Abstract: | Several studies have previously shown that shoot removal of forage species, either by cutting or herbivore grazing, results
in a large decline in N uptake (60%) and/or N2 fixation (80%). The source of N used for initial shoot growth following defoliation relies mainly on mobilisation of N reserves
from tissues remaining after defoliation. To date, most studies investigating N-mobilisation have been conducted, with isolated
plants grown in controlled conditions. The objectives of this study were for Lolium perenne L., grown in a dense canopy in field conditions, to determine: 1) the contribution of N-mobilisation, NH4
+ uptake and NO3
- uptake to growing shoots after defoliation, and 2) the contribution of the high (HATS) and low (LATS) affinity transport
systems to the total plant uptake of NH4
+ and NO3
-. During the first seven days following defoliation, decreases in biomass and N-content 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 reserves or uptake) was similar to data reported for isolated plants.
Both HATS and LATS contributed to the total root uptake of NH4
+ and NO3
-. The Vmax of both the NH4
+ and NO3
- HATS increased as a function of time after defoliation, and both HATS systems were saturated by substrate concentrations
in the soil at all times. The capacity of the LATS was reduced as soil NO3
- and NH4
+ concentrations decreased following defoliation. Data from 15N uptake by field-grown plants, and uptake rates of NH4
+ and NO3
- estimated by excised root bioassays, were significantly correlated, though uptake was over-estimated by the later method.
The results are discussed in terms of putative mechanisms for regulating N uptake following severe defoliation.
This revised version was published online in June 2006 with corrections to the Cover Date. |