Impact of defoliation intensity and frequency on N uptake and mobilization in Lolium perenne

Abstract

The aim of the study was to evaluate the impact of defoliation intensity, defoliation frequency, and interactions with N supply on N uptake, N mobilization from and N allocation to roots, adult leaves, and growing leaves. Plants of Lolium perenne were grown under two contrasted N regimes. Defoliation intensity treatments consisted of a range of percentage leaf area removal (0, 25, 50, 75, or 100%). These treatments were applied in parallel to a set of plants previously undefoliated, and to a second set of plants which had been defoliated several times at a constant height. A (15)N tracer technique was used to quantify N uptake, mobilization, and allocation over a 7 d period. A significant reduction in plant N uptake was observed with the removal of more than 75% of lamina area, but only with high N supply. As defoliation intensity increased, the amount of N taken up and subsequently allocated to growing leaves during the labelling period was maintained at the expense of N allocation to roots and adult leaves. Increasing defoliation intensity increased the relative contribution of roots supplying mobilized N to growing leaves and decreased the relative contribution of adult leaves. Defoliation frequency did not substantially alter N uptake, mobilization, and allocation between roots, adult and growing leaves on a plant basis. However, tiller number per plant was largely increased under repeated defoliation, hence indicating that allocation and mobilization of N to growing leaves, on the basis of individual tillers, was decreased by defoliation frequency.

Adaptation of the C4 grass Panicum maximum to defoliation is related to plasticity of N uptake, mobilisation and allocation patterns

“…Lestienne et al (2006) observed no effect of final defoliation intensity on labelled N allocated to growing leaves of Lolium perenne, but also found a decrease in labelled N allocated to roots when 100 % leaf area was removed.…”

“…In the present experiment, roots were source of N for mobilisation just when plants were defoliated on day 0 and became a sink for mobilized N when plants initially cut to a 0.30 m height were not subjected to a final defoliation (0 % leaf area removal on day 0). Lestienne et al (2006) observed that roots were a net source of N and that increasing defoliation severity of L. perenne did not change the amount of N mobilised to young leaves, reliant on an increase in the contribution of roots supplying mobilised N to growing leaves. Taken together results of Lestienne et al (2006) and the current study suggest a different effect of defoliation on source/sink relations of P. maximum and L. perenne.…”

“…On the other hand, Lestienne et al (2006) observed an increased specific uptake of N when defoliation severity increased from 0 to 100 % leaf area removal and suggested that changes in root morphology could partially explain these results. These results differ from those observed in the present experiment, where complete defoliation of P. maximum plants reduced specific uptake of N. A negative effect of defoliation on root growth and/or senescence rate may in part explain the reduced specific uptake of N observed on completely defoliated plants, as young roots seems to have an important role on nutrients uptake (Taiz and Zeiger, 2010).…”

“…Labelled N of whole plants was lower when the plants were completely defoliated (100 % final defoliation treatment), probably due to a negative effect of defoliation on root dry mass and specific uptake of N. Additionally, since all leaf area was removed, low soil N flux induced by lower plant transpiration may also have contributed to a reduction of N uptake of plants from the 100 % defoliation treatment (Durand et al, 2010). Lestienne et al (2006) observed a reduction in N uptake of Lolium perenne with the removal of more than 75 % leaf area when N supply was high and suggested this was related to a negative effect of defoliation intensity on root dry mass. In the present experiment, negative effects of complete defoliation on root dry mass of P. maximum plants receiving a high supply of N were also observed.…”

“…Nitrogen availability for regrowth may be reduced after defoliation due to a decrease in the N pool from senescing leaves, lower root growth or a reduction in soil N flux induced by lower crop transpiration (Lestienne et al, 2006;Gastal et al, 2010;Durand et al, 2010). On the other hand, plants can adapt to defoliation intensity by changing their shoot and root morphology and growth rate (Matthew et al, 2000;Gastal et al, 2010) and, consequently, changing both the amount of shoot material and N stores bellow cutting height and the exploitation of the soil by roots for water and nutrients uptake.…”