It’s summer time once again in the Northern Hemisphere. And like every summer, expect the occasional heat wave to set off a fury of news stories hyping the claim that today’s heat waves are caused or made worse by anthropogenic global warming.
Of course there is no clear evidence to support such assertions (sorry, climate model projections are not evidence!). Yet, the warmer temperatures that have finally reached the cold part of the country in which I live got me thinking about my next post exploring the many biological benefits of rising atmospheric CO2. So read on if you want to learn about how higher levels of CO2 help alleviate many of the stresses plants experience during heat waves.
Heat stress can cause a multitude of challenges to plant growth and survival, including dehydration and oxidative damage to biomembranes from elevated reactive oxygen species (ROS). Yet elevated levels of CO2 have been shown to lessen the severity of this stress. Thus, there is interest in determining the interactive effects of heat stress and atmospheric CO2 on plants given predictions of future elevated temperatures and CO2 concentrations.
Pan et al. (2018), in particular, studied the interactive effects of elevated CO2 and heat stress on a range of photosynthetic and chlorophyll fluorescence parameters, as well the cellular redox state, of tomato plants. The work was conducted in environmentally controlled growth chambers, where tomato seedlings (Solanum lycopersicum cv. Hezuo 903) were exposed to CO2 concentrations of either 380 ppm or 800 ppm. Then, following an acclimation period of 48 hours, half of the seedlings in each CO2 treatment were subjected to 24 hours of heat stress (42°C, compared to unstressed day/night temperatures of 26/22°C in control plants), followed by a 24 hour recovery period. After 24 hours of heat stress and again after the 24 hour recovery period to control temperatures, a range of measurements were taken to evaluate the ability of elevated CO2 to mitigate temperature stress. The results are depicted in the two figures below.
Figure 1. Effects of elevated CO2 and heat stress on the net photosynthetic rate (Pn) of tomato plants. The red text shows the change in Pn due to elevated CO2 during the control, heat and recovery stage. Source: Pan et al. (2018).
Figure 1 presents the CO2-induced response of the tomato plants on net photosynthesis during the control, heat and recovery period. At normal temperatures, elevated CO2 increased net photosynthesis by 45%. Not surprisingly, regardless of CO2 concentration, heat stress reduced net photosynthesis, which parameter increased during the recovery period but not quite back to its pre-stressed condition. Nevertheless, elevated CO2 caused a relative increase in net photosynthesis of 116% and 96% during heat stress and recovery, which values were not significantly different than that observed in the control treatment under normal CO2. Thus, elevated CO2 was able to fully ameliorate the negative effects of heat stress on net photosynthesis of the tomato plants.
Figure 2. Effects of elevated CO2 and heat stress on the photosynthetic apparatus of tomato. The left panel displays the maximum photochemical efficiency of photosystem II (Fv/Fm) shown in pseudo color images, the false color code depicted in the image ranges from 0 (black) to 1 (purple). The right panel shows the actual Fv/Fm values, with the percentages in red text indicating the change in Fv/Fm values due to elevated CO2 during the heat and recovery period. Source: Pan et al. (2018).
A similar finding is noted in Figure 2, which presents the effects of elevated CO2 and heat stress on the maximum photochemical efficiency of photosystem II (Fv/Fm). Although elevated CO2 had no effect on Fv/Fm under normal temperature conditions, it increased this parameter by 60% and 14% in response to heat stress and at recovery, respectively, compared with that observed in the ambient CO2 treatments.
In commenting on these and other of their findings, Pan et al. noted “heat-induced excessive production of ROS caused damage to photosynthetic apparatus as evidenced by decreased Fv/Fm, low electron transport rate and altered oxidized and reduced states of PSII and PSI. On the other hand, elevated CO2 remarkably attenuated heat-induced damage to photosynthetic apparatus and promoted electron transport in PSII and PSI by maintaining proper redox balance.”
