Triose phosphate utilization in leaves is modulated by whole-plant sink-source ratios and nitrogen budgets in rice

Abstract

Triose phosphate utilization (TPU) is a biochemical process indicating carbon sink-source (im)balance within leaves. When TPU limits leaf photosynthesis, photorespiration-associated amino acid exports probably provide an additional carbon outlet and increase leaf CO2 uptake. However, whether TPU is modulated by whole-plant sink-source relations and nitrogen (N) budgets remains unclear. We address this question by model analyses of gas-exchange data measured on leaves at three growth stages of rice plants grown at two N levels. Sink-source ratio was manipulated by panicle pruning, by using yellower-leaf variant genotypes, and by measuring photosynthesis on adaxial and abaxial leaf sides. Across all these treatments, higher leaf N content resulted in the occurrence of TPU limitation at lower intercellular CO2 concentrations. Photorespiration-associated amino acid export was greater in high-N leaves, but was smaller in yellower-leaf genotypes, panicle-pruned plants, and for abaxial measurement. The feedback inhibition of panicle pruning on rates of TPU was not always observed, presumably because panicle pruning blocked N remobilization from leaves to grains and the increased leaf N content masked feedback inhibition. The leaf-level TPU limitation was thus modulated by whole-plant sink-source relations and N budgets during rice grain filling, suggesting a close link between within-leaf and whole-plant sink limitations.

Keywords: Oryza sativa; Adaxial versus abaxial measurement; panicle pruning; photorespiration-associated nitrogen assimilation; sink limitation; triose phosphate utilization; yellower-leaf modification.

Fig. 1.

Effects of adaxial versus abaxial measurement on the photosynthetic parameters in four rice genotypes (data measured at the tillering stage in the 2019 experiment). (A–D) Light-response curves of photosynthesis (A) at the CO₂ concentration of 400 μmol mol^─1. (E–H) CO₂-response curves at the light intensity of 1000 μmol m^─2 s^─1, for rice control (C) genotypes (circles) and their yellower-leaf (Y) variant genotypes (triangles). Data shown as the mean of four replicates (±SE) for each genotype, with black symbols representing measurement on adaxial surface of leaves and red symbols representing measurement on abaxial surface of leaves. WYJ and ZF are the abbreviations of two genetic backgrounds Wuyunjing 3 and Zhefu 802, respectively. For (A–D), the curves are drawn from Equation 6 using fitted parameter values. The estimated maximum linear electron transport under saturating light (J_max, μmol m^─2 s^─1), net photosynthesis rate under light intensity of 2000 μmol m^─2 s^─1 (A₂₀₀₀, μmol m^─2 s^─1), and the percentage difference in the J_max and A₂₀₀₀ (calculated as [(Adaxial−Abaxial)/Adaxial]×100) are listed. For (E–H), the curves representing A_c- (dashed curve), A_j- (dotted curve), and A_p-limited (full curve) parts are drawn from Equation 6 using fitted values of the parameters: the estimated maximum rate of Rubisco carboxylation (V_cmax, µmol m^─2 s^─1), rate of triose phosphate utilization (T_p, µmol m^─2 s^─1), and the proportion of glycolate carbon exported from the photorespiratory pathway in the form of serine (α_S). The different letters indicate statistical significance at the P<0.05 level for the estimated parameters between adaxial and abaxial measurements, and the asterisks represent significant differences (P<0.05) between C genotype and its Y variant.

Fig. 2.

Effects of altered sink–source ratios on parameter T_p (the 2022 experiment). (A–D) The rate of triose phosphate utilization (T_p, based on measurements on the adaxial leaf surface) for rice control (C) genotypes and their yellower-leaf (Y) variant genotypes of intact or panicle-pruned plants at tillering (TS), flowering (FS), and 15 days after flowering (DAF) stages under low-nitrogen (N1, white bars) and high-nitrogen (N2, dashed bars) levels. The value of each bar representing the mean ±SE of four replicates was estimated by fitting curves to CO₂ exchange data (see Supplementary Fig. S2). For intact plants (no pruning), different letters indicate statistical significance at the P<0.05 level between three stages within each genotype–nitrogen combination, and the asterisks in black represent significant differences (P<0.05) between N1 and N2 levels within each genotype and stage. The asterisk in red represents significant difference (P<0.05) for a given genotype–nitrogen combination between pruned and un-pruned plants at 15 DAF stage. WYJ and ZF are the abbreviations of two genetic backgrounds: cv. Wuyunjing 3 and cv. Zhefu 802. (E) Comparisons of the values of T_p measured at adaxial surface versus those measured at abaxial surface. The percentage is the average of adaxial relative to abaxial parameters and the diagonal line is the 1:1 line. Data represented by different colours and symbols are from C genotypes (circles) and Y-variant genotypes (triangles) of intact (open symbols) and panicle-pruned (filled symbols) plants at TS (blue), FS (red), and 15 DAF (green) stage. Each point represents the mean of three or four replicates.

Fig. 3.

