Sowing date and nitrogen rate effects on dry matter and nitrogen partitioning in bread and durum wheat

Grain yield and protein in bread and durum wheat are determined by the plant efficiencies in partitioning dry matter (harvest index, HI) and nitrogen harvest index (NHI) to the grain. Responses of HI and NHI to two rates of preplant N supply (105 and 170 kg N ha(-1)) were characterized for nine bread and five durum wheat genotypes along with a triticale genotype in three sowing dates. Genotype x N input interaction was significant for several characters in early sowing (26 October 1993), for shoot biomass at anthesis, grain yield, and straw yield in optimum sowing (6 December 1993), and for grain yield in late sowing (5 February 1993). Although plants removed more N from the soil in early than in optimum sowing, mean HI and NHI in the former, 38.5 and 66.3%, were lower than those in the latter, 45.5 and 71.2, respectively. On average, plants lost more N from anthesis to maturity at early (16 kg ha(-1)) than at optimum sowing date (6 kg ha(-1)). Mean HI and NHI in optimum and late sowing dates were similar, but mean grain yield in the former (4.9 t ha(-1)) was greater than that in the latter (4.0 t ha(-1)). The high rate of N supply reduced mean HI and NHI at all sowing dates. Mean HI and NHI were highly positively correlated (r = 0.89). Depending upon sowing date and N input, negative significant correlation was found between grain yield and N concentration ranging from -0.59 to -0.66. Grain yield and shoot biomass at anthesis were highly correlated (r = 0.74), indicating the importance of preanthesis assimilated carbohydrates in grain filling in most genotypes. Mean postanthesis N accumulation by plants was relatively small, indicating that most of grain N was translocated from preanthesis assimilated N. A substantial amount of N remained in the straw. Despite substantial genotypic variation, only a few genotypes had mean grain yield, grain N concentration, Hl, and NHI greater than the overall mean. Wheat germplasm is needed that can easily accumulate N before and after anthesis and efficiently translocate it to the grain with a minimum of N loss from plants after anthesis. Further improvement in N uptake and partitioning should be under-taken in wheat under low N input. (C) 2001 Published by Elsevier Science B.V.