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D. Savvas, H.C. Passam, C. Olympios, E. Nasi, E. Moustaka, N. Mantzos, and P. Barouchas

Two successive lettuce crops were grown in spring 2005 in a completely closed hydroponic system. The ratio of ammonium to total nitrogen (Nr) in the fresh nutrient solution (FNS) introduced into the closed system to compensate for plant uptake was 0, 0.1, 0.2 and 0.3 on a molar basis. In all Nr treatments, the concentrations of total N, K, Ca, Mg, P, and micronutrients in the FNS were identical, but that of SO4 2– increased as Nr increased, to compensate electrochemically for the enhanced NH4 + and decreased NO3 supply. The highest fresh and dry weights per plant were attained with the highest ammonium supply (Nr = 0.3) but, even when no NH4 + was included in the FNS as an N source, the plants were healthy without apparent nutritional disorders. The ammonium concentration in the drainage solution dropped to nearly zero in all treatments some days after the initiation of recycling, which implies a preferential uptake of NH4-N over NO3-N. The root zone pH, as indicated by the values measured in the drainage solution, decreased slightly as Nr increased, and ranged from 6.5 to 8.0 in all treatments. The leaf K, Ca, Mg, and Fe concentrations were not influenced, whereas those of P, Mn, Zn, and Cu were enhanced by the increasing NH4 + supply. The increased ammonium supply did not enhance the utilization of N in plant metabolism, although it reduced the nitrate concentration of the internal leaves in the early spring experiment. The leaf micronutrient concentrations were clearly more than critical levels even when NO3 was the sole N source for lettuce, whereas the P concentration approached the lowest critical level when Nr was 0 or 0.1. The stimulation of lettuce growth as Nr was increased to 0.3 may be a consequence of enhanced P uptake resulting from better control of pH in the root zone.

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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.