Effects of Ammonium Nitrogen on Lettuce Grown on Pumice in a Closed Hydroponic System

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 amolar basis. In allNr 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 whenNrwas 0 or 0.1. The stimulation of lettuce growth asNrwas increased to 0.3 may be a consequence of enhanced P uptake resulting from better control of pH in the root zone. Nitrogen is the only nutrient that can be supplied to plants via fertigation in both anionic (NO3 ) and cationic (NH4 ) form, whereas the uptake rates of both N forms are influenced by their external concentrations (Forde and Clarkson, 1999). However, the manipulation of NH4-N/NO3-N in the nutrient solution supplied to the plants, while maintaining the same total-N concentration, may considerably modify the total cation-toanion uptake ratio. Changes in the total cation-to-anion uptake ratio have a profound impact on the pH in the root zone (Imas et al., 1997; Lea-Cox et al., 1996). Indeed, the imbalance of total cation over anion uptake in the rhizosphere originating from enhanced NH4 + uptake is electrochemically compensated for by the release of protons (H), which results in a lowering of the medium pH (Bolan et al., 1991; Imas et al., 1997). Similarly, the excess of anion over cation uptake by the plants resulting from the supply of NO3 – as the sole or dominant N form is compensated for by H influx or equivalent anion extrusion (Barber, 1984; Imas et al., 1997; Magalhaes and Wilcox, 1983), which increases the pH of the external solution. The pH of the external medium may influence the uptake of P and micronutrients, thereby inducing serious nutritional disturbances (Adams, 2002; Imas et al., 1997; Islam et al., 1980; Savvas et al., 2003). Furthermore, the form of N supplied may influence the uptake of other macronutrients resulting from ion antagonism (Marschner, 1995). The N form also influences plant metabolism, resulting from differences in the intracellular assimilation paths (Gerendás et al., 1997; Raab and Terry, 1994) and the accumulation of nitrate (Blom-Zandstra, 1989). As a rule, the use of ammonium as the sole or principal N source results in impaired growth and yield reductions (Chaillou et al., 1986; Chance et al., 1999; Claussen and Lenz, 1999; Guo et al., 2002; Raab and Terry, 1994). This is ascribed to the high toxicity of ammonia at an intracellular level (Barker and Mills, 1980; Givan, 1979; Raab and Terry, 1994). The current recommendation for soilless culture is that NH4-N should not exceed 25% of the total-N supply (Sonneveld, 2002). However, plant species differ in their response to the NH4-N/total-N supply ratio and root zone pH (Akl et al., 2003; Beevers and Hageman, 1983; Claussen and Lenz, 1999; Islam et al., 1980). The changes in nutrient concentrations and pH in the root zone of soilless-grown plants may be rapid as a result of the restricted volume of the rooting medium and a lack of buffering capacity. Moreover, the impact of a particular NH4-N/ total-N supply ratio may be quite different in closed hydroponic systems than in open ones (Savvas and Gizas, 2002). Hence, to prevent nutrient disturbances and to optimize growth in crops grown in closed hydroponic systems, specific knowledge is required for each particular plant species concerning its response to the ratio of NH4-N to total-N supply via the nutrient solution. High NO3 – concentrations in edible plant parts constitute a potential threat for human health and, therefore, many countries have set maximum permissible values by legislation (Blom Zandstra, 1989; Van der Boon et al., 1990). Many plants tend to accumulate nitrate in their leaves, and lettuce is one of these (Gent, 2003). To restrict the accumulation of NO3 – in lettuce, a partial substitution of NH4 + for NO3 – has been tested in crops grown in recirculating nutrient solution (Ikeda and Osawa, 1981; Van der Boon et al., 1990) and in pots filled with an organic substrate (Scaife et al., 1986). The previously mentioned investigations focused mainly on plant growth and tissue nitrate levels. The current study was designed to investigate the responses of lettuce to NH4-N/total-N when the plants are grown on an inert substrate (pumice) under Mediterranean climatic conditions and the drainage solution is completely recycled. To understand better the mechanisms underlying the effects of NH4 + and NO3 – on lettuce growth, our study focuses not only on plant growth and tissue nitrate concentrations, but also on the nutrient status in the root zone and the uptake of nutrients. Materials and Methods Two successive experiments with lettuce (Lactuca sativa L., cv. Vardac) were carried out in a glasshouse located in Arta (lat. 39 7#N, long. 20 56#E), Greece. The experiments were conducted in 12 independent hydroponic installations enabling complete recycling of the drainage solution, which corresponded to 12 experimental units. Each experimental unit comprised two channels, 5 m in length and 0.25 m in width. The channels were filled with pumice (particle size range, 0–5 mm) to a depth of 8 cm. Each channel accommodated 40 plants and, hence, the volume of substrate per plant was 2.5 L. All channels were covered with black–white polyethylene sheets to avoid water evaporation. Nutrient solution was automatically applied via a drip irrigation system at a rate Received for publication 26 Apr. 2006. Accepted for publication 14 July 2006. This investigation was supported by the Greek Ministry of Education (YPEPTH) within the frame of the ‘‘ARCHIMEDES’’ research project, which was partly funded by EU.We thank LAVAS.A. for donating the substrate (pumice). To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 41(7) DECEMBER 2006 1667 JOBNAME: horts 41#7 2006 PAGE: 1 OUTPUT: October 24 22:05:25 2006 tsp/horts/127877/01588 SOIL MANAGEMENT, FERTILIZATION, AND IRRIGATION