Effect of elevated CO₂, temperature and drought on photosynthesis of nodulated alfalfa during a cutting regrowth cycle

Rising atmospheric CO 2 may increase potential net leaf photosynthesis under short‐term exposure, but this response decreases under long‐term exposure because plants acclimate to elevated CO 2 concentrations through a process known as downregulation. One of the main factors that may influence this phenomenon is the balance between sources and sinks in the plant. The usual method of managing a forage legume like alfalfa requires the cutting of shoots and subsequent regrowth, which alters the source/sink ratio and thus photosynthetic behaviour. The aim of this study was to determine the effect of CO 2 (ambient, around 350 vs. 700 µmol mol −1 ), temperature (ambient vs. ambient + 4° C) and water availability (well‐irrigated vs. partially irrigated) on photosynthetic behaviour in nodulated alfalfa before defoliation and after 1 month of regrowth. At the end of vegetative normal growth, plants grown under conditions of elevated CO 2 showed photosynthetic acclimation with lower photosynthetic rates, V cmax and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (rubisco) activity. This decay was probably a consequence of a specific rubisco protein reduction and/or inactivation. In contrast, high CO 2 during regrowth did not change net photosynthetic rates or yield differences in V cmax or rubisco total activity. This absence of photosynthetic acclimation was directly associated with the new source‐sink status of the plants during regrowth. After cutting, the higher root/shoot ratio in plants and remaining respiration can function as a strong sink for photosynthates, avoiding leaf sugar accumulation, the negative feed‐back control of photosynthesis, and as a consequence, photosynthetic downregulation.