Patterns of plant biomass partitioning depend on nitrogen source

Abstract

Nitrogen (N) availability is a strong determinant of plant biomass partitioning, but the role of different N sources in this process is unknown. Plants inhabiting low productivity ecosystems typically partition a large share of total biomass to belowground structures. In these systems, organic N may often dominate plant available N. With increasing productivity, plant biomass partitioning shifts to aboveground structures, along with a shift in available N to inorganic forms of N. We tested the hypothesis that the form of N taken up by plants is an important determinant of plant biomass partitioning by cultivating Arabidopsis thaliana on different N source mixtures. Plants grown on different N mixtures were similar in size, but those supplied with organic N displayed a significantly greater root fraction. ¹⁵N labelling suggested that, in this case, a larger share of absorbed organic N was retained in roots and split-root experiments suggested this may depend on a direct incorporation of absorbed amino acid N into roots. These results suggest the form of N acquired affects plant biomass partitioning and adds new information on the interaction between N and biomass partitioning in plants.

Figure 1. Biomass (a) and fraction of biomass in roots (b) of Arabidopsis thaliana grown on either NO₃ ⁻ or on NH₄NO₃ or on different combinations of glutamine and NO₃ ⁻.

All media had a total N concentration of 6 mM. Plants were grown on sterile agar plates for 21 days. Bars represent average values ± SE, n = 8. Different lower-case letters indicate differences at p≤0.05 between N treatments.

Figure 2. Origin of root N, shoot N and plant N, in Arabidopsis thaliana plants grown on 3 mM NH₄NO₃ (a) or a mixture of 1.5 mM glutamine+3 mM NO₃ ⁻ (b).

Fractions of N derived from individual N sources in the mixtures were calculated from N contents and rates of ¹⁵N abundance in plant parts. Plants were grown on sterile agar plates for 21 days. Bars represent average values ± SE, n = 5. Different lower-case and capital letters indicate differences at p≤0.05 between plant parts and between N sources, respectively.

Figure 3. Split-root experiment with Arabidopsis thaliana.

Plants were grown on agar plates that were divided into two identical compartments by a plastic rib. The growth medium was identical on both sides of the rib and with N supplied as a mixture of 1.5 mM glutamine+3 mM NO₃ ⁻ but on one side, one of the N sources (either glutamine or NO₃ ⁻) was ¹⁵N-labelled. Bars indicate the fraction of N derived from each source and represent average ± SE, n = 6–7. Different lower-case and capital letters indicate differences at p≤0.05 between plants parts and between N sources, respectively.

Figure 4. Split-root experiment with Arabidopsis thaliana.

Plants were grown on agar plates that were divided into two identical compartments by a plastic rib. The two compartments contained either 1.5 mM glutamine or 3 mM NO₃ ⁻ as N sources. For each plate, one of the N sources (either glutamine or NO₃ ⁻) was ¹⁵N-labelled. Bars indicate the fraction of N derived from each source for the shoot and for roots growing in the NO₃ ⁻ compartment and the glutamine compartment. Bars represent average ± SE, n = 5. Different lower-case and capital letters indicate differences at p≤0.05 between plant parts, and between N sources, respectively.