Photosynthesis and protein metabolism associated with elevated CO2-mitigation of heat stress damages in tall fescue

Heat stress is a primary factor limiting the growth of cool-season (C3) perennial grass species during summer months. Elevated CO2 may alleviate heat stress damage in C3 plants. The objective of this study was to investigate mechanisms underlying elevated CO2-mitigation of adverse effects due to heat stress in C3 perennial grass species by examining effects of elevated CO2 on major photosynthetic components and proteins for tall fescue (Festuca arundinacea) subjected to heat stress. Plants of tall fescue (cv. 'Rembrandt') were grown under ambient CO2 (400 μmol mol−1) or elevated CO2 (800 μmol mol−1) and subjected to ambient temperature (25/20 °C day/night) or heat stress (35/30 °C day/night). Elevated CO2 enhanced photosynthetic rate under both ambient temperature and heat stress in tall fescue. The improved photosynthesis under elevated CO2 was associated with the increase in the abundance of proteins involved in photosynthetic light reactions (chlorophyll a-b binding protein), electron transport carrier molecule (ferredoxin), and ATP generation enzyme (adolase), as well as higher carbon assimilation efficiency and carboxylation enzyme activities of the Calvin cycle [higher carbon:nitrogen ratio (C:N), maximal rate of photosynthetic electron transport (Jmax), Rubisco activity and Rubisco activation]. Elevated CO2 also induced the accumulation of proteins involved in antioxidant metabolism (ascorbate peroxidase and 2-Cys peroxiredoxin). Elevated CO2 induced stomatal closure and chlorophyll content decline under both ambient temperature and heat stress, which could have limited the positive effects of elevated CO2 on the photosynthetic capacity. It would be useful to select cultivars of C3 perennial grass species with decreased stomatal sensitivity to elevated CO2 to achieve maximal benefits of elevated CO2 on photosynthesis and whole-plant growth. Our results suggested that the increased photosynthetic efficiency and activities, as well as protein abundance involved in photosynthesis and antioxidant metabolism could play important roles in elevated CO2-mitigation of heat stress damage in C3 perennial grass species.