NITROGEN SUPPLY EFFECTS ON PARTITIONING OF DRY-MATTER AND NITROGEN TO GRAIN OF IRRIGATED WHEAT

Partitioning efficiencies of plant dry matter (harvest index, or HI) and N (NHI) to grain are determinants of economic yield and protein content of wheat (Triticum aestivum L.). Response of HI and NHI to a range of early- and late-season N-supply environments was characterized at three sites with two hard red spring wheat cultivars (Year 1), and at one site with a third cultivar (Year 2). Treatments combined preplant N rates (0-250 kg N ha-1) with additional N applied at anthesis (0-65 kg N ha-1). The N-supply environment had contrasting effects on the HI and NHI. Greater early-season N supply increased both grain and vegetative yields in approximately the same proportion so that the HI was relatively constant or increased slightly over most of the response to N. Greater postanthesis N supply resulted in a small grain yield increase and a small increase in HI. In contrast, the NHI decreased linearly for all cultivars with increasing early-season N supply. The slope and y intercepts for these regressions differed between cultivars in Year 1 due to a significant cultivar x preplant N interaction. Late-season N addition increased or decreased the NHI depending on whether the early-season N supply limited grain yield. Additional plant N derived from supplemental N supplied at anthesis was partitioned between grain and straw in a ratio that was predetermined by the early-season N environment. Of the individual grain, straw, or whole-plant parameters from which the NHI is calculated, the straw N concentration seems most useful for referencing the NHI in relation to grain N demand and plant N supply across diverse N-regime and site environments.