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Soilless zucchini (Cucurbita pepo L.) crops were grown in two distinct cropping periods (spring-summer; SS and autumn-winter; AW) using irrigation water with different Ca2+ concentrations (1.5, 3, 4.5 and 6 mM). The objectives of this study were to: (i) mathematically correlate the accumulation of Ca2+ in the root environment with the respective Ca2+/water uptake ratio (namely uptake concentration; UC), and (ii) determine the UC of macronutrients (i.e., N, P, K, Ca and Mg) under these conditions. Equations of the literature, initially developed to predict NaCl accumulation in a closed hydroponic system, were further extended to fit experimental results. The evolution of Ca2+ accumulation in the drainage exhibited a sigmoid pattern with time and the relationship between the concentration of Ca2+ in the root zone and the corresponding uptake ratio Ca2+/water was better described by curvilinear functions. Validation of the model showed a very good agreement between simulated and measured values. Increasing Ca2+ levels affected both tissue concentrations and UC of Ca and N, but this was not the case for P, K, and Mg. Photosynthesis, growth, yield and plant water uptake were restricted (avg. 15% decrease) at high external Ca2+ levels, due to high total salt concentration (EC) in the recycled solution (4.2–5.5 dS m−1). Fruit quality attributes, however, remained unaffected by treatments with the exception of fruit nitrate content. The empirical model parameterized and tested in this work may serve as a tool to predict Ca2+ ion concentrations in the root environment of zucchini crops as relationships of the water absorbed by the crop. Finally, the results showed that in soilless zucchini crops with zero discharge of fertigation effluents, there is no compelling reason not to use irrigation water resources with Ca2+ concentration up to 3 mM.

Partial substitution of nitrate (NO3-) with chloride (Cl-) in the nutrient solution supplied to tomato crops (beefsteak and cherry types), grown in a closed hydroponic system, maintained nitrogen (N)- and carbon (C)- assimilation status in plants despite N-supply limitations. Lowering NO3- ions (90% of total N) supply to 2/3 of the standard recommendations, which was electrochemically compensated for by an equivalent increase of the Cl- concentration in the replenishment nutrient solution (RNS), increased N use efficiency (kg produce kg−1 N supply) and decreased NO3- in the drainage without compromising growth, yield and nutritional quality. Tomato plants supplied with Cl--amended RNS increased leaf Cl- content to macronutrient level (35 mg g−1 dry weight), retaining photosynthetic rates and crop yield potential at lower stomatal conductivity and transpiration. Nutrient to water uptake ratios (mass of nutrient per water volume absorbed), which are commonly termed ‘uptake concentrations’, were defined in different cropping seasons and ranged as follows: 12.1–13.5 (Nitrogen- NO3-+NH4+), 1.25–1.35 (Phosphorus-P), 6.1–6.3 (Potassium-K), 3.6–4.0 (Calcium-Ca), 1.0–1.3 (Magnesium-Mg, mmol L−1), 13.0–14.3 (Iron-Fe), 7.6–8.4 (Manganese-Mn), 5.1–5.2 (Zinc-Zn) and 0.7–0.9 (Copper-Cu, μmol L−1). The levels of N and Cl- supply had no impact on the uptake concentrations. This study suggests that replacing 1/3 of the standard NO3- supply by Cl- in closed hydroponic tomato crops enhances N use by two-fold and eliminates NO3- losses to one-half, with no significant effects on assimilation processes and fruit biomass production, suggesting that Cl- at appropriate concentrations is not only an essential micronutrient but also a beneficial macronutrient. The obtained uptake concentrations may be used through on-line operating decision support systems to optimize nutrient supply in hydroponic cultivations in Mediterranean greenhouses.

To optimize nutrient supply in melon (Cucumis melo L.) cultivated in closed-loop hydroponic systems under Mediterranean climatic conditions, the process of salinity build-up has to be better understood. To attain this objective, two experiments were conducted in two cropping seasons (winter–spring and spring–summer) in order to: (i) establish relationships between Na+ and Cl− concentrations in the root zone and uptake concentrations (UC) of Na+ and Cl−, respectively, i.e., Na+/water and Cl−/water uptake ratios, and (ii) test whether macronutrient UC in melon grown in closed hydroponic systems are influenced by the gradual salinity build-up. Three different NaCl concentrations in the irrigation water used to prepare nutrient solutions, i.e., 0.7, 2.5, and 5 mM, were applied. The UC of Na+ and Cl− increased over time but at a certain time point in the cropping cycle they converged to a plateau corresponding to the salinity treatment. Exponential relationships between the Na+ and Cl− concentrations in the root zone and the UC of Na+ and Cl−, respectively, were fitted to experimental results in both experiments. However, parameterization of the model with data from the high-transpiration season revealed superiority. The established model parameters corresponded well over the whole melon cultivation cycle and a wide range of climatic conditions. The NaCl-salinity up to the tested level had no significant effect on the UC of macronutrients (i.e., N, P, K, Ca and Mg). The mean UC of Ca and N were higher than those reported under northern-European climatic conditions. The obtained results may be used through on-line operating decision support systems to optimize nutrient supply and minimize salinity impacts in melon grown in closed hydroponic systems when the quality of the irrigation water is sub-optimal.

The aim of the present investigation was to simulate the uptake concentrations (weights of ion per volume of water absorbed) of Na+ and Cl− in hydroponic tomato crops as a function of the NaCl concentration in the root zone. An empirical model was calibrated and validated, which can be incorporated into on-line operating decision support systems aimed at optimizing the nutrient supply and minimizing the discharge of drainage solution in tomato crops grown in closed-cycle hydroponic systems. Three experiments were conducted, of which one was carried out to calibrate the model using irrigation water with NaCl concentration ranging from 0 to 14.7 mol m−3 while the other two experiments were commissioned to validate the model within either a low (0.5–2 mol m−3) or a high (1.2–12 mol m−3) concentration range. The model could successfully predict the uptake concentration of Na+, but Cl− could not be simulated by this model at external Cl− concentrations lower than 10 mol m−3. The results indicate that Na+ is excluded actively and effectively by the tested tomato cultivar even at low external Na+ concentrations, while Cl− is readily taken up at low concentrations, particularly during the initial growing stages. Due to the efficient exclusion of Na+ by tomato, the Na+ concentration in the root environment increased rapidly to extremely high levels even when the Na+ concentration in the irrigation water was relatively low. These results indicate that tomato genotypes characterized by high salt-exclusion efficiency, require irrigation water with a very low NaCl concentration, if they are grown in closed hydroponic systems and the drainage water is not flushed periodically. To maintain Na+ at levels lower than 19 mol m−3 in the root zone of the tomato hybrid ‘Formula’ in closed hydroponics, a maximum acceptable Na+ concentration of 0.53 mol m−3 was estimated for the irrigation water.

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