Elevated CO2 alleviates the negative impact of heat stress on wheat physiology but not on grain yield

Abstract

Hot days are becoming hotter and more frequent, threatening wheat yields worldwide. Developing wheat varieties ready for future climates calls for improved understanding of how elevated CO2 (eCO2) and heat stress (HS) interactively impact wheat yields. We grew a modern, high-yielding wheat cultivar (Scout) at ambient CO2 (aCO2, 419 μl l -1) or eCO2 (654 μl l-1) in a glasshouse maintained at 22/15 °C (day/night). Half of the plants were exposed to HS (40/24 °C) for 5 d at anthesis. In non-HS plants, eCO2 enhanced (+36%) CO2 assimilation rates (Asat) measured at growth CO2 despite down-regulation of photosynthetic capacity. HS reduced Asat (-42%) in aCO2- but not in eCO2-grown plants because eCO2 protected photosynthesis by increasing ribulose bisphosphate regeneration capacity and reducing photochemical damage under HS. eCO2 stimulated biomass (+35%) of all plants and grain yield (+30%) of non-HS plants only. Plant biomass initially decreased following HS but recovered at maturity due to late tillering. HS equally reduced grain yield (-40%) in aCO2- and eCO2-grown plants due to grain abortion and reduced grain filling. While eCO2 mitigated the negative impacts of HS at anthesis on wheat photosynthesis and biomass, grain yield was reduced by HS in both CO2 treatments.

Keywords: Climate change; elevated CO2; grain yield; heat stress; photosynthetic acclimation; temperature response; wheat.

Fig. 1.

Photosynthetic response of wheat cultivar Scout to eCO₂ measured 13 and 17 weeks after planting (WAP) at 25 °C leaf temperature and two CO₂ concentrations. Bar plot of means for light-saturated CO₂ assimilation rate (a, b, and c) and stomatal conductance (d, e, and f) calculated using two-way ANOVA. The error bars indicate the SE of the mean (n=9–10). Ambient and elevated CO₂-grown plants are depicted in black and grey, respectively. Grouping is based on measurement CO₂ (400 μl l⁻¹ or 650 μl l⁻¹). Bars sharing the same letter in the individual panels are not significantly different according to Tukey’s HSD test at the 5% level. Statistical significance levels (t-test) for eCO₂ effect are shown: *P<0.05; **P<0.01: ***P<0.001.

Fig. 2.

In vivo Rubisco properties and temperature response of V_cmax and J_max measured 13 weeks after planting (WAP). Maximum velocity of carboxylation, V_cmax (a), maximum velocity of RuBP regeneration, J_max (b), and J_max/V_cmax ratio (c) determined using the response of CO₂ assimilation to variation in chloroplastic CO₂ (C_c) at five leaf temperatures (15, 20, 25, 30, and 35 °C) in wheat cultivar Scout (n=6). The ratio of J_max/V_cmax (c) is plotted using the visreg package in R. Regression lines are means with 95% confidence intervals. The lower panel is a bar plot showing in vivo V_cmax at 25 °C (n=6) (d) and Rubisco sites (n=5) (e) measured in flag leaf discs harvested at the same time point. For (a), (c), and (d), values are means ±SE. Ambient and elevated CO₂-grown plants are shown in black and grey, respectively.

Fig. 3.

Response of V_cmax and J_max to growth at eCO₂ and heat stress (HS) measured 13 and 17 weeks after planting (WAP) at the recovery stage of the HS cycle. Bar plot of means ±SE for V_cmax (a and b), J_max (c and d), and V_cmax/J_max (e and f) using two-way ANOVA. Leaf gas exchange was measured at 25 °C in ambient (black) and elevated (grey) CO₂-grown plants exposed (HS) or not exposed (Control) to a 5 d HS. Bars sharing the same letter in the individual panels are not significantly different according to Tukey’s HSD test at the 5% level. The error bars indicate the SE of the mean (n=9–10). Statistical significance levels (t-test) for eCO₂ effect are shown: *P<0.05; **P<0.01: ***P<0.001.

Fig. 4.

Photosynthesis and chlorophyll fluorescence response of aCO₂- and eCO₂-grown wheat cv. Scout measured before, during, after, and at the recovery stage of the heat stress cycle. CO₂ assimilation rates (a), of the F_v/F_m ratio in dark-adapted leaves (b), stomatal conductance (c), and of the F_v'/F_m' ratio in light-adapted leaves (d) measured at growth CO₂ (aCO₂-grown plants measured at 400 μl l⁻¹ and eCO₂-grown plants measured at 650 μl l⁻¹). Values are means ±SE (n=9–10). Ambient and elevated CO₂-grown plants are depicted in black and grey, respectively. Filled and open circles represent control and heat-stressed plants, respectively. The circle and star symbols depict CO₂ assimilation rates measured at 25 °C and 35 °C, respectively.

Fig. 5.

Response of biomass and ears (or tillers) to eCO₂ and HS across the life cycle of wheat cv. Scout. Response of total biomass (a) and spike number (b) to eCO₂ and HS at three time points; before HS (B), after recovery from HS (R), and at the final harvest after maturity (M). Ambient and elevated CO₂-grown plants are depicted in black and grey, respectively. Solid and dotted lines represent control and heat-stressed plants, respectively. Filled and open circles represent control and heat-stressed plants, respectively. Vertical black dotted lines show the timing of HS. Symbols are means per plant ±SE (n=9–10).

Fig. 6

Response of plant total biomass and grain yield to elevated CO₂ and heat stress (HS) at the final harvest. Bar plot of means ±SE for total biomass (a), grain yield (b), grain yield of tillers (c), and grain yield of the main shoot (d) using two-way ANOVA measured in ambient (black) and elevated (grey) CO₂-grown plants exposed (HS) or not exposed (Control) to a 5 d HS. Bars sharing the same letter in the individual panels are not significantly different according to Tukey’s HSD test at the 5% level. Values are means ±SE (n=9–10). Statistical significance levels (t-test) for eCO₂ effect are shown: *P<0.05; **P<0.01: ***P<0.001.