Turning to one other example of this incredible benefit, Chavan et al. (2019) examined the interactive effects of elevated CO2 and heat stress on the photosynthesis, biomass and grain yield of wheat. The CO2 concentrations examined in their study included ambient (419 ppm) and elevated (654 ppm). Temperatures were maintained at 22/15 °C (day/night) in the control treatment. Then, thirteen weeks after planting heat stress was enacted on half the plants in each CO2 treatment by raising the day/night temperatures to 40/24 °C for five days. Thereafter, the heat-stressed plants were returned to the control temperatures. Adequate water was supplied to all treatments throughout the experiment so as to avoid confounding effects of water stress.
Chavan et al. found elevated CO2 enhanced net photosynthesis by 36% in non-heat stressed plants, whereas high temperature stress reduced this parameter by 42%. In the combined elevated CO2 and heat stress treatment, net photosynthesis was not reduced because, in the words of the authors, “elevated CO2 protected photosynthesis by increasing ribulose biphosphate regeneration capacity and reducing photochemical damage [caused by] heat stress.”
Figure 3. Total biomass (a) and grain yield (b) of wheat plants at harvest in response to elevated CO2 and heat stress (HS). Values represent means ± SE using two-way ANOVA. Means 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 as follows: ** P < 0.01: *** P < 0.001. The percentages in red text indicate the change in biomass or grain yield due to elevated CO2 under control or heat stress conditions. Source: Chavan et al. (2019).
With respect to biomass and yield, as shown in Figure 3, elevated CO2 stimulated these two parameters by 36% and 31%, respectively, in the control treatment. Heat stress alone, in contrast, induced a small non-significant reduction in total biomass and a 44% reduction in grain yield. When elevated CO2 and heat stress were combined, total biomass increased by 46% over the control treatment (ambient CO2 and non-heat stress) and by 58% relative to the heat stress treatment under ambient CO2. Grain yield, on the other hand, experienced a 23% decline in the combined elevated CO2 and heat stress treatment relative to control conditions, but a positive 37% increase relative to heat stress alone at ambient CO2. Thus, in the future, elevated CO2 may well be able to ameliorate a large portion of the negative impact of high temperature stress on grain yield for the particular wheat variety examined in this study.
Commenting on the important benefits of CO2 observed in their study, Chavan et al. state “heat stress caused irreversible photosynthetic damage at ambient CO2, while growth at elevated CO2 mitigated the negative impact of heat stress on photosynthesis.” Additionally, they found “plant biomass completely recovered from heat stress under both CO2 treatments due to the development of additional late tillers and ears; yet these did not fully develop and fill grains,” which explains the drop in grain yield observed under heat stress. Consequently, they advocate for more research and breeding programs designed to improve grain filling and translocation of plant resources to the grain at high temperatures and elevated CO2 to protect future food production.
Many more studies have investigated the interactive effects of elevated CO2 and heat stress (see, for example, a list of articles reviewed on my CO2 Science website on this topic under the headings Temperature x CO2 Interaction (Plant Growth Response: Agricultural Crops), Temperature x CO2 Interaction (Plant Growth Response: Grassland Species), and Temperature x CO2 Interaction (Plant Growth Response: Trees)). In nearly every instance these studies demonstrate the air’s rising CO2 content is helping plants better cope with and endure high temperature stresses. And these benefits are being realized now, courtesy of the approximate 50% increase in atmospheric CO2 that the world has experienced since the Industrial Revolution began, which benefits will only continue to accrue in the future as the air’s CO2 content continues to rise.
So when the next summer heat wave arrives along with all the negative spin stories demonizing CO2 as its cause, I hope you will remember this post and the numerous scientific studies proving rising CO2 levels helps plants better withstand and recover from temperature-induced stresses. And when you do remember this, please share it with others!
Dr. Craig D. Idso
Chavan, S.G., Duursma, R.A., Tausz, M. and Channoum, O. 2019. Elevated CO2 alleviates the negative impact of heat stress on wheat physiology but not on grain yield. Journal of Experimental Botany 70: 6447-6459.
Pan, C., Ahammed, G.J., Li, X. and Shi, K. 2018. Elevated CO2 improves photosynthesis under high temperature by attenuating the functional limitations to energy fluxes, electron transport and redox homeostasis in tomato leaves. Frontiers in Plant Science 9: Article 1739, doi: 10.3389/fpls.2018.01739.
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July 20, 2022 at 06:02AM