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George Gizas and Dimitrios Savvas

Four successive trials with gypsophila, rose, cucumber, and lettuce grown on different pumice grades were conducted in a heated glasshouse. In the gypsophila experiment, three grades of pumice (0 to 2, 0 to 5, and 0 to 8 mm) were tested, whereas in the rose, cucumber, and lettuce experiments, four pumice grades (0 to 2, 0 to 5, 0 to 8, and 4 to 8 mm) were compared. In each experiment, these pumice grades were combined with two growing systems, specifically placement of the same media volume either in pots or in bags. Furthermore, the physical properties of the tested pumice grades were determined. Overall, gypsophila and cucumber gave the highest yields when grown in pots filled with the two finest pumice grades, whereas the yield obtained from plants grown in bags was poor regardless of particle size range. Coarse pumice restricted gypsophyla and cucumber yield in pot culture to nearly the same degree as in bags. In contrast, lettuce, and to a greater degree roses, exhibited a weaker response to the different pumice grades and growing systems. The two finer pumice grades were characterized by relatively low air-filled porosity, which presumably restricted plant growth and yield as a result of poor root aeration when the media were placed in bags with a height of 8 cm. The coarsest pumice grades were characterized by a steep drop in the water content as the suction increased only a few centimeters above zero, which imposed an appreciable lowering of the overall water content in the pots in comparison with the bags (≈12.5%), as a result of the increased height of the former. Our results indicate that gypsophila and cucumber respond more strongly than roses and lettuce to the variations in the air-to-water ratio in the root zone originating from differences in the physical properties of the growing media.

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Fidanka Trajkova, Nicolas Papadantonakis and Dimitrios Savvas

The objective of this study was to discriminate among Na, Cl, and Ca salinity effects on cucumber (Cucumis sativus L.). Cucumber plants grown in perlite were exposed for 134 days to low and moderate levels of salinity induced by the addition of either NaCl or CaCl2 at equal rates (on a chemical equivalent basis) to a standard nutrient solution for cucumber up to two target electrical conductivity (EC) levels (3.0 and 5.0 dS·m–1). The experimental treatments included also a control, which was irrigated with the standard nutrient solution without additional salt. The mean EC values in the drainage solution were 2.35, 3.94, 4.2, 6.31, and 6.35 dS·m–1 for the control, low NaCl, low CaCl2, high NaCl, and high CaCl2 treatments, respectively. The fresh and dry weights of stems and leaves were reduced only under conditions of high NaCl salinity, whereas root mass was not affected. Fruit yield decreased in proportion to the increase in NaCl salinity, while CaCl2 salinity reduced yield only at the high EC, to a level that corresponded to the low NaCl salinity. The suppression of yield with increasing salinity resulted mainly from a decrease in fruit size, while the number of fruit per plant was reduced to a lesser extent. These changes caused a reduction in the number of Class I fruit and an increase in nonmarketable produce. Both salinity sources enhanced the total soluble solids and the fruit chlorophyll concentration. NaCl salinity appreciably raised the concentrations of Na and Cl in young and old leaves, and suppressed the K concentration. CaCl2 salinity increased leaf Cl and Ca levels and diminished Mg and K. It is concluded that cucumber is more susceptible to NaCl salinity than to equal EC levels of CaCl2 salinity.

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Dimitrios Savvas, Gerasimos Meletiou, Spiridoula Margariti, Ioannis Tsirogiannis and Anastasios Kotsiras

In a completely closed hydroponic system, Na and Cl commonly accumulate in the root zone, at rates depending on the concentration of NaCl in the irrigation water (rate of Na and Cl inlet) and the Na to water and Cl to water ratios at which they are taken up by the plants (rates of Na and Cl outlet). However, while the concentration of NaCl in the irrigation water is commonly a constant, the Na to water and Cl to water uptake ratios are variables depending on the concentrations of Na and Cl in the root zone and, hence, on the rates of their accumulation. To quantify this feed-back relationship, a differential equation was established, relating the rate of Na (or Cl) accumulation to the rate of water uptake. This equation was solved according to the classical Runge-Kutta numerical method using data originating from a cucumber experiment, which was conducted in a fully automated, closed-loop hydroponic installation. Four different NaCl concentrations in the irrigation water, 0.8, 5, 10 and 15 mm, were applied as experimental treatments. The theoretically calculated curves followed a convex pattern, with an initially rapid increase of the Na and Cl concentrations in the root zone and a gradual leveling out as the cumulative water consumption was rising. This was ascribed to the gradual approaching of the Na to water and Cl to water outlet ratios via plant uptake, which were increasing as NaCl was accumulating in the root zone, to the constant NaCl to water inlet ratio (NaCl concentration in irrigation water). The model could predict the measured Na and Cl concentrations in the drainage water more accurately at 10 and 15 mm NaCl than at 0.8 and 5 mm NaCl in the irrigation water. Possible explanations for these differences are discussed. Plant growth and water uptake were restricted as salinity was increasing, following a reverse pattern to that of Na and Cl accumulation in the root zone. The leaf K, Mg and P concentrations were markedly restricted by the increasing salinity, while that of Ca was less severely affected.

