Chemical composition and yield of onion under different fertilizer regimes

Onion is a very important vegetable crop throughout the world, while in Greece it is the most important cultivated Allium species. Although fertilization requirements have been more or less evaluated regarding crop yield, little information is available regarding their effect on chemical composition of onion dry bulbs. In the present study, we examined the effect of fertilization and zeolite on yield and chemical composition of two onion genotypes [‘Sturon’ (commercial cultivar) and ‘Vatikiotiko’ (local landrace). Five fertilizer treatments were applied, namely: 1) control (C), 2) farm yard manure (M), 3) standard fertilizer (F1), 4) zeolite + standard fertilizer (Z) and 5) slow release nitrogen fertilizer (F2). Crop was established by using sets of medium size for both genotypes (14/21 for ‘Sturon’ and sets of similar size for ‘Vatikiotiko’) in a plant density of 22 plants m(‑2). Harvest took place on 14-15/07/2017. After harvest total yield, bulb firmness, dry matter and total soluble solids (TSS) contents were recorded. Chemical composition analyses included proximate composition, sugars, organic acids, tocopherols, carotenoids, chlorophylls and fatty acids contents and antioxidant activity. The results of the study showed that fertilizer treatments (F1, Z and F2) had a beneficial effect on the yield of ‘Sturon’ genotype, whereas for ‘Vatikiotiko’ only standard fertilizer (F1) resulted in higher yield. Firmness, dry matter and TSS contents were also affected by fertilizer regime. Control treatment was more beneficial for carbohydrates and energy content for both genotypes, as well as for sugars content in ‘Vatikiotiko’ and organic acids content in ‘Sturon’. Alpha-tocopherol and sugars content increased for F1 treatment in ‘Sturon’, while the same treatment showed better results for antioxidant properties of ‘Vatikiotiko’. In conclusion, chemical composition and quality were affected in a genotype and fertilizer treatment dependent manner, while yield increased for all fertilizer treatments only in the case of ‘Sturon’. 2