Abstract
The effects of elevated CO2 (750 ppm vs. 390 ppm) were evaluated on nitrogen (N) acquisition and assimilation by three Medicago truncatula genotypes, including two N-fixing-deficient mutants (dnf1-1 and dnf1-2) and their wild-type (Jemalong). The proportion of N acquisition from atmosphere and soil were quantified by 15N stable isotope, and N transportation and assimilation-related genes and enzymes were determined by qPCR and biochemical analysis. Elevated CO2 decreased nitrate uptake from soil in all three plant genotypes by down-regulating nitrate reductase (NR), nitrate transporter NRT1.1 and NR activity. Jemalong plant, however, produced more nodules, up-regulated N-fixation-related genes and enhanced percentage of N derived from fixation (%Ndf) to increase foliar N concentration and N content in whole plant (Ntotal Yield) to satisfy the requirement of larger biomass under elevated CO2. In contrast, both dnf1 mutants deficient in N fixation consequently decreased activity of glutamine synthetase/glutamate synthase (GS/GOGAT) and N concentration under elevated CO2. Our results suggest that elevated CO2 is likely to modify N acquisition of M. truncatula by simultaneously increasing N fixation and reducing nitrate uptake from soil. We propose that elevated CO2 causes legumes to rely more on N fixation than on N uptake from soil to satisfy N requirements.
Highlights
Global atmospheric CO2 concentrations have been increasing at an accelerating rate [1]
The dilution hypothesis, which has received the most attention, considers that N content is diluted under elevated CO2 by accumulation of more total non-structural carbohydrates (TNC), which results in a greater biomass for a given quantity of N [8]
The notion that elevated CO2 can increase plant biomass and TNC content in plant tissues is widely accepted [8]. This concept was further supported by the current report, our results indicate that the key element in the increase in biomass of M. truncatula under elevated CO2 is the availability of N (Fig. S2)
Summary
Global atmospheric CO2 concentrations have been increasing at an accelerating rate [1]. The effects of elevated CO2 on C3 plants are generally characterized by increased photosynthesis, growth and yield in plant tissues [2]. Besides increases in biomass and productivity, a common characteristic of non-leguminous C3 plants in an elevated CO2 environment is a 10–15% decrease in N concentration (g of N per g of plant tissue ) [4]. According to the reduced uptake hypothesis, N content is reduced because decreased stomatal conductance and transpiration under elevated CO2 reduces N uptake by roots [6]. The N loss hypothesis presumes that N losses increase under elevated CO2 because of increasing NH3 volatilization or increasing root exudation of organic N [7]. The dilution hypothesis, which has received the most attention, considers that N content is diluted under elevated CO2 by accumulation of more total non-structural carbohydrates (TNC), which results in a greater biomass for a given quantity of N [8]
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