Abstract
Elevated atmospheric CO2 improves leaf photosynthesis and plant tolerance to heat stress, however, the underlying mechanisms remain unclear. In this study, we exposed tomato plants to elevated CO2 (800 μmol mol-1) and/or high temperature (42°C for 24 h), and examined a range of photosynthetic and chlorophyll fluorescence parameters as well as cellular redox state to better understand the response of photosystem II (PSII) and PSI to elevated CO2 and heat stress. The results showed that, while the heat stress drastically decreased the net photosynthetic rate (Pn), maximum carboxylation rate (Vcmax), maximum ribulose-1,5-bis-phosphate (RuBP) regeneration rate (Jmax) and maximal photochemical efficiency of PSII (Fv/Fm), the elevated CO2 improved those parameters under heat stress and at a 24 h recovery. Furthermore, the heat stress decreased the absorption flux, trapped energy flux, electron transport, energy dissipation per PSII cross section, while the elevated CO2 had the opposing effects that eventually decreased photoinhibition, damage to photosystems and reactive oxygen species accumulation. Similarly, the elevated CO2 helped the plants to maintain a reduced redox state as evidenced by the increased ratios of ASA:DHA and GSH:GSSG under heat stress and at recovery. Furthermore, the concentration of NADP+ and ratio of NADP+ to NADPH were induced by elevated CO2 at recovery. This study unraveled the crucial mechanisms of elevated CO2-mediated changes in energy fluxes, electron transport and redox homeostasis under heat stress, and shed new light on the responses of tomato plants to combined heat and elevated CO2.
Highlights
Since the initial era of plant establishment in the terrestrial ecosystem, photosynthesis has been serving as a key process for sustaining any life forms on the earth (Cousins et al, 2014; Brestic et al, 2018)
The results of this study suggest that elevated CO2 could alleviate the heat stress-induced functional limitations to photosynthesis in tomato leaves
The results showed that heat stress drastically decreased the photosynthetic rate (Pn) by 57% and it was not fully restored to the control level at recovery (Figure 1)
Summary
Since the initial era of plant establishment in the terrestrial ecosystem, photosynthesis has been serving as a key process for sustaining any life forms on the earth (Cousins et al, 2014; Brestic et al, 2018). Carbon dioxide (CO2) is the basic input for the photosynthesis in green plants; excess or low CO2 has diverse effects on plant growth and productivity (Amthor, 1995; Sage and Coleman, 2001). Over the last couple of centuries, the concentration of atmospheric CO2 has increased tremendously. Due to sessile life style, plants have to endure unfavorable weather events such as high temperature, cold, drought, flood and salinity (Ahuja et al, 2010; Ohama et al, 2017). Elevated atmospheric CO2 concentrations can improve plant growth and productivity, and enhance plant tolerance to a range of abiotic stresses including high temperature and drought (AbdElgawad et al, 2015; Zinta et al, 2018). The mechanisms of plant responses to combined heat and elevated CO2 remain poorly understood (Cassia et al, 2018; Zhang et al, 2018; Zinta et al, 2018)
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