Effects of Severity of Defoliation on Root Functioning in Grasses

Abstract

Grass shoots after defoliation can be supplied with the nitrogen required for regrowth by either root uptake or remobilization of stores. Whilst it is accepted that after a single defoliation inhibition of root uptake and remobilization from root occurs, it has not been established how the capability of roots to supply nitrogen by uptake and from storage is affected with differing severities of regular defoliation, as experienced by grazed swards. The objective was to examine this question using Agrostis castellana Boiss et Reut., Festuca rubra L., Lolium perenne L. and Poa trivialis L., grasses associated with sites of differing fertility, grown in sand culture and defoliated weekly at a height of either 4 or 8 cm. Nitrogen was supplied as NH4NO3 in a complete nutrient solution. The use of 15N as a tracer allowed the nitrogen supplied to the shoot by root uptake and remobilization to be discriminated over a 35 day period. An increased severity of defoliation resulted in decreased root mass, and increased nitrogen uptake per unit root weight for all species. Increased severity of defoliation did not affect uptake on a per plant basis for A. castellana, 0.54 mg N (plant)-1 (week)-1 and P. rubra, 0.40 mg N (plant)-1 (week)-1, whilst mg N (plant)-1(week)-1 decreased from 0.54 to 0.14, and 0.54 to 0.34 for L perenne and P. trivialis respectively. For plants clipped at 4 or 8 cm, over 88% and 77% respectively of uptake appeared in the shoot. Nitrogen was remobilized from roots to the shoot for A. castellana and F. rubra when clipped at 4 cm, and for A. castellana, L. perenne and P. trivialis when clipped at 8 cm. Uptake by roots was more important than remobilization from roots in supplying nitrogen to the shoot. The ability to maintain the supply of nitrogen by uptake and remobilization to the shoot with increased severity of defoliation was species dependent.