Photosynthesis of C3 C3–C4 and C4 grasses at glacial CO2

摘要

Most physiology comparisons of C3 and C4 plants are made under current or elevated concentrations of atmospheric CO2 which do not reflect the low CO2 environment under which C4 photosynthesis has evolved. Accordingly, photosynthetic nitrogen (PNUE) and water (PWUE) use efficiency, and the activity of the photosynthetic carboxylases [Rubisco and phosphoenolpyruvate carboxylase (PEPC)] and decarboxylases [NADP-malic enzyme (NADP-ME) and phosphoenolpyruvate carboxykinase (PEP-CK)] were compared in eight C4 grasses with NAD-ME, PCK, and NADP-ME subtypes, one C3 grass, and one C3–C4 grass grown under ambient (400 μl l^–1) and glacial (180 μl l^–1) CO2. Glacial CO2 caused a smaller reduction of photosynthesis and a greater increase of stomatal conductance in C4 relative to C3 and C3–C4 species. Panicum bisulcatum (C3) acclimated to glacial [CO₂] by doubling Rubisco activity, while Rubisco was unchanged in Panicum milioides (C₃–C₄), possibly due to its high leaf N and Rubisco contents. Glacial CO₂ up-regulated Rubisco and PEPC activities in concert for several C₄ grasses, while NADP-ME and PEP-CK activities were unchanged, reflecting the high control exerted by the carboxylases relative to the decarboxylases on the efficiency of C₄ metabolism. Despite having larger stomatal conductance at glacial CO₂, C₄ species maintained greater PWUE and PNUE relative to C₃–C₄ and C₃ species due to higher photosynthetic rates. Relative to other C₄ subtypes, NAD-ME and PEP-CK grasses had the highest PWUE and PNUE, respectively; relative to C₃, the C₃–C₄ grass had higher PWUE and similar PNUE at glacial CO₂. Biomass accumulation was reduced by glacial CO₂ in the C₃ grass relative to the C₃–C₄ grass, while biomass was less reduced in NAD-ME grasses compared with NADP-ME and PCK grasses. Under glacial CO₂, high resource use efficiency offers a key evolutionary advantage for the transition from C₃ to C₄ photosynthesis in water- and nutrient-limited environments.