Responses of cucumber plant to NH4+ and NO3− nutrition: The relative addition rate technique vs. cultivation at constant nitrogen concentration

Different N sources (NO3−, NH4+, or NH4NO3) at different relative addition rates (RAR) were supplied to cucumber (Cucumis sativus L.), a species sensitive to NH4+ toxicity. For comparison, cucumber plants were also grown at constant concentrations of 1 and 5 mM NH4+ or 5 mM NO3−. The fresh weight of NH4+-fed plants at RAR 0.15 and RAR 0.25 day−1 was similar to that of NO3−-fed plants, while at RAR 0.35 or RAR 0.45 day−1 growth reduction occurred. When available as a constant concentration, NH4+ decreased plant growth at 5 mM. It is concluded that at low rates of N supply the relative addition rate technique can be used for growing cucumber plants with NH4+ as sole N source without deleterious effects.