Plant organic nitrogen nutrition: costs, benefits, and carbon use efficiency

Abstract

Differences in soil mobility and assimilation costs between organic and inorganic nitrogen (N) compounds would hypothetically induce plant phenotypic plasticity to optimize acquisition of, and performance on, the different N forms. Here we evaluated this hypothesis experimentally and theoretically. We grew Arabidopsis in split-root setups combined with stable isotope labelling to study uptake and distribution of carbon (C) and N from l-glutamine (l-gln) and NO₃ ^- and assessed the effect of the N source on biomass partitioning and carbon use efficiency (CUE). Analyses of stable isotopes showed that 40-48% of C acquired from l-gln resided in plants, contributing 7-8% to total C of both shoots and roots. Plants grown on l-gln exhibited increased root mass fraction and root hair length and a significantly lower N uptake rate per unit root biomass but displayed significantly enhanced CUE. Our data suggests that organic N nutrition is linked to a particular phenotype with extensive growth of roots and root hairs that optimizes for uptake of less mobile N forms. Increased CUE and lower N uptake per unit root growth may be key facets linked to the organic N phenotype.

Keywords: Arabidopsis thaliana; amino acids; carbon use efficiency; glutamine; organic nitrogen; root hair.

Fig. 1

Setup of experiment 1. Shoots were positioned on the middle rib of the plate and roots were divided equally between two growth compartments in which N was supplied as either: (a) 1.5 mM l‐gln on both sides, (b) as 3 mM NO₃ ⁻ on both sides or (c) as 1.5 mM l‐gln on one side and 3 mM NO₃ ⁻ on the other side. Results shown in Figs 3, 4, 5 and in Table 1 are derived from this experimental system.

Fig. 2

Setup of experiment 2. ¹³C, ¹⁵N labelling to study uptake of C and N from l‐gln. Seedlings precultivated on vertical plates and moved to plates with air flow. N was supplied as (a) 1.5 mM U¹⁵N₂ ¹³C₅ – l‐gln (10 atom% enrichment) or on (b) 1.5 mM U¹⁵N₂ ¹³C₅ – l‐gln + 3 mM NO₃ ⁻. Control seedlings grown on (c) nonlabelled l‐gln or on (d) nonlabelled l‐gln + NO₃ ⁻, to account for re‐fixation of respired ¹³CO₂ were included in each plate. Results shown in Figs 6, 7, 8 and Supporting Information Figs S1 and S2 are derived from this experimental system.

Fig. 3

Shoot and root biomass of Arabidopsis thaliana plants grown on axenic split‐root systems. Roots were divided equally between two growth compartments containing agar media with N administered either as 3 mM nitrate in both root compartments (NO₃ ⁻/NO₃ ⁻), n = 5, as 3 mM nitrate in one of the root compartments and 1.5 mM l‐gln in the other compartment (l‐gln/NO₃ ⁻), n = 10, or as 1.5 mM l‐gln (l‐gln/l‐gln) in both root compartments, n = 5. Green, upper part of the bars correspond to shoots, light grey, middle part of the bars to l‐gln root compartment in the NO₃ ⁻/l‐gln treatment and lower grey part of the bars correspond to roots in the NO₃ ⁻ compartment in the l‐gln/NO₃ ⁻ treatment. Bars represent average ± SE. Statistical significance was calculated using one‐way ANOVA and Tukey post hoc test. Different lower‐case and upper‐case letters indicate significant differences at P‐value ≤ 0.05 in shoot and root biomass between treatments, respectively. The * indicates a statistical difference in root biomass between root compartments in the l‐gln/NO₃ ⁻ treatment. DW, dry weight.

Fig. 4

Root systems of Arabidopsis thaliana plants grown in split‐root systems with N supplied as (a) 1.5 mM l‐gln supplied on both sides (l‐gln/l‐gln), (b) 3 mM NO₃ ⁻ on both sides (NO₃ ⁻/NO₃ ⁻), as or as N supplied as (c) 1.5 mM l‐gln on one side and supplied (d) 3 mM NO₃ ⁻ on the other side. Pictures were taken using a Leica DC300 digital camera coupled to a Leica MZ9₅ stereomicroscope.

Fig. 5

Root hair length of Arabidopsis thaliana plants with N supplied as 1.5 mM l‐gln supplied on both sides (l‐gln/l‐gln), 3 mM NO₃ ⁻ on both sides (NO₃ ⁻/NO₃ ⁻), as or as N supplied as 3 mM NO₃ ⁻ on one side and 1.5 mM l‐gln supplied on the other side. Pictures of roots from 18 to 20 plants of each treatment were analysed and root hair length was measured using the program ImageJ. Values indicate average ± SE (n = 18–20). Statistical significance was calculated using one‐way ANOVA and Tukey post hoc test. Different capital letters indicate statistical differences at P‐value ≤ 0.05 in root hair length between N treatments.

Fig. 6

Regression analysis of excess ¹³C vs excess ¹⁵N content in Arabidopsis thaliana plants grown on 1.5 mM U¹⁵N₂U¹³C₅‐l‐gln (10 atom%; a, c) or a mixture of 0.75 mM, U¹⁵N₂U¹³C₅‐l‐gln (10 atom%) and 1.5 mM nitrate (nonlabelled; b, d). Dotted lines with slope 2.5 indicate theoretical regressions corresponding to all ¹³C acquired through uptake of l‐gln remaining in tissues. Regression equations (a) y = 1.017x + 0.015 (R ² = 0.99); (b) y = 1.062x + 0.03 (R ² = 1.0); (c) y = 1.075x + 0.03 (R ² = 0.99); (d) 1.016 + 0.012 (R ² = 1.0). Slopes correspond to (a) 41%; (b) 42%; (c) 43% and (d) 41% of carbon derived from uptake of l‐gln remaining in plant biomass.

Fig. 7

Carbon use efficiency (CUE) modelled based on N assimilation costs vs observed CUE for Arabidopsis thaliana plants grown on 1.5 mM l‐gln (blue symbols) or mixed l‐gln and NO₃ ⁻ (0.75 and 1.5 mM, respectively; red symbols). The growing times were 1 d (circles), 3 d (triangles) and 6 d (squares). (a) All observations, R ² = 0.66, (b) only observations at day 6, R ² = 0.89. The shaded areas indicate a 95% confidence band of the mean.

Fig. 8

N uptake vs root mass for Arabidopsis thaliana plants grown on 1.5 mM l‐gln (blue symbols) or mixed l‐gln and NO₃ ⁻ (0.75 and 1.5 mM, respectively; red symbols), for 1 d (circles), 3 d (triangles) and 6 d (squares). The shaded area indicates a 95% confidence band of the mean.