Growth of Seedling in Relation to Composition of Seed

Abstract

1. Different kinds of seedlings produce very different growth responses when exposed to the same external influences. The responses are often related to the chemical composition of the seed. 2. If the nitrogen content of the seed is low in proportion to the carbohydrates, the response as measured by an increase in total growth in terms of green weight is greater when nitrates but no CO2 (except traces given off in respiration) are utilized than when CO2 but no nitrates are utilized. 3. If the nitrogen content of the seed is high in proportion to the carbohydrates, the response as measured by an increase in total growth is greater when CO2 but no nitrates are utilized than when nitrates but no CO2 (except for traces given off in respiration) are utilized. 4. The greatest total amount of growth, however, is produced by seedlings from both high and low-nitrogen seeds by allowing the seedlings to utilize both nitrates and CO2. 5. The utilization of both nitrates and CO2 tends to result in a shifting of the relative amounts of shoots and roots, that is, there are qualitative as well as quantitative responses to these external influences. The proportion of shoots to roots of low-protein seeds tends to be increased because of the relatively larger increase in the amount of shoots, if nitrates but no CO2 is utilized; whereas the proportions of shoots to roots of seedlings of high-protein seeds are not increased and in some cases are even decreased by this treatment. 6. The shoot to root ratios of seedlings of low-protein seeds are not noticeably affected if CO2 but no nitrates are utilized, but the shoot to root ratios of high-protein seeds tend to be greatly reduced if CO2 but no nitrates are utilized. This shifting of ratios is a result of a greater increase in the amount of roots than of shoots. 7. The proportions of shoots to roots of seedlings of low-protein seeds tend to be slightly increased if both CO2 and NO3 are utilized, but the proportions of shoots to roots of seedlings of high-protein seeds are very greatly decreased by such treatment, because the weight and number of roots increase much more than the shoots. 8. The modification of type of growth is correlated with extent of modification in quantity of growth. Those treatments which result in the greatest increase in quantity of growth are the ones which produce the greatest change in type of growth. (The foregoing statements apply to growth of seedlings in the light.) 9. A high-protein, high-oil food supply appears to be the most efficient in producing growth both in darkness and in light. This is not surprising, since both types of foods are constituted of a relatively large proportion of the condensed compounds of their respective types. For example, high-protein seeds contain relatively much more of basic nitrogen and consequently have a great amount of nitrogen in proportion to their weight; oils contain a great amount of carbon in proportion to their weight. 10. The shoot to root ratios of seedlings grown in darkness appear to vary with the proportion of nitrogen to carbohydrates in the food reserves of the seed, the higher the nitrogen in proportion to carbohydrate content the higher the shoot to root ratios of the seedlings. There is some evidence that addition of nitrates to the culture medium of the seedlings grown in darkness can modify the type of growth (increase shoot to root ratios), but in only a few cases do nitrates increase the amount of growth. 11. Since wide variations in growth responses are associated with variations in the types and quantities of storage materials in the seed, it seems inadvisable to draw conclusions as to the effect of mineral nutrients or carbon dioxide treatment upon the subsequent growth of the plants when the observations are based on growth responses during the first three weeks of growth only. Before studying the effect of an element or compound on growth (in general), it would seem desirable to conduct experiments in which the storage supply of the element or compound in question has been at least partially exhausted, by allowing the seedlings to grow to maximum size without that substance. This method has been employed by animal workers for some time. 12. Although contradictory results have not been found in the different series of experiments, the results are not considered conclusive but will be repeated and elaborated.