Interactions between salinity and irrigation frequency in greenhouse pepper grown in closed-cycle hydroponic systems

Two different irrigation regimes with two different salinity levels were applied to peppers (Capsicum annum L.) grown in closed hydroponic systems in a glasshouse. The two salinity levels were attained by adding NaCl to the irrigation water used to prepare nutrient solution to obtain concentrations of 0.8 and 6 mol m−3, and allowing the salts to progressively accumulate in the recycled nutrient solution. The two salinity levels were combined with two different levels of irrigation frequency in a two-factorial experimental design. Initially, the Na and Cl concentrations increased rapidly in the recycled effluents, but nearly three months after treatment initiation they converged gradually to maximal levels depending on the NaCl treatment. The low irrigation frequency imposed a more rapid salt accumulation in the root zone, which was ascribed to restriction of the volume of drainage solution. However, the maximal salt concentrations in the root zone were independent of the watering schedule. This finding agrees with previous research revealing that the maximal salt accumulation in the root zone of plants, grown in closed hydroponics, is dictated merely by the NaCl concentration in the irrigation water. Total and Class I yields were suppressed by salt accumulation but the high irrigation frequency significantly mitigated the deleterious salinity effects. At low salinity, the low irrigation frequency raised significantly the weight percentage of fruits with blossom-end rot (BER), whereas at high salinity the incidence of BER was further increased without significant differences due to the irrigation regime. Frequent irrigation resulting in high drainage fractions in closed hydroponic systems may delay the rate of salt accumulation in the root zone, thereby enhancing yield and improving fruit quality, without increasing the discharge of polluting fertigation effluents to the environment.