The role of N-remobilisation and the uptake of NH4+ and NO3- by Lolium perenne L. in laminae growth following defoliation under field conditions

Several studies have previously shown that shoot removal of forage species, either by cutting or herbivore grazing, results in a large decline in N up take (60%) and/or N-2 fixation (80%). The source of N used for initial shoo t growth following defoliation relies mainly on mobilisation of N reserves from tissues remaining after defoliation. To date, most studies investigati ng N-mobilisation have been conducted, with isolated plants grown in contro lled conditions. The objectives of this study were for Lolium perenne L., g rown in a dense canopy in field conditions, to determine: 1) the contributi on of N-mobilisation, NH4+ uptake and NO3- uptake to growing shoots after d efoliation, and 2) the contribution of the high (HATS) and low (LATS) affin ity transport systems to the total plant uptake of NH4+ and NO3-. During th e first seven days following defoliation, decreases in biomass and N-conten t of roots (34% and 47%, respectively) and to a lesser extent stubble (18% and 43%, respectively) were observed, concomitant with mobilisation of N to shoots. The proportion and origin of N used by shoots (derived from reserv es or uptake) was similar to data reported for isolated plants. Both HATS a nd LATS contributed to the total root uptake of NH4+ and NO3-. The V(m)ax o f both the NH4+ and NO3- HATS increased as a function of time after defolia tion, and both HATS systems were saturated by substrate concentrations in t he soil at all times. The capacity of the LATS was reduced as soil NO3- and NH4+ concentrations decreased following defoliation. Data from N-15 uptake by field-grown plants, and uptake rates of NH4+ and NO3- estimated by exci sed root bioassays, were significantly correlated, though uptake was over-e stimated by the later method. The results are discussed in terms of putativ e mechanisms for regulating N uptake following severe defoliation.