Producing nitrate-free endive heads: effect of nitrogen form on growth, yield, and ion composition of endive

In a growth chamber, endive (Cichorium endivia L. var. crispum Hegi) plants were grown using a solution culture method to evaluate the influence of four ammonium : nitrate (NH4-N : NO3-N) percentage ratios (100:0, 70:30, 30:70, and 0:100) on growth (leaf area, dry mass, crop growth rate, relative growth rate, and net assimilation rate), yield characteristics (head and root fresh mass and root length), quality (dry matter, nitrogen, and nitrate), and inorganic ion content. No symptoms of NH(+) toxicity were detected in endive plants 8 weeks after beginning nutrient treatments. Moreover, by feeding N in mixed form, the growth indices increased compared to indices from feeding with any of the two N forms alone. Ammonium-fed plants produced nitrate-free heads with a fresh mass (171 g) similar to nitrate-fed plants. Compared to the other treatments, the heads of NH(+)-fed plants were darker green and more succulent. Mixed N improved yield but caused a remarkable accumulation of nitrate in heads. Following an increase in NO3-N from 30% to 70% in the nutrient solution, head fresh mass rose from 196 to 231 g and NO(-) concentration more than doubled (from 2.4 to 6.1 g.kg-1 fresh mass). With 100% of NO3N, NO(-) concentration was 5.5 g.kg-1 fresh mass. With higher NO3-N percentages in the nutrient solution, the difference in the concentration of inorganic cations and anions increased, but K(+) concentration was also high in ammonium-fed plants (on average 77 g.kg-1 dry mass). Head total N accumulation was increased by the presence of NH4(+) in the nutrient solution and decreased with 100% NO3-N. From the commercial viewpoint, the produce obtained from 100% NH4-N was good, with the value-added factor of the absence of nitrate. This may be an extremely remarkable factor because of the commercial limits on the allowable nitrate content in leafy vegetables already enforced by many European countries and those the European Union is going to adopt in a directive.