Changes in Whole-Plant Metabolism during the Grain-Filling Stage in Sorghum Grown under Elevated CO2 and Drought

Abstract

Projections indicate an elevation of the atmospheric CO2 concentration ([CO2]) concomitant with an intensification of drought for this century, increasing the challenges to food security. On the one hand, drought is a main environmental factor responsible for decreasing crop productivity and grain quality, especially when occurring during the grain-filling stage. On the other hand, elevated [CO2] is predicted to mitigate some of the negative effects of drought. Sorghum (Sorghum bicolor) is a C4 grass that has important economical and nutritional values in many parts of the world. Although the impact of elevated [CO2] and drought in photosynthesis and growth has been well documented for sorghum, the effects of the combination of these two environmental factors on plant metabolism have yet to be determined. To address this question, sorghum plants (cv BRS 330) were grown and monitored at ambient (400 µmol mol(-1)) or elevated (800 µmol mol(-1)) [CO2] for 120 d and subjected to drought during the grain-filling stage. Leaf photosynthesis, respiration, and stomatal conductance were measured at 90 and 120 d after planting, and plant organs (leaves, culm, roots, prop roots, and grains) were harvested. Finally, biochemical composition and intracellular metabolites were assessed for each organ. As expected, elevated [CO2] reduced the stomatal conductance, which preserved soil moisture and plant fitness under drought. Interestingly, the whole-plant metabolism was adjusted and protein content in grains was improved by 60% in sorghum grown under elevated [CO2].

Figure 1.

A, OTCs used during the experiment to grow sorghum ‘BRS 330’ at ambient and elevated CO₂. B, Experimental design.

Figure 2.

Soil moisture (cm³ water cm⁻³ soil) during the experiment with sorghum ‘BRS 330’ at ambient and elevated CO₂. n = 8. Water deficit treatment started at 60 d after planting (DAP). From 90 to 93 DAP, soil moisture sensors from pots at elevated CO₂ did not log the data (gray dashed line).

Figure 3.

Light-saturated rate of leaf photosynthesis (A; A), Stomatal conductance (g_s; B), and dark leaf respiration (Rd; C) of the youngest fully expanded leaf of sorghum ‘BRS 330’ at ambient and elevated CO₂ at 90 and 120 DAP. Bars represent means ± sd of biological replicates. n = 3. The asterisk indicates significantly statistical differences between treatments (P < 0.05).

Figure 4.

Plant height without panicle (A) and panicle size (B) of sorghum ‘BRS 330’ at ambient and elevated CO₂ during the experiment. Data points are means ± sd of biological replicates. n = 3. Asterisks indicate significant statistical differences between treatments (P < 0.05).

Figure 5.

PCA of the metabolites identified by LC-MS/MS of sorghum ‘BRS 330’ at ambient (open symbols) and elevated (closed symbols) CO₂ at 90 DAP (squares) and 120 DAP (circles). P values indicate significant statistical differences of the PCs.

Figure 6.

PCA of the metabolites identified by LC-MS/MS of leaves, culm, roots, prop roots, and grains of sorghum ‘BRS 330’ at ambient and elevated CO₂ at 90 and 120 DAP. P < 0.05 indicates significant statistical differences of the PCs.

Figure 7.

Heat maps showing the variation in the amount of metabolites in sorghum ‘BRS 330’ grown under ambient and elevated [CO₂] at 90 and 120 DAP in leaves (A), culm (B), roots (C), prop roots (D), and grains (E). 2/3PGA, 2- or 3-phosphoglycerate; 6PG, 6-phosphogluconate; AKG, α-ketoglutarate; cis-aco, cis-aconitate; Deoxyxyl5P, deoxyxylulose-5-phosphate; E4P, erythrose-4-phosphate; F1,6bP, Fru-1,6-bisP; Fru6P, Fru-6-P; GABA, γ-aminobutyric acid; Gal1p, Gal-1-P; Glc6P, Glc-6-P; GlycerolP, glycerol-3-phosphate; Man6P, Man-6-P; Man/Gal-ol, mannitol or galactitol; Man/Glc1P, Man- or Glc-1-P; OHPro, Hyp; Pentoses P, pentose-phosphates; PEP, phosphoenolpyruvate; Ribul1,5bP, ribulose-1,5-bisphosphate; S7P, sedoheptulose-7-phosphate; Suc6P, Suc-6-P; trans-aco, trans-aconitate; Tre6P, trehalose-6-phosphate.