Nitrogen rate, rather than foliar biostimulants, drives marketable yield of two eggplant (Solanum melongena L.) genotypes under Mediterranean open-field conditions

In eggplant ( Solanum melongena L.), the nitrogen (N) rate that maximizes marketable yield need not maximize agronomic N efficiency or fruit quality, and whether foliar biostimulants modify the N response has rarely been tested in the field. A two-year open-field experiment in Sicily compared two genotypes (Black Bell, BB, and Sciacca), four N rates (0, 60, 120 and 240 kg N ha –1 ) and three foliar treatments (control, Auxym and Trainer) in a factorial design. Marketable yield rose from 37.98 t ha –1 at N0 to 68.13 t ha –1 at N240 (p < 0.001), mainly through more marketable fruits (13.6 to 22.0 m –2 ) and, secondarily, heavier fruit (281.5 to 315.2 g). Agronomic efficiency tended to decline from N60 to N240, with overlapping bootstrap confidence intervals, while the marginal agronomic return between N120 and N240 remained positive. Ascorbic acid showed a year- and genotype-dependent response to N, with the highest numerical values at N120 in three of four combinations. Foliar treatments had no significant main effect on yield, and no differences in N-response slopes were detected (ANCOVA, p = 0.269). A Genotype × N rate × Biostimulant interaction (p = 0.039) mainly reflected a positive Trainer response in Sciacca at N240 that survived multiple-testing correction but was significant in only one season. The yield-component responses are consistent with N limitation of reproductive output, although the underlying physiological mechanisms were not measured. N supply governed productivity; no stable modulatory effect of foliar biostimulants was detected.