NaCl accumulation in a cucumber crop grown in a completely closed hydroponic system as influenced by NaCl concentration in irrigation water
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Four different NaCl concentrations in the irrigation water, 0.8, 5, 10 and 15 mM, were applied as experimental treatments to cucumber (Cucumis sativus L.) grown in a closed hydroponic system. These treatments were attained by automatically injecting the required amounts of NaCl into irrigation water containing 0.8 mM NaCl, whenever water was mixed with fertilizers and drainage solution to prepare fresh irrigation solution. Initially, the Na+ and Cl- concentrations increased rapidly in both the fresh nutrient solution supplied to the crop and the drainage water, but they were stabilized to maximal levels depending on the treatment 45-55 days after initiation of solution recycling. It was concluded that the Na+ and Cl- concentrations in the root zone were maximized as soon as the Na/water and Cl/water uptake ratios reached equal levels with the NaCl concentration in the irrigation water. Based on these data, relationships between the Na/water or Cl/water uptake ratios and the NaCl concentration in the root zone were established. The leaf Na+ and Cl- concentrations were influenced by both the external Na+ and Cl- concentrations and the season. The Cl:Na uptake ratio (mol basis) was higher than 1 at low external NaCl concentrations but decreased below 1 as salinity increased, thereby indicating a more rapid decline in the ability of the plant to exclude Na+ from the leaves as compared to that for Cl-. Verlag Eugen Ulmer KG.