Characteristics of direct uptake of organic nitrogen by tomato roots associated with plant growth and water uptake were analyzed quantitatively using a root chamber equipped with a high-pressure flowmeter. The rate of root nutrient uptake was evaluated using the nutrient concentration in the xylem sap collected from the stem stump and the water uptake rate measured by the flowmeter. There was a difference in nutrient content in the xylem sap of the roots grown with or without organic nitrogen (amino acids), suggesting the direct uptake of them. The rate of organic nitrogen uptake was different depending on the type of organic nitrogen used, and was increased with plant growth through water uptake. Furthermore, the increase in water uptake supposed to be daytime transpiration induced a higher rate of organic nitrogen uptake, whereas the rate of nitrate nitrogen uptake either remained the same or increased slightly. As a result, the ratio of organic nitrogen uptake to total nitrogen uptake increased from approximately 15% to 40% with the water uptake increase.
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