Effects of nitrogen fertilization on the mineral status of plants and the chemical composition of scallop squash fruits

A field study was conducted to evaluate the effects of increased doses of nitrogen supplied as a preplant (60, 120, 180, 240 kg N ha-1) or split (120 + 60, 120 + 60 + 60 kg N ha-1) fertilization treatment on the content of macronutrients in index plant parts and fruits of scallop squash. Leaf petioles used as the index parts were collected 3 times during the growing period, while fruits with a diameter of 3-6 cm, 6.1-12 cm and >12 cm were harvested at the maximum fruit setting. The data indicate that nitrogen fertilization strongly affected the nitrate content in leaf petioles of scallop squash at the onset of fruit harvest, while its influence was much weaker in later stages of the growth period. The content of phosphorus, potassium, calcium and magnesium was not dependent on either the nitrogen dose or the size of harvested fruits. Heavy N fertilization caused substantial enhancement of the total N and nitrate accumulation in fruits, but did not change the concentrations of P, K, Ca and Mg. Fruits harvested when having the 3-6 cm diameter contained higher amounts of total N, phosphorus, potassium, magnesium and nitrates than the ones which had a diameter of 6.1-12 cm and >12 cm. However, even at the maximum concentration of nitrates in fruits harvested early, which was 317 mg NO3 - kg-1 f.w., scallop squash should be treated a minor contributor to dietary nitrate intake by consumers. The data indicate that tissue nitrate analysis may be successfully used as the basis for determination of the crop’s nitrogen requirement during the early growth period.