Elevated carbon dioxide and chronic warming decrease nitrogen (N) uptake and levels of N-uptake and N-assimilatory proteins in tomato provided different forms of inorganic N

Background/Question/Methods

Atmospheric CO₂ enrichment is expected to benefit plant growth, despite causing global warming and nitrogen (N) dilution in plants. Global warming is expected to increase both chronic and acute heat stress in plants. Most plants procure most of their N as inorganic nitrate (NO₃^-) or ammonium (NH₄⁺), using membrane-localized transport proteins in roots, which are key targets for improving N-use efficiency. Though interactive effects of elevated CO₂, chronic warming, and N form on N relations are expected, these have not been studied. In this study, tomato (Solanum lycopersicum) plants were grown at two levels of CO₂ (ambient=400 ppm vs. elevated=700 ppm) and under two temperature regimes (30^oC vs. 37^oC), while providing either NO₃^- (1.5 mM) or NH₄⁺ (1.5 mM) as the sole N source. To assess plant N relations under these conditions, plant biomass, %N, root N-uptake rate, root total-protein concentration, and the concentration of N-uptake and N-assimilatory proteins in roots were measured.

Results/Conclusions

Elevated CO₂ and chronic warming acted synergistically to severely inhibit the growth of plants, regardless of N form, while individually, these factors slightly increased growth. Although %N in roots among all treatments was similar, the combination of elevated CO₂ and warming decreased (1) the rate of N uptake by roots, (2) the concentration of total protein in roots, indicating an inhibition of N assimilation (especially in NH₄⁺-treated plants), and (3) shoot %N, indicating a possible negative effect on N translocation from roots to shoots (especially in NO₃^--treated plants). Under elevated CO₂ and chronic warming, the decrease in N uptake rate was concomitant with a decrease in the concentration of N-uptake proteins, while inhibition of N assimilation was correlated with reduced concentration of N-assimilatory proteins. Together, these results indicate that elevated CO₂ and chronic warming will act synergistically to decrease plant N uptake and assimilation through reduction of N-uptake and N-assimilation protein concentrations, respectively, thereby decreasing growth and total protein concentration. Hence, future global warming with increasing CO₂ may potentially decrease both crop production and its food quality.