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

Abstract

Rising atmospheric CO2 may increase potential net leaf photosynthesis under short‐term exposure, but this response decreases under long‐term exposure because plants acclimate to elevated CO2 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 CO2 (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 CO2 showed photosynthetic acclimation with lower photosynthetic rates, Vcmax 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 CO2 during regrowth did not change net photosynthetic rates or yield differences in Vcmax 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.