

 of the aerial organs of the plant which is favored by such metabolic conditions 

 results in the consumption of more carbohydrates as well as more proteinaceous 

 foods by the aerial meristems. Because of the vigorous vegetative develop- 

 ment of the shoot system the proportion of the carbohydrate foods which 

 are translocated to the roots may be relatively small. Hence, relative to 

 the shoots, the roots are likely to be deficient in both carbohydrate and pro- 

 tein foods, since synthesis of the latter requires carbohydrates as well as 

 nitrates, and will grow at a relatively slower rate than the tops. The net 

 result will be a higher shoot-root ratio than when the plants are grown in 

 a soil which is deficient in nitrates. 



Similarly, a decrease in the supply of carbohydrates within the plant, 

 due to diminution in the rate of photosynthesis, or any other cause, influences 

 the shoot-root ratio of plants. In general, diminution in the quantity of car- 

 bohydrate foods available in the tops results in an increased shoot-root ratio, 

 and vice versa. Plants grown in the shade, for example, have higher shoot- 

 root ratios than other plants of the same species grown in full sunlight. Simi- 

 larly, pruning commonly results in increasing the shoot-root ratio of woody 

 plants, since the new growth following pruning is usually especially vigorous, 

 resulting in monopolization of most of the available carbohydrates by the 

 shoots. The explanation of such effects follows a line of reasoning similar 

 to that just presented in explanation of the relative influence of high and low 

 nitrate supply on the shoot-root ratios of plants. 



The shoot-root ratio is also influenced by the available soil water content. 

 In general, a relatively low soil water content and adequate soil aeration 

 favor relatively low shoot-root ratios, while the opposite conditions favor 

 relatively high ones (Table 59). 



The shoot-root ratios were computed in this investigation on a fresh weight 

 basis but undoubtedly would show essentially the same relations if expressed 

 on a dry weight basis. The results indicate clearly that the shoot-root ratio 

 increases with increase in the percentage of water in the soil. The absolute 

 weight of the shoots increases consistently with increase in soil water content, 

 while the absolute weight of the roots increases to a maximum at a soil water 



