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Abstract
Determination of the nitrogen (N) concentration, nitrogen form and nitrate to ammonium ratio is one of the most important principles of plant nutrition, because of its considerable effects on growth parameters, plant metabolism, yield and quality of horticultural crops. This experiment was conducted to evaluate the effects of N concentrations (170 and 200 ppm) and different nitrate to ammonium ratios (100:0, 90:10, 80:20 and 70:30) on morpho-physiological characteristics of calendula (Calendula officinalis) such as flower number and number, plant height, flower diameter, flowers fresh and dry weight, chlorophyll a, b and total chlorophyll, carotenoids, leaf relative water content, proline, hydrogen peroxide, soluble sugars, total phenols and essential oil) in a factorial experiment based on the completely randomized design with four replications. Results showed that higher nitrogen concentration has significant effects on flower and leaf number, fresh and dried flowers weight, carotenoid content, soluble sugars and essential oils, but had no significant effects on other characteristics. Different nitrate to ammonium ratios showed significant effects on the leaves and flowers number, plant height, flower diameter, flower fresh weight, carotenoids, proline, soluble sugar and essential oils. Interaction effects between nitrogen concentration and nitrate to ammonium ratios ratio showed significant effects on physio-morphological traits, except chlorophyll a, hydrogen peroxide and total phenol. Accordingly, the highest y
ield was observed at 80:20 nitrate to ammonium ratios. Based on the results, the nitrogen concentration nitrate to ammonium ratios in the nutrient solution of calendula play a significant role in growth parameters, yield and quality of essential oil.
Key words: Nitrogen, nitrate to ammonium ratios, calendula, essential oil quality, soilless culture.

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