Interannual variability in root growth of spring wheat (Triticum aestivum L.) at low and high nitrogen supply

Little is known about the spatial and temporal characteristics of the root growth of spring wheat and its modification by nitrogen (N) supply. For 4 years the cultivar Toronit was fertilized with 20 (LN) or 270 kg N ha-1 (HN) in lysimeters. The shoot traits were measured at harvest, while root growth was screened regularly at 10 soil depths in minirhizotrons between 0.05 and 1.00 m. The cumulative number of roots cm-2 (CNR) was fitted to a logistic equation to study the course of root growth at each soil depth. Furthermore, the vertical patterns of CNR were examined at the beginning of tillering, stem elongation, anthesis and physiological maturity by a non-parametric regression (splines). The parameters of the logistic and non-parametric models were influenced by all the factors; thus the root system was highly plastic. Whereas the N off take was similar at LN in 1999, 2001 and 2002, the period of linear increase in CNR in the subsoil was 7 d longer in 2001 than in 2002. At HN, the N off take was higher in 1999 than in 2001 but the reverse was true for root growth. There was also variation among years in the total duration of root growth, with differences up to 20 d. The percentage of roots grown after anthesis ranged from 1 to 22% of the total roots grown by physiological maturity, demonstrating that the root growth of spring wheat can be high and variable after anthesis. This percentage differed among years more in the subsoil and supported the evidence provided by the time parameters of the logistic equation that the impact of climatic and soil conditions on root growth seems to become stronger with time. At all levels of N supply, the vertical pattern of CNR was characterized by an exponential decrease at the beginning of tillering in all the years. Such a clear pattern was not found at later developmental stages. Though the basic knowledge of the variability of root growth of spring wheat increased, the interannual variability in root dynamics was not explained fully by climatic differences among the growing seasons.