Fertilization strategies for lowering nitrate content in leafy vegetables. Chicory and rocket salad cases

Studies have been carried out to evaluate whether chicory (Cichorium intybus L.) and rocket salad [Eruca vesicaria (L.) Car. subsp. sativa (Mill)] nitrate (NO3) content can be reduced by decreasing nitrate-nitrogen (NO3-N) in the nutrient solution or by partially replacing it with ammonium-nitrogen (NH4-N) (chicory only) few days before harvest. Soilless culture method was used applying two levels of NO3 for chicory (2 and 4 mM NO3) and only one for rocket (2 mM NO3). Five (rocket) and six (chicory) days before harvesting, half of the plants were grown always in the same solutions, while the others were grown in a solution with 0.25 mM N (rocket and chicory transferred from 2 mM N) or in a solution where 3/4 of NO3-N were replaced with NH4-N (chicory transferred from 4 mM N). Two cultivars of chicory were used. 'Clio' hybrid had larger leaf area, but lower dry mass and NO3 concentration than 'Frastagliata' chicory. For rocket and chicory alike, the change of nutrient solution five or six days before harvesting did not affect relative growth rate and water-use efficiency, even if plants transferred in the solution with 0.25 mM NO3 absorbed all the available N, whereas plants kept in the initial solution with 2 mM NO3 only took 62% (chicory) or 32% of N (rocket). Chicory plants transferred from 4 mM NO3-N to 1 mM NO3N plus 3 mM NH4-N absorbed 60% of available N, thanks to the uptake of 15.7 mg NH4-N per plant day, while those grown always with 4 mM NO3 absorbed 52% of the available N. Nitrate content, expressed on fresh matter (f.m.) basis, resulted to be remarkably high in both species when nutrient solution was kept unchanged, but it was more than halved when available NO3 was cut by 3/4 in the nutrient solution five or six days before harvesting. In chicory, it decreased by 36% when NO3-N:NH4-N ratio passed from 4:0 to 1:3.