Petiole sap nitrate and leaf nitrogen critical values in melon plants grown in unheated greenhouse and field conditions

Two experiments with melon plants were carried out with similar procedures; one in unheated greenhouse and another in field conditions to establish petiole sap NO3- and leaf N critical values. Each experiment consisted of four randomized blocks containing five plots. Different Nitrogen (N) rates (0, 75, 150, 300, and 450 kg ha(-1)) as urea were placed in furrows (30%) before seedling transplanting and the remaining 70% were trickle-applied through the plant cycle. Petiole sap NO3- (NS) and leaf N (NT) concentrations were determined in the 5th leaf from the apex sampled at 25 (first fruit set) and 52 DAT (second fruit set). Both NS and NT at the first and second sampling were increased with increasing N applications in both greenhouse and field. There was a high correlation between NS and NT. NS and NT critical values were higher in plants grown in greenhouse than in the field being more accentuated at the first than at the second sampling. In greenhouse, at the first melon fruit set, the petiole sap NO3- and the 5th leaf from the apex N concentrations should be 5360 mg L-1 and 4.38 day kg(-1), respectively, or slightly lower at the second fruit set and under field conditions.