The influence of the plant nutrition status on bleeding and salt uptake

When tomato plants were kept in a high-salt condition by changing the nutrient solution daily, the rate of bleeding after decapitation and the nitrate concentration of the bleeding sap d inimished rapidly. Practically all the nitrate taken up from the nutrient solution was transported to the xylem vessels. When decapitation was preceded by a period during which the nutrient solution was replaced by tap water the rate of bleeding was low immediately after cutting but it increased rapidly to a more or less constant value, which declined again after some time. The nitrate concentration of the sap after a short initial period of low concentration was much higher than with high-salt plants. The rate of nitrate uptake from the solution was greater than the nitrate excretion into the vessels; the size of this difference increased with the length of the pretreatment period. When the content of water soluble carbohydrates was determined in the roots at the end of the experiment, this was always much higher in plants with a pretreatment on tap water than with the high-salt controls. With high-salt plants both the nitrate concentration of the bleeding sap as well as the rate of bleeding could be increased by adding sugars to the external solution. Glucose was more effective than sucrose, which was better than fructose. As isotonic mannitol solution had no effect. These results are discussed in relation to earlier findings and results of other workers in this field. It is concluded that the rate of bleeding is dependent on the rate of salt excretion into the vessels and that this excretion, in turn, depends on the amount of metabolites present in the root. High salt roots have practically no sugar reserves and are dependent on transport from the shoot. A pretreatment on tap water causes an accumulation of sugars in the root and a depletion of nitrate. For these reasons salt excretion after decapitation can go on for a much longer period than with high salt plants, while also the nitrate level in the root tissue is restored again, so that the nitrate intake from the solution is larger than the excretion into the vessels.