Nitrogen limitation and source alter the effects of low CO2 on plant performance

Background/Question/Methods

Low atmospheric CO₂ concentrations dominated the recent evolutionary history of plants. However, our knowledge of plant performance in low CO₂ environments is poor, and that of how low CO₂ interacts with nitrogen source and availability is even poorer. Here, we investigated growth, photosynthetic traits and nitrogen isotopic signature in Elymus canadensis, grown from seed at either current CO₂concentrations (400 ppm) or values representative of the last glacial maximum (180-190 ppm), in combination with either high or low nitrogen supply, with the high nitrogen treatment supplied as nitrate, ammonium, or a mix of both.

Results/Conclusions

Low CO₂ reduced total biomass by almost half, except when nitrogen was limited, where growth was unaffected by CO₂. Plants grown with nitrate, ammonium, or at low nitrogen up-regulated photosynthesis at low CO₂, presenting higher maximum electron transport rates (J_max), but not maximum Rubisco carboxylation rates (V_cmax), than plants grown at ambient CO₂. Growth at low CO₂ also increased leaf nitrogen concentrations. Compared to plants from ambient CO₂, low CO₂ plants were enriched in ¹⁵N (1.2‰) under nitrate fertilization, but 0.8‰ depleted under low nitrogen availability. Our work demonstrates that the effects of low CO₂ on plants varies depending on nitrogen source and availability; such responses should be considered when inferring data about plant responses to past climates, especially when carbon resources were most limiting.