IMPROVED NITROGEN MANAGEMENT IN IRRIGATED DURUM-WHEAT USING STEM NITRATE ANALYSIS .2. INTERPRETATION OF NITRATE-NITROGEN CONCENTRATIONS

Attempts to characterize the N status of irrigated spring wheat (Triticum aestivum L.) using basal stem nitrate-N (NO3-N) tissue tests have shown contradictory results due to the narrow range of agronomic conditions existing in most studies. Five field experiments were conducted in southern Arizona to examine the effects of N rate, mobility of N fertilizer form, residual soil N, soil texture, and two contrasting cultivars on basal stem NO3-N concentrations, yield and quality of irrigated durum wheat (Triticum turgidum L. var. durum). Fertilizer N treatments were broadcast at planting, then at Feekes 5, 10, and 10.5 growth stages (GS) to simulate fertilization in conjunction with the first four basin irrigation events. Stem tissue samples were taken from all plots at GS 2, 5, 6, 10, and 10.5 for NO3-N analysis. A critical level of 2000 mg NO3-N kg-1 in durum wheat stem tissue at GS 2 through 5 was suggested. Critical wheat stem tissue NO3-N concentrations of 1000 mg kg-1 at GS 6 through 10 and 500 mg kg-1 at GS 10.5 were also defined. Significant grain yield reductions due to excessive N fertilizer applications resulted when stem tissue NO3-N concentrations exceeded 6000, 5000, 4000, 3000, and 2000 mg kg-1 at GS 2, 5, 6, 10, and 10.5, respectively. Optimum durum wheat grain yield and quality occurred when basal stem tissue NO3-N concentrations ranged from 3000 to 4000, 2500 to 3500, 1000 to 1500, 500 to 1000, and 200 to 500 mg kg-1 at GS 2, 5, 6, 10, and 10.5, respectively. These results should prove useful in predicting the N needs of irrigated durum wheat grown under arid and semi-arid conditions.