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Constantinos Tzerakis, Dimitrios Savvas, Nick Sigrimis and Georgios Mavrogiannopoulos

The objective of the present experiment was to estimate the uptake of manganese (Mn) and zinc (Zn) by cucumber in closed hydroponic systems at different Mn and Zn concentrations in the recycled nutrient solution under Mediterranean climatic conditions. The obtained data might be used to manage Mn and Zn supply in closed hydroponic crops of cucumber grown in Mediterranean greenhouses and avoid their accumulation to toxic levels. Four Mn levels (10, 40, 80, 120 μM) at a standard Zn concentration (6 μM) and four Zn levels (6, 20, 40, 60 μM) at a standard Mn concentration (10 μM) in the solution supplied to compensate for nutrient and water uptake by plants were applied as experimental treatments. The actual uptake concentrations of Mn and Zn were estimated by applying two different methods. The first method was based on the removal of Mn, Zn, and water from the recycling nutrient solution, whereas the second method was based on the total quantities of Mn and Zn that were recovered in plant biomass in combination with the total water uptake. Both methods gave similar uptake concentrations for Mn in the low-Mn supply level and Zn in all Zn levels. However, in the three higher Mn supply levels, the values estimated on the basis of nutrient removal from the recirculating nutrient solution were significantly higher than those found by measuring the total Mn content in plant biomass. These discrepancies in the three high-Mn treatments were possibly caused by partial immobilization of Mn by oxidizing bacteria in the nutrient solution.

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George Gizas, Ioannis Tsirogiannis, Maria Bakea, Nikolaos Mantzos and Dimitrios Savvas

The residues of the aquatic plant Posidonia oceanica that are washed ashore, thereby causing environmental problems in coastal areas, can be used as growing media in horticulture. In the present study, the hydraulic characteristics of raw or composted Posidonia residues, coir, and their 1:1 blends (v/v) with pumice were determined, and their agronomic performance was evaluated in a lettuce crop. The mixture of all three substrates with pumice reduced their effective pore space and increased their bulk density. Furthermore, the water and air capacity (determined at a suction of 10 cm) and the easily available water were also reduced by mixing the three tested media with pumice. The relative hydraulic conductivity (Kr) decreased with increasing suction (ψ) in all of the tested media. The highest and the lowest rates of Kr decrease with increasing ψ were observed in the mix of non-composted Posidonia with pumice and in 100% composted Posidonia, respectively. Blending composted or non-composted Posidonia with pumice at a 1:1 ratio raised the rate of Kr decrease with increasing ψ in comparison with 100% composted or 100% non-composted Posidonia, respectively. In contrast, blending coir with pumice reduced the rate of Kr decrease with increasing ψ in comparison with 100% coir. The differences in the mean fresh weight between lettuce plants grown on the six growing media were similar with those in the rate of Kr decrease with increasing ψ. These results indicate that the crucial factor for the yield performance of lettuce grown on the tested growing media was not the air but the water availability. Furthermore, the present results indicate that the actual water availability to plants grown on the tested substrates depends much more on water flux toward roots and concomitantly on their hydraulic conductivity than on the easily available water (i.e., the difference in water content between 10 and 50 cm suction).

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Dimitrios Savvas, Dimitrios Papastavrou, Georgia Ntatsi, Andreas Ropokis, C. Olympios, Hagen Hartmann and Dietmar Schwarz

Tomato (Solanum lycopersicum L. cv. Belladona F1) plants were either self-grafted or grafted onto the rootstock ‘He-Man’ and grown in recirculating nutrient solution with low, standard, or high manganese (Mn) concentrations (2, 15, and 100 μM, respectively). The concentrations of all nutrients except Mn were identical in all treatments. The objectives of the experiment were to test whether grafted tomato plants have a higher or lower ability to withstand deficient or toxic levels of Mn in the root zone and to study the effects of grafting on nutrient uptake and translocation to the aerial organs. Both excessive and insufficient Mn concentrations in the root zone significantly reduced the number of fruit per plant, whereas mean fruit weight was unaffected by external Mn concentrations ranging from ≈1 to 100 μM. The excessive external Mn concentration caused the leaf Mn concentration to increase beyond the critically high level at the expense of leaf and root iron and zinc concentrations but without significant differences between the grafting treatments. The fruit yield of plants grafted onto ‘He-Man’ was significantly lower than that of self-grafted plants when the Mn concentration in the root zone was excessively high. This response might be associated with the lower translocation of magnesium (Mg) to the leaves of plants grafted onto ‘He-Man’ in comparison with the self-grafted plants, resulting in lower Mg/Mn ratios in the leaves. Grafting onto ‘He-Man’ also restricted the leaf and root iron and copper concentrations but enhanced those of potassium. Overall, tomato cv. Belladona proved to be more tolerant to excess Mn than to Mn deficiency in terms of vegetative growth and fruit yield.

