effects of nitrogen fertilizer rates and application time on root characteristics of dryland wheat genotypes

understanding of wheat root system is important in dryland conditions. root development, important factor in wheat production under water limiting conditions, can be affected by environmental factors especially nitrogen fertilizer. a field experiment was carried out to study the nitrogen rates and application times on root morphology of three rainfed wheat genotypes, using a split-splitplot based on a randomized complete block design (rcbd) with 3 replications in maragheh dryland agricultural research institute (dari) during 2010-2011. treatments were two nitrogen application times (all in fall, and two-third in fall + one-third in spring) assigned in main plot; four nitrogen rates (0, 30, 60 and 90 kg/ha) assigned in sub-plots; and three wheat genotypes (azar2, cereal-3 and cereal-4) assigned in sub-subplots. root samples were collected at stem elongation stage (gs22) to study root characteristics. the results showed that nitrogen applications resulted in significantly increasing plant height, crown root number, shoot fresh weight, root fresh weight, root dry weight, above-ground plant biomass, total root volume, ratio of root volume per soil volume compared with control (no nitrogen). the increasing trends in all parameters were found linearly upward to the last level of nitrogen treatment used. nitrogen application in fall significantly increased crown root length (p<0.05) by 11 percent compared to split application. also, nitrogen fall application significantly affected crown root and coleoptile lengths of genotypes. cereal-4 genotype had better root morphological characteristics compared to other two genotypes. nitrogen fertilization could affect rainfed wheat root characteristics in order to produce desirable traits. in general, it was concluded that fall application of 60 kg n/ha could increase rainfed wheat grain yield due to improvement of  root systems.