Nitrogen fertilization form and energetic status as target points conditioning rice responsiveness to elevated [CO2]

Abstract

The nitrogen (N) fertilization form and plant energy status are known to significantly influence plant responses to elevated atmospheric carbon dioxide (CO₂) concentrations. However, a close examination of the interplay between N sources under contrasting light intensity has been notably absent in the literature. In this study, we conducted a factorial experiment with rice plants involving two different light intensities (150 and 300 µmol m^-2 s^-1), inorganic N sources [nitrate (N-NO₃) or ammonium nitrate (N-NH₄NO₃)] at varying CO₂ levels (410 and 700 parts per million, ppm). The aim was to examine the individual and combined effects of these factors on the allocation of biomass in whole plants, as well as on leaf-level photosynthetic characteristics, chloroplast morphology and development, ATP content, ionomics, metabolomics, and hormone profiles. Our research hypothesis posits that mixed nutrition enhances plant responsiveness to elevated CO₂ (eCO₂) at both light levels compared to sole N-NO₃ nutrition, due to its diminished energy demands for plant assimilation. Our findings indicate that N-NO₃ nutrition does not promote the growth of rice, its photosynthetic capacity, or N content when exposed to ambient CO₂ (aCO₂), and is significantly reduced in low light (LL) conditions. Rice plants with N-NH₄NO₃ exhibited a higher carboxylation capacity, which resulted in larger biomass (total C, tiller number, and lower root-shoot ratio) supported by higher Calvin-cycle-related sugars. The lower leaf N content and overall amino acid levels at eCO₂, particularly pronounced in N-NO₃, combined with the lower ATP content (lowest at LL and N-NO₃), may reflect the higher energy costs of N assimilation at eCO₂. We also observed significant plasticity patterns in leaves under eCO₂. Our findings highlight the importance of a thorough physiological understanding to inform innovative management practices aimed at mitigating the negative effects of climate change on plant N use efficiency.

Keywords: ATP; elevated CO2; low light intensity; nitrate; nitrogen source; photosynthesis; plasticity; rice.

Figure 1

Represents the combined effect of nitrogen (N) form, irradiance, and CO2 concentration on rice plant growth. (A) shows representative images of rice plants grown under different combinations of ninitrate (NO3) or ammonium nitrate (NH4NO3), at two light intensities (150 µmol m^-2 s^-1 , white bars; or 300 μmol m^-2 s^-1, grey bars) and under ambient (aCO2) or elevated (eCO2) CO2 concentrations. (B, C) quantify the fresh weight (FW) and dry matter (DM), respectively, of the above-ground tissues (shoots) of these plants; similarly, (D, E) present the FW and DM of the root tissues. Bar graphs within (B–E) display means ± SD derived from 9 replicates. The lowercase letters indicate statistically significant differences within the LL (Low Light) treatment, while the uppercase letters denote significant differences within the CL (control Light) treatment.

Figure 2

Represents the combined effect of N form, irradiance, and CO2 concentration on rice plant growth parameters. (A) A displays the plant height (cm) under various conditions, while (B) shows root length (cm). (C) represents the number of leaves per plant. In all panels, rice plants were grown with either nitrate (NO3) or ammonium nitrate (NH4NO3), at two light intensities (150 and 300 μmol m^-2 s^-1) represented by white and gray bars respectively, and under either ambient (aCO2) or elevated (eCO2) CO2 conditions. Bar graphs within (A-C) display means ± SD derived from 9 replicates. The lowercase letters indicate statistically significant differences within the LL (Low Light) treatment, while the uppercase letters denote significant differences within the CL (control Light) treatment.

Figure 3

Represents the combined effect of N form, irradiance, and CO² concentration on leaf length of rice plants. (A) shows representative images of the top, second, and third leaves of rice plants grown under different conditions. (B-D) present the length (cm) of the top, second, and third leaves respectively. In all panels, rice plants were grown with either nitrate (NO³) or ammonium nitrate (NH⁴ NO³), at two light intensities (150 and 300 μmol m^-2 s^-1) represented by white and gray bars respectively, and under either ambient (aCO²) or elevated (eCO²) CO² conditions. Bar graphs within (A-C) display means ± SD derived from 9 replicates. The lowercase letters indicate statistically significant differences within the LL (Low Light) treatment, while the uppercase letters denote significant differences within the CL (control Light) treatment.

Figure 4

Represents the combined effect of N form, irradiance, and CO₂ concentration on leaf length of rice plants. (A, B) shows assimilation rate (Asat) at growing CO₂. (C, D) the maximum carboxylation velocity (Vmax) and themaximum electron transport rate (Jmax) repectively. (E) represents adenosine triphosphate (ATP) levels. In all panels, rice plants were grown with either nitrate (NO₃) or ammonium nitrate (NH₄NO₃), at two light intensities (150 and 300 μmol m^-2 s^-1) represented by white and gray bars respectively, and under either ambient (aCO₂) or elevated (eCO₂) CO₂ conditions. Bar graphs within (A-D) display means ± SD derived from 5replicates while from ATP was derived from 3-4 replicates. The lowercase letters indicate statistically significant differences within the LL (Low Light) treatment, while the uppercase letters denote significant differences within the CL (control Light) treatment.

Figure 5

Illustrates the impact of various N forms (NO₃ or NH₄NO₃) on selected leaf minerals (A–H) and phytohormones (I) contents of rice plants grown under contrasting irradiances (150 µmol m⁻² s⁻¹ or 300 mmol m⁻² s⁻¹) and cultivated in either ambient (aCO₂) or elevated (eCO₂) CO₂ conditions. The values presented are means ± SD derived from 3 replicates.

Figure 6

Illustrates the impact of various nitrogen forms (nitrate or ammonium nitrate) on selected leaf metabolites varying between treatments using (A) Venn diagram, (B) heatmap, and (C–E) the top 25 varying metabolites of rice plants grown under irradiances of 150 µmol m^-2 s^-1 (depicted by white bars) and 300 µmol m^-2 s^-1 (represented by gray bars), and cultivated in either ambient (aCO₂) or elevated (eCO₂) CO₂ conditions. The values presented are means ± SD derived from 3 replicates.