Effects of various growth stages and nitrogen levels on parameter α_S, the proportion of glycolate carbon exported from the photorespiratory pathway in the form of serine (the 2022 experiment). (A–D) α_S (based on measurements on the adaxial leaf surface) for rice control (C) genotypes and their yellower-leaf (Y) variant genotypes of intact or panicle-pruned plants at tillering (TS), flowering (FS), and 15 days after flowering (DAF) stages under low-nitrogen (N1, white bars) and high-nitrogen (N2, dashed bars) levels. The value of each bar representing the mean ±SE of four replicates was estimated by fitting curves to CO₂ exchange data (see Supplementary Fig. S2). For intact plants (no pruning), different letters indicate statistical significance at the P<0.05 level between three stages within each genotype–nitrogen combination, and the asterisks in black represent significant differences (P<0.05) between N1 and N2 levels within each genotype and stage. The asterisks in red represent significant differences (P<0.05) for a given genotype-nitrogen combination between pruned and un-pruned plants at 15 DAF stage. WYJ and ZF are the abbreviations of two genetic backgrounds: cv. Wuyunjing 3 and cv. Zhefu 802. (E) Comparison of the values of α_S measured at adaxial surface versus those measured at abaxial surface. The percentage is the average of adaxial relative to abaxial parameters and the diagonal line is the 1:1 line. Data represented by different colours and symbols are from C genotypes (circles) and Y-variant genotypes (triangles) of intact (open symbols) and panicle-pruned (filled symbols) plants at TS (blue), FS (red), and 15 DAF (green) stage. Each point represents the mean of three or four replicates.

Fig. 4.

Relationship between photosynthetic parameters and leaf nitrogen content (based on measurements on the un-pruned plants in the 2022 experiment). (A–H) Relationship between triose phosphate utilization rate (T_p) and specific nitrogen content (SLN). (I–P) Relationship between the proportion of glycolate carbon exported from photorespiratory pathway in the form of serine (α_S) and SLN. Data represented by different colours and symbols are from tillering (TS, blue), flowering (FS, red), and 15 days after flowering (DAF) (green) stage under low-nitrogen (N1, open symbols) and high-nitrogen (N2, filled symbols) levels, with circles for rice control (C) genotypes and triangles for their yellower-leaf (Y) variant genotypes. Linear regressions were fitted for each genotype with four or five replicates across two nitrogen levels and three stages. The significance of each correlation is shown by asterisks: *P<0.05, **P<0.01, ***P<0.001. WYJ and ZF are the abbreviations of two genetic backgrounds: cv. Wuyunjing 3 and cv. Zhefu 802.

Fig. 5.

The threshold C_i values in relation to leaf physiological parameters. (A) Relationship between threshold C_i and the rate of triose phosphate utilization (T_p) (data based on measurements on the adaxial surface of leaves in the 2022 experiment). (B) Relationship between threshold C_i and specific leaf nitrogen (SLN). The threshold C_i represents the transition point where the photosynthesis-limiting process changed from electron transport to TPU, derived from CO₂ response curves (see Supplementary Fig. S2). Data represented by different colours and symbols are from tillering (TS, blue), flowering (FS, red), and 15 days after flowering (DAF) (green) stage under low-nitrogen (N1, open symbols) and high-nitrogen (N2, filled symbols) levels, with circles for rice control (C) genotypes and triangles for their yellower-leaf (Y) variant genotypes. Linear regressions were fitted for overall data with the significance of each correlation shown by asterisks: ***P<0.001.

Fig. 6.

Relationship between the rate of triose phosphate utilization (T_p, based on measurements on the adaxial leaf surface) and single-culm sink–source ratio. Here, following Fabre et al. (2020), the ratio of flag leaf area (source) to the fertile spikelet number of the panicle (sink) on the culm was used as an indicator of the single-culm sink–source ratio (also see the text). Data are for rice control (C) genotypes (circles) and yellower-leaf (Y) variant genotypes (triangles) from grain-filling stage under low-nitrogen (N1, open symbols) and high-nitrogen (N2, filled symbols) levels in the 2022 experiment, with cv. Wuyunjing 3 (WYJ) in black and cv. Zhefu 802 (ZF) in red. For those plants with panicle pruning, we define their sink–source ratio to be zero, so all their data points fall on the y-axis. Linear regression was fitted for data (representing no pruning) with the significance of the correlation shown by asterisks: ***P<0.001.

Fig. 7.

The effects of panicle pruning on leaf photosynthetic parameters (based on measurements on the adaxial leaf surface in the 2022 experiment). (A–D) Relationship between the rate of triose phosphate utilization (T_p) and specific leaf nitrogen (SLN) for the intact and panicle-pruned plants. (E–H) Effect of panicle pruning on SLN. (I–M) Leaf photosynthetic nitrogen-use efficiency (PNUE) at tillering (TS), flowering (FS), and 15 days after flowering (DAF) stage (see Supplementary Table S2 for the definition of PNUE). Linear regressions in (A–D) were fitted for each genotype with four or five replicates under two nitrogen levels; the significance of each correlation is shown by asterisks: *P<0.05, **P<0.01, ***P<0.001. Data in (E–M) represent the mean ±SE of four replicates; the asterisks represent significant differences (P<0.05) within each genotype–nitrogen combination between unpruned and pruned plants. The data in (A–H) are from the 15 DAF stage, and the data in (A–D) and (I–M) represent the values for rice control (C) genotypes (circles) and their yellower-leaf (Y) variant genotypes (triangles) of the intact (black) and panicle-pruned (red) plants under low-nitrogen (N1, open symbols) and high-nitrogen (N2, filled symbols) levels. WYJ and ZF are the abbreviations of two genetic backgrounds: cv. Wuyunjing 3 and cv. Zhefu 802.