Nitrogen Effect on Fruit Quality and Yield of Muskmelon and Sweet Pepper Cultivars

Yield and fruit quality are two of the most important parameters for the profitability of vegetable crops. In commercial vegetable production, nitrogen (N) is commonly applied in excess, which is associated with nitrate (NO₃⁻) leaching loss. In addition, excess N application may affect yield and fruit quality. The aim of this study was to evaluate the effect of N applications of very deficient N (N1, 2 mmol L⁻¹), deficient N (N2, 8 mmol L⁻¹), and conventional N (N3, 14 mmol L⁻¹), according to local fertigation practices, in soil-grown muskmelon and sweet pepper crops in Almeria, South-Eastern Spain. The evaluation was conducted in three cultivars of each species. The yield and the fruit quality parameters of firmness, colour, total soluble solids (TSS) and morphometric variables were evaluated in two years for each species. For most parameters in both species, the effects of N, when significant, occurred regardless of cultivar. In muskmelon and sweet pepper, application of 8.2 mmol N L⁻¹ (i.e., N2) was sufficient to achieve a maximum yield of 6.7 and 7.4 kg m⁻², respectively. In muskmelon, very deficient N application led to an increase of 58% in the percentage of fruit discarded, mostly due to malformed and undersized fruits. Fruit firmness and red–green axis coordinate (a*) were not consistently affected by N in any of the crops. However, the fruit lightness (L*) increased with N addition in both species, likely because of increased chlorophyll pigments. With N addition, fruit TSS slightly decreased in muskmelon and slightly increased in sweet pepper. However, fruit TSS of both species were within reference values for commercialisation in the three N treatments. There were differences in yield between cultivars in muskmelon but not in sweet pepper, likely due to differences in fruit number in muskmelon. There were differences between cultivars in TSS and colour a* coordinate in both muskmelon and sweet pepper. Cultivars with higher TSS and a* coordinate will likely be more desirable for consumers because of the sweeter taste and more intense orange colour in muskmelon and reddish colour in sweet pepper. Overall, our manuscript showed that N application can be reduced, relative to the conventional N application, without reducing yield or fruit quality in muskmelon, yet additional studies should be conducted in sweet pepper to complement the results of the 2020 crop.