Influence of NaCl concentration in the irrigation water on salt accumulation in the root zone and yield in a cucumber crop grown in a closed hydroponic system
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The effects of four different NaCl concentrations in the irrigation water used to replenish the transpiration losses in a cucumber (Cucumis sativus L.) crop grown in a closed hydroponic system were investigated for 4 months in a glasshouse experiment. Four different NaCl concentrations in the irrigation water, particularly 0.8, 5, 10 and 15 mM, were compared. These were obtained by automatically injecting the required amounts of NaCl to irrigation water containing 0.8 mM NaCl. During the experiment, no drainage solution was discharged. Initially, the electrical conductivity (EC) increased rapidly in the root environment, as indicated by the values measured in the drainage solution. However, 6-7 weeks after recycling initiation the EC of the drainage water approached asymptomatically a maximal level depending on the treatment. The concentration of some macronutrients showed also an increasing tendency with time, but this increase was relatively small and could not account for the observed rise of EC. Hence, this pattern of EC increase was ascribed to accumulation of Na and Cl, which was initially rapid but tended to be minimized as uptake concentrations of Na and Cl were approaching the corresponding concentrations in the irrigation water used to compensate for the transpiration losses. The yield was suppressed by the progressive increase of EC at a rate of 12.3% per unit of EC increase above 3.02 dS m-1. The yield suppression was due to decrease of both the mean fruit weight and the number of fruits per plant. The mean length of the cucumber fruit was also affected by the progressively increasing salinity.