Temperature constraints on rates of symbiotic nitrogen fixation (SNF) have been hypothesized as one of the primary determinants of low woody N fixer abundance at high latitudes. The temperature hypothesis posits that because SNF is an enzymatic process, optimal rates of SNF cannot be achieved in colder climates. This, coupled with the carbon cost of SNF, may act as a filter against N fixers. Here we evaluate the temperature hypothesis and the role that adaptation and acclimation play on SNF and CO2 exchange rates at different temperatures. Actinorhizal (Alnus rubra and Morella cerifera) and rhizobial (Robinia pseudoacacia and Gliricidia sepium) plants from high and low latitudes were inoculated with field collected nodules and grown under high (31:25°C day:night) or low (21:15°C day:night) temperatures. Adaptation was evaluated by comparing SNF and CO2 exchange responses to temperature in plants from high vs. low latitudes. Acclimation was evaluated by making intra-species comparisons on plants grown at different temperatures. To build temperature response curves of SNF and CO2 exchange, we developed a novel system that measures SNF rates non-destructively, continuously and in real-time on N fixer seedlings (Acetylene Reduction Assay by Cavity ring-down laser Absorption Spectroscopy) which we coupled to a CO2/H2O analyzer.
Results/Conclusions
Optimal rates of SNF ranged from 26 to 37°C and, due to high whole-plant respiration rates at high temperatures, were generally higher than optimal temperatures for net plant CO2 exchange. However, optimal rates of photosynthesis were at similar temperatures to SNF rates. Signs of adaptation and acclimation to temperature were apparent in both actinorhizal and rhizobial N fixers, sometimes with a shift in optimal temperature and even more so at low and high temperatures (i.e. 15 and 40°C). For example, at 15°C, M. cerifera sustained only 2% of its maximum SNF rate, while A. rubra sustained 35% of its maximum at the same temperature, suggesting a role of adaptation. Examples of acclimation included a 7°C shift in the optimum SNF temperature for R. pseudoacacia due to growing temperature. Our results suggest that optimal SNF rates in woody plants may be higher than reported for other N fixing symbioses (generally 25-26°C) and to our knowledge are the first such measurements in woody N fixers. This has implications for estimating SNF inputs in earth system models and in response to climate change. Furthermore, our results suggest that the roles of adaptation and acclimation to temperature on SNF rates cannot be ignored.