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

• Published in: Plant and soil 2000-01, Vol.220 (1-2), p.175-187
• Main Authors: LOUAHLIA, S, LAINE, P, THORNTON, B, OURRY, A, BOUCAUD, J
• Format: Article
• Language: English
• Subjects: Absorption. Translocation of ions and substances. Permeability; Agronomy. Soil science and plant productions; Bioassays; Biological and medical sciences; Biomass; Defoliation; Economic plant physiology; Fundamental and applied biological sciences. Psychology; Life Sciences; Lolium perenne; Nitrogen; Nutrition. Photosynthesis. Respiration. Metabolism; Plant growth; Shoots; Soils; Vegetal Biology
• ISSN: 0032-079X; 1573-5036
• DOI: 10.1023/a:1004728327955

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 N^sub 2^ 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, NH^sub 4^ ^sup +^ uptake and NO^sub 3^ ^sup -^ 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 NH^sub 4^ ^sup +^ and NO^sub 3^ ^sup -^. 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 NH^sub 4^ ^sup +^ and NO^sub 3^ ^sup -^. The V^sub max^ of both the NH^sub 4^ ^sup +^ and NO^sub 3^ ^sup -^ 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 NO^sub 3^ ^sup -^ and NH^sub 4^ ^sup +^ concentrations decreased following defoliation. Data from ^sup 15^N uptake by field-grown plants, and uptake rates of NH^sub 4^ ^sup +^ and NO^sub 3^ ^sup -^ 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.[PUBLICATION ABSTRACT]