The response of photosynthesis to future projected warming will dramatically influence the rate and magnitude of climate change. In the short term (seconds to minutes), photosynthetic enzymes are stimulated by warming, which increases photosynthetic rates. However, concurrent increases in leaf respiration and decreases in stomatal conductance at high temperatures can reduce photosynthesis. The shape of this short-term response can shift over longer time periods (days to years) as the result of acclimation and adaptation. These longer-term processes are important for understanding future biosphere-atmosphere feedbacks, but are difficult to reliably predict. Previously developed empirical models have improved model realism, but come at a cost of additional parameter estimation. Here, we developed a theoretical framework for understanding and predicting C3 photosynthetic temperature acclimation. Our theory is based on the hypothesis that, optimally, plants will strive to achieve a photosynthetic rate at the lowest possible cost, which is associated with investments in photosynthetic capacity and water transport, and coordinate photosynthetic investment such that photosynthesis is neither limited by Rubisco carboxylation nor electron transport. We present the theory, test it against observations, and examine its predictions for future global photosynthesis.
Results/Conclusions
Using the least cost hypothesis and the concept of photosynthetic coordination, we developed a theory that predicts photosynthetic acclimation to the environment, including temperature. In response to long-term warming, our theory predicts an increase in photosynthetic and respiratory capacity, but to a lesser degree than would be expected from their short-term temperature responses alone. Our theory also predicts greater photosynthetic investment in Rubisco carboxylation relative to electron transport with warming. Finally, the theory predicts an increase in leaf intracellular to atmospheric CO2 ratio with warming as a result of both increased stomatal conductance and a lower Rubisco affinity to CO2. Each of these responses is consistent with empirical data from climate gradient and direct temperature manipulation studies. In fact, the theory alone can account for >90% of the variability in the acclimation responses from 22 species grown under various warming conditions (15-35°C). Combined, our theory suggests that net photosynthesis should increase with long-term warming up to ~32°C, beyond which respiratory carbon loss limits this increase. As a result, our theory predicts an increase in photosynthesis across the globe in response to future warming alone, suggesting that future increases in temperature will not inhibit, and may instead magnify, plant carbon uptake.