Effect of PGPR application and nitrogen doses on baby leaf lettuce grown in a floating system

The floating system is one of the easiest and cheapest hydroponic methods used to produce baby leaf vegetables. Among these products, the baby leaf lettuce has grown in popularity in recent years as a ready-to-eat vegetable. The aim of this work was to study the effect of application of two plant growth promoting rizhobacterias (PGPRs: Bacillus subtilis and B. velezensis) and two concentrations of nitrogen (4 and 12 mM) in the nutrient solution on yield, quality and nitrate content of two baby leaf lettuce cultivars grown in a floating system. Two sowings were carried out in December 2009 and February 2010 directly into styrofloat trays containing peat media. Plant density was 1700 plants m-2. The duration of the crop cycles was 36 and 30 days, respectively. The experiment was a randomized complete block design with two factors: N concentration (two levels) and bacterial inoculant (three levels). In both cultivars and crop cycles plant height was affected by the level of N and the PGPR application, while leaf area, fresh and dry weight were only affected by the level of N. The use of the nutrient solution containing 12 mM of nitrogen increased the accumulation of nitrate in leaves. The application of B. velezensis in nutrient solution provoked a decrease of nitrate content in 'Diveria' leaves respect to control. Finally, root growth was not affected by neither nutritive solution nor bacterial inoculants in both cultivars.