2024-B1 Quantifying the net effect of photorespiration on plant carbon uptake, under ambient and elevated atmospheric CO2 levels

Overview

Photorespiration is an essential metabolic pathway that occurs in all organisms that undertake photosynthesis (Eisenhut et al., 2019). It is thought that photorespiration lowers the efficiency of C3 photosynthesis, the dominant photosynthetic pathway of most plants in the UK, by ~25% through the release of previously fixed carbon as CO2 (Timm and Hagemann, 2020). Photorespiration can negatively impact carbon uptake under high temperatures and low CO2 concentrations, but it can also have a positive impact under contrasting conditions (Busch, 2020). It is therefore not immediately obvious what the net impact of photorespiration is on the carbon gain of plants. The magnitude of this effect is currently highly debated: on one hand, the impact of photorespiration is considered large, due to its ‘wasteful’ consumption of ATP and NAD(P)H, and release of CO2 and ammonia that requires refixing inside the plant (Eisenhut et al., 2019). An increase in atmospheric CO2 concentration would thus result in a large CO2 fertilisation effect. On the other hand, Busch (2020) showed that CO2 released from photorespiration does not actually account for large losses in net CO2 uptake, as CO2 diffusion resistances moderate the effect of the photorespired CO2 as some of this CO2 is refixed. This, in turn, would limit the CO2 fertilisation effect under future climates. It is therefore important to accurately quantify the impact of photorespiration on net photosynthesis under current and future CO2 concentrations to capture both the basic physiology and the capacity of plants to acclimate to new environmental conditions.  

To date, the net effect of photorespiration on carbon uptake has not been quantified across the environmental conditions a leaf operates under. By integrating over conditions when photorespiration is beneficial and when it is detrimental, this research will determine if overall photorespiration has a positive or negative effect on carbon uptake. Ultimately, it will improve our understanding of photosynthetic efficiency under elevated and current atmospheric CO2 concentrations. The research will be conducted in controlled glasshouse conditions to improve our mechanistic understanding of photorespiration and at the BIFoR-FACE facility to quantify the real-life impact of photorespiration on plants in the natural environment. 

Figure 1: Aerial view of the BIFoR-FACE facility. The experimental plots are surrounded by towers emitting CO2 to expose the plants inside the plots to atmospheric CO2 concentrations expected in about 50 years. This experiment simulates a climate change scenario to study the response of a forest ecosystem to future CO2 concentrations (https://www.birmingham.ac.uk/research/bifor/face/index.aspx).