 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com J. Plant Nutr. Soil Sci. 2012, 175, 125–134 DOI: 10.1002/jpln.201100035 125 Responses of cucumber grown in recirculating nutrient solution to gradual Mn and Zn accumulation in the root zone owing to excessive supply via the irrigation water Constantinos Tzerakis 1 , Dimitrios Savvas 2 *, and Nick Sigrimis 3 1 National Agricultural Research Foundation, Institute of Subtropical and Olive Crops of Chania, Agrokipio, 73100 Chania, Greece 2 Agricultural University of Athens, Department of Crop Science, Iera Odos 75, 11855 Athens, Greece 3 Agricultural University of Athens, Department of Agricultural Engineering, Iera Odos 75, 11855 Athens, Greece Abstract A standard and a high manganese (Mn) level (10 and 160 lM) were combined with a standard and a high zinc (Zn) level (4 and 64 lM) in the nutrient solution supplied to cucumber in closed- cycle hydroponic units to compensate for nutrient uptake. The concentrations of all nutrients except Mn and Zn were identical in all treatments. The objectives of the experiment were to establish critical Zn and Mn levels in both nutrient solutions and leaves of cucumber grown hydroponically, to assess the impact of gradual Zn and/or Mn accumulation in the external solu- tion on nutrient uptake and gas exchange, and to find whether Mn and Zn have additive effects when the levels of both ions are excessively high in the root zone. The first symptoms of Mn and Zn toxicity appeared when the concentrations of Mn and Zn in the leaves of cucumber reached 900 and 450 mg kg –1 in the dry weight, respectively. Excessively high Mn or/and Zn concentra- tions in the leaves reduced the fruit biomass production due to decreases in the number of fruits per plant, as well as the net assimilation rate, stomatal conductance, and transpiration rate, but increased the intercellular CO 2 levels. Initially, the Mn or Zn concentrations in the recirculating nutrient solution increased rapidly but gradually stabilized to maximal levels, while the corre- sponding concentrations in the leaves constantly increased until the end of the experiment. The uptake of Mg, Ca, Fe, and Cu was negatively affected, while that of K and P remained unaf- fected by the external Mn and Zn levels. The combination of high Mn and Zn seems to have no additive effects on the parameters investigated. Key words: cucumis sativus / nutrient toxicity / hydroponics / soilless culture / photosynthesis Accepted October 17, 2011 1 Introduction Manganese (Mn) and zinc (Zn) are essential trace elements for plant growth and development. However, excessive con- centrations of Mn 2+ and Zn 2+ in the root zone of cultivated plants have been frequently associated with growth and yield restrictions (Hannan and Ohki, 1988; Osawa and Ikeda, 1974; Osawa and Ikeda, 1979; Gupta and Gupta, 1998; Le Bot et al., 1990a). Elevated concentrations of trace elements in greenhouse soils may develop with time due to prolonged use of organic fertilizers composed of sewage sludge and poultry manure or due to irrigation with wastewater (Marsch- ner , 1995; He et al., 2005; Arora et al., 2008). Also, micronu- trients present as impurities in fertilizers may make a signifi- cant contribution to micronutrient supply to crops (Procopiou and Wallace, 1981; Rengel et al., 1999). In soilless culture, the Mn concentration in the root environment may increase to undesirably high levels if the pH decreases to levels below 6 (Silber et al., 2008). Furthermore, toxic Mn 2+ and Zn 2+ levels in the root zone may develop with time in closed-loop sys- tems due to progressive accumulation originating from exces- sive concentrations in the nutrient solution used to compen- sate for plant uptake (Adams, 2002). The latter may arise either from inadvertent overdosing or from excessive Mn 2+ and Zn 2+ concentrations in the irrigation-water source used to prepare nutrient solution. Manganese toxicity is a major factor limiting plant growth in acidic and waterlogged soils. Under such conditions, exces- sively high amounts of Mn can become plant-available, and this can reduce plant growth by interfering with some meta- bolic processes (Le Bot et al., 1990b; Lidon and Teixeira, 2000; Hauck et al., 2003). Excess Mn induces inhibition of chlorophyll biosynthesis and a decline in the photosynthetic rate (Macfie and Taylor , 1992; Hauck et al., 2002). The responses of plants to excessive Mn availability depend not only on the exact Mn concentration in the root zone, but also on temperature (Rufty et al., 1979), levels of other nutrients in the root zone (Wang et al., 1994), and light intensity (Nable et al., 1988). It has been reported that cucumber plants grown under high-light conditions display lower net photo- synthesis and develop more serious symptoms of Mn toxicity than plants grown under low-light conditions (Shi et al., 2006). The normal Mn concentration in cucumber leaves amounts to 100–300 mg kg –1 (Adams, 2002). Different plant species and cultivars of the same species may exhibit quite * Correspondence: Dr. D. Savvas; e-mail: dsavvas@aua.gr