Effects of Nitrogen Supply on Rates of Photosynthesis and Respiration in Plants

On the basis of the foregoing data it is evident that the composition of any given plant is in a marked degree correlated with its respiratory activity. Thus tomato plants very high in carbohydrates but low in their nitrate or soluble nitrogen content may have a relatively low respiratory rate, whereas one which is higher in its nitrate content will have an appreciably higher one. Or any given plant high in its carbohydrate reserves will have its respiratory rate increased if nitrates are supplied to it in abundance, other conditions remaining the same. Although all the plants used in the various experiments were very high in reserve carbohydrates, a relatively slight change in the amount of this reserve had a great effect upon the time required for nitrates to make their appearance in the tops of the plants after they had been applied in the nutrient solution. In every case the addition of nitrates to plants high in reserve carbohydrates caused an increase in respiration, the greater such reserve the more nearly immediate the response and the greater in degree. In the case of plants extremely high in reserve carbohydrates the response was almost immediate and very marked. In the first and last experiments respiration of the plants to which nitrates had been added increased to three times that of the plant which received no nitrate. In the case of plants slightly lower in reserve carbohydrates the response was not so great, nor did it occur at once. In the second experiment a response was noticeable only after twenty-four hours, and the increase in respiration was about 100 per cent. In the third experiment the response became evident in twenty-four hours, and the total increase was 120 per cent. In each case the increase in rate of respiration seemed to take place subsequent to the appearance of nitrates in the top of the plant; in no instance did it occur prior to such appearance, whatever the length of time elapsing after their application in the nutrient solution.