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Georgia Ntatsi, Dimitrios Savvas, Georgia Ntatsi, Hans-Peter Kläring and Dietmar Schwarz

Tomato (Solanum lycopersicum cv. Kommeet) plants were either self-grafted, grafted onto the cold-sensitive cultivar Moneymaker or onto LA 1777, a cold-tolerant accession of Solanum habrochaites. Plants were grown at three different temperatures (T): optimal (19.4 °C), intermediate (17 °C), or low (14.6 °C). Grafting tomato ‘Kommeet’ onto LA 1777 increased shoot growth at intermediate and optimal T and root growth at low or intermediate T in comparison with self-grafting or grafting onto ‘Moneymaker’. Reducing T significantly suppressed fruit yield as a result of reduced fruit number per plant without any interaction with grafting treatments. Grafting ‘Kommeet’ onto LA 1777 remarkably restricted fresh and dry fruit mass in comparison with self-grafting of ‘Kommeet’ as a result of a reduction in the fruit number per plant while not influencing flower number per plant and mean fruit mass, regardless of T regime. Negative effect of LA 1777 on fruit setting points to impairment of pollen fertility indicating signals originating from the root. At low and intermediate T, plants grafted onto LA 1777 were capable of increasing soluble carbohydrates, total amino acids, and guaiacol peroxidase activity in roots to higher levels than those grafted onto S. lycopersicum rootstocks while maintaining a significantly lower malondialdehyde content. These differences point to a much weaker oxidative stress in roots of plants grafted onto S. habrochaitis than those grafted onto S. lycopersicum rootstocks when exposed to low and intermediate T. Furthermore, plants grafted onto LA 1777 exhibited significantly higher levels of soluble carbohydrates, total amino acids, and guaiacol peroxidase activity in leaves and fruit and superoxide dismutase in fruit at low and intermediate T, which indicates that LA 1777 improved the level of antioxidant compounds in tomato shoots, thereby enhancing its adaptation to lower T than optimal.

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Charis-Konstantina Kontopoulou, Sofia Giagkou, Efthalia Stathi, Dimitrios Savvas and Pietro P.M. Iannetta

To date, few attempts have been made to assess the impact of Rhizobium inoculation on N2 fixation and plant yield in soilless cultivations of common bean. In the present study, common bean (P. vulgaris L.) grown on an inert medium (pumice) was inoculated with either Rhizobium tropici CIAT899 or a commercial product containing a mix of N2-fixing bacteria, specifically rhizobia, and Azotobacter sp. The plants treated with both inoculants were supplied with nitrogen (N)-free (0% N) nutrient solution (NS) throughout the cropping period. A third treatment with non-inoculated plants, which were supplied with a standard (100% N) NS was applied as a control. Inoculation with R. tropici significantly increased the total number of root nodules (80 nodules per plant on average) in comparison with the other two treatments (nine nodules per plant on average). The supply of N-free NS restricted markedly both total plant biomass and pod yield, whereas the inoculation with R. tropici mitigated this effect. The aboveground tissues of plants fed with N-free NS contained appreciably less N than those fed with standard solution when they were inoculated with the commercial inoculant (1.7 vs. 29 mg·g−1 dry weight, respectively). The shoot total N concentration 45, 65, and 90 days after transplanting (32, 31, and 29 mg·g−1 dry weight, respectively) was not reduced by the supply of N-free NS when the plants were inoculated with R. tropici. This finding indicates that, at least from the first sampling date onward, the tissue N level was not a limiting factor for growth and yield in plants inoculated with R. tropici. The supply of N-free NS restricted appreciably the potassium (K), magnesium (Mg), and zinc (Zn) levels in the aboveground plant biomass, regardless of inoculation treatment. The impaired growth and yield in plants fed with N-free NS and inoculated with R. tropici is ascribed to both a N shortage at early growth stages and a reduced K+ uptake aimed at electrochemically balancing the anion-to-cation uptake ratio under conditions of no external NO3 supply.