Effects of increasing salinity due to progressive NaCl accumulation in the nutrient solution on French beans grown in a closed hydroponic system

Beans (Phaseolus vulgaris L.) were grown hydroponically using irrigation water with four different NaCl concentrations (0.8, 3.0, 6.0 and 9.0 mM) to compensate for transpiration losses. The experiment was carried out in 12 completely independent closed units, each of which contained two channels accommodating 80 plants per channel, and with 3 replicates per NaCl treatment. The amounts of nutrients supplied to compensate for plant uptake were identical in all experimental units. The different treatments were applied by automatically injecting the calculated amounts of NaCl into the irrigation water, which already contained 0.8 mM NaCl. During the experiment, no drainage solution was discharged. Initially, the recycling of the leachate resulted in a progressive increase in the electrical conductivity (EC) within the root zone (indicated by the values measured in the drainage solution) due principally to NaCl accumulation. However, 60-70 days after the initiation of recycling, the rate of EC increase declined and finally approached zero level, indicating that the EC had asymptomatically reached a maximum, the level of which depended on the NaCl concentration in the irrigation water. The maximum EC level was established as soon as the Na/water and Cl/water uptake ratios (uptake concentrations) had reached the concentration of NaCl in the irrigation water of the particular treatment. The gradual increase of the EC in the root zone imposed a corresponding decrease on water uptake. Due to the progressive increase of the EC, the early fruit yield of bean was hardly affected, but subsequently the yield losses imposed by increasing salinity were considerable. Yield suppression resulted from a decrease in both the mean fruit (pod) weight and the number of pods per plant. The progressive increase in EC did not depress the K, Ca and Mg concentrations in the leaves, but the leaf chlorophyll content was